Approach to LV ground fault protection on feeders

[philly]

What is the general approach to including ground fault protection on feeder breakers. I know the NEC requirement for over 1000A but didn't know if there was a standard approach for feeders less than 1000A.

For instance if I have a 800A 480V SWBD with an 800A main and (3) 500A feeder breakers feeding chiller units does it make sense to provide ground fault on these feeder breakers. Now days with electronic trip units its easy to include this function and to me its always been about a selective means to isolating ground faults to keep process running.

 

[paulengr]

What is the general approach to including ground fault protection on feeder breakers. I know the NEC requirement for over 1000A but didn't know if there was a standard approach for feeders less than 1000A.

For instance if I have a 800A 480V SWBD with an 800A main and (3) 500A feeder breakers feeding chiller units does it make sense to provide ground fault on these feeder breakers. Now days with electronic trip units its easy to include this function and to me its always been about a selective means to isolating ground faults to keep process running.

High reliability plants use high resistance grounds. So for instance we can use a resistor roughly the size of a briefcase to hold the ground fault current to say 10 A, low enough that the fault load can be carried by even the smallest conductors. Damage is minimized, and the system can run “indefinitely” with a ground fault present. Transients are almost as low as grounded wye unlike ungrounded systems. So we have to alarm by Code but we can trip too. So at the loads (motors especially) set the relays to trip quickly, say within 1-2 seconds. Then at the distribution level(s) add 2-4 seconds per level. All relays see exactly the same fault current so we coordinate using time delays rather than inverse time. In the alarm only version you can start switching loads to find it like in ungrounded systems but since there is a current you can trace it using a flexible current probe wrapped around all 3 phases. This is the lowest cost system to maintain because damage is minimized and faults are located rather than tripping immediately to minimize damage.

Outside of this system ground fault tripping is nice to have at motor loads. Inverse time tripping can be done but coordinating with overcurrent protection is often challenging.