Bias circuit requirements.

The bias circuit stabilizes the operating point of the transistor for variations in transistor characteristics and operating temperature. The gain of a transistor can vary significantly between different batches, which results in widely different operating points for sequential units in serial production or after replacement of a transistor. By the Early effect, the current gain is affected by the collector–emitter voltage. Both gain and base–emitter voltage depend on the temperature. The leakage current also increases with temperature. A bias network is selected to reduce effects of device variability, temperature, and voltage changes.  A bias circuit may be composed of only resistors, or may include elements such as temperature-dependent resistors, diodes, or additional voltage sources, depending on the range of operating conditions expected.

Signal requirements for class-A amplifiers.

For analog operation of a class-A amplifier, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) when input is applied. For digital operation, the Q-point is placed so the transistor does the contrary – switches from the "on" (saturation) to the "off" (cutoff) state. Often, the Q-point is established near the center of the active region of a transistor characteristic to allow similar signal swings in positive and negative directions.

Thermal considerations.

At constant current, the voltage across the emitter–base junction "V" of a bipolar transistor "decreases" by 2 mV (silicon) and 1.8 mV (germanium) for each 1 °C rise in temperature (reference being 25 °C). By the Ebers–Moll model, if the base–emitter voltage "V" is held constant and the temperature rises, the current through the base–emitter diode "I" will increase, and thus the collector current "I" will also increase. Depending on the bias point, the power dissipated in the transistor may also increase, which will further increase its temperature and exacerbate the problem. This deleterious positive feedback results in thermal runaway. There are several approaches to mitigate bipolar transistor thermal runaway. For example, The circuits below primarily demonstrate the use of negative feedback to prevent thermal runaway.

Types of bias circuit for class-A amplifiers.

The following discussion treats five common biasing circuits used with class-A bipolar transistor amplifiers:

Fixed bias (base bias).

This form of biasing is also called "base bias or fixed resistance biasing". In the example image on the right, the single power source (for example, a battery) is used for both collector and base of a transistor, although separate batteries can also be used.  In the given circuit,  Therefore,  For a given transistor, V does not vary significantly during use. As V is of fixed value, on selection of R, the base current I is fixed. Therefore, this type is called "fixed bias" type of circuit.  Also, for the given circuit,  Therefore,  The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β on this page.  Because  we can obtain I as well. In this manner, operating point given as (V,I) can be set for given transistor.  Advantages:  Disadvantages:  Usage:  Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits (i.e., those circuits which use the transistor as a current source). Instead, it is often used in circuits where the transistor is used as a switch. However, one application of fixed bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a DC signal derived from the AC output of a later stage.

Collector feedback bias.

This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. In this form of biasing, the base resistor formula_1 is connected to the collector instead of connecting it to the DC source formula_2. So any thermal runaway will induce a voltage drop across the formula_3 resistor that will throttle the transistor's base current.  From Kirchhoff's voltage law, the voltage formula_4 across the base resistor formula_1 is  By the Ebers–Moll model, formula_7, and so  From Ohm's law, the base current formula_9, and so  Hence, the base current formula_11 is  If formula_13 is held constant and temperature increases, then the collector current formula_14 increases. However, a larger formula_14 causes the voltage drop across resistor formula_16 to increase, which in turn reduces the voltage formula_4 across the base resistor formula_1. A lower base-resistor voltage drop reduces the base current formula_11, which results in less collector current formula_14. Because an increase in collector current with temperature is opposed, the operating point is kept stable.  Advantages:  Disadvantages:  which is the case when  Usage: The negative feedback also increases the input impedance of the amplifier as seen from the base, which can be advantageous. Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted.

Fixed bias with emitter resistor.

The fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point. From Kirchhoff's voltage law, the voltage across the base resistor is  From Ohm's law, the base current is  The way feedback controls the bias point is as follows. If V is held constant and temperature increases, emitter current increases. However, a larger I increases the emitter voltage V = IR, which in turn reduces the voltage V across the base resistor. A lower base-resistor voltage drop reduces the base current, which results in less collector current because I = β I. Collector current and emitter current are related by I = α I with α ≈ 1, so the increase in emitter current with temperature is opposed, and the operating point is kept stable.  Similarly, if the transistor is replaced by another, there may be a change in I (corresponding to change in β-value, for example). By similar process as above, the change is negated and operating point kept stable.  For the given circuit,  Advantages:  The circuit has the tendency to stabilize operating point against changes in temperature and β-value.  Disadvantages:  which is approximately the case if  Usage:  The feedback also increases the input impedance of the amplifier when seen from the base, which can be advantageous. Due to the above disadvantages, this type of biasing circuit is used only with careful consideration of the trade-offs involved.  Collector-Stabilized Biasing.

Voltage divider biasing or emitter bias.

"The voltage divider is formed using external resistors" R and R. The voltage across R forward biases the emitter junction. By proper selection of resistors R and R, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current, provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. By manipulating the resistors in certain ways you can achieve more stable current levels without having β value affect it too much.  In this circuit the base voltage is given by:  formula_40 voltage across formula_41 formula_42  Also formula_45  For the given circuit,  Advantages:  Disadvantages:  which is approximately the case if  where R || R denotes the equivalent resistance of R and R connected in parallel.  Usage:  The circuit's stability and merits as above make it widely used for linear circuits.

Voltage divider with AC bypass capacitor.

The standard voltage divider circuit discussed above faces a drawback – AC feedback caused by resistor R reduces the gain. This can be avoided by placing a capacitor (C) in parallel with R, as shown in circuit diagram. The result is that the DC operating point is well controlled, while the AC-gain is much higher (approaching β), rather than the much lower (but predictable) value of formula_49 without the capacitor.

Emitter bias.

When a split supply (dual power supply) is available, this biasing circuit is the most effective, and provides zero bias voltage at the emitter or collector for load. The negative supply V is used to forward-bias the emitter junction through R. The positive supply V is used to reverse-bias the collector junction. Only two resistors are necessary for the common collector stage and four resistors for the common emitter or common base stage.  We know that,  V − V = V  If R is small enough, base voltage will be approximately zero. Therefore, emitter current is,  Ie = (V − V)/R  The operating point is independent of β if Re » R/β  Advantages:  Disadvantages:  This type can only be used when a split (dual) power supply is available.

Class-B and AB amplifiers.

Signal requirements.

Class B and AB amplifiers employ 2 active devices to cover the complete 360 deg of input signal flow. Each transistor is therefore biased to perform over approximately 180 deg of the input signal. Class B bias is when the collector current I with no signal is just conducting (about 1% of maximum possible value). Class-AB bias is when the collector current I is about 1/4 of maximum possible value. The class-AB push–pull output amplifier circuit below could be the basis for a moderate-power audio amplifier. <br> A practical amplifier circuit  Q3 is a common emitter stage that provides amplification of the signal and the DC bias current through D1 and D2 to generate a bias voltage for the output devices. The output pair are arranged in class-AB push–pull, also called a complementary pair. The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimized. That is, the diodes push the output stage into class-AB mode (assuming that the base-emitter drop of the output transistors is reduced by heat dissipation).  This design automatically stabilizes its operating point, since overall feedback internally operates from DC up through the audio range and beyond. The use of fixed diode bias requires the diodes to be both electrically and thermally matched to the output transistors. If the output transistors conduct too much, they can easily overheat and destroy themselves, as the full current from the power supply is not limited at this stage.  A common solution to help stabilize the output device operating point is to include some emitter resistors, typically an ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amplifier.

Bipolar transistors must be properly biased to operate correctly. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. By selecting the proper resistor values, stable current levels can be achieved that vary only little over temperature and with transistor properties such as β.

Die Grundschaltungen einer Verstärkerstufe sind nach der Elektrode benannt, welche auf einem fest definierten elektrischen Potential liegt. Das ist jene Elektrode, die Eingangs- und Ausgangskreis gemein ist. Im Falle eines Bipolartransistors mit seinen drei Elektroden "Emitter, Kollektor" und "Basis" ergeben sich so die Emitterschaltung, die Kollektorschaltung und die Basisschaltung. Aufgrund ihrer Eigenschaften wird die Kollektorschaltung meistens Emitterfolger genannt. Die Transistor-Grundschaltungen unterscheiden sich prinzipiell in ihren elektrischen Eigenschaften und daher im Verwendungszweck.


Payment for unfree labour.

If payment occurs, it may be in one or more of the following forms:  Unfree labour is often more easily instituted and enforced on migrant workers, who have traveled far from their homelands and who are easily identified because of their physical, ethnic, linguistic, or cultural differences from the general population, since they are unable or unlikely to report their conditions to the authorities.  According to the Marxian economics, under capitalism, workers never keep all of the wealth they create, as some of it goes to the profit of capitalists. By contrast with modern subjective theory of value (as used by neoclassical economists), the wages offered necessarily represent the marginal utility of the labour, and any profit (or loss) is also due to other inputs provided, such as capital, time value of money, or risk.

Industrial involvement.

In many contexts, the use of unfree labour is prohibited under the law and is mainly associated with the black economy. In other contexts, established industries have embraced the use of unfree labour as a socially accepted practice in that time and place. Use of compelled labour is especially common when the labour involved can not be performed without risk of death, disfigurement, disability, or diminished life expectancy; in the extreme, these detriments render the voluntary labour market uneconomic, and the industry in question is forced to either adopt compelled labour or discontinue operations altogether.  Industries which continue to employ unfree labour worldwide include agriculture, domestic work, manufacture, and hospitality. Mining, defense, the merchant marine and transportation infrastructure, which employed questionable practices during the heydays of railway track construction (often involving the use of high explosives or constructing high wooden trestle bridges in sheer mountain canyons), and of canal excavation (sometimes in conditions of permafrost) also have historical ties.

The present situation.

Unfree labor re-emerged as an issue in the debate about rural development during the years following the end of the Second World War, when a political concern of Keynesian theory was not just economic reconstruction (mainly in Europe and Asia) but also planning (in developing "Third World" nations). A crucial aspect of the ensuing discussion concerned the extent to which different relational forms constituted obstacles to capitalist development, and why.  During the 1960s and 1970s unfree labor was regarded as incompatible with capitalist accumulation, and thus an obstacle to economic growth, an interpretation advanced by exponents of the then-dominant semi-feudal thesis. From the 1980s onwards, however, another and very different Marxist view emerged, arguing that evidence from Latin America and India suggested agribusiness enterprises, commercial farmers and rich peasants reproduced, introduced or reintroduced unfree relations.  However, recent contributions to this debate have attempted to exclude Marxism from the discussion. These contributions maintain that, because Marxist theory failed to understand the centrality of unfreedom to modern capitalism, a new explanation of this link is needed. This claim has been questioned by Tom Brass (2014), ‘Debating Capitalist Dynamics and Unfree Labour: A Missing Link?’, The Journal of Development Studies, 50:4, 570–82. He argues that many of these new characteristics are in fact no different from those identified earlier by Marxist theory and that the exclusion of the latter approach from the debate is thus unwarranted.  The International Labour Organization (ILO) estimates that at least 12.3 million people are victims of forced labour worldwide; of these, 9.8 million are exploited by private agents and more than 2.4 million are trafficked. Another 2.5 million are forced to work by the state or by rebel military groups. From an international law perspective, countries that allow forced labor are violating international labour standards as set forth in the Abolition of Forced Labour Convention (C105), one of the fundamental conventions of the ILO.  According to the "ILO Special Action Programme to Combat Forced Labour" (SAP-FL), global profits from forced trafficked labour exploited by private agents are estimated at US$44,3 billion per year. About 70% of this value (US$31.6 billion) come from trafficked victims. At least the half of this sum (more than US$15 billion) comes from industrialized countries.

Trafficking.

Trafficking is a term to define the recruiting, harbouring, obtaining and transportation of a person by use of force, fraud, or coercion for the purpose of subjecting them to involuntary acts, such as acts related to commercial sexual exploitation (including forced prostitution) or involuntary labour.

Forms of Unfree Labour.

Slavery.

The archetypal and best-known form of unfree labour is chattel slavery, in which individual workers are "legally" owned throughout their lives, and may be bought, sold or otherwise exchanged by owners, while never or rarely receiving any personal benefit from their labour. Slavery was common in many ancient societies, including ancient Egypt, Babylon, Persia, ancient Greece, Rome, ancient Israel, ancient China, classical Arab states, as well as many societies in Africa and the Americas. Being sold into slavery was a common fate of populations that were conquered in wars. Perhaps the most prominent example of chattel slavery was the enslavement of many millions of black people in Africa, as well as their forced transportation to the Americas, Asia, or Europe, where their status as slaves was almost always inherited by their descendants.  The term "slavery" is often applied to situations which do not meet the above definitions, but which are other, closely related forms of unfree labour, such as debt slavery or debt-bondage (although not all repayment of debts through labour constitutes unfree labour). Examples are the Repartimiento system in the Spanish Empire, or the work of Indigenous Australians in northern Australia on sheep or cattle "stations" (ranches), from the mid-19th to the mid-20th century. In the latter case, workers were rarely or never paid, and were restricted by regulations and/or police intervention to regions around their places of work.  In late 16th century Japan, "unfree labour" or slavery was officially banned; but forms of contract and indentured labour persisted alongside the period's penal codes' forced labour. Somewhat later, the Edo period's penal laws prescribed "non-free labour" for the immediate families of executed criminals in Article 17 of the "Gotōke reijō" (Tokugawa House Laws), but the practice never became common. The 1711 "Gotōke reijō" was compiled from over 600 statutes that were promulgated between 1597 and 1696.  According to Kevin Bales, in "" (1999), there are now an estimated 27 million slaves in the world.

Truck system.

A truck system, in the specific sense in which the term is used by labour historians, refers to an unpopular or even exploitative form of payment associated with small, isolated and/or rural communities, in which workers or self-employed small producers are paid in either: goods, a form of payment known as truck wages, or tokens, private currency ("scrip") or direct credit, to be used at a company store, owned by their employers. A specific kind of truck system, in which credit advances are made against future work, is known in the U.S. as debt bondage.  Many scholars have suggested that employers use such systems to exploit workers and/or indebt them. This could occur, for example, if employers were able to pay workers with goods which had a market value below the level of subsistence, or by selling items to workers at inflated prices. Others argue that truck wages, at least in some cases, were a convenient way for isolated communities to operate, when official currency was scarce.  By the early 20th century, truck systems were widely seen, in industrialised countries, as exploitative; perhaps the most well-known example of this view was a 1947 U.S. hit song "Sixteen Tons". Many countries have Truck Act legislation that outlaws truck systems and requires payment in cash.

Mandatory services due to social status.

Corvée.

Though most closely associated with Medieval Europe, governments throughout human history have imposed regular short stints of unpaid labor upon lower social classes. These might be annual obligations of a few weeks or something similarly regular that lasted for the laborer's entire working life. As the system developed in the Philippines and elsewhere, the laborer could pay an appropriate fee and be exempted from the obligation.

Vetti-chakiri.

In Vetti-chakiri and begar lower castes have only had obligations or duties to render free services to the upper caste community also called as Vetti or Vetti chakiri.

Penal labour.

Labour camps.

Another historically significant example of forced labour was that of political prisoners, people from conquered or occupied countries, members of persecuted minorities, and prisoners of war, especially during the 20th century. The best-known example of this are the concentration camp system run by Nazi Germany in Europe during World War II, the "Gulag" camps run by the Soviet Union, and the forced labour used by the military of the Empire of Japan, especially during the Pacific War (such as the Burma Railway). Roughly 4,000,000 German POWs were used as "reparations labour" by the Allies for several years after the German surrender; this was permitted under the Third Geneva Convention provided they were accorded proper treatment. China's "Laogai" ("labour reform") system and North Korea's Kwalliso camps are current examples.  About 12 million forced labourers, most of whom were Poles and Soviet citizens "(Ost-Arbeiter)", were employed in the German war economy inside Nazi Germany. More than 2000 German companies profited from slave labour during the Nazi era, including Daimler, Deutsche Bank, Siemens, Volkswagen, Hoechst, Dresdner Bank, Krupp, Allianz, BASF, Bayer, BMW, and Degussa.  In Asia, according to a joint study of historians featuring Zhifen Ju, Mark Peattie, Toru Kubo, and Mitsuyoshi Himeta, more than 10 million Chinese were mobilized by the Japanese army and enslaved by the Kōa-in for slave labour in Manchukuo and north China. The U.S. Library of Congress estimates that in Java, between 4 and 10 million "romusha" (Japanese: "manual laborer") were forced to work by the Japanese military. About 270,000 of these Javanese laborers were sent to other Japanese-held areas in South East Asia. Only 52,000 were repatriated to Java, meaning that there was a death rate of 80%. , was the term for forced labor in Indonesia during Dutch colonisation.  The Khmer Rouge attempted to turn Cambodia into a classless society by depopulating cities and forcing the urban population ("New People") into agricultural communes. The entire population was forced to become farmers in labour camps.

Prison labour.

Convict or prison labour is another classic form of unfree labour. The forced labour of convicts has often been regarded with lack of sympathy, because of the social stigma attached to people regarded as "common criminals". In some countries and historical periods, however, prison labour has been forced upon people who have been victims of prejudice, convicted of political crimes, convicted of "victimless crimes", or people who committed theft or related offences because they lacked any other means of subsistence—categories of people who typically are entitled to compassion according to current ethical ideas.  Three British colonies in Australia—New South Wales, Van Diemen's Land (Tasmania) and Western Australia—are three examples of the state use of convict labour. Australia received thousands of convict labourers in the eighteenth and nineteenth centuries who were given sentences for crimes ranging from those now considered to be minor misdemeanors to such serious offences as murder, rape and incest. A considerable number of Irish convicts were sentenced to transportation for 'treason' while fighting for Irish independence from British rule.  More than 165,000 convicts were transported to Australian colonies from 1788 to 1868. Most British or Irish convicts who were sentenced to transportation, however, completed their sentences in British jails and were not transported at all.  It is estimated that in the last 50 years more than 50 million people have been sent to Chinese "laogai" camps.

Indentured and bonded labour.

A more common form in modern society is indenture, or "bonded labour", under which workers sign contracts to work for a specific period of time, for which they are paid only with accommodation and sustenance, or these essentials in addition to limited benefits such as cancellation of a debt, or transportation to a desired country.

Permitted exceptions of unfree labour.

As mentioned above, there are several exceptions of unfree or forced labour recognized by the "International Labour Organization":

Civil Conscription.

Some countries practice forms of civil conscription for different major occupational groups or inhabitants under different denominations like "civil conscription", "civil mobilization", "political mobilisation" etc. This obligatory services on the one hand has been implemented due to long-lasting labour strikes, during wartimes or economic crisis, to provide basic services like medical care, food supply or supply of the defense industry. On the other hand this service can be obligatory to provide recurring and inevitable services to the population, like fire services, due to lack of volunteers.

Temporary civil conscription.

Between December 1943 and March 1948 young men in the United Kingdom, the so-called Bevin Boys, had been conscripted for the work in coal mines. In Belgium in 1964, in Portugal and in Greece from 2010 to 2014 due to the severe economic crisis, a system of civil mobilization was implemented to provide public services as a national interest. In Greece these obligatory services have been called "political mobilization") and "civil conscription".

Recurring civil conscription.

In Switzerland in most communities for all inhabitants, no matter if they are Swiss or not, it is mandatory to join the so-called "Militia Fire Brigades", as well as the obligatory service in Swiss civil defense and protection force. Conscripts in Singapore are providing the personnel of the country's fire service as part of the national service in the Civil Defence Force. In Austria and Germany citizens have to join a compulsory fire brigade if a volunteer fire service can not be provided, due to lack of volunteers. In 2018 this regulation is executed only in a handful of communities in Germany and currently none in Austria.

Conscription for military service and security forces.

Beside the conscription for military services, some countries draft citizens for paramilitary or security forces, like internal troops, border guards or police forces. While sometimes paid, conscripts are not free to decline enlistment. Draft dodging or desertion are often met with severe punishment. Even in countries which prohibit other forms of unfree labour, conscription is generally justified as being necessary in the national interest and therefore it is not contrary to the exceptions of the Forced Labour Convention, signed by the most countries in the world.

Mandatory community service.

Community services.

Community service is a non-paying job performed by one person or a group of people for the benefit of their community or its institutions. Community service is distinct from volunteering, since it is not always performed on a voluntary basis. Although personal benefits may be realized, it may be performed for a variety of reasons including citizenship requirements, a substitution of criminal justice sanctions, requirements of a school or class, and requisites for the receipt of certain benefits.

"De facto" obligatory community work.

During the Cold War in some communist countries like Czechoslovakia, the German Democratic Republic or the Soviet Union the originally voluntary work on Saturday for the community called "Subbotnik", "Voskresnik" or "Akce Z" became "de facto" obligatory for the members of a community.

Hand and hitch-up services.

In some German states it is feasible for communities to draft citizens for public services, called Hand and hitch-up services. This mandatory service is still executed to maintain the infrastructure of small communities.

Mandatory work to receive social benefits.

In several countries recipients of social benefits are required to work in special programs (such as usually unpaid or low-paid work) to continue to receive social or welfare benefits. For example the workfare programm is realized in many countries.  In Australia the "Work for the Dole" program was implemented in 1998. It is one means by which job seekers can satisfy what the government calls "mutual obligation requirements". Other "mutual obligation" measures are accredited study, part-time work, Army Reserves and volunteer work. "Work for the Dole" continues as of 2018.  In Germany the "Bürgerarbeit" (German for "citizen ́s work") is a similar concept, but politically discussed from time to time only.  In the United Kingdom different workfare programs like the "Mandatory Work Activity were implemented by different governments.

Unfree labour is any work relation, especially in modern or early modern history, in which people are employed against their will with the threat of destitution, detention, violence (including death), compulsion, or other forms of extreme hardship to themselves or members of their families.

Als Zwangsarbeit werden Arbeiten bezeichnet, zu denen Menschen unter Androhung einer Strafe oder eines sonstigen empfindlichen Übels gegen ihren Willen gezwungen werden. Sie ist – mit verschwimmenden Übergängen – die schärfste Form der „Arbeitspflicht“. Die Sklaverei und die Leibeigenschaft beschreiben ähnliche Abhängigkeitsverhältnisse, bei denen allerdings Menschen als Eigentum bzw. Objekte des Menschenhandels im Vordergrund stehen. Zwangsprostitution ist eine der aktuell verbreitetsten Formen von Zwangsarbeit.


Description.

Large numbers of people pass through airports every day. This presents potential targets for terrorism and other forms of crime because of the number of people located in one place. Similarly, the high concentration of people on large airliners increases the potentially high death rate with attacks on aircraft, and the ability to use a hijacked airplane as a lethal weapon may provide an alluring target for terrorism (such as during the September 11 attacks).

Airport enforcement authority.

While some countries may have an agency that protects all of their airports (such as Australia, in which the Australian

Process and equipment.

Some incidents have been the result of travelers carrying either weapons or items that could be used as weapons on board aircraft so that they can hijack the plane. Travelers are screened by metal detectors and/or millimeter wave scanners. Explosive detection machines used include X-ray machines and explosives trace-detection portal machines (a.k.a. "puffer machines"). In the United States, the TSA is working on new scanning machines that are still effective searching for objects that are not allowed in the airplanes but that do not depict the passengers in a state of undress that some find embarrassing. Explosive detection machines can also be used for both carry-on and checked baggage. These detect volatile compounds given off from explosives using gas chromatography.  Computed tomography and walk-through body scanning (Thz radiation) may also be done. Artificial intelligence systems are also being used, for example for translation service on information stations around the airport and for reducing the time airplanes spend at the gate between flights (by monitoring and analyzing everything that happens after the aircraft lands). In the future, it may also be used in conjunction with CT machines and Thz radiation detectors. It may also be used for use with biometric deployment across touchpoints and of new solutions, such as risk-based screening and intelligent video analytics.  A recent development is the controversial use of backscatter X-rays to detect hidden weapons and explosives on passengers. These devices, which use Compton scattering, require that the passenger stand close to a flat panel and produce a high resolution image. A technology released in Israel in early 2008 allows passengers to pass through metal detectors without removing their shoes, a process required as walk-through gate detectors are not reliable in detecting metal in shoes or on the lower body extremities. Alternately, the passengers step fully shoed onto a device which scans in under 1.2 seconds for objects as small as a razor blade. In some countries, specially trained

Notable incidents.

The world's first terrorist attack while in flight was Cubana Flight 455 on October 6, 1976, when the airliner flying from Barbados to Jamaica was brought down by two time bombs, killing 73 people. Evidence implicated several Central Intelligence Agency-linked anti-Castro Cuban exiles and members of the Venezuelan secret police DISIP, including Luis Posada Carriles.  The single deadliest airline catastrophe resulting from the failure of airport security to detect an onboard bomb was Air India Flight 182 in 1985, which killed 329 people.  Another onboard bomb that slipped through airport security was the one on Pan Am Flight 103 in 1988, which killed 270 people; 259 on the plane, and 11 residents of Lockerbie, Scotland.  Another notable failure was the 1994 bombing of Philippine Airlines Flight 434, which turned out to be a test run for a planned terrorist attack called Operation Bojinka. The explosion was small, killing one person, and the plane made an emergency landing. Operation Bojinka was discovered and foiled by Manila police in 1995.  On May 30, 1972, three members of the Japanese Red Army undertook a terrorist attack, popularly called the Lod Airport massacre, at the Lod Airport, now known as the Ben Gurion International Airport, in Tel Aviv. Firing indiscriminately with automatic firearms and throwing grenades, they managed to kill 24 people and injure 78 others before being neutralized (one of them through suicide). One of the three terrorists, Kozo Okamoto, survived the incident.  The Rome and Vienna airport attacks in December 1985 were two more instances of airport security failures. The attacks left 20 people dead when gunmen threw grenades and opened fire on travelers at El Al airline ticket counters.  The September 11 attacks are the most widely recognized terrorist attacks in recent times involving air travel. On the morning of September 11,

Airport security by country.

Canada.

All restrictions involving airport security are determined by Transport Canada and some are implemented by the Canadian Air Transport Security Authority (CATSA) in conjunction with the Airport Operator. Since the September 11 attacks, as well as the Air India bombing in 1985 and other incidents, airport security has tightened in Canada in order to prevent any attacks in Canadian Airspace.  CATSA uses x-ray machines to verify the contents of all carry-ons as well as metal detectors, explosive trace detection (ETD) equipment and random physical searches of passengers at the pre-board screening points. X-ray machines, CTX machines, high-resolution x-rays and ETDs are also used to scan checked bags. All checked baggage is always x-rayed at all major commercial airports.  CATSA launched its Restricted Area Identity Card

European Union.

Regulation (EC) No 300/2008 of the European Parliament and of the Council establishes common rules in the European Union to protect civil aviation against acts of unlawful interference. The regulation's provisions apply to all airports or parts of airports located in an EU country that are not used exclusively for military purposes. The provisions also apply to all operators, including air carriers, providing services at the aforementioned airports. It also applies to all entities located inside or outside airport premises providing services to airports. The standards of regulation 300/2008 are implemented by Commission Regulation (EU) 2015/1998.  The regulation no 2320/2002 from 2002 introduced the requirement to have security checks for all passenger flights, also domestic. Some EU countries had no checks for domestic flights until around 2005 (introducing full security checks took some time since terminals might need expansion).

Finland.

Passenger, luggage and freight security checking and security guard duties are outsourced to contractors. General public security is the responsibility of the Finnish Police, which has an airport unit at Helsinki Airport. The airport unit has a

France.

French security has been stepped up since terrorist attacks in France in 1986. In response France established the Vigipirate program. The program

Iceland.

As a member of the European Economic Area, Iceland has adopted EC regulation No 300/2008 into national law and thus complies with EU standards on airport security for all international

Netherlands.

Airport security in the Netherlands

Spain.

Airport security in Spain is provided by police forces, as well as private security guards. The Policía Nacional provides general security as well as passport (in international airports) and documentation checking. In Catalonia and Basque Country, the Mossos d'Esquadra and

Sweden.

Airport security is handled by security guards provided by the airport itself, with police assistance if needed. Airport fire fighters are also security guards. The Swedish Transport Agency decides the rules for the check, based on international regulations. Airports are generally defined by law as "protected objects", which give guards

United Kingdom.

The Department for Transport (DfT) is the governing authority for airport security in the United Kingdom, with the Civil Aviation Authority (CAA) also responsible for certain security regulatory functions. In September 2004, with the Home Office, DfT started an initiative called the "Multi Agency Threat and Risk Assessment" (MATRA), which was piloted at five of the United Kingdom's major airports — Heathrow, Birmingham, East Midlands Airport, Newcastle and Glasgow. Following successful trials, the scheme has now been rolled out across all 44 airports.  Since the September 11 attacks in New York,

Hong Kong.

The Hong Kong International Airport is secured by the Hong Kong Police Force and Aviation Security Company (AVSECO). Within the police force, the Airport District is responsible for the safety and security of the airport region. Airport Security Units are deployed around the airport and are armed with H&K MP5 A3 sub-machine guns and Glock 17 pistols. The security of the restricted area is

India.

India stepped up its airport security after the 1999 Kandahar hijacking. The Central Industrial Security Force, a paramilitary organisation, is in charge of airport security under the regulatory framework of the Bureau of Civil Aviation Security (Ministry of Civil Aviation). CISF formed an Airport Security Group to protect Indian airports. Every airport has now been given an APSU (Airport Security Unit), a trained unit to counter unlawful interference with civil aviation.

Israel.

El Al Airlines is headquartered in Israel. The last hijacking occurred on July 23, 1968, and no plane departing Ben Gurion Airport, just outside Tel Aviv, has ever been hijacked.  It was in 1972 that terrorists from the Japanese Red Army launched an attack that led to the deaths of at least 24 people at Lod Airport. Since then, security at the airport relies on a number of fundamentals, including a heavy focus on what Raphael Ron, former director of security at Ben Gurion Airport, terms the "human factor", which may be generalized as "the inescapable fact that terrorist attacks are carried out by people who can be found and stopped by an effective security methodology."  On December 27, 1985, terrorists simultaneously attacked El Al ticket counters

Pakistan.

In Pakistan Airports Security Force (ASF) is responsible for protecting the airports and the facilities and the planes.

Singapore.

Security for the country's two international passenger airports comes under the purview of the Airport Police Division of the Singapore Police Force, although resources are concentrated at Singapore Changi Airport where scheduled passenger traffic dominate. Seletar Airport, which specializes in handling non-scheduled and training flights, is seen as posing less of a security issue. Since the September 11 attacks, and the naming of Changi Airport as a terrorism target by the Jemaah Islamiyah, the airport's security has been stepped up.  Roving patrol teams of two soldiers and a police officer armed with automatic weapons patrol the terminals at random. Departing passengers are checked at the entrance of the gate rather than after immigration clearance unlike Hong Kong International Airport. This security measure is easily noticed by the presence of X-ray machines and metal detectors at every gate, which is not normally seen at other airports.  Assisting

United States.

Prior to the 1970s, American airports had minimal security arrangements to prevent acts of terrorism. For some flights with unassigned seating, no reservations were required, and fares were collected in the air. Federal security personnel started serving on high-risk flights in 1962 as the FAA Peace Officers Program; sky marshals began working out of the Miami field office of the United States Marshals Service in 1969. (Hijackers typically wanted to defect to Cuba, making flights from Florida common targets.) There were insufficient numbers to protect every flight, and hijackings continued to take place. The first hijacking resulting in a fatality occurred on Eastern Air Lines Shuttle Flight 1320 in 1970. Later in 1970, sky marshall staffing was increased, and some passengers fitting a behavioral profile were screened for weapons (including with handheld metal detectors).  On November 10, 1972, a trio of hijackers threatened to fly Southern Airways Flight 49 into a nuclear reactor at Oak Ridge National Laboratory. As a direct response to this incident, the Federal Aviation Administration required all airlines to begin screening passengers and their carry-on baggage by January 5, 1973. This screening was

See also.

US specific:

Airport security refers to the techniques and methods used in an attempt to protect passengers, staff, aircraft, and airport property from accidental/malicious harm, crime, terrorism, and other threats.

Unter Flughafensicherheit versteht man alle Maßnahmen, die der Vorbeugung gegen Verbrechen, insbesondere Terroranschläge auf einem Flughafen, also am Boden, dienen. Da sich eine große Zahl von Personen auf relativ geringem Raum aufhalten, sind Flughäfen ein potenzielles Ziel für den Terrorismus. Die meisten großen Flughäfen haben eigene Sicherheitskräfte, die von Polizeibeamten unterstützt werden. In einigen Ländern schützen paramilitärische Kräfte oder Soldaten die Flughäfen vor Bedrohungen.


Composition.

The composition of raw water is naturally variable, but commonly contains one or more of the following significant contaminants, in the form of dissolved ions, particles and living organisms: Other, less common, contaminants of raw water include: These contaminants can be harmful to humans if they drink raw water containing them, or if the water is not treated properly before consumption. The contaminants can also have an effect on the local ecosystem. For example, endocrine disrupting chemicals can have harmful effects on fish populations living in natural lakes and rivers. These growing health concerns have led to the development of guidelines, such HACCP (Hazard Analysis and Critical Control Points), for the production of safe drinking water from wastewater and raw water, and research into sustainable water supply alternatives

Treatment.

Raw water can be used for many purposes, such as cooling water, water for rinsing and chemical production, purified water, and drinking water. However, due to the possible contaminants, raw water must be treated before it is allowed for human consumption or industrial use. There are several steps involved in the treatment of raw water, and different methods in which it can take place.

Reverse osmosis.

Reverse osmosis has been used for the production of demineralised water for over 30 years. In reverse osmosis, water is transported across a membrane under high pressure, leaving a product that consists mainly of water and a concentrate containing most other components such as minerals and unwanted residues. Many contaminants, including iron, manganese, ammonium, traces of pesticides and medicines, organic micro pollutants, and radioactive particles can be removed with reverse osmosis alone. This efficiency has made reverse osmosis the primary method of water purification, often being used in combination with other methods, as a final treatment. A downside to reverse osmosis is that the removing of most minerals from water can have negative effects on its taste. Minerals are sometimes added to drinking water that was already filtered to improve its taste.

Conventional pre-treatment.

The conventional treatment method for water purification is a complex, multistage process that was used for many years. It generally consists of five primary steps. First, raw water is adjusted for alkalinity and pH with the addition of hydrated lime and carbon dioxide. Second, particulate matter is congregated with aluminum sulphate and other coagulants, such as polymers, which the water flows in a cascade that mixes the chemicals and raw water with the coagulants. Third, the water is slowly mixed in clarifiers where larger particles settle down to the bottom and are periodically removed (sedimentation). Fourth, water is directed from the clarifiers to the filters (e.g. anthracite and sand filter) to entrap any smaller particles that survived the clarification process. Finally, sodium hydroxide is added to adjust the final pH/alkalinity, sodium hypochlorite for disinfection and fluoride for fluoridation. This process is often used as a pre-treatment method while another processes, such as reverse osmosis, is used for the final treatment. A disadvantage of this method is its use of chemicals, such as ozone, flocculents, hydrogen peroxide, lime, and chlorine for the filtration process. These chemicals could be dangerous if used incorrectly or if they remained in the water after complete treatment. This requires special precautions, and each step of the process has to be controlled to achieve an overall optimal performance. The need for a complex control system for conventional treatment can result in it being financially costly. This has led to the development of alternative pre-treatment and treatment methods for raw water.

Ultrafiltration.

Ultrafiltration is a membrane filtration process and provides an alternative to conventional pre-treatment. In this method water is only pre-filtered with a common screen filter before being filtered at high pressure through a membrane, separating the water from contaminants within it. Ultrafiltration can be used on its own for water purification or as a pre-treatment for reverse osmosis. Its advantages over conventional pre-treatment include: very high water quality independent of the contaminants in the initial raw water, a plant that is simpler in design and more flexible, which makes it easier to automate, lower use of chemicals, and final product free of viruses and microorganisms.

Biofilm pre-treatment and Bio-diatomite Dynamic Membrane Reactor.

The biofilm membrane filtration method is used to remove harmful chemical by-products, such as NH4+-N, total nitrogen, and dissolved organic matter, that could form from disinfection processes. Biofilm Pre-treatment is a necessary step for water treatment in many parts of world due to its low operation cost and effectiveness at pollutant removal. In this process microorganisms remove contaminants in raw and wastewater that are harmful to humans, but nutrients to them. Bio-diatomite dynamic membrane reactor filtration combines dynamic membrane technology for wastewater treatment, and microbial colonies that form diatomite particles to purify water. These methods are under development in China as part of addressing the country's pollution problems.

Turbidity.

Turbidity is how murky or hazy water seems due to suspended particles. The more suspended particles, the higher the turbidity. Turbidity is used to visually measure water quality. In raw water, turbidity is very common. High amounts of turbidity in raw water is due to:

Turbidity and health concerns.

During high turbidity peaks, pathogens are more commonly found in raw water. This can contribute to the spread of illnesses. When turbidity increases, reported hospital visits for the elderly (65+) with gastrointestinal illnesses also increase. Even places with water filtration systems that meet standards can have an endemic of gastrointestinal illness, or waterborne infectious diseases. Those hospitalized represent a small percentage of total morbidity caused by these illnesses.

Controlling turbidity.

Controlling the land surrounding raw water sources is key to reducing turbidity. Areas of high sediment resuspension and erosion need attending regularly, and screens and other devices are needed to catch suspended particles. Screens that trap particles and debris must be cleaned consistently. Flushing out water pumps should be done when demand is lowest. This gives time for the water to settle, and decrease turbidity before is it needed in high amounts. "Turbidity reduction is best achieved when the water is run through a series of chemical and physical treatment methods before reaching the filter".

Bottled products.

Around 2017 untreated spring water, described as "naturally probiotic" raw water, became a commercial product distributed through health food shops, but experts have raised public health concerns about the possible occurrence of harmful bacteria, viruses, and parasites such as "Giardia". One raw water business in the United States has even claimed that methods used by bottled and spring water producers could change the molecular structure of water. The fad was mocked on "The Daily Show" in April 2018 in a segment that included an interview with Marion Nestle.

Raw water is water found in the environment that has not been treated and does not have any of its minerals, ions, particles, bacteria, or parasites removed. Raw water includes rainwater, ground water, water from infiltration wells, and water from bodies like lakes and rivers.

Betriebswasser (oft auch als Brauch- oder als Nutzwasser bezeichnet) ist Wasser, das einer spezifischen technischen, gewerblichen, landwirtschaftlichen oder hauswirtschaftlichen Anwendung dient. Betriebswasser ist anders als Trinkwasser nicht für den menschlichen Genuss vorgesehen, sollte jedoch einer gewissen Mindesthygiene entsprechen. In jedem Fall muss es den technologischen Anforderungen des jeweiligen Prozesses genügen. Beispielsweise muss Kühlwasser so beschaffen sein, dass die Kühlaggregate sich nicht mit Algen oder Kalk zusetzen. Teilweise muss das Wasser sehr weit aufgereinigt werden (z. B. vollentsalztes Wasser für den Betrieb von Dampfturbinen).


History.

Breguet recommended the use of reduced-section conductors to protect telegraph stations from lightning strikes; by melting, the smaller wires would protect apparatus and wiring inside the building. A variety of wire or foil fusible elements were in use to protect telegraph cables and lighting installations as early as 1864.  A fuse was patented by Thomas Edison in 1890 as part of his electric distribution system.

Construction.

A fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-combustible housing. The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the current flow. The size and construction of the element is (empirically) determined so that the heat produced for a normal current does not cause the element to attain a high temperature. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit.  The fuse element is made of zinc, copper, silver, aluminum, or alloys to provide stable and predictable characteristics. The fuse ideally would carry its rated current indefinitely, and melt quickly on a small excess. The element must not be damaged by minor harmless surges of current, and must not oxidize or change its behavior after possibly years of service.  The fuse elements may be shaped to increase heating effect. In large fuses, current may be divided between multiple strips of metal. A dual-element fuse may contain a metal strip that melts instantly on a short-circuit, and also contain a low-melting solder joint that responds to long-term overload of low values compared to a short-circuit. Fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments.  The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc. Silica sand or non-conducting liquids may be used.

Characteristics.

Rated current I.

A maximum current that the fuse can continuously conduct without interrupting the circuit.

Speed.

The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases. Fuses have different characteristics of operating time compared to current. A standard fuse may require twice its rated current to open in one second, a fast-blow fuse may require twice its rated current to blow in 0.1 seconds, and a slow-blow fuse may require twice its rated current for tens of seconds to blow.  Fuse selection depends on the load's characteristics. Semiconductor devices may use a fast or "ultrafast" fuse as semiconductor devices heat rapidly when excess current flows. The fastest blowing fuses are designed for the most sensitive electrical equipment, where even a short exposure to an overload current could be very damaging. Normal fast-blow fuses are the most general purpose fuses. A "time-delay fuse" (also known as an "anti-surge" or "slow-blow" fuse) is designed to allow a current which is above the rated value of the fuse to flow for a short period of time without the fuse blowing. These types of fuse are used on equipment such as motors, which can draw larger than normal currents for up to several seconds while coming up to speed.  Manufacturers can provide a plot of current vs time, often plotted on logarithmic scales, to characterize the device and to allow comparison with the characteristics of protective devices upstream and downstream of the fuse.

The It value.

The It rating is related to the amount of energy let through by the fuse element when it clears the electrical fault. This term is normally used in short circuit conditions and the values are used to perform co-ordination studies in electrical networks. It parameters are provided by charts in manufacturer data sheets for each fuse family. For coordination of fuse operation with upstream or downstream devices, both melting It and clearing It are specified. The melting It is proportional to the amount of energy required to begin melting the fuse element. The clearing It is proportional to the total energy let through by the fuse when clearing a fault. The energy is mainly dependent on current and time for fuses as well as the available fault level and system voltage. Since the It rating of the fuse is proportional to the energy it lets through, it is a measure of the thermal damage from the heat and magnetic forces that will be produced by a fault.

Breaking capacity.

The breaking capacity is the maximum current that can safely be interrupted by the fuse. This should be higher than the prospective short-circuit current. Miniature fuses may have an interrupting rating only 10 times their rated current. Fuses for small, low-voltage, usually residential, wiring systems are commonly rated, in North American practice, to interrupt 10,000 amperes. Fuses for commercial or industrial power systems must have higher interrupting ratings, with some low-voltage current-limiting high interrupting fuses rated for 300,000 amperes. Fuses for high-voltage equipment, up to 115,000 volts, are rated by the total apparent power (megavolt-amperes, MVA) of the fault level on the circuit.  Some fuses are designated high rupture capacity (HRC) or high breaking capacity (HBC) and are usually filled with sand or a similar material. Low-voltage high rupture capacity (HRC) fuses are used in the area of main distribution boards in low-voltage networks where there is a high prospective short circuit current. They are generally larger than screw-type fuses, and have ferrule cap or blade contacts. High rupture capacity fuses may be rated to interrupt current of 120 kA.  HRC fuses are widely used in industrial installations and are also used in the public power grid, e.g. in transformer stations, main distribution boards, or in building junction boxes and as meter fuses.  In some countries, because of the high fault current available where these fuses are used, local regulations may permit only trained personnel to change these fuses. Some varieties of HRC fuse include special handling features

Rated voltage.

The voltage rating of the fuse must be equal to or, greater than, what would become the open-circuit voltage. For example, a glass tube fuse rated at 32 volts would not reliably interrupt current from a voltage source of 120 or 230 V. If a 32 V fuse attempts to interrupt the 120 or 230 V source, an arc may result. Plasma inside the glass tube may continue to conduct current until the current diminishes to the point where the plasma becomes a non-conducting gas. Rated voltage should be higher than the maximum voltage source it would have to disconnect. Connecting fuses in series does not increase the rated voltage of the combination, nor of any one fuse.  Medium-voltage fuses rated for a few thousand volts are never used on low voltage circuits, because of their cost and because they cannot properly clear the circuit when operating at very low voltages.

Voltage drop.

The manufacturer may specify the voltage drop across the fuse at rated current. There is a direct relationship between a fuse's cold resistance and its voltage drop value. Once current is applied, resistance and voltage drop of a fuse will constantly grow with the rise of its operating temperature until the fuse finally reaches thermal equilibrium. The voltage drop should be taken into account, particularly when using a fuse in low-voltage applications. Voltage drop often is not significant in more traditional wire type fuses, but can be significant in other technologies such as resettable (PPTC) type fuses.

Temperature derating.

Ambient temperature will change a fuse's operational parameters. A fuse rated for 1 A at 25 °C may conduct up to 10% or 20% more current at −40 °C and may open at 80% of its rated value at 100 °C. Operating values will vary with each fuse family and are provided in manufacturer data sheets.

Markings.

Most fuses are marked on the body or end caps with markings that indicate their ratings. Surface-mount technology "chip type" fuses feature few or no markings, making identification very difficult.  Similar appearing fuses may have significantly different properties, identified by their markings. Fuse markings will generally convey the following information, either explicitly as text, or else implicit with the approval agency marking for a particular type:

Packages and materials.

Fuses come in a vast array of sizes and styles to serve in many applications, manufactured in standardised package layouts to make them easily interchangeable. Fuse bodies may be made of ceramic, glass, plastic, fiberglass, molded mica laminates, or molded compressed fibre depending on application and voltage class.  Cartridge (ferrule) fuses have a cylindrical body terminated with metal end caps. Some cartridge fuses are manufactured with end caps of different sizes to prevent accidental insertion of the wrong fuse rating in a holder, giving them a bottle shape.  Fuses for low voltage power circuits may have bolted blade or tag terminals which are secured by screws to a fuseholder. Some blade-type terminals are held by spring clips. Blade type fuses often require the use of a special purpose extractor tool to remove them from the fuse holder.  Renewable fuses have replaceable fuse elements, allowing the fuse body and terminals to be reused if not damaged after a fuse operation.  Fuses designed for soldering to a printed circuit board have radial or axial wire leads. Surface mount fuses have solder pads instead of leads.  High-voltage fuses of the expulsion type have fiber or glass-reinforced plastic tubes and an open end, and can have the fuse element replaced.  "Semi-enclosed fuses" are fuse wire carriers in which the fusible wire itself can be replaced. The exact fusing current is not as well controlled as an enclosed fuse, and it is extremely important to use the correct diameter and material when replacing the fuse wire, and for these reasons these fuses are slowly falling from favour.  These are still used in consumer units in some parts of the world, but are becoming less common. While glass fuses have the advantage of a fuse element visible for inspection purposes, they have a low breaking capacity (interrupting rating), which generally restricts them to applications of 15 A or less at 250 V. Ceramic fuses have the advantage of a higher breaking capacity, facilitating their use in circuits with higher current and voltage. Filling a fuse body with sand provides additional cooling of the arc and increases the breaking capacity of the fuse. Medium-voltage fuses may have liquid-filled envelopes to assist in the extinguishing of the arc. Some types of distribution switchgear use fuse links immersed in the oil that fills the equipment.  Fuse packages may include a rejection feature such as a pin, slot, or tab, which prevents interchange of otherwise similar appearing fuses. For example, fuse holders for North American class RK fuses have a pin that prevents installation of similar-appearing class H fuses, which have a much lower breaking capacity and a solid blade terminal that lacks the slot of the RK type.

Dimensions.

Fuses can be built with different sized enclosures to prevent interchange of different ratings of fuse. For example, "bottle style" fuses distinguish between ratings with different cap diameters. Automotive glass fuses were made in different lengths, to prevent high-rated fuses being installed in a circuit intended for a lower rating.

Special features.

Glass cartridge and plug fuses allow direct inspection of the fusible element. Other fuses have other indication methods including:  Some fuses allow a special purpose micro switch or relay unit to be fixed to the fuse body. When the fuse element blows, the indicating pin extends to activate the micro switch or relay, which, in turn, triggers an event.  Some fuses for medium-voltage applications use two or three separate barrels and two or three fuse elements in parallel.

Fuse standards.

IEC 60269 fuses.

The International Electrotechnical Commission publishes standard 60269 for low-voltage power fuses. The standard is in four volumes, which describe general requirements, fuses for industrial and commercial applications, fuses for residential applications, and fuses to protect semiconductor devices. The IEC standard unifies several national standards, thereby improving the interchangeability of fuses in international trade. All fuses of different technologies tested to meet IEC standards will have similar time-current characteristics, which simplifies design and maintenance.

UL 248 fuses (North America).

In the United States and Canada, low-voltage fuses to 1 kV AC rating are made in accordance with Underwriters Laboratories standard UL 248 or the harmonized Canadian Standards Association standard C22.2 No. 248. This standard applies to fuses rated 1 kV or less, AC or DC, and with breaking capacity up to 200 kA. These fuses are intended for installations following Canadian Electrical Code, Part I (CEC), or the National Electrical Code, NFPA 70 (NEC).  The standard ampere ratings for fuses (and circuit breakers) in USA/Canada are considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses are 1, 3, 6, 10, and 601.  UL 248 currently has 19 "parts". UL 248-1 sets the general requirements for fuses, while the latter parts are dedicated to specific fuses sizes (ex: 248-8 for Class J, 248-10 for Class L), or for categories of fuses with unique properties (ex: 248-13 for semiconductor fuses, 248-19 for photovoltaic fuses). The general requirements (248-1) apply except as modified by the supplemental part (240-x). For example, UL 248-19 allows photovoltaic fuses to be rated up to 1500 volts, DC, versus 1000 volts under the general requirements.  IEC and UL nomenclature varies slightly. IEC standards refer to a "fuse" as the assembly of a fusible link and a fuse holder. In North American standards, the "fuse" is the replaceable portion of the assembly, and a "fuse link" would be a bare metal element for installation in a fuse.

Automotive fuses.

"Automotive fuses" are used to protect the wiring and electrical equipment for vehicles. There are several different types of automotive fuses and their usage is dependent upon the specific application, voltage, and current demands of the electrical circuit. Automotive fuses can be mounted in fuse blocks, inline fuse holders, or fuse clips. Some automotive fuses are occasionally used in non-automotive electrical applications. Standards for automotive fuses are published by SAE International (formerly known as the Society of Automotive Engineers).  Automotive fuses can be classified into four distinct categories:  Most automotive fuses rated at 32 volts are used on circuits rated 24 volts DC and below. Some vehicles use a dual 12/42 V DC electrical system that will require a fuse rated at 58 V DC.

High voltage fuses.

Fuses are used on power systems up to 115,000 volts AC. High-voltage fuses are used to protect instrument transformers used for electricity metering, or for small power transformers where the expense of a circuit breaker is not warranted. A circuit breaker at 115 kV may cost up to five times as much as a set of power fuses, so the resulting saving can be tens of thousands of dollars.  In medium-voltage distribution systems, a power fuse may be used to protect a transformer serving 1–3 houses. Pole-mounted distribution transformers are nearly always protected by a fusible cutout, which can have the fuse element replaced using live-line maintenance tools.  Medium-voltage fuses are also used to protect motors, capacitor banks and transformers and may be mounted in metal enclosed switchgear, or (rarely in new designs) on open switchboards.

Expulsion fuses.

Large power fuses use fusible elements made of silver, copper or tin to provide stable and predictable performance. High voltage "expulsion fuses" surround the fusible link with gas-evolving substances, such as boric acid. When the fuse blows, heat from the arc causes the boric acid to evolve large volumes of gases. The associated high pressure (often greater than 100 atmospheres) and cooling gases rapidly quench the resulting arc. The hot gases are then explosively expelled out of the end(s) of the fuse. Such fuses can only be used outdoors.  These type of fuses may have an impact pin to operate a switch mechanism, so that all three phases are interrupted if any one fuse blows.  "High-power fuse" means that these fuses can interrupt several kiloamperes. Some manufacturers have tested their fuses for up to 63 kA short-circuit current.

Comparison with circuit breakers.

Fuses have the advantages of often being less costly and simpler than a circuit breaker for similar ratings. The blown fuse must be replaced with a new device which is less convenient than simply resetting a breaker and therefore likely to discourage people from ignoring faults. On the other hand, replacing a fuse without isolating the circuit first (most building wiring designs do not provide individual isolation switches for each fuse) can be dangerous in itself, particularly if the fault is a short circuit.  High rupturing capacity fuses can be rated to safely interrupt up to 300,000 amperes at 600 V AC. Special current-limiting fuses are applied ahead of some molded-case breakers to protect the breakers in low-voltage power circuits with high short-circuit levels.  "Current-limiting" fuses operate so quickly that they limit the total "let-through" energy that passes into the circuit, helping to protect downstream equipment from damage. These fuses open in less than one cycle of the AC power frequency; circuit breakers cannot match this speed.  Some types of circuit breakers must be maintained on a regular basis to ensure their mechanical operation during an interruption. This is not the case with fuses, which rely on melting processes where no mechanical operation is required for the fuse to operate under fault conditions.  In a multi-phase power circuit, if only one fuse opens, the remaining phases will have higher than normal currents, and unbalanced voltages, with possible damage to motors. Fuses only sense overcurrent, or to a degree, over-temperature, and cannot usually be used independently with protective relaying to provide more advanced protective functions, for example, ground fault detection.  Some manufacturers of medium-voltage distribution fuses combine the overcurrent protection characteristics of the fusible element with the flexibility of relay protection by adding a pyrotechnic device to the fuse operated by external protective relays.

Fuse boxes.

United Kingdom.

In the UK, older electrical consumer units (also called fuse boxes) are fitted either with semi-enclosed (rewirable) fuses or cartridge fuses (Fuse wire is commonly supplied to consumers as short lengths of 5 A-, 15 A- and 30 A-rated wire wound on a piece of cardboard.) Modern consumer units usually contain miniature circuit breakers (MCBs) instead of fuses, though cartridge fuses are sometimes still used, as in some applications MCBs are prone to nuisance tripping.  Renewable fuses (rewirable or cartridge) allow user replacement, but this can be hazardous as it is easy to put a higher-rated or double fuse element (link or wire) into the holder ("overfusing"), or simply fitting it with copper wire or even a totally different type of conducting object (coins, hairpins, paper clips, nails, etc.) to the existing carrier. One form of fuse box abuse was to put a penny in the socket, which defeated overcurrent protection and resulted in a dangerous condition. Such tampering will not be visible without full inspection of the fuse. Fuse wire was never used in North America for this reason, although renewable fuses continue to be made for distribution boards.  The "Wylex standard" consumer unit was very popular in the United Kingdom until the wiring regulations started demanding residual-current devices (RCDs) for sockets that could feasibly supply equipment outside the equipotential zone. The design does not allow for fitting of RCDs or RCBOs. Some Wylex standard models were made with an RCD instead of the main switch, but (for consumer units supplying the entire installation) this is no longer compliant with the wiring regulations as alarm systems should "not" be RCD-protected. There are two styles of fuse base that can be screwed into these units: one designed for rewirable fusewire carriers and one designed for cartridge fuse carriers. Over the years MCBs have been made for both styles of base. In both cases, higher rated carriers had wider pins, so a carrier couldn't be changed for a higher rated one without also changing the base. Cartridge fuse carriers are also now available for DIN-rail enclosures.

North America.

In North America, fuses were used in buildings wired before 1960. These "Edison base" fuses would screw into a fuse socket similar to Edison-base incandescent lamps. Ratings were 5, 10, 15, 20, 25, and 30 amperes. To prevent installation of fuses with an excessive current rating, later fuse boxes included rejection features in the fuse-holder socket, commonly known as "Rejection Base (Type S fuses)" which have smaller diameters that vary depending on the rating of the fuse. This means that fuses can only be replaced by the preset (Type S) fuse rating. This is a North American, tri-national standard (UL 4248-11; CAN/CSA-C22.2 NO. 4248.11-07 (R2012); and, NMX-J-009/4248/11-ANCE). Existing Edison fuse boards can easily be converted to only accept Rejection Base (Type S) fuses, by screwing-in a tamper-proof adapter. This adapter screws into the existing Edison fuse holder, and has a smaller diameter threaded hole to accept the designated Type S rated fuse.  Some companies manufacture resettable miniature thermal circuit breakers, which screw into a fuse socket. Some installations use these Edison-base circuit breakers. However, any such breaker sold today does have one flaw. It may be installed in a circuit-breaker box with a door. If so, if the door is closed, the door may hold down the breaker's reset button. While in this state, the breaker is effectively useless: it does not provide any overcurrent protection.  In the 1950s, fuses in new residential or industrial construction for branch circuit protection were superseded by low voltage circuit breakers.  Fuses are widely used for protection of electric motor circuits; for small overloads, the motor protection circuit will open the controlling contactor automatically, and the fuse will only operate for short-circuits or extreme overload.

Coordination of fuses in series.

Where several fuses are connected in series at the various levels of a power distribution system, it is desirable to blow (clear) only the fuse (or other overcurrent device) electrically closest to the fault. This process is called "coordination" or "discrimination" and may require the time-current characteristics of two fuses to be plotted on a common current basis. Fuses are selected so that the minor, branch, fuse disconnects its circuit well before the supplying, major, fuse starts to melt. In this way, only the faulty circuit is interrupted with minimal disturbance to other circuits fed by a common supplying fuse.  Where the fuses in a system are of similar types, simple rule-of-thumb ratios between ratings of the fuse closest to the load and the next fuse towards the source can be used.

Other circuit protectors.

Resettable fuses.

So-called self-resetting fuses use a thermoplastic conductive element known as a polymeric positive temperature coefficient (PPTC) thermistor that impedes the circuit during an overcurrent condition (by increasing device resistance). The PPTC thermistor is self-resetting in that when current is removed, the device will cool and revert to low resistance. These devices are often used in aerospace/nuclear applications where replacement is difficult, or on a computer motherboard so that a shorted mouse or keyboard does not cause motherboard damage.

Thermal fuses.

A "thermal fuse" is often found in consumer equipment such as coffee makers, hair dryers or transformers powering small consumer electronics devices. They contain a fusible, temperature-sensitive composition which holds a spring contact mechanism normally closed. When the surrounding temperature gets too high, the composition melts and allows the spring contact mechanism to break the circuit. The device can be used to prevent a fire in a hair dryer for example, by cutting off the power supply to the heater elements when the air flow is interrupted (e.g., the blower motor stops or the air intake becomes accidentally blocked). Thermal fuses are a 'one shot', non-resettable device which must be replaced once they have been activated (blown).

Cable limiter.

A cable limiter is similar to a fuse but is intended only for protection of low voltage power cables. It is used, for example, in networks where multiple cables may be used in parallel. It is not intended to provide overload protection, but instead protects a cable that is exposed to a short circuit. The characteristics of the limiter are matched to the size of cable so that the limiter clears a fault before the cable insulation is damaged.

Unicode symbol.

The Unicode character for the fuse's schematic symbol, found in the Miscellaneous Technical block, is (<unk>).

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, it must be replaced or rewired, depending on type.

Eine Schmelzsicherung ist eine Überstromschutzeinrichtung, die durch das Abschmelzen eines Schmelzleiters den Stromkreis unterbricht, wenn die Stromstärke einen bestimmten Wert während einer ausreichenden Zeit überschreitet.


Biography.

Hobrecht was born as the son of the estate owner Ludolph Hobrecht and his wife Isabella (born Johnson) in East Prussian Memel. His elder brother Arthur Hobrecht would later become the mayor of Berlin. In 1834 his father was called to the royal economic council and the family moved to Königsberg. In 1841, Hobrecht broke off his school education and began an apprenticeship as geodesist (professional land surveyor) for which he passed examination in 1845. Until 1847 he was engaged in separation work (clearing the pathway of transportation projects from rocks and hills) in East Prussia as well as the Cologne-Minden Railway. During the German revolutions of 1848 he served in the student guards at the Berlin Palace.  By 1847 he had started studies at the Berlin Bauakademie (one of the institutes that would later form the Technical University of Berlin). While at the building academy he was an active member of the "Akademischer Verein 'Motiv'" student association. The fraternity was originally a loose group of students interested in arts and philosophy founded in 1847 as a chorus. Along with the rise of the academy it quickly grew by size and influence on contemporary architecture. Being at the academy he attended a variety of courses until his examination as "Bauführer" (academic site manager) in 1849. Right after examination he joined the "Architekten- und Ingenieur-Verein zu Berlin" professional association of architects in Berlin. He held different management positions in the circle until he was called off to the infantry regiment in the Electorate of Hesse in 1850.  His first professional work is recorded with the building of the "Packhof" in Königsberg in 1851. In 1852 there are records of managing the Dirschkeim estate in Samland. On 4 February 1853 he married Henriette Wolff. The couple had three sons and four daughters in the coming years.  James Hobrecht continued his studies in civil engineering with records of field placements at the Prussian Eastern Railway in 1857. In 1858 he passed examination in transportation planning (Wasser-, Wege- und Eisenbahnbaumeister) and found an employment at the Royal Prussian urban planning administration (Baupolizei) in the same year. In that role he was commanded in 1859 to head the commission on creation of a land-use plan for Berlin and its environs. As part of his job he traveled to Hamburg, Paris and London in 1860 to learn about the contemporary development status in urban planning especially their sewer systems. Before the plan was finished the city came to an amalgamation of its suburbs on 1. January 1861. Based on the just finished land surveys and existing land-use proposals James Hobrecht constructed a map showing a possible land-use for a city at a projected size of 1.5 to 2 million inhabitants. The map of building lines was soon to be called the Hobrecht-Plan but he was not able to continue on the details due to his dismissal on 15. December 1861, even before the resolution of the planning works on 18. July 1862.  The reasons for the job termination are unknown. He went to Stettin to construct the water supply system for the city and to create plans for a modern sewer system Which was started to be built in 1870. Before the project was realized in Stettin, he was able to return to Berlin in 1869 where he was commissioned to build a sewer system for the city. This was enabled by his brother Arthur Hobrecht, who became lord mayor of Berlin in 1872, and Rudolf Virchow, who had already been a famous medical doctor and influential liberal politician. He laid out plans for a radial system of 12 main routes of canalization from the city to new sewage farms on the outskirts of Berlin. Soon after his brother took office the grand pipes were constructed from 1873 until the last one in 1893. While the works were ongoing he was called to help with the planning of the sewer systems of 30 German cities and the sewer systems in Moscow, Tokyo and Cairo.  From 1872 to 1874 he had a teaching assignment at the Bauakademie. In 1885 he was elected to head the municipal urban planning departement, a position that he held for 12 years. Most of the embankments of the river Spree were built under his leadership allowing larger ships to pass through the city. In 1897 he retired for health reasons. In the same year he was honored with the title of "Stadtältester von Berlin". He remained in the city until his death in 1902. His grave can be found on the Friedhof II der Sophiengemeinde Berlin.

Influence and Legacy.

When James Hobrecht was commanded to head the urban planning commission for Berlin in 1859 he was just 34 years old and he had only minor experience compared to the size of the project which included 14 chapters. Actually he was just replacing a colleague who had fallen sick at the time. The shortcomings of the crowded Mietskaserne tenement blocks were largely attributed to that fact by historians in the early 20th century. Even the deadly street fights of fascist and communist squadrons in those area have been traced back to the Hobrecht-Plan that he created at that time. Modern historians are more favourable as his writings do also include chapters on estimates of the social consequences and his planning was good enough for later reformations that allowed Berlin to grow successfully.  In 1868 he published the influential paper "Ueber öffentliche Gesundheitspflege und die Bildung eines Central-Amts für öffentliche Gesundheitspflege im Staate" (on the public health issues and the founding of a central office in the government) that was spawned by a Cholera epidemic at the time. He is still favouring the "Durcheinanderwohnen" (mixed class housing) in the tenement areas but criticizes the catastrophic housing conditions that had developed in the Wilhelmine Ring of Berlin. He finds support from Rudolph Virchow who had the same agenda on improving public health and housing conditions. With his wide knowledge on sewer systems and land-use for its pipes, he is the technician being in need to bring these reform ideas into reality.  Soon after his return to Berlin he is one of the founders of the "Deutsche Vierteljahrsschrift für öffentliche Gesundheitspflege" (Quarterly Magazine on Public Health) in 1869. Between 1870 and 1879 he publishes 13 issues of "Reinigung und Entwässerung von Berlin. Einleitende Verhandlungen und Berichte über mehrere auf Veranlassung des Magistrats der Königlichen Haupt- und Residenzstadt Berlin angestellte Versuche und Untersuchungen" (water treatment and sewer systems of Berlin. Introduction and reports on experiments and studies on request of the municipal government of Berlin). When the Berlin sewer system started to work successfully, with much of an improvement to inhabitants of Berlin, he became a well-known expert in the field - helping later with the planning in Moscow in 1880, in Tokyo in 1886 and in Cairo in 1892.

James Friedrich Ludolf Hobrecht (31 December 1825 in Memel – 8 September 1902 in Berlin) was a Prussian director for urban planning. His development plan of 1862 for a million-sized Berlin was soon to be simply called the Hobrecht-Plan. His main focus was on modern sewer systems for which he was well known in the late 19th century.

James Friedrich Ludolf Hobrecht (* 31. Dezember 1825 in Memel; † 8. September 1902 in Berlin) war ein preußischer Stadtplaner und für Berlins ersten perspektivischen Bebauungsplan, den Hobrecht-Plan von 1862, verantwortlich. Als Stadtbaurat organisierte er ab 1885 die Einführung der Stadtentwässerung und machte sich damit um die Gesundheit der Berliner Bevölkerung verdient.