ATA 24

Circuit Breaker Problem:
During the last aircraft inspection, DC electrical system circuit breaker CB2-4 was replaced due to high resistance across the contacts. This condition was detected during the routine inspection/test. The circuit breaker was replaced with a new unit. However, after the CB was replaced, the right STARTER & GEN ½ amp circuit breaker on the cockpit overhead panel opens immediately whenever DC power is applied to the bus system. Attempts to reset the CB are unsuccessful.
You suspect that the problem may be related to replacement of circuit breaker CB2-4 in the right DC contactor box. A visual inspection shows the CB is closed, and everything appears normal inside and outside of the DC contactor box.

Answer:
Circuit breaker CB2-4 located on the right DC contactor box was replaced due to high resistance. The circuit breaker contains auxiliary contacts that, when closed, provide a ground to the generator trip relay (GTR). This condition would exist if the circuit breaker opened for any reason, such as a short on the generator field wiring. When the circuit breaker was replaced, a small bit of solder flowed across this grounding circuit connection on the rear of the circuit breaker, shorting the GTR monitor circuit to ground. Whenever the distribution bus is powered, the GTR coil has bus power. This short is providing a ground on the fault monitor circuit to the GTR. The GTR senses the “fault” and does its job of opening the generator control circuit breaker on the overhead panel, protecting the field wiring from further damage. The lesson here is to pay particular attention to your soldering techniques when replacing either CB1-4 or CB2-4.