Ungrounded 480x240 delta to delta 3ph 3w voltage to ground reading


Senior Member
Agreed with most of what was said. But here’s another potential issue even if you fix the ground faults. An intermittent (arcing) ground fault on an ungrounded system acts like a charge pump. It jacks up the fault voltage until it finds the weakest insulation anywhere in the system. I mean anywhere. This is the only thing that keeps the voltage from going infinitely high. Typically something burns up at 6 to 8 times system voltage so around 1500-2000 V in your case. I suspect this might be what is burning up everything even without double ground faults. This is typically what happens. Once you fix this, expect electrical issues to drop to about 25% of what you are experiencing now.


A brief intro of this application history.
Information provided, this application was working fine and we had many more welders running so what’s the problem now.
Fuses blowing, breakers tripping & failing, melting plugs, receptacles, extension cord connectors and wiring at disconnects.
Someone just didn’t do it right, could be just like a loose wire.
Add more welder receptacles also fix plugs, receptacles and extension cords.

Stress from the magnitude of welders running on the same overamped circuits. With two and three welders running on the same disconnects with oversized fuses, and so many running at the same time. Possibility of an unbalanced load on one phase at the panels causing failure to one leg on 3-pole breakers. Exceeding the load on the panel, transformer and only a primary breaker.
Suggested Solution:
Wiring from the 240v panel to the disconnects needs to be tested for faults
Removal of the delta xmfr and install a smaller wye to provide necessary 230v single phase for only three welders all other welders are a dual voltage. Install 480V 3Ph MLO panelboard adjacent to location of welders.
All of the manufacture’s manuals show individual disconnect adjacent to the welders. Art. 630 also indicates disconnection means for each welder. Plan to install the 480 panel within 50’ of all welders and add safety switches to comply with 630.13.

Thank you all for the support


Senior Member
Sounds good. They have to spend something substantial just to repair and get back where they were. Moving the 480 loads to 480 will have a lot of advantages, new breakers and connectors being just one. Reliability and safety would be the big one, especially if they did not know how to care for the floating delta.

Once some of the load moves, you may end up with more good used spares from the old 240 V connectors and disconnects.

The 112 kVA delta secondary is a big monster. But other than the larger idling losses, I don't see any driving need to change it. Idling loss would be the number I would want to know, If the idling losses will pay for a new properly sized transformer, it may be justified. If the idling losses are never going to pay for a new transformer, I would certainly be planning on keeping it. Adding the corner ground to the delta and repairing the reduced size 240 V distribution system with spare parts from the converted loads, is only a small bit of work from where they are now.

If they are going to continue as a production welding shop, reducing and adding load to the 240 V (corner grounded) delta could happen routinely.They may have load now and in the future for it.