On a dead short, live to earth, the fuse or MCB will almost always operate before the motor overload.
If however an earth fault occurs part way along a motor winding, then the fault current will be much reduced by the impedance of the sound part of the winding, under these circumstances then the motor overload will probably operate first.
Another and rather dangerous possibility is that the earth loop impedance at the motor is very high, perhaps due to a missing or damaged earth connection. An excessive earth loop impedance might limit the earth fault current to say twice the full load current. That will eventually trip the overload, but probably not operate the MCB, which may be sized at more than twice the running current.
This is very dangerous since during any future earth fault, a dangerous voltage might persist for some time on the motor casing.
I would strongly advise an earth loop test at the motor.
On a dead short, live to earth, the fuse or MCB will almost always operate before the motor overload.
If however an earth fault occurs part way along a motor winding, then the fault current will be much reduced by the impedance of the sound part of the winding, under these circumstances then the motor overload will probably operate first.
Another and rather dangerous possibility is that the earth loop impedance at the motor is very high, perhaps due to a missing or damaged earth connection. An excessive earth loop impedance might limit the earth fault current to say twice the full load current. That will eventually trip the overload, but probably not operate the MCB, which may be sized at more than twice the running current.
This is very dangerous since during any future earth fault, a dangerous voltage might persist for some time on the motor casing.
I would strongly advise an earth loop test at the motor.
