Circuit Breakers 1

[kimseunghee]

It is found that circuit breaker shall satisfy that the r.m.s symetrical breaking current for which the device is rated is to be not less than the r.m.s value of the a.c component of the prospective fault current, at the first half cycle.(previously at the instant of contact separation).
*Revision*
"Instant of contact separation --> the first half cycle"

It looks that circuit breaker should be selected as more breaking capacity in selecting Circuit breaker as per the above revision. IS that right ? What does that mean ?

 

[waross]

Although equipment is rated for symmetrical current, it is tested to interrupt the asymmetrical current that is expected to be associated with the available fault current which is the symmetrical component of a fault current (by definition).
"Instant of contact separation" sounds as if it would apply to a special class of equipment known as "current limiting".
What code are you referring to?

Bill
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"Why not the best?"
Jimmy Carter

 

[Marmite]

This was a change in the IEC standards. The concept of dc component at contact separation was used in IEC60056 and the first edition of 62271-100. It was found that the testing leads to stresses different from those expected under service conditions. This method was changed to allow equivalence of asymetrical tests at different laboratories using test circuits with different time constants, and to be able to use the results from one test to cover more than one dc time constant rating. The equivalence is obtained by introducing the concept of dc component at current zero and a standard curve for %age dc component against time from short circuit initiation. The required dc component is determined by adding a half power frequency cycle to the circuit breaker opening time to represent the relay operating time. The normal time constant for distribution switchgear is 45ms. Armed with the standard curve, if you had a circuit breaker with an opening time of 40ms on a 50Hz system the total time is 50ms, which corresponds to 30% dc component. There are defined time constants for 45,60,75 and 120ms. High DC applications requiring the 120ms curve are associated with locations near to generators and rotating plant, such as oil rigs.
Regards
Marmite