My company does audio visual services. When we use a single 120/208 VAC Three-phase "Y" power source for lighting, audio and video we get noise caused by the lighting dimmers injected into the audio and video. We purchased a 30 KVA Isolation transformer. We tie-in power and run it first to the lighting dimmers then put the isolation transformer in line before powering up the audio and video equipment. It eliminates buzz in the audio and interference lines in the video. The transformer has three hot and one ground connections on the input side (no neutral on the input side) and on the output side a ground, neutral and three hot connections. Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?

  • \$\begingroup\$ Not enough information here to draw a safe conclusion. Did you consider that posting a link to the transformer's data sheet (or equivalent) unnecessary? \$\endgroup\$
    – Andy aka
    Commented Mar 29, 2015 at 21:30
  • \$\begingroup\$ Ask yourself this... what is the primary need for the tansformer? is it to step down the voltage,to allow something to float or to provide a VA limitation ? \$\endgroup\$
    – user16222
    Commented Mar 29, 2015 at 22:03
  • \$\begingroup\$ It's not really possible to say anything for sure without a schematic/datasheet of the transformer. It sounds like you have a Delta/Wye Isolation transformer even though the utility feed is Y (meaning you should have a neutral). In any case connecting neutrals to ground is normally done through a neutral grounding resistor.. \$\endgroup\$ Commented Mar 30, 2015 at 2:42
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    \$\begingroup\$ @michaelyoyo: Depending on your jurisdiction, and engineering design, it may be equally valid to use a solidly-earthed or resistance-earthed system. In Australia, the overwhelming preference is for solid earthing, with earthing resistors only used in special circumstances i.e. underground coal mining. \$\endgroup\$ Commented Mar 30, 2015 at 4:51
  • \$\begingroup\$ The OP is describing a standard problem (and solution) common in live events. Phase-controlled lighting dimmers are incredibly noisy and the incoming Neutral conductor bears the brunt of all that noise. Eliminating the connection to that incoming Neutral gets rid of the majority of the noise. \$\endgroup\$ Commented Mar 30, 2015 at 5:25

2 Answers 2


Your situation is standard. Unless you have a specific permit allowing a floating Neutral on the secondary of the transformer, you MUST bond the Neutral connection on the transformer secondary to Earth ground. This is a simple bond to the case of the iso transformer and the incoming Earth conductor on the feeder provides the ground.

There are specific conditions where the Neutral is allowed to float. Primarily in recording studios and only with single-phase 120V secondary windings.

The reason that the isolation transformer is helping you with your audio and video power is that you are creating your own Neutral on the secondary of the iso transformer. The incoming Neutral is polluted with dimmer noise. Your isolation transformer allows you to create your own Neutral which doesn't have all that noise on it.

  • \$\begingroup\$ +1 Correct, and the neutral noise isolation is well explained. \$\endgroup\$ Commented Mar 30, 2015 at 10:28

If you were in Australia, the earthing arrangement below would be the minimum standard for safety.

enter image description here

Do not compromise on electrical earthing. It may be the last mistake you ever make. If in doubt, consult a qualified electrician.


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