Indeed, subcircuits that are small in relation to the generator capacity will be protected against short circuits/gross overloads by fuses or circuit breakers in the usual way.


Mains and submains, may not be properly protected, but that is is many circumstances a price worth paying, as has already been said "better than no electricity"


Large final subcircuits may not be properly protected and a bad fault may trip or stop the generator, again a price probably worth paying.


A rule of thumb is that standard types of fuses or circuit breakers should be rated at no more than about 10% of the generator capacity.


Example, a medium sized diesel generator with a full load capacity of 540*/670* amps per phase used for 100% backup to an office building.

Most final circuits are 32 amps or less and will be protected in the usual way without any special considerations.

Submains of 100 amps are NOT protected in generator mode and a bad fault might trip or stop the generator. Unlikely in practice since all such circuits had the ORIGINAL HRC fuses fitted from decades before.

Large circuits to lifts and chillers also not protected, but such machines have various protective devices and the fuses only in practice protect the cable. Faults on correctly specified large SWA cables with buildings are almost unknown.


*540 amps "prime power rating" for long hour use such as base load generation in locations without utility service. 670 amps "standby power rating" for short term use in case of utility failure or for peak hour use only.

Indeed, subcircuits that are small in relation to the generator capacity will be protected against short circuits/gross overloads by fuses or circuit breakers in the usual way.


Mains and submains, may not be properly protected, but that is is many circumstances a price worth paying, as has already been said "better than no electricity"


Large final subcircuits may not be properly protected and a bad fault may trip or stop the generator, again a price probably worth paying.


A rule of thumb is that standard types of fuses or circuit breakers should be rated at no more than about 10% of the generator capacity.


Example, a medium sized diesel generator with a full load capacity of 540*/670* amps per phase used for 100% backup to an office building.

Most final circuits are 32 amps or less and will be protected in the usual way without any special considerations.

Submains of 100 amps are NOT protected in generator mode and a bad fault might trip or stop the generator. Unlikely in practice since all such circuits had the ORIGINAL HRC fuses fitted from decades before.

Large circuits to lifts and chillers also not protected, but such machines have various protective devices and the fuses only in practice protect the cable. Faults on correctly specified large SWA cables with buildings are almost unknown.


*540 amps "prime power rating" for long hour use such as base load generation in locations without utility service. 670 amps "standby power rating" for short term use in case of utility failure or for peak hour use only.