Neutral Bonding Issue with ATS, Generator, and UPS 1

[fieldy]

I was listening to a conversation recently regarding the following situation (as I understand it).

Location - USA
Incoming 13.2kV:480/277Y Utility Feed to Normal side of an ATS, Emergency side of ATS is fed from a 480/277Y diesel powered genset.
ATS feeds a breaker, followed by a UPS.

The issue seems to be with the bonding of the neutral. The utility has their neutral bonded to ground out at the street, and the generator neutral is bonded to ground at the generator. There is also a bonding point between neutral and ground at the UPS.

Folks here are wondering if that UPS neutral to ground bonding point is a second bonding point that will now violate code, and create circulating currents under normal situations (bonded neutral at utility/generator as well as bonded neutral at UPS), and they want to break the neutral to ground bonding point at the UPS.

If I understand what I read in a post from last year:

http://www.eng-tips.com/viewthread.cfm?SQID=53912&SPID=242&newpid=242&page=2

- as well as hearing what the UPS manufacturer had to say, the UPS is actually a separately derived source, so it needs to have its own bonding point. Additional information on the UPS is that the incoming neutral is directly connected to the output neutral.

Another issue with lifting the bond at the UPS is that the 4-pole ATS switch is a break-before-make arrangement. During a swap from Normal to Emergency, the UPS would be running without a ground reference for any 4-wire loads.

I would like to know what is the proper solution for this situation. Your time and effort is greatly appreciated.

 

[robertb]

Fieldy,

Please inform whether the UPS has integral transformer and maintenance bypass.

Regards

 

[alehman]

Many UPS's can be configured either way. Some models can be reconfigured in the field as you are suggesting. Others cannot. As mentioned in the previous post, it depends on whether or not there is an integral transformer and how it's connected. UPS's with matching input and output voltages commonly are not configured as separately derived systems. In this case, the static bypass source neutral is usually carried through to the output and there is no neutral-ground bond. You need to carefully read your UPS instruction manual or consult the manufacturer's engineers to figure this out. If there is a neutral-ground bond, you should have only 3-wire circuits going to the UPS.

 

[fieldy]

In response:

I've been told that the UPS has a wraparound maintenance bypass external to the UPS. When the system is in bypass, there is no transformer in the circuit.

Further review has lead the UPS manufacturer to state that the neutral to ground bond can be removed at the UPS as there is no transformer on the bypass circuit, there are no 4 wire loads, and that any internal control power is not referenced to ground as it is received from the internal DC link.

I think that about does it, thanks for your input - but if there is something else that's glaring out at you, please feel free to post.

 

[jbartos]

Comment on the original posting: The 480V/277V power supply implies that the neutral is installed downstream to some power distribution hardware. The load may be three phase ungrounded and without neutral, i.e. 3-phase, 3-wire. Since there are two 480V/277V solidly grounded sources, ground fault protection should be installed to each. This will require to have the three pole ATS with the neutral switching contact make-before-break. This is what is required upstream from UPS by NFPA 70 NEC. Downstream from ATS is UPS with bypass switch. There are various principles of ATS. However, if the output of UPS is 3-phase, 3-wire load, the power supply may be ungrounded or high-resistance grounded or medium resistance-grounded. The ungrounded power supply is less used and frequently replaced by the high-resistance system grounding to suppress the transient overvoltages the ungrounded 480V system can produced during arcing. This needs to be further investigated over the manufacturer literature and tech support.

 

[rbulsara]

The bad item you have is the ATS.

You have three sources of power: utility, generator and the UPS.

The 4-pole ATS permits generator to be configured as a separately derived source and the generator is correctly grounded.

For the UPS, the inverter output has to be grounded, just like any other source (like generator) but whether it should be a separately derived system or not is the function of the configuration of the UPS loads, if all UPS loads are 3 wire not requiring a neutral, the UPS is a seprately derived system and its neutral is sepratley grounded, say the building steel. You should be not be even be bringing the system neutral to the UPS in this case.

If the UPS output loads requires a neutral, then it HAS to be configured as 'not separately derived' system, that is UPS inverters (output) neutral is solidly connceted to the bypass input supply neutral and not separately grounded.

Your fear of ATS is correct, if it is a 'break-before-make' switch. In that case the problem is the ATS and not anything else. You should not be using 'break-before make' neutral switching to begin with.

 

[alehman]

As an example, various grounding arrangements for this model are shown starting on page 19 of this document. Figure 1 is not separately derived. The neutral is not grounded at the UPS. Figure 2 is "separately derived" and the neutral is grounded at the UPS. In all cases where the neutral from the supply is brought to the UPS, it is not grounded and therefore not a separately derived system.

http://www.liebert.com/assets/produ...hase/ser610/60hz/instman/acrobat/sl_25140.pdf

I agree that the open transition neutral ATS could be a problem, although a lot of systems have been installed that way. I couldn't find any NEC requirements for overlapping neutral.

 

[jbartos]

Suggestion: Contact ASCO for ASCO FACTS

http://www.asco.com