Very few electrical failures are of the "bolted fault" type. With any other type of fault the arc itself will limit the fault current. This impedance will increase the trip time of the OCPD. This is especially true of 480/277 Y systems. The tendency for the arc to limit the current is exactly why the code requires GFP for OCPDs rated 1000 amps or more on these types of systems. The arc limits the current to the point that the breaker "thinks" that it is just seeing a "normal load" current, but in reality the switchgear in "melting down".
In the case that started this thread, assuming a typical 20 amp circuit breaker is used as the OCPD, a total impedance in the fault circuit of 1.6 ohms could result in a breaker clearing time of 5 seconds.
Don
[This message has been edited by resqcapt19 (edited 04-08-2004).]
