Iso Xfmr Questions

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Joined May 18, 2020
I work in an industrial sized hammer forge. Really neat industrial complex and processes. I have the privilege of working on high power-low frequency heating furnaces. All powered by induction. We have 2 IGBT driven 600kW 3.3 kc units and several other SCR driven power supplies that range from 3kc to 10kc. All for heating billets to a forging temp of ~2300 deg F. It is in essence, a scaled down radio frequency generator. These units and their output power circuits are what drives my learning about electronics. The logic in all of the units use PLL for control of the output.

Anyways, I have added light fixtures in all of our induction heaters except the 2 IGBT units. I tried to add one yesterday via a seperate circuit fed off the 480vac line of the power supply, to a 2 amp fuse to a step down 250VA xfmr; strictly for this one light and a convenience outlet for scopes and measurement equipment. I bonded the X2 per NEC and I couldn't get the light to come on. This proceeded to blow my mind. Checked voltage at the light and I was getting 5vac between X2 and case ground. Checked voltage at the X1 of the xfmr. 122vac. WTF!? I removed the light from the circuit and my voltage returned at the end of the circuit as would be expected. After a few mins of scratching my head I began to look at the other transformers that were shipped with the power supply. Lo and behold there is an isolation xfmr present. I have been an industrial electrician for 13-14 years. I'm not going to toot my horn but I am very capable, pardon my ego trip (DISCLAIMER: I feel the term electrician is used too loosely nowadays and I have worked extremely hard for my title). I have limited experience with isolation transformers. My only real knowledge of them is that we have some on the output circuits of a few of our power supplies and that they raise the ground plane to a different potential to prevent any harm to an operator who may ground themselves (inadvertently) and touch the bus bars. As soon as I cut the bonding jumper on the X2 my light came on. Now, X2 is bonded to the housing of the power supply, as is the EGC from the switchgear. What the heck is going on? I'll find my electrical schematic and crop a picture of the ISO circuit if needed. Thanks in advance.


Joined Jul 18, 2013
In the case of a isolated supply via transformer, it is allowed to re-reference the grounded neutral by taking one terminal of the secondary to the earth GND point of the system.
If you have any concerns as to safety, this may be an option in this case.
See NEC and NFPA79.
POst the schematic if possible.
If this was a typical isolation transformer, I don't see where you would have had a problem?
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