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Interview Question of circuit breaker and Switchgear


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1.What are the functions of a circuit breaker ?

Ans : The function of a circuit breaker is to make and break an electric circuit under normal and abnormal conditions.

2. What do you mean by the term “Switchgear” ?

Ans : A switchgear is a general term used for an electrical apparatus intended for switching and interrupting the current in the power system under normal and abnormal operating conditions covering the combination of switching devices and a wide range of associated auxiliaries e.g. controlling, measuring, protecting, regulating equipments along with their associated of switching devices and a wide range of associated auxiliaries e.g. controlling, measuring, protecting, regulating equipments along with their associated interconnection, accessories, enclosures and supporting structures. It comprises the following elements viz, switches, fuses, circuit breakers, busbar connection, relays, control panel, metering panel, current transformers, potential transformers etc.

3. What do you mean by the term “Opening time”, “Arcing time”, “Total break time”, “Make time” of a circuit breaker ?

Ans : Opening time – It is the time interval from the energisation of the trip coil to the instant of contact separation.

Arcing time – It is the time interval from the separation of contact to the extinction of the arc.

Total break time – It is the sum of the opening time interval and the arcing time interval.

Make time – It is the time interval from the energisation of the solenoid to the instant of making contact for current establishment during closing of circuit breaker.

Relay time – It is the time interval from the initiation of fault to the energisation of the trip coil.

4. What are the specified rating of an a.c. circuit breaker ?

Ans : The specified ratings of an a.c. circuit breaker are as following :-

Symmetrical breaking capacity – The breaking capacity in symmetrical condition is known as symmetrical breaking capacity which is generally expressed in terms of MVA and is equal to the product of rated symmetrical breaking current (k A), rated service voltage (kV) and a factor depending upon the number of phases (1 for single phase and √3 for three phase).

Asymmetrical breaking capacity – The breaking capacity in asymmetrical condition is as asymmetrical breaking capacity which is also expressed in terms of MVA and is equal to the product of rated asymmetrical breaking current (kA), rated service voltage (kV) and a factor depending upto the number of phases (1 for single phase and √3 for three phase).

Making capacity – The making capacity is a term used in connection with the rating of a switchgear to denote the capability of a switch, circuit breaker or similar apparatus to make a circuit by closing under short circuit condition. Making capacity is approximately equal to 2.55 times the symmetrical breaking capacity.

Short time rating – The short time rating of a circuit breaker is the highest permissible value of the current which the fully closed circuit breaker can carry for a specified time of about 1 to 3 seconds without excessive temperature rise and electrodynamic effects capable of causing injury.

Rated service voltage – The rated service voltage of a circuit breaker is the r.m.s. value of the operating voltage at which the circuit breaker shall be able to carry the normal current continuously at normal frequency.

Rated operating sequence –
0 (open) – t – Co (close open) – T – Co
(t = 0.3 or 0.5 second, T = 3 minutes)

5. Why are circuit breakers designed to have a short time rating ?

Ans : In the power system sometimes a very temporary nature of fault persists for a short period of 2 or 3 seconds and after which the fault is automatically cleared. To maintain continuity of supply the breaker should not trip in such condition. Therefore, breaker should be able to carry high current safely for some for some specified period during running i.e. the circuit breakers should have a short time rating.


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