Part Three

In Part 3 of this article we are going to learn about marine circuit protection. Circuit protection on a boat is there to protect wires from overheating during an overload condition. While your wires should be sized for normal usage in regards to current flow and voltage tolerance, conditions can cause too much current to flow in a circuit causing the wire to overheat very quickly and potentially start a fire. When this occurs, circuit protection is what 'opens' the circuit before the wire overheats. We will learn about overloaded circuits and the devices used to protect your wiring and devices from fire and damage.

Fuses and Circuit breakers – Protectors of the Wire

By now you should be comfortable with marine wire and how to choose the sizes for your boat project. Your wires should be able to handle normal current as well as deliver close to 12 V DC (within 3 or 10 percent) to your boats equipment. But… what if something happens that causes too much current to flow through your carefully sized wires? This excessive current flow will likely cause a fire and/or may ruin an expensive piece of equipment. How do you prevent this from occurring? You use fuses and/or circuit breakers (circuit protection devices) in your circuits to sense the current flow and OPEN the circuit before fire and damage occurs.

Abnormal Circuit Conditions

In Part 2 you have already been introduced to the normal electrical circuit. Before we start our discussion on circuit protection devices we need to look at circuits that no longer function correctly. There are three basic abnormal circuit conditions that can affect the electrical circuit. The first is known as a SHORT circuit, the second is known as an OPEN circuit. These conditions mainly have to do with wire connections between the devices in a circuit such as the battery and a light or a switch. The third is a failure of a component in a circuit. For our purposes here, the component could be a wire or termination, light fixture, a piece of electronics, a switch, even the battery.

 We will begin with discussing the OPEN CIRCUIT. Study and compare the normal circuit diagram with the open circuit diagram.

Normal Circuit

Open Circuit

This OPEN circuit diagram shows a break in the positive power lead. As mentioned above the break can be a broken wire (positive or negative wire), bad terminations or corrosion. This results in an OPEN CIRCUIT. With an open circuit there is no way for current to flow from the battery to the device your powering and also no voltage at the power terminals. This not a fire causing scenario but it can result in equipment not operating that you are depending on to safely operate your boat.  How do these conditions occur? A physical break is usually caused by poorly routed wires that are in the way of people moving about or moving around with equipment. Many times nobody sees or feels the wire breaking when it happens. Another common cause is bad terminations due to using cheap crimp tools. Your terminations may look connected, but in fact may not be; they may work for a while then vibration opens the circuit right inside the wire terminal resulting in very intermittent operation or complete failure of the circuit. In addition terminations can have CORROSION buildup; this creates a very high RESISTANCE connection which is similar to an open circuit. Too much resistance will not let enough current flow  as well as reduces the voltage at the device power terminals so the device fails to operate. Corrosion happens more often with poorly crimped terminal as well as using un-tinned wire. These two reasons alone are why it is imperative that you buy the best crimping tool for your marine terminations and to make sure you use only tinned marine wire.

The best type of crimpers for terminations

The second abnormal circuit condition is the SHORT CIRCUIT. Study the "SHORT CIRCUIT" diagram. The diagram shows that the two power wires are connected together; when you activate the system the battery current goes from the positive connection of the battery back to the negative battery connection through the connection point, no current flows through the device.

The current that does flow will be the maximum that the battery can deliver. This can easily be 500 amps or more from a single battery to thousands of amps if you have a large number of batteries feeding your boats electrical system. This unwanted connection will instantly melt wire insulation, overheating everything in the wires path and start a fire. In addition there is a good chance the battery(s) could explode. This connection of the positive wire to the negative wire of the battery is called a SHORT, or SHORTED CONNECTION. A short is nothing more than two wires that are connected together accidentally that should not be connected together. A short can be between battery power wires or any other wires that get connected together when they SHOULD NOT be connected together. In a new installation for example, wires can be connected to the wrong points such as mistakenly connecting a positive power wire to a ground connection. Another example is where two power wires( + and a -) routed right next to each other (as they normally are) and passing through a metal hole or other sharp edge that cuts the insulation and with some vibration (which all boats have) causes the two wires with insulation ribbed off to connect together. The effect is the same as an improper wiring connection. You will get excessive current; wire overheating, fire and possible battery explosion. In a poorly installed system where terminations are physically uncovered, dropping a metal tool on the connections could cause a SHORT with the same high current effects. That is why terminations must always be protected by physical covers or in enclosures. But why the excessive current flow in a shorted circuit? In some shorts, it is simply two wires inadvertently connected together through defective electronic components. This usually occurs inside of electronic devices, a place where you will not normally have access to and this usually results in the device not working properly. However shorts in power wires, which are the common place for shorted connections in marine systems, the device is bypassed, the device no longer has two power connections, it is effectively out of the circuit.(see the shorts diagram) What is left is a battery connected to itself with NO RESISTANCE other than the wire resistance itself. This wire resistance is extremely small allowing the maximum current from the battery to flow in the wires which will certainly exceed the safe current carrying capability of the wire. By now you should know the effect of this condition. Shorts can also cause other problems not involving excessive current. A switch may operate more than one device for example. So be aware of strange operation of your controls and switches, there could be a short somewhere in all of the connections.

The third abnormal circuit condition is a failure in a device that is part of the circuit. This type of failure can cause the device (radio, relay, etc.) to draw much more current than normal, up to possibly a full shorted condition. And of course an internal failure can also cause the device not to fully operate or not operate at all with no excessive current draw. While this is not as common as the other two abnormal circuit conditions, (shorts and opens) it does occur. In addition all electronic devices and many electrical devices have a fuse or circuit breaker mounted inside their housings that is supposed to isolate the device from the outside circuit power in case of internal failure, but these circuit protection devices do not always work.

Enter The Circuit Protection Device

Now that we have learned about circuit conditions that cause excessive current flow, how do we prevent this from happening? You add a circuit protection device to your circuit. This device can be in the form of a fuse or a circuit breaker. These devices come in different current ratings that will open the circuit when the current meets or exceeds the rating, thereby protecting your wires, your battery(s), your devices and ultimately your boat. There is no protection for open circuits, when they happen you have to troubleshoot the circuit and find the open, but there is no fire danger from an open. As a matter of fact when a fuse or circuit breaker ‘blows' (opens up) it is actually creating an OPEN circuit so no more current can flow, making the circuit safe until it can be repaired.

As you can see in this diagram, the fuse is in line with the positive lead of your circuit. When this device opens up due to excessive current flow, the whole circuit is OPENED, meaning no current can flow thereby stopping any potential for fire and damage. All of this occurs in less than a second without any action on your part. Now you can identify the problem (troubleshoot), make the repair, replace the fuse or reset the circuit breaker and you are back in business again.

Fuses or Circuit Breakers?

Now that we have seen where the circuit protection device fits in, what are they called? Most people have some idea about what a fuse or a circuit breaker is all about but we will go over them here just to be sure. Fuses and circuit breakers are two very different devices with the same function, to break open an electrical circuit that's in a shorted condition. A fuse is a simple device that has a precisely designed "resistance wire" that melts away when the current flowing through it meets or exceeds the designed current value or CURRENT RATING. Once the fuse has opened, it is no longer re-usable and must be discarded. There are a few types of these fuses available to the marine electrician. We will cover these later.

Circuit breakers are more complex than fuses and are mechanical in nature and open by two methods. The first and most common type of breaker opens by sensing the current MAGNETICALLY. When the current flow through the breaker reaches its CURRENT RATING, the magnetism created by the current flow causes a switch like connection to open and interrupt the circuit. This open condition stays that way until you fix the problem and reset the circuit breaker switch by putting it back into its normal operating position. This means of course that the circuit breaker is re-usable. This is the type of breaker you should use in the boating environment. The other method of breaker operation is THERMAL. When current flows through the thermal breaker it heats up and the wire mechanism distorts and pulls apart the current connection. Again, once you fix the problem, you move the breaker switch back to its operating position and you are back in operation. This is the breaker normally used for motor protection. We will cover the circuit breaker and its variants later.

Which Device Is Better For The Job?

The question usually comes up as to which device is better for the job of circuit protection, the fuse or circuit breaker. They both do the job equally when it comes to protecting your circuits and your boat. The obvious advantage of breakers is that you do not need a box of spares to put your circuit back into operation once you repair the circuit defect. With fuses you need to keep spare fuses of every CURRENT RATING on your boat. With that said, you should still have a couple of breaker spares on board just in case, particularly if you are doing offshore cruising. Circuit breakers can fail. Circuit breakers and their holders/panels are far more expensive than their fuse cousins. Circuit breakers are can be used as switches to turn various electrical devices on such as bilge pumps, deck lighting, and electronics. If these breakers are being used as SWITCHES, they must be rated as switches and not just circuit breakers. With fuses you can buy switch panels or fuse holders that have switches built in so the fuse panel acts like a breaker panel, of course the cost goes up but it is still cheaper than breakers. For simple electrical systems needing only a few fuses a simple FUSE HOLDER panel may be all you need but make sure all of your electrical devices have their own power switch. So, with some of these thoughts in mind, which device is better for the job? It depends on your boat complexity, your budget and what type of cruising you are planning. Large boats with plenty of gear and maybe some long distance cruising, breakers would be my choice. There will be some areas fuses are still needed but breakers are the main protection. Small boats on the other hand can use fuses even if you have to keep spares. Small boats usually don't have lots of gear and usually stay close to some sort of marina facilities. So the choice is yours. Look at your boat and your cruising profile then pick your circuit protection devices.


We will now look at some of the more common examples of the fuses and breakers available to the marine electrician/boat builder. Those discussed here are not the only devices available but they are the most common ones in use. We will start out by discussing the most important fuse specifications and some of the various fuses available. We will do the same for circuit breakers.


Knowing fuses by what they look like is only part of the deal. You need to understand specifications first, then decide on the fuse style you want or need. Here are the most important specs with their explanations:

A few remarks about the specs shown. The typical voltage rating for marine fuses is 32VDC. If you find a fuse with a higher rating feel free to use it. Glass fuses are rated for 250VDC or VAC. The Ampere-Interrupt Current spec is often overlooked. The reason being is most boats only have a couple of batteries feeding the electrical system (not including starting battery). If there is a short you may get up to the two cranking amps worth of current in your wiring where the short is. This could equal a little over a thousand amps. Pretty much all fuses can handle that. BUT… if you have three or more batteries connected in parallel, you are looking at over two thousand amps or much more, enough to weld with. When a short occurs this current will JUMP the fuse and holder from one side to the other regardless of the fact that the wire melted. This means the current will continue to flow through your wiring and it WILL cause a fire and possible battery explosion. When you have that many batteries make sure you are using fuses and holders that can withstand all of your battery current at one time!  

The Common Fuse Types

A fuse type not shown here is the PIGTAIL fuse. This fuse is an inline glass or sometimes a blade fuse that is attached to electronics and other equipment. This fuse is part of the power wiring that sticks out of the equipment. You attach your distribution wire to this PIGTAIL to supply power to the equipment. Keep this in mind when working with pigtails… you could use this pigtail fuse to protect your power wire from the distribution panel to the pigtail but it is not good practice in my opinion. Fuse your whole circuit at the panel and use the pigtail as backup. The pigtail wire is usually smaller in size than your wiring so you need to protect the pigtail wire with a smaller fuse the equipment manufacturer calls for. The smaller fuse may also prevent catastrophic damage to a piece of equipment that has failed internally. Pigtail fuses will be in both breaker and fused protection systems. Some equipment will have a RESET ONLY type of breaker instead of a pigtail fuse. We will discuss the RESET ONLY breaker later.

Circuit Breakers

As with fuses you need to understand breaker specifications in order to use breakers properly. There are many specifications for breakers but these are the important ones for marine electricians.

A couple of remarks about breaker specs. The most important spec is the TRIP FREE spec. Every marine breaker must be a TRIP FREE breaker. Luckily breakers made by marine manufacturers are TRIP FREE and I believe all breakers for the house and boat are TRIP FREE. But watch out, you may come across a few old breakers at a yard sale and may get them for pennies. Don't use them unless you know for sure they are of the TRIP FREE type. The second spec is the OPEN AND CLOSE CYCLES spec. Many times breakers are used like power switches, to turn equipment on and off with. Breakers were originally never designed to be used as power switches. However in the marine field using them as switches is common enough that breaker manufacturers construct their breakers with enough strength to be able to be used as switches. But there are some that are really for breaker use only. Make sure you install the right breaker type for the job. Switch rated breakers are more expensive but if you are not sure how you are going to use the breaker INSTALL SWITCH RATED BREAKERS.

One last area that needs to be discussed, and this applies to fuses as well as circuit breakers. Circuit protection devices have current rating. As you know this is the rating that tells you when the device will open. HOWEVER… there is a tolerance in this rating that is in the data sheet (usually) but not catalog pages. This tolerance can be as much as 10%. This means a 10 amp fuse could open between 9 or 11 amps. So do not run your circuit protection devices to their theoretical rating. Allow room for the tolerance. For example if your wire is good for 25 amps and you are loading that wire to 25 amps (which you shouldn't do) then choosing a circuit protection device is difficult. A 25 amp fuse may open at 28 amps which is risky, or it may open at 23 amps which makes it a nuisance because it is blowing out without a circuit defect. So… don't fully load your wires, and don't size your fuses for exact fuse ratings. Again, these statements apply equally to circuit breakers.

Some Common Breaker Examples

All of the breakers above except the RESET ONLY breaker work similar as far as the user goes. The breakers open and you reset it when repairs are made or you can open the circuit manually if you need to like a power switch. However the RESET ONLY breaker opens in a over current situation and when repairs are made you push the breaker button in and you are running again. BUT… you cannot open the circuit manually if you need to. You cannot grab the button and pull it out to open the circuit. These are used on many types of electrical equipment to protect the equipment more than your wiring and are mounted on or in the equipment. They shouldn't be used as power distribution breakers.


We have discussed circuit protection and the reasons for the need of these devices. We have also discussed some of the devices available to the marine electrician. In the last article of this 4-part article we will discuss putting all of this wiring and circuit protection knowledge together to design a reliable, safe electrical installation for your boat. In PART 4 we will go further into fuse and breaker sizing and selection. After you have chosen your equipment to be installed, you will chose and size your wires, set-up your main power distribution panel and then choose the right sized circuit protection devices to protect your system.

By Edward Scott, Bayside Marine Design
February 2016

Ed is owner of Bayside Marine Design, a firm that specializes in marine electrical system design as well as general small craft boat design. Ed was an instructor and former Director of the Eastport Boat School that was located in Eastport, Maine. You can contact Ed at:

Loading article data ...

All articles by {author}:


by {author} {month}, {year}


the Boatbuilding Ring - Click to Join
[ Prev ] [ Random ] [ List ] [ Next ]

Boat Design Net