120v By 3phase leg and ground - how bad ?

[mmiller]

So the machine shop got in a new lathe and wanted me to add a box like the old one had, so the operator could have 120v receptacles.

I went and looked and said okay, where did the old one plug in ?

They told me oh it was hooked up to the power coming in from the lathe.

That is 240 3 phase - no neutral

I told them no, and that I would be back to fix all the other ones.

So my question what is the potential safety hazard here ?

I'm assuming that if I were to touch one machine and then the next in line, I could possibly become a conductor.

The neutral sides of the receptacle were hooked right to the ground.

Mike

 

[iwire]

It's violation plain and simple and under the right conditions will energize all the enclosures connected to the circuit.

Install a 240 to 120 volt transformer on the machine to supply the outlet.

 

[mdshunk]

I guess it's possible the old lathe had a control power transformer inside they tapped off of. Lathe workers like 120V recs close by to use magnetic base worklights.

 

[jim dungar]

That is 240 3 phase - no neutral

The neutral sides of the receptacle were hooked right to the ground.

I assume your service is 240/120 3PH 4W and not simply 240V 3PH 3W ungrounded.

 

[mmiller]

Power

Power

The transformer feeding this system is a 480/240 delta no 120 tap at all.

basically they grabbed one of the legs and hooked it up to a 120v receptacle hot side and the neutral side is just tied to the chasis of the machine.

There are 3 machines all in a row that are like this.

They don't have any control transformers that they are pulling it from as well.

All the grinders are setup that way though, obvioulsy the grinder companies realized that people were doing something wrong and gave them a nice little transformer to make things right - or so it seems all the surface grinders have a 120v receptacle done correctly

 

[gar]

080627-1202 EST

mmiller:

I believe you are describing a delta system that includes one secondary with a center tap for 120-0-120 single phase. The center tap will be grounded and have a neutral for the 120 single phase loads.

What your customer is doing is to use the EGC as a neutral, or maybe the machines are not explicitly grounded and some other path carries this current. Totally wrong for two reasons. The first is very obvious --- the improper use of the EGC. The second is the circuit protection might be 30 to 40 A. One more is that they could easily get on the high leg.

iwire's suggestion of an isolation transformer on the machine is the best solution. Much better than bringing a separate circuit to the machine.

If you had a floating delta there would be problems with the lights.

.

 

[jim dungar]

The transformer feeding this system is a 480/240 delta no 120 tap at all.

Are you positive that there is no 120V tap on the transformer? I would not expect you to have a usable 120V line to ground on a 240V floating delta system.