Circuit breaker ratings explained | Breaking and Making capacity

Circuit breaker ratings and specifications: 

A circuit breaker may be called upon to operate under all conditions. However, major duties imposed on the circuit breaker when there is a fault on the system in which it is connected. Under fault conditions, a circuit breaker is required to perform the following three duties.
  1. Must be capable of opening the faulty circuit and breaking the fault current.
  2. Must be capable of being closed on  to a fault.
  3. Must be capable of carrying fault current for a short time, while the other circuit breaker is clearing the fault.
According to the duties, the circuit breaker is to perform, it has the ratings namely
  • Breaking capacity
  • Making capacity 
  • Short time duty
In addition to the above ratings,a circuit breaker should be specified in terms of number of poles, service voltage, normal current, frequency and operating duty.

Circuit breaker ratings calculation:

Breaking capacity of cb:

It is defined as the r.m.s current that a circuit breaker is capable of breaking at given recovery voltage and under specified conditions.
The breaking capacity is generally expressed in terms of MVA and is equal to the product of rated breaking current in kilo-amperes, rated voltage in kV and a factor which depends upon the number of phases i.e. breaking capacity calculation formula is given by
Breaking capacity in MVA = √3 x rated service voltage (kV) x rated symmetrical current (kA)
The agreed international standard of specifying breaking capacity is defined as the rated symmetrical breaking current at a rated voltage.

Making capacity of circuit breaker:

It is defined as the peak value of current (including d.c component) during the first cycle of current wave after the closure of circuit breaker.
There is always a possibility that the circuit breaker is closed under short - circuit conditions. The making capacity of the circuit breaker depends upon its ability to withstand the effects of electromagnetic forces which are proportional to the square of the peak value of the making current. The peak making current of a circuit breaker when closed on a short circuit is the peak value of the maximum current wave in the first cycle of the current after the circuit is closed by the circuit breaker. According to BSS116 the maximum value of asymmetrical current is 2.55 times symmetrical current. The making capacity calculation formula is expressed as 
Making capacity = 2.55 X symmetrical breaking capacity
Rated making capacity = 1.8 x √2 x rated symmetrical breaking capacity

Short time rating of circuit breaker:

It is the period for which the circuit breaker is able to carry fault current while remaining closed.
The short time rated current is the current that can be safely applied, with the circuit breaker in its normal condition, for 3 seconds, if the ratio of symmetrical breaking current to normal current is less than 40, or for 1 second otherwise.
The short time rating of circuit breaker depends upon its ability to withstand
  • The electro-magnetic force effects and 
  • The temperature rise.
Short time rating = 3 seconds
Short time rating = Symmetrical rating current / Rated normal current

Normal current rating of a circuit breaker:

The rated normal current of the circuit breaker is the r.m.s value of the current which the circuit breaker shall be able to carry continuously as its rated frequency under specified conditions. The important condition for normal working of an OCB is that the temperature of oil should not be more than 40°C and that of contacts should not exceed 35°C.

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