November 2010|Online electrical engineering & Electrical Interview Questions

Saturday, November 20, 2010


What is the difference between Relay,Circuit Breaker,Isolator?

Relay is a protective device which senses the faulty conditions in a power system line and initiates the operation of circuit breaker to isolate the faulty part from the rest of the healthy power system.Examples of relays are Over Current relay,Directional relay,etc..

Circuit Breaker is a device which can
  • Make or break circuit under normal conditions either manually or through remote control.
  • Break circuit automatically under faulty conditions.
  • Make or break circuit under faulty conditions either manually or through remote control.
Some types of circuit breakers are Air blast circuit breakers,Vaccum Circuit Breakers ,SF6 Circuit Breakers,etc.You can notice the circuit breakers near to the transformers.Next time when you see a transformer search for the circuit breaker.Mostly it will be SF6 Circuit Breakers.

Isolator or Isolating Switch is a device which is used to disconnect a part of the system for maintanance purpose and repairs.Isolator can open a circuit under no load condition only.If two isolators are connected on either sides of circuit breaker,then the circuit breaker must be opened before opening the isolators.
After perfoming maitanancee work on the circuit breaker,isolators must be closed first and then the circuit breaker must be closed to bring the system into operation.

Wednesday, November 17, 2010


Hey Amit first of all thanks for asking your doubts.Let me apologize for not posting the answer for your questions for a long time.I couldn't post because of internet problems and other work issues.

1.When there is a sudden loss in the demand what happens to the transformer and generation? 
Consider a train running at 50 km/hr with 20 coaches.Assume that 5 coaches detaches when the train is running.What happens at this time is train will be experiencing an increase in speed and the engine driver has to reduce the speed to bring the train speed to 50 km/hr.

Similarly in a Power System,when there is a sudden loss in load or demand the speed of turbine(steam turbine, hydraulic turbine..etc)  driving the generator increases.So a SPEED GOVERNOR is employed to reduce the speed of the turbine by limiting the input to the turbine.

Consumers need a constant voltage profile for proper functioning of the electrical equipments.But when there is a sudden loss in load, generator terminal voltage is increased.We know that field coils are used instead of permanent magnets in generator.The field coils are excited using a exciter.If we adjust the input to the exciter, the magnetic field changes thereby changing the generator's terminal voltage.this is done by using AUTOMATIC VOLTAGE REGULATOR(AVR).

Speed Governor controls Real Power generation and the frequency of supply.
AVR controls Reactive Power generation and the voltage.