GF Protection 517-17a (1999 NEC)

[ed downey]

I have a hospital that was designed with two levels of GF protection per 517-17a (1999 NEC) the incoming service is 4W with GF Protection but (2) of the (5) branch breakers are only fed with 3W and feed a 3 pole transfer switch (The breakers are set up with GF Protection). Since there is not a neutral conductor the GF will not work.

My question is this: How far do we need to carry the neutral conductor to make this system operate correctly. Do we need to change the transfer switch out to a 4 Pole since there is currently (1) 600A 4 pole transfer switch in the distribution system.

Here is how it is currently set up: Main switchboard 125A GF Circuit breaker feeds a 3 Pole 125A transfer switch (3 Phase 3 Wire). The transfer switch then feeds a 75KVA dry type transformer (3 Phase 3 Wire). Which then feeds a 225A 3P 4W Panelboard. {Also the feeder from the generator switchboard to the 125A 3 pole transfer switch is 3 phase 3 wire}

Thanks,
Ed

 

[jim dungar]

Re: GF Protection 517-17a (1999 NEC)

Why do you say that the GF will not work on a 3 wire circuit?

Most external (to breakers) GF systems are of the zero sequence type so they do not care how many wires are being used. Most internal (to breakers) ones are designed for either 3 wire or 4 wire. Of course, there are complicated differential schemes used on some systems but even these will work without a neutral conductor.

 

[ed downey]

Re: GF Protection 517-17a (1999 NEC)

Jim,

I do not know that I have ever seen GF protection on a 3 phase 3 wire 125A circuit. I have a call in to my local GE Rep to let me know if this will work. Currently called out as a GE #SGPB1 With integral ground fault protection.

If you have any more information I would be interested in how this works.
-Ed