Reading 120VAC wire to gnd when disconnected

[Doug_W]

Greetings,

This is my first post here. Trying to figure out why I read 120VAC from a control wire to ground with a switch off. Output 13 from the PLC in CP-1 energizes CR236, 24VDC coil relay. Wire 41 goes to CP-2 through a switch, (SS136), and energizes a starter (M136). I am trying to figure out why I read 120VAC from wire 42 to ground even with SS136 open. I don't read 120VAC to Neutral with the switch open.

I would appreciate any thoughts on this situation

Thanks,
Doug

AB 5069-OB16
24VDC PLC OUTPUT
output 13 --------------313------------------------------------------(CR236)--------| 24VDC

CR236 \ SS136
32----------| |--------------41------------0___|___0----------42------(M136)--------| 120VAC


CR236 - WAGO 859-304
M136 - Telemecanique LD1-LB030

 

[Doug_W]

My ASCI attempt at a diagram above shifted some characters a bit; CR236 should be above the contact and \SS136 should be over 0___|___0 to look like a selector switch.

 

[ptonsparky]

Does the 120v have one lead grounded?

 

[don_resqcapt19]

Is the 120 volt power from a grounded system?

 

[Doug_W]

Is the 120 volt power from a grounded system?

Yes, the 120VAC is from a grounded system. The other thing about the second panel where M236 is mounted, is that there are 2 different 208VAC sources coming in. They both have individual disconnects.

 

[Doug_W]

Does the 120v have one lead grounded?

Yes

 

[ptonsparky]

It appears that you have the Line & Neutral reversed. You're switching the neutral and not the hot.

 

[jim dungar]

Are you using a high impedance meter?

 

[Doug_W]

I just tried to edit some of my earlier responses but the time had run out.

EDIT
Looking at the original drawings from the panel with the 120V source, I now see that the 120V comes from a 480 to 120 control transformer. The Neutral of the control transformer is grounded but the secondary of the transformer is wired to an outlet which feeds a UPS. The neutral of the UPS system does not show a grounded neutral and that is where the 120 is coming from.

Is that the problem?

 

[Doug_W]

Are you using a high impedance meter?

Not sure, I have a KLEIN TOOLS MM400

 

[Doug_W]

It appears that you have the Line & Neutral reversed. You're switching the neutral and not the hot.

I will double check this on Friday, out next non-production day.

 

[don_resqcapt19]

I just tried to edit some of my earlier responses but the time had run out.

EDIT
Looking at the original drawings from the panel with the 120V source, I now see that the 120V comes from a 480 to 120 control transformer. The Neutral of the control transformer is grounded but the secondary of the transformer is wired to an outlet which feeds a UPS. The neutral of the UPS system does not show a grounded neutral and that is where the 120 is coming from.

Is that the problem?

Very likely, voltage measurements to ground on an ungrounded system have no meaning.

 

[ptonsparky]

Go back to post 3 & 4.
We wanted to know if the secondary of the transformer feeding coil M136 has a grounded conductor.

A 2 wire 120 volt control circuit does not mean it has one leg grounded. Don't call a white wire a neutral or automatically expect it to be grounded. It may not be.

 

[Doug_W]

Go back to post 3 & 4.
We wanted to know if the secondary of the transformer feeding coil M136 has a grounded conductor.

A 2 wire 120 volt control circuit does not mean it has one leg grounded. Don't call a white wire a neutral or automatically expect it to be grounded. It may not be.

The secondary of the control transformer from the Control Panel where the 120VAC comes from has a grounded Neutral, however that circuit feeds an outlet which a UPS is plugged into. The UPS circuit is the source for the 120VAC circuit in question. From the drawings it appears that the neutral from the UPS is not grounded but I am not sure what actually goes on in the UPS so I won't be able to check that panel until this Friday.

Friday I will report back and let everyone know what I find.

Thanks for the suggestions,
Doug

 

[ptonsparky]

FWIW, my UPS outlets float when it's on battery. Ungrounded.

 

[zooby]

FWIW, my UPS outlets float when it's on battery. Ungrounded.

Tom...not looking to hijack this but---could you elaborate a bit please?

 

[ptonsparky]

When the battery backed up receptacles are in normal utility mode they have the normal polarity with EG. At power failure it supplies 120 volts but the EG has no voltage potential to either circuit conductor.

APC BackUPS 1300.

 

[Jraef]

Not sure, I have a KLEIN TOOLS MM400

Get yourself an inductive voltage tester. Digital MMs can pick up induced voltages that are otherwise meaningless because the meter has no significant burden. As soon as as you put the load of an inductive voltage tester (like a Knopp or Wiggy), I’m betting you read zero…

 

[tortuga]

I'd bypass the UPS for testing, they put out a horrible square wave.
I dont think UPS's are allowed to be 'ungrounded' AC systems but they are allowed to be center grounded 60/120V. A while back I tested one to confirm this it put out ~60V to equipment ground on both hot and 'neutral' (white wire), a fun test is with just a incandescent lamp from white wire to equipment ground with a voltmeter in parallel, the lamp will glow dim and youll still read ~ 55V from white wire to equipment ground.

 

[don_resqcapt19]

When the battery backed up receptacles are in normal utility mode they have the normal polarity with EG. At power failure it supplies 120 volts but the EG has no voltage potential to either circuit conductor.

APC BackUPS 1300.

So their is no ground fault clearing path? Using any of the equipment shown in 250.114 with that UPS would be a code violation