Door Interlock for MCCB Incomers

The engineering reason for this interlock is to prevent the maintenance person from arc fault exposure or electrocution caused by live components exposed in the enclosure.

It has been shown that at 480 volts, arc faults can be self sustaining at typical load currents, so that the breaker might never trip.

The enclosure when complete (door closed) should withstand an internal fault without involving adjacent items.

Before arc fault awareness, this type of interlock would be used to inherently protect disconnect switches that did not have a load break rating. There is a key interlock on the door of the switch (usually high voltage) so that the key can only be retrieved from the main circuit breaker when the breaker is proven open. With that key, then the disconnect switch can be operated to the open position.

The effects of a failure of any type should not cause the failure of any adjacent components, whether electrical or other. This might preclude one from constructing an enclosure of paper mache, for an extreme example.

Arc plasma, thermal limits of insulation and conductors before upstream clearing, magnetic forces due to high current are a few of the engineering considerations.

I think that UL or CSA, or perhaps IEC would have caveats on that. I have only tangential experience with those organizations, but they generally offer guidance that is based on practical conditions and especially past experience.