Can thermal magnetric breaker be used to protect a ground fault?

[anbm]

I know the thermal magnetic breaker shall be tripped when detect a short circuit current exceed its AIC rating...

What's about the ground fault? Can it do same thing or we have to install GFCI breaker to protect personnel from
both short circuit and ground fault incident? Am I missing anything here? I am talking about branch circuits like
20A, 40A, etc.

 

[kwired]

I know the thermal magnetic breaker shall be tripped when detect a short circuit current exceed its AIC rating...

What's about the ground fault? Can it do same thing or we have to install GFCI breaker to protect personnel from
both short circuit and ground fault incident? Am I missing anything here? I am talking about branch circuits like
20A, 40A, etc.

Low level ground faults do not effect a standard thermal magnetic device, not enough fault current flows to get into trip curve. To detect such faults you do need differential current sensing type of components - A GFCI.

A "bolted" ground fault or other instance with low resistance and available high current will have a rapid rise in current and that is what the magnetic trip function is for on thermal magnetic breakers.

If the source can't deliver high enough current then the fault will last longer and possibly will depend on the "thermal" function of the breaker to interrupt current - but on thermal function trip time varies with how much "overload" is present.

 

[ron]

As mentioned, it depends on the amount of GF current. The breaker is dumb. It just sees current, it doesn't know the difference between a GF (L-G fault) or a L-L or a 3 phase fault (L-L-L) current.

 

[adamscb]

Depends on if the system is grounded or not. If the system is solidly grounded then a ground fault will trip a thermal-mag on magnetic action. If the system is ungrounded then a ground fault might or might not trip a thermal mag on thermal action, like k-wired said.

(in the three-phase world at least)

 

[Jraef]

Even if a standard Thermal-Mag breaker trips on a grounded fault, it’s not likely going to save lives. If you have a 20A breaker it will trip in approximately 1 minute at 40A of current flowing to ground. By then, if you were in the current path, your organs would be cooked from the inside. A Ground Fault breaker intended to protect humans uses a separate mechanism or algorithm that will trip within 5 seconds at no more than 6mA here in North America, or 30mA elsewhere in the world.

And by the way, your assertion of a T-M breaker tripping on short circuit current at its AIC rating is incorrect. AIC rating has nothing to do with where or when a breaker trips, it only means that the breaker will not explode in the attempt up to that level of available current. During a fault, all available current in a system attempts to flow into the fault. The breaker will try to trip at what it is set to trip at, usually a fraction of what is available. But if the breaker is not strong enough to withstand the total current attempting to flow, it can literally explode when it tries to stop it. That level where it will NOT explode is the AIC (Amperes Interrupting Capacity).