Sie sind auf Seite 1von 4


LAB No. 11
Fault :
Electrical fault is an abnormal condition, caused by equipment failures such as transformers and
rotating machines, human errors and environmental conditions. Theses faults cause interruption to
electric flows, equipment damages and even cause death of humans, birds and animals.

Types of Faults:
Electrical fault is the deviation of voltages and currents from nominal values or states. Under
normal operating conditions, power system equipment or lines carry normal voltages and currents
which results in a safer operation of the system.
But when fault occurs, it causes excessively high currents to flow which causes the damage to
equipments and devices. Fault detection and analysis is necessary to select or design suitable switchgear
equipments, electromechanical relays, circuit breakers and other protection devices.
By nature of electrical systems, at the basic level, electrical faults can be categorized as:
1. Short-Circuit Faults: This is caused when there is a failure of insulation causing a short-circuit
condition. This is by far the most common cause of failure.
2. Open Circuit Faults: This fault occurs when a failure happens in the conduction path of
Besides this, there could be combination (Simultaneous) fault situations as well and equipment level
winding faults.
Since Short-Circuit Faults are the most common causes of faults in Electrical distribution systems, let
us study them in detail:
There are mainly two types of faults in the electrical power system. Those are symmetrical and
unsymmetrical faults.

1. Symmetrical faults:
These are very severe faults and occur
infrequently in the power systems. These are
also called as balanced faults and are of two
types namely line to line to line to ground (LL-L-G) and line to line to line (L-L-L).
Only 2-5 percent of system faults are
symmetrical faults. If these faults occur,
system remains balanced but results in severe
damage to the electrical power system
This figure shows two types of three phase
symmetrical faults. Analysis of these faults is easy and usually carried by per phase basis. Three phase

fault analysis or information is required for selecting set-phase relays, rupturing capacity of the circuit
breakers and rating of the protective switchgear.

2. Unsymmetrical faults:
These are very common and less severe than symmetrical faults. There are mainly three types
namely line to ground (L-G), line to line (L-L) and double line to ground (LL-G) faults.

Line to ground fault (L-G) is most common fault and 65-70 percent of faults are of this type.
It causes the conductor to make contact with earth or ground. 15 to 20 percent of faults are
double line to ground and causes the two conductors to make contact with ground. Line to line faults
occur when two conductors make contact with each other mainly while swinging of lines due to winds
and 5- 10 percent of the faults are of this type.
These are also called unbalanced faults since their occurrence causes unbalance in the system.
Unbalance of the system means that that impedance values are different in each phase causing
unbalance current to flow in the phases. These are more difficult to analyze and are carried by per phase
basis similar to three phase balanced faults.

Causes of Electrical Faults :

Weather conditions: It includes lighting strikes, heavy rains, heavy winds, salt deposition on overhead
lines and conductors, snow and ice accumulation on transmission lines, etc. These environmental
conditions interrupt the power supply and also damage electrical installations.
Equipment failures: Various electrical equipments like generators, motors, transformers, reactors,
switching devices, etc causes short circuit faults due to malfunctioning, ageing, insulation failure of
cables and winding. These failures result in high current to flow through the devices or equipment which
further damages it.
Human errors: Electrical faults are also caused due to human errors such as selecting improper rating
of equipment or devices, forgetting metallic or electrical conducting parts after servicing or
maintenance, switching the circuit while it is under servicing, etc.
Smoke of fires: Ionization of air, due to smoke particles, surrounding the overhead lines results in
spark between the lines or between conductors to insulator. This flashover causes insulators to lose
their insulting capacity due to high voltages.


Circuit Diagram:
The following circuit is a 9 bus system designed in ETAP. It has three generators and 1 slack bus.
Load is being tapped off at different buses:

Fault at bus 4:


Fault at bus 1:

Fault at bus 9:

The most severe fault occurs on bus 9. The voltage of bus 9 becomes 0 V and on the other
buses, voltages and currents are greatly reduced due to which the whole system goes into the most
critical conditions.