J
jojo
Ronald,
from the details you have given, I am inclined to think that there was some looseness between the contact rose of the circuit breaker and the fixed contact of the switchgear assembly. This resulted in the overheating of the said contact, and the ensuing heat damaging melting the adjacent assemblies. In such a situation the actual current passing through the circuit breaker remains the same and so the protective device will detect the failure, until an insulation part somewhere in the assembly fails leading to an earth fault. Fortunately it seems that you just managed to get to the point where things started failing, but before the final catastrophic flash-over.
Suggest two things, especially if you have similar circuit breakers in your plant:
1. check the internal resistance of the circuit breaker, obviously with the CB racked out but in closed position. Compare the readings you get with manufacturer specification.
2. insert dummy fixed contacts (which should be an exact replica of the fixed contact you have in the switchgear assembly) in the circuit breaker roses, and check again CB resistance. Compare with manufacturer specification. In the event that there is weakening of the rose springs this test should indicate a higher than spec resistance.
As a final check, ensure that the CB racks in parallel and in line with the fixed contacts, and that there is no lateral stress on the rose. This will also increase the contact resistance.
from the details you have given, I am inclined to think that there was some looseness between the contact rose of the circuit breaker and the fixed contact of the switchgear assembly. This resulted in the overheating of the said contact, and the ensuing heat damaging melting the adjacent assemblies. In such a situation the actual current passing through the circuit breaker remains the same and so the protective device will detect the failure, until an insulation part somewhere in the assembly fails leading to an earth fault. Fortunately it seems that you just managed to get to the point where things started failing, but before the final catastrophic flash-over.
Suggest two things, especially if you have similar circuit breakers in your plant:
1. check the internal resistance of the circuit breaker, obviously with the CB racked out but in closed position. Compare the readings you get with manufacturer specification.
2. insert dummy fixed contacts (which should be an exact replica of the fixed contact you have in the switchgear assembly) in the circuit breaker roses, and check again CB resistance. Compare with manufacturer specification. In the event that there is weakening of the rose springs this test should indicate a higher than spec resistance.
As a final check, ensure that the CB racks in parallel and in line with the fixed contacts, and that there is no lateral stress on the rose. This will also increase the contact resistance.