All…
My garage had only one outlet so I extended it using surface raceways. Like a dummy, I did not turn off the breaker when I started the installation of the raceway. "I know what I'm doing and I wrapped electrical tape around the existing outlet, and I'll be careful". (Stupid and lazy!) Of course, I created a short. No injuries – just my pride.
When I went down to the basement to check things out I discovered something odd. I'm not sure if this is something I should be concerned about…
The 15A garage circuit is in a subpanel. Our automatic backup generator (Generac) is not large enough to power everything in the house. The subpanel contains a transfer switch. Commercial power comes into the transfer switch via a 50A breaker in the main panel.
The 15A garage circuit breaker tripped, as expected. What I did not expect was to find that the 50A breaker in the main panel had also tripped. The generator and transfer switch did their thing, firing up the generator and throwing the transfer switch. I reset the 50A breaker and everything went back to normal, again as expected.
Is the tripping of the 50A breaker in the main panel something I should be concerned about? I would have expected that tripping the 15A breaker would have "protected" the 50A breaker. Is there a test I should do on the two breakers to make sure they are functioning correctly? They seems to be but…
TIA for your help…
Best Answer
Grats on your successful generator test :)
Whenever you have a bolted fault, you flow hundreds of amps. The only thing that impedes flow is the resistance of the wires themselves, which you can look up on the internet. "Hundreds of amps" is well within the range of the "instant trip" functionality of every breaker in the panel except perhaps the main.
Due to manufacturing tolerances and environment changes, "instant trip" has an acceptable range. The acceptable range runs between 4x and 10x, where it becomes a guaranteed trip.
So for instance, a 500A bolted fault is a guaranteed trip for a 15A and 50A breaker, and a possible trip for a 100A main breaker. Probably will not trip a 200A main.
If your short had flowed only 90 amps, both breakers would operate in thermal-trip mode (most likely). The 15A breaker would see a 6x overload, and the 50A breaker would see a 1.8x overload. The 15A breaker would trip in a few seconds at most, and the 50A breaker would trip in a few minutes, based on the usual trip curves for breakers. In that case, the "selective coordination" you were hoping for would work as you'd expect.
However, all bets are off in a bolted-fault/instant-trip mode.