Guy's,

I have an ex-colleague who share his recent experince to me about major tripping incident.

what happen is one of the equipments had an earth leakage and cause the sub pdu tripped. It also cause the upstream main pdu tripped as well.

Any idea why is that so? I thought the sub pdu should be able to "fence" off before it manage to affect upper stream? If the ELR setting wrong?

I suspect the sub pdu ELR time delay setting was slower then the main pdu. Do i make any sense?

The designer got the discrimination wrong or you were running the load to close to the breaker settings.

What was the loads on each breakers and what are the types of breakers and their trip settings, do any of the breakers have GF protection?

Lots of questions unfortunately.