Isolation Transformer Sets Off Alarm on Battery Backup


Thread Starter


In the US we have a machine with a transformer that takes 208/240 power and converts it to 120v power. The neutral on the secondary is floating. I get 120 between T1 and Neutral and around 60 from Neutral to ground or T1 to ground as expected. We have a simple 120v APC UPS downstream in the system. IT shows alarm due to neutral not being bonded.

Should neutral be bonded?

What are the issues if it is? Ground loops etc?

Where should it be grounded? Run all neutrals to one spot then bond to PE at exit?

If not bonding neutral in the system how to we keep the UPS from alarming?

thanks in advance
The neutral should always be grounded at the transformer, I believe in systems above 50v and less than 600v the NEC requires it. See Article 250 of the NEC.

The primary of the transformer should be a floating connection line-to-line; the secondary with a grounded neutral; and if you're fusing the secondary, it should only be 1 fuse for the hot leg.

You shouldn't have any problems in this configuration.
NEC 250 does not apply to industrial equipment in this context. A Isolation transformer has a shield that is ran to PE but neutral is not grounded. IF you ground neutral on the secondary side you break isolation. The neutral on the primary side would be bonded at the main panel per NEC guidelines.

I have read dozens of posts on this subject and everyone has a different OPINION. No facts to be found. So I still do not have a definitive answer.

All panels and a components in the machine have chassis grounds. Neutral is just floating.

The fusing is a interesting point. Currently both legs are fused Hot and neutral. I would like to find some documentation to detail proper fusing for Isolated transformer secondaries. I can find sizing but nothing about not fusing or not fusing the neutral.


The fact is... NEC Article 240.22 prohibits installation of a fuse in series with any conductor intentionally grounded, except in one situation.

Article 420.36 states (paraphrasing) where fuses are used for motor overload protection in 3-phase, 3-wire systems having one one conductor grounded, such as the Mid-point Grounded Delta (Hi-Leg) or Corner-grounded Delta.

Phil Corso
What you have isn't really an isolation transformer, it's just transforming the incoming power to a voltage required by some of the sub components of the machine, so isolation isn't really its purpose. By the alarms on the device, for some reason the device requires a grounded neutral 120v circuit, which is common everywhere 120v is used, it's what everybody is expecting. The primary of your transformer should always be floating, 120v neutral should always be grounded. Otherwise it's not a neutral, you would have 2 hot legs 120v which is unusual.

I've seen isolation transformers used for drives installed in a facility that is not TT or TN, it's always a floating delta primary, and a wye secondary with the neutral grounded at the transformer. It's important that the primary be floating because you can't know what's going on with the power upstream, the transformer needs to float with the incoming power.

240v secondaries it's common to have two hot legs with two fuses, single phase 240v devices are designed with this in mind.

120v single phase devices are sometimes designed to rely on the neutral being grounded for safety reasons. Look in your house, two prong plugs with one prong bigger, that's probably a good example of this.

What you have is a UPS, which contains a DC battery, that probably relies on chassis grounded for short circuit protection. If it's throwing alarms about an ungrounded neutral, the manual probably states somewhere that this is a requirement. Are there other devices on the machine's 120v circuit that would conflict with a grounded neutral? I can't imagine anything that would
What I have is a Isolation Transformer. Its not a Autotransformer. There is no path from the Secondary to the Primary. Not sure why you say its not. Can you explain. Maybe I am missing something. See

Wiring diagram 20 for how this thing is wired.
Primary Voltage is 208 (H2 to H7 interconnect) H1 is L1 and H8 is L2
Secondary Voltage 120 (X1 to X3 X2 to X4 interconnect) T1 is X1 and Neutral is X4.

Provides no ground. Neutral is Floating. This is not a house wiring problem. It is in a industrial piece of equipment.

I have read they use this type of transformer in the medical industry for safety and in controls to help prevent ground loops. All AC switches in the machine are double pole. Both lines from transformer are switched. Both lines are fused. I looked at NEC 240 and I cannot find provision for fusing or not the neutral in a isolated setup. This is not a delta or wye 3 phase setup its single phase power.

I want it to be safe and comply with code. I am not the one that set it up this way but do have to verify it is correct.

I believe the battery backup will never be happy since its never seeing ground and if I tie its neutral to ground I will lose isolation defeating the purpose of a isolation transformer.

Still lost in a sea of misinformation.

Just to clarify, I brought up the 3-phase isolation transformer, and household single-phase neutral wiring, just as some common examples for background on concepts of isolation transformers and common-practice neutral wiring.

In my experience, there are 3-phase systems (e.g. certain drives) that can require isolation transformers, and as an example I bring up a delta-wye to illustrate that grounding the wye-neutral does not "break isolation", in fact in my example grounding the neutral is the whole point of the isolation transformer. In that example. Notice on the 3-phase wiring diagrams of the transformer you are using, none of them show the wye-neutral being grounded: and yet, it is common experience with motors and drives that the neutral is grounded at the transformer. They do not show it on the datasheet for your transformer, because it is not required by the transformer, but you would have to ground that neutral if the devices you are using require it.

I am aware that you are transforming 208 to 120 single phase, but I'm not sure why you think that grounding the 120v neutral would break isolation. Are there other devices on the 120v circuit that would require a floating circuit? The only reason I would suggest grounding the neutral is because the UPS seems to require it. Can you provide a link to the UPS you are using? With more information on the UPS and any other devices on that 120v circuit, we can look at why or why not it would require a floating neutral.
Its just a APC 750watt UPS. Right off the shelf. It needs neutral to be grounded. The tech there had not a clue except what was on his cheat sheet.

There is nothing special in the machine that needs a isolated system that I can see. I think the guy that set it up just did it for ground loops but can't be sure. It also might be a bit safer while working on it hot since you can touch phase and ground without lighting up.

As for whats in it:
There are several PLC's, 7 power supplies, 14 stepper drives, 2 motion controllers, 31 sensors, 40 solenoids, 32 SSRs, numerous relays, contactors and switches and a couple DC motors. Those are the basics.

So if there is nothing that requires Isolation is there a problem tying Neutral to ground? If no isolation do I tie it on the way out of the machine or at the transformer?
And then remove the fuses on the neutrals?

Is there any NEC UL or IEE standards that cover proper use of Isolation transformers when used with a floating neutral?


Bob Peterson

We don't typically refer to such a transformer as "isolated". They are typically however, separately distributed systems which means there is no direct contact between the input and output except via the grounding means.

There are transformers used with drives that are often referred to a isolation transformers but they are actually really often used to convert from an ungrounded delta to a wye system because many drives in the past did not like ungrounded delta.

What you are calling a "floating neutral" just plain is not allowed by the NEC.

This is what the code says:

250.20 Alternating-Current Systems to Be Grounded.
Alternating-current systems shall be grounded as provided for in 250.20(A), (B), (C), or (D).
(B) Alternating-Current Systems of 50 Volts to 1000 Volts.
Alternating-current systems of 50 volts to less than 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor

(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor

250.21 covers systems that are not required to be grounded and your system is not there.