In the present modern era, the demand of electrical energy has greatly increased. Most of this energy is utilized in lighting, heating, household equipments, industrial machinery and in electric traction. The demand of electric supply is so high that in case of a faulty situation, the healthy system must be isolated from the fault in order to ensure continuity of the supply. For this purpose, switch gears are used to test generators, transmission lines, distributors and other electrical equipments during normal and abnormal conditions. In other words, switch gears are used for the protection of electrical power system.

All the apparatus used for switching, controlling and regulating an electric circuit are called switch gears. A switch gear basically consists of switches, protective devices, fuses, isolators, circuit breakers and relays etc.

 In normal operation, a switch gear is used to turn the generators, transmission lines and distributors on or off. But whenever a fault occurs on a power system, heavy currents flow and if the system is not isolated then it causes damage and the supply of other systems, which are not connected to the faulty system, also gets affected. That is why automatic switch gears are needed for the protection of generators and lines. Whenever a fault occurs in a system, the protective relay operates and signals the circuit breaker to trip. This way the circuit breaker disconnects the supply to that system and protects our main power system.

Tumbler switch used in houses and offices is a fine example of a simple switch gear. A fuse is connected to it which protects and controls lights and other equipments. For higher ratings, High Rupturing Capacity (HRC) fuses are used. However such a switch is not beneficial for much high currents and voltages because when the fuse melts, it takes a lot of time to repair it.

With the development of the power system, now lines and other equipments can be operated on high voltages and currents. Heavy currents passing through a system in faulty conditions, damages the equipment. To stop this from happening, automatic circuit breakers are used. Circuit breaker is an equipment which opens or closes according to normal or abnormal conditions.

Switch gears are normally of two types:

  1. Outdoor type
  2. Indoor type

Above 66kV, outdoor circuit breakers are used. The spacing between the conductors increases and the size of insulators also increases due to which the installation cost increases.For a switch gear below 66kV, a stand is constructed. Moreover, such types of switch gears are metal clad type which takes less space and are also less expensive.

From the above discussion, we can conclude that a switch gear is used for the following purposes:

  1. To isolate the system.
  2. To disconnect the faulted system from the rest of the system.
  3. To provide Instantaneous protection against fault current.
  4. To keep the consumers supply from being interrupted.


As the circuit breaker has to make or break the current according to its rated short circuit capacity therefore, before choosing a circuit breaker and other equipments the current in the system due to normal and abnormal conditions, must be known. Generators, bus bars, circuit breakers and isolators are also chosen according to their short circuit ratings.

For the selection of protective relaying scheme, the currents due to faulty and unfaulty conditions should be known. The protection scheme cannot be designed without it. For steady state DC fault calculations, the steady state reactances of the respective components are used. For AC calculations, RMS values are used.

Following points are to be considered for calculating the fault current:

  1. Resistances are neglected as they are less than the inductive reactances.
  2. Capacitance of transmission lines is neglected.
  3. Saturation effect of generators, motors and transformers is also neglected.
  4. Generated voltage is kept constant.

As the impedance of the faulted area is very less, therefore the MVA depends on the reactance of the system. Also, due to the low value of impedance, the voltages of the system during fault, also gets unstable. Therefore system stability is also kept in check during different types of faults in a system. Usually fault MVA is much greater as compared to normal MVA of the system.

Fault current levels in transmission and distribution systems:

Steady state current level (kA)                                                        System voltage (kV)

40                                                                                                        11

25                                                                                                        33

30                                                                                                        132

40                                                                                                        220

60                                                                                                        400

In an interconnected power plant, if generated MVA is greater, then fault currents will also be greater.

Fault calculations provide the basics for selecting the protection schemes. For fault calculations, first, we make a single line diagram. It provides the important information of a power system. If load flow study is to be done through the diagram then circuit breakers and relays are not shown. As in the following figure:

one line diagram of power system
one line diagram of power system

On the other hand, if we have to study the stability of the system, then circuit breakers are also included in the one line diagram.


one line diagram another way
one line diagram another way

Leave a Comment