Fault Current

[winnie]

Jon,

I'm not familiar with the term 'series rated' Could you shed some light?

The idea is that the _combination_ of two OCPD in series gets rated for higher short circuit current than the individual devices.

The most common situation is a panel where the main OCPD has a higher rating than the branch OCPD, but the 'series combination' can handle the higher fault current.

The devices have to actually be tested and demonstrate proper protection against the short circuit current, and you lose things like coordination; the main might open before the branch breaker. But it lets you use less expensive hardware.

-Jon

 

[kingpb]

They call it series rating but it's basically doing the AIC rating based on through fault calculations. I have done this at the MV level where old gear was having new motor loads added to it, and didn't want to have to do a complete switchgear changeout. Panelboards, don't see why it would ever be considered.

Series rating is ill advised and although may seem to be an initial cost effective way to keep the price down, it does a dis-service to the end user and because it is not well understood creates a hidden danger to maintenance personnel.