MB Trip

[A/A Fuel GTX]

What would cause a ground fault on a 240V, 20A circuit to take out the main breaker ( 200A ) versus the breaker feeding this circuit? I had an HVAC guy that was too lazy to go downstairs and shut the breaker feeding a condensing unit off so he caused an intentional ground fault at the condensor disconnect to kill the power. Now the HO is on my case because the main tripped.

 

[luckylerado]

is it an instantaneous trip MB and an inverse time 20 amp?

 

[A/A Fuel GTX]

is it an instantaneous trip MB and an inverse time 20 amp?

Good point.......Not sure.

 

[jim dungar]

Unless the equipment was special ordered, both breakers are thermal magnetic, which means they will overlap in the instantaneous portion of their curves.

If the fault current was sufficiently high (about 1000A) then both breakers may have tried to trip at once, and the 200A simply went first. This is a prime example of why emergency systems (Art 700, 701) need to be specifically 'selectively coordinated'.

 

[kwired]

The more available fault current the more likely that can happen.

As Jim mentioned the trip curves of the two breakers can overlap.

This does not happen as often in most dwellings because often times the available fault current is not high enough to trip the main before the smaller breaker opens the fault. The main would still trip if the fault were to last for a longer time.

Commercial and industrial often have transformers located closer to service equipment and/or larger service capacity than residential. Shorter service drops or laterals with everything else the same will increase available fault current at the service equipment.