MARS 190 - August 2008
Main breaker malfunction
Most air circuit breakers used for generator and power switching systems are provided with an automatic release mechanism. Those that do not have an ‘undervoltage’ or ‘novolt’ automatic release (or where one is fitted but is not working) may accidentally remain closed even when a ‘trip’ condition exists. In such a case, there is a serious fire hazard due to intense heating and/or arcing.
It is usual for shipboard generators also to be fitted with remote interlocks that open the main breakers under ‘no-volt’ or ‘undervoltage’ conditions. The first indication of a defective ‘no-volt’ or ‘undervoltage’ release is often that the remote release system of the breaker does not operate, requiring manual tripping of the release to keep the breaker open. This condition should be recognised as potentially dangerous, in that other breakers will probably be inoperative as well, and arrangements must be made for immediate checking, overhaul or renewal, as appropriate.
Such an accident happened recently on board one of our vessels, which resulted in serious damage to the main switchboard. Fortunately no one was hurt in the incident.
After withdrawing any breaker, make sure that the switching contacts are open before returning it to the operating position. Do not rely on built-in trips or mechanical stops. While the breaker is withdrawn, check that the indicator is showing true by sighting the switching contacts, or by using an ohmmeter. Check the indicator is showing open again before returning the breaker to the operating position.