Main Breaker

[the man]

Ok I have been wondering this for a while now. The main breaker is set up for 120 amps at 48 volts. Does the voltage matter in this aplication? Or will it still trip at 120 amps at 12 volts?

[jason701802]

The voltage is a maximum rating, the breaker will trip at 120Amps at 48Volts or less.

[Sparks333]

The main breaker is a bimetallic device with extremely low resistance across it when closed. Since it does not go directly to ground, the voltage it drops between main leads and breaker output is equal to the current being passed times the resistance of the breaker - notice that this number is the same regardless of input voltage (unless you exceed the maximum voltage rating of the breaker, at which point insulators become weak conductors and you are no longer guaranteed a broken circuit). As a result, the breaker is dependent entirely on current flow, and will break at 120 amps regardless of input voltage (again, with the caveat that it must be within its operating region to guarantee this).

Since it's bimetallic, it trips based on heat - this leads to an interesting phenomenon many teams are aware of (but shouldn't take advantage of). Since the bimetallic strip represents a thermal mass that doesn't change temperature instantaneously, it is possible to get more current than rated through the breaker for short bursts of time - however, this does weaken the metal strip and will make it easier to trip on subsequent runs.

I hope this is helpful.

Sparks

[JamesCH95]

As I have just learned from Chris_Is_Me, the main breaker is good for more than 120A in a time-dependent relationship. Check out the spec sheet:

http://team358.org/files/electrical/120aMainBreaker.pdf

[the man]

Cool. Thanks, I wasn't sure.