Transfer Switch and Neutral Connections

[Roger Crawford]

I have a system that has a generator with a standby transfer switch and a life safety transfer switch. Both transfer switches are 3-pole, so not a separately derived system. The life safety transfer switch feeds a 4-wire + ground panelboard, and the standby transfer switch feeds only a 3-wire 480 volt 3-phase panelboard. The question is, with a non-seperately derived system is a neutral conductor required to be brought to the transfer switch from both the generator and normal power feed? If the transfer switch is in the standby position and a phase-ground fault ocurs how will the breaker at the generator see enough current to trip, without the neutrals tied together?

 

[benaround]

Roger,

I agree with you, for the same reason we are required to bring in a neutral

on any 480/277vac service at least to the first disconnect, even if all loads

are three phase three wire. This is your neutral/ground connection, MBJ.

 

[dbuckley]

The real question here is where is the neutral earth bond? If it is before the transfer switches (as I believe it will be), the the generator neutral is already permanently bonded to ground. Thus bringing a neutral to the three wire panel will have no functional purpose, it'll just end up on an empty busbar. There is already a permanent path from ground to neutral for both utility supply and generator supply (and, though perhaps irrelevantly, no supply at all) situations which is where the fault current for a phase/ground short will return through.

If the neutral earth bond is after the transfer switches, I think you have a problem, as you should have four pole transfer switches, and then it all gets much more confusing.

 

[benaround]

dbuckley,

The neutral of a non-SDS can not be bonded to ground at the generator as

this would cause a Load side neutral-to-case bond. The neutral from the Gen.

must be brought to the transfer switch and tie to the system neutral.

 

[dbuckley]

I agree, thats exactly what I said.

You noted that "The neutral from the Gen. must be brought to the transfer switch and tie to the system neutral."

Thus "There is already a permanent path from ground to neutral for both utility supply and generator supply" - I didn't say it as it's implied, but it is, as you rightly note, the "system neutral", which is where the N/E bond occurs, which is the hallmark of three pole transfers: the neutral and ground stay solid at all times.

Assuming of course, that it's all wired the way we both think it is

 

[RUWired]

dbuckley,

The neutral from the Gen.must be brought to the transfer switch and tie to the system neutral.

If the gen set is a non sds, then i can be thought of like any other piece of 3 phase equipment with out a neutral.The equipment ground conductor will carry the fault current back through the service equipment.
Rick

 

[nakulak]

I don't understand why they even sell ats's that don't switch the neutral. There's so many things that can go wrong with the power supply while its still attached at the poco neutral.

 

[benaround]

RUWired,

How will the fault current get back to the gen. neutral without tieing it in

with the system neutral??

 

[RUWired]

RUWired,

How will the fault current get back to the gen. neutral without tieing it in

with the system neutral??

The fault current would travel back through the main bonding at the service equipment and then through the transfer switch that has the neutral.If this was a stand alone 3 wire system then it would need to have a switched neutral xfr switch so the neutral could be bonded at the generator.
Rick

 

[dbuckley]

I don't understand why they even sell ats's that don't switch the neutral. There's so many things that can go wrong with the power supply while its still attached at the poco neutral.

Having specified it both ways over the years, I'm now of the opinion that solid neutral is less bad than switched neutral. Of course, your common neutral bar better be made of strong stuff, for when the neutral becomes a hot (failed overhead wire, for example) and you've got not only the load on the bar, but the fault current as well, but in terms of delivering a safe and electrically quiet environment solid beats switched (almost) every time.