Jay see 1997 GU build up

[PeeBee]

.

Those circuit breakers ( guessing based on another brand of Automotive C/B's ) probably have an instantaneous trip (<0.4sec) of 10×Rated current.
That is 10x120A=1200A. If your circuit is undersize, poorly jointed or whatever , the short circuit current wont be high enough to trip the breaker instantly,

I am sure they will trip at a point well before the insulation is compromised is perhaps the message. They are a cheap generic design and I have them in use and they certainly have worked for me when i have replicated the short circuit fault.