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Switchgear Riddle No.8 - Replace of circuit breaker
Operation group of an industrial network as illustrated figure below decide to replace one of incoming circuit breaker of main distribution switchgear which failed due to a problem in its control mechanism, they don't access to any reserve unit and replace it with one of switchgear outgoing circuit breaker with similar rated current; because they think continuous power supplying of network is important than having of all outgoings, but after a time operation switchgear front to a sever busbar fault current and all of incoming circuit breaker except mentioned replaced one trip successfully.
How do you can explain the reason of that breaker mal-function?

Author : Hamid - From: Iran- Firouzabad in Fars
Thu, April 29th, 2010 - 07:54
In general α is selected as 2.5  for ordinary circuit breakers . But for points very close to generator and as GCB it will be 3.0 . It is probable this C.B. has had malfunction due to high peak of subtransient short circuit  current .
On the other hand these types of CBs has the advantage of current limiting ,namely , in case of short circuit the down stream CB will be fronted with less short circuit current . Probably , the designer has sized the outgoing feeders according to this phenomenon . But , now the function and duty of CB has changed and it is not outgoing feeder any longer . It is incoming breaker . 
Author : amir khashayar - From: IRAN
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