120/208V w/ 208V Phase to Ground

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bwgd26

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I have a 120/208V transformer feeding a panel that feeds a PLC. At the PLC I have A Phase, Neutral and Ground. A Phase to ground is 208V, Neutral to Ground is 120V, A Phase to Neutral is 120V. What would cause this?
 
bwgd26 said:
I have a 120/208V transformer feeding a panel that feeds a PLC. At the PLC I have A Phase, Neutral and Ground. A Phase to ground is 208V, Neutral to Ground is 120V, A Phase to Neutral is 120V. What would cause this?
Click to expand...
What would cause this is connecting one of the transformer endpoints to ground and connecting the wye point to neutral, and the next end point to hot.
The correct wiring is the wye point to neutral and bonded to ground, with any two of the winding endpoints connected to the two hot lines (L1 and L2) in the single phase three wire panel.


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bwgd26 said:
I have a 120/208V transformer feeding a panel that feeds a PLC. At the PLC I have A Phase, Neutral and Ground. A Phase to ground is 208V, Neutral to Ground is 120V, A Phase to Neutral is 120V. What would cause this?
Click to expand...

Welcome to the forum.

Miswiring. or you actually have 2 hots and a ground. Say your ground is actually B (or C) phase, then all your readings make sense. A to B (ground) would be 208V, neutral to B (ground) is 120V, and A to neutral is 120V. That would be 208V 1ph. The other possibility is that those 3 conductors are A, B, and C phase, there is no neutral and ground is handled via metal raceway, and you've lost one phase.

Why were you checking voltages there? That's not normally not done unless there is a problem.

Can you post pics/video of the xfmr, the panel, and the programmable logic controller? Also, I'd take measurements in the panel, 10 in all: phase to phase (AB, AC, BC), phase to neutral (AN, BN, CN), phase to ground (AG, BG, CG), and neutral to ground (NG). That will give you and us a much clearer picture of what is going on
 
120/208V w/ 208V Phase to Ground

Where I was going with this, a (Multi Tap / Auto) Machine Tool Transformer if used could be configured to Isolate the PLC from the Power Line. If the Manufacturer / Builder of the PLC did the Cabinet like that, the Secondary is probably not Bonded to Chassis Ground on one Leg. This would allow the Secondary to Float and you would see odd Voltages if the measurements are taken from the Incoming Power to the Floating Secondary. The only measurement you can do with an Isolated Secondary and a DVM is across the Secondary to be certain you have selected the Transformer Tap Voltage correctly.

If a Customer requests an AC Input Voltage PLC I don't use a Machine Tool Transformer in an Isolated configuration unless required. Even a 24 VAC Secondary when left to float can be above the 'safe' 50 Volt limit to Ground. Bonding to Chassis on one Secondary Leg and Fusing the other Secondary Leg creates a circuit that is much more understandable to view and more predictable for measurements.
JimO
 
JFletcher said:
Welcome to the forum.

Miswiring. or you actually have 2 hots and a ground. Say your ground is actually B (or C) phase, then all your readings make sense. A to B (ground) would be 208V, neutral to B (ground) is 120V, and A to neutral is 120V. That would be 208V 1ph. The other possibility is that those 3 conductors are A, B, and C phase, there is no neutral and ground is handled via metal raceway, and you've lost one phase.

Why were you checking voltages there? That's not normally not done unless there is a problem.

Can you post pics/video of the xfmr, the panel, and the programmable logic controller? Also, I'd take measurements in the panel, 10 in all: phase to phase (AB, AC, BC), phase to neutral (AN, BN, CN), phase to ground (AG, BG, CG), and neutral to ground (NG). That will give you and us a much clearer picture of what is going on
Click to expand...
I'm having dame issue. Here are my readings
an 120
bn 120
cn 120
ab 208
ac 208
bc 208
ag 202
be 203
cg 008
ng 120
Transformer looks like its wired properly
 
This thread is 4 years old and a new thread has been started HERE

Roger
 
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