Why would we need isolation transformers before and/or after for a UPS system? What are we trying to protect, and from what? I have read that it has to do with grounding and neutral but I didn't understand.

This is 100KVA UPS. The inputs are 3 phase, neutral, and ground, coming from the service panel which has the neutral connected to the ground inside the panel. The output 3 phase, neutral and a ground while only the 3phase and ground are connected to the load (no neutral is connected to the load). When the UPS-company-man saw the installation he said that we must use a isolation transformer before the UPS.

Here is a simple schematic of the this configuration :


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Need more details. What kind of UPS are you talking about, that you need to put it outside the UPS? \$\endgroup\$ Commented Feb 16, 2016 at 18:16
  • \$\begingroup\$ @StephenCollings sorry for the stupid question but what exactly you mean by "kind of UPS" ( what info should I add ) \$\endgroup\$
    – Chebhou
    Commented Feb 16, 2016 at 18:19
  • 3
    \$\begingroup\$ You've added it. When most people hear "UPS" they think the little box that keeps your PC running during an outage. 100KVA is a whole different thing! \$\endgroup\$ Commented Feb 16, 2016 at 18:35
  • \$\begingroup\$ If you're running 100 kVA, I'm guessing you're running a motor and variable frequency drive. If so, you might consider this alternate solution. bonitron.com/m3460.html \$\endgroup\$ Commented Feb 16, 2016 at 18:37
  • \$\begingroup\$ @StephenCollings no it is only for a rectifier ( 200A at 300V ) \$\endgroup\$
    – Chebhou
    Commented Feb 16, 2016 at 18:38

4 Answers 4


Most high-power installations require an isolation transformer. Every variable frequency drive manual I've ever seen requires the same thing. So it's not a UPS-specific requirement.

In general, the isolation transformer limits your available fault current, and keeps electrical disturbances generated by your load from propagating quite so easily through your entire facility. Limiting the fault current is particularly important, because fuses and breakers are only rated for some maximum current they are able to interrupt. And that, in turn, gets listed in the product's UL file. (I'm assuming this UPS is UL listed.) So it's not necessarily that the UPS won't work without the transformer. It's more that if something goes horribly wrong, they can't guarantee it won't catch fire.

I found this resource that explains more about drive isolation transformers, which is pretty consistent with the above: http://www.eecoonline.com/drive-isolation-transformer/

If the selected transformer has a Wye configured secondary, then a secondary ground can be gained with the installation of the isolation transformer. This ground is isolated from the primary input and provides a couple of distinct benefits:

  • Grounding prevents the transfer of common-mode noise and transients, both from the primary source to the motor drive, and from the drive to the power system. This can reduce "bearing currents" that often cause fluting.

  • Introducing a grounded, drive isolation transformer localizes the high-frequency induced ground currents and prevents them from extending upstream of the transformer, minimizing "noise" and related problems often associated with drives.

Consider an isolation transformer when power quality and fault currents are the primary consideration.

Another optional benefit of isolation transformers is that of electrostatic shielding. This provides a shield between the primary and secondary winding, which can provide in the range of 40-60 dB of common mode noise reduction.

What you're talking about may not be a VFD, but many of the same concerns apply to a UPS or any other switching power supply.

  • 2
    \$\begingroup\$ I don't understand "Every variable frequency drive manual I've ever seen requires the same thing [an isolation transformer]." In every situation I've seen VFDs run from the factory supply and are transformerless. \$\endgroup\$
    – Transistor
    Commented Feb 16, 2016 at 18:59
  • \$\begingroup\$ @transistor The manuals I've seen typically give a spec for an external isolation transformer... \$\endgroup\$ Commented Feb 16, 2016 at 19:29
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    \$\begingroup\$ How big are those VFDs in question, are they some >1 MVA monsters? I've never seen dedicated isolation transformers on VFDs of <100 kVA, be they from Omron, ABB, Siemens, Vacon or Eaton. Do you work somewhere where very high power quality is critical, or are things somehow different outside Europe? \$\endgroup\$
    – jms
    Commented Sep 29, 2016 at 9:07

If you are converting AC mains voltages to 12V DC for the battery, the most efficient way to do this is via a transformer and you get the added benefit of the transformer giving you isolation thus, if you touch a battery terminal (say during maintenance) you don't necessarily get electrocuted.

Stepping up a sinewave from several volts peak to peak to AC line voltages is also done quite effectively using a transformer.

  • \$\begingroup\$ all the work is done in transformer-less ups the transformer I'm referring to would be outside the UPS \$\endgroup\$
    – Chebhou
    Commented Feb 16, 2016 at 18:07

What is the earthing system being installed? TT/TN?

if TT, then you need the isolating transformer since the neutral is opened during a fault on the supply side. In such a case, since the UPS still has power, you need to give the load the neutral reference, thus the need for an isolating transformer.

In a TN system, the neutral is not opened, thus you do not need the isolating transformer.

  • \$\begingroup\$ it's a TN system \$\endgroup\$
    – Chebhou
    Commented Sep 29, 2016 at 19:05

This is probably due to the fact that output voltage is refferenced to the ground in the star connection Y. An AC/DC to DC/AC converter, like VFD, the bridge output voltage has no reference to neutral.

  • \$\begingroup\$ and what could go wrong if we don't use the isolation transformer ? \$\endgroup\$
    – Chebhou
    Commented Feb 16, 2016 at 18:59
  • \$\begingroup\$ @Chebhou Iif you would use one phase and ground/neutral (like single phase load) then the inverter can't work. The load has to be always connected between two output phases of the inverter, therefore such UPS could only run a 3ph motor or 3ph heater, not a computer. \$\endgroup\$ Commented Feb 16, 2016 at 19:15
  • \$\begingroup\$ the inverter has a neutral output but it is not used in our case \$\endgroup\$
    – Chebhou
    Commented Feb 16, 2016 at 19:19
  • \$\begingroup\$ @Chebhou You should upload some schematics or picture abou your UPS, but I guess the output of inverter can't have a neutral output. \$\endgroup\$ Commented Feb 16, 2016 at 19:34
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    \$\begingroup\$ A isolation transformer or an inverter is like a power utility. You need to take care of the neutral to earth bonding after that point. An isolation transformer can add some warm fuzzy feeling regarding neutral/ground point but is not automatically required in your case based on the details so far provided. \$\endgroup\$
    – KalleMP
    Commented Feb 16, 2016 at 22:03

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