240 3ø totally confused...

[TRM0002]

So I have a panel box in which I have (roughly) 240v across all three legs A-B B-C A-C which is good BUT A-ground is 128, C-ground is 128, while B-ground is 178mv (like nothing). Should't they either all be the same OR one be a high leg? (Delta vs WYE is confusing as hell to me). Before you say "You're missing a leg" I have 3 machines running off that box right now and one machine that the drives suddenly won't power up on; which is what triggered me to check incoming power. After I saw nothing on 1 leg at the machine breaker, I went right to the panel box to verify what was there and that's when I recorded the values at the beginning of this...

 

[ActionDave]

Please update your profile to indicate your connection to the electrical industry.

You most likely have a corner grounded system.

 

[infinity]

I agree with Dave you have a corner grounded Delta system.

 

[__dan]

If it was corned grounded delta, A and C would be 240 to ground. The 128 to ground makes no sense.

Are you sure B is 178 milliVolts and not 178 Volts. 178 Volts on B would indicate a red leg delta.

You need to know two things.

You have to look at the source transformer wiring diagram for the secondary windings. That will tell you if you have delta, Y, or red leg delta. Any of those would be common.

Next you would have to open the transformer or up to the first disconnect scondary side and physically "see" the main or system bonding jumper. You don't know it's grounded until you physically see the bonding jumper. No bonding jumper for separately derived systems would be one of the most common violations I see.

Until you see the bonding jumper for all you know it could be a floating system with one leg shorted.

After you verify which system you have and if it is properly grounded, if it is a delta you would want to check with the drive manufacturer about running their drive on a delta supply. The drive may not like a delta supply.

 

[infinity]

If it was corned grounded delta, A and C would be 240 to ground. The 128 to ground makes no sense.

Are you sure B is 178 milliVolts and not 178 Volts. 178 Volts on B would indicate a red leg delta

The B phase to ground voltage should be 1/2 of the line voltage. He said that it's about 240 volts so 128 volts to ground isn't that far off for a corner grounded Delta.

 

[Tulsa Electrician]

If that's the case ( cornor grounded)
Why have voltage to ground on all three lines. Would one line be zero to ground under normal operating conditions.

I will read again to be sure I read it correctly. My arms are sore from the last set of push ups.

 

[__dan]

The B phase to ground voltage should be 1/2 of the line voltage. He said that it's about 240 volts so 128 volts to ground isn't that far off for a corner grounded Delta.

For a red leg delta with one winding center tapped and grounded at the center tap, yes. A and C to ground could be 120 (ish), 128. B would be 180 (ish) to ground.

He is saying B is 0 Volts to ground (178 milliVolts), A to B is 240, A to ground would be 240, for a corner grounded delta, B grounded.

The sum of what was said does not add up.

I suspect the B Voltage was misread as mV and not V. Reading the transformer nameplate for the secondary windings is the only place I would start. Unless you get B as 180 to ground then the red leg delta would be confirmed.

 

[Tulsa Electrician]

I see the MV now. Thanks dan

 

[TRM0002]

Update.. so long story short, went right to the step-down transformer (old- and I mean OLD- Federal Pacific FPE type FH) and NO (well, very little) voltage on the middle output leg. (3) 480V legs incoming and that same 240/0/240 outgoing. So it appears the old gal finally gave out. Not sure how the other machines (older lathes, jig bore, etc) are still functioning on it as is unless getting just enough feedback across the legs for them to run. It's a Haas CNC on that line that went down and started all the troubleshooting.

 

[jaggedben]

Hmmm.
What are the line-line voltages?

 

[ptonsparky]

Hmmm.
What are the line-line voltages?

I agree. Motors don't give diddly squat about line to ground. They want L - L.

 

[Joethemechanic]

For a red leg delta with one winding center tapped and grounded at the center tap, yes. A and C to ground could be 120 (ish), 128. B would be 180 (ish) to ground.

He is saying B is 0 Volts to ground (178 milliVolts), A to B is 240, A to ground would be 240, for a corner grounded delta, B grounded.

The sum of what was said does not add up.

I suspect the B Voltage was misread as mV and not V. Reading the transformer nameplate for the secondary windings is the only place I would start. Unless you get B as 180 to ground then the red leg delta would be confirmed.

High leg delta, B leg should be 208 volts to ground

 

[LarryFine]

High leg delta, B leg should be 208 volts to ground

Agreed. It was 190v back when we had 110/220v.

 

[Joethemechanic]

Agreed. It was 190v back when we had 110/220v.

First time I encountered a high leg I read 190 volts to ground. I was like WTH is this? That was over 40 years ago and supposedly long after the voltage was raised to 120/240. I don't remember checking phase to phase, I just remember standing there looking at my Simpson 260 and wondering if the leg was dead and I was seeing capacitive coupling or something.

 

[winnie]

and that same 240/0/240 outgoing. So it appears the old gal finally gave out. Not sure how the other machines (older lathes, jig bore, etc) are still functioning

In the original report you said 128,128,0 L-G and 240, 240, 240 L-L.

What are you now reading as 240/0/240?

Jon

 

[jim dungar]

For a red leg delta with one winding center tapped and grounded at the center tap, yes. A and C to ground could be 120 (ish), 128. B would be 180 (ish) to ground.

He is saying B is 0 Volts to ground (178 milliVolts), A to B is 240, A to ground would be 240, for a corner grounded delta, B grounded.

The sum of what was said does not add up.

I suspect the B Voltage was misread as mV and not V. Reading the transformer nameplate for the secondary windings is the only place I would start. Unless you get B as 180 to ground then the red leg delta would be confirmed.

The high leg of a center tapped winding must be 1.732x the line to neutral, or 221V in the case of the OP.

If the system is corner grounded it is possible for a meter to show .178V to ground, likely from noise in the meter and testing procedure.

If the system is actually ungrounded, or without a solid connection, then all line to ground voltages would be dependent on coupling capacitance and could be close to the readings of a wye system

 

[kwired]

If it were corner ground delta he should have 240 volt to ground on the two he measured 128.

Sounding like he possibly has 240 volt wye system with a open leg. that would have 138 volts nominal to neutral on each leg. If this is a transformer for powering VFD's - even more likely.

 

[winnie]

My _hunch_ is an ungrounded delta with a ground fault causing it to appear corner grounded.

The across the line motor driven devices would just see the L-L voltage and be perfectly happy.

The VFD driven devices would see the imbalance overloading any L-G protective devices or filters.

If the fault isn't solid, then voltage readings could be all over the place and variable.

-Jon

 

[kwired]

My _hunch_ is an ungrounded delta with a ground fault causing it to appear corner grounded.

The across the line motor driven devices would just see the L-L voltage and be perfectly happy.

The VFD driven devices would see the imbalance overloading any L-G protective devices or filters.

If the fault isn't solid, then voltage readings could be all over the place and variable.

-Jon

But shouldn't a ground fault on an ungrounded delta effectively turn it into a corner ground and line to ground volts on the ungrounded conductors would read ~240?

 

[winnie]

But shouldn't a ground fault on an ungrounded delta effectively turn it into a corner ground and line to ground volts on the ungrounded conductors would read ~240?

Yup. Look at post #9.

In the original post the OP measured 240V L-L on all phases, and 138V L-N on 2 phases, 178mV L-N on one phase.

Then in post 9 the measurements were 240/0/240.

I can't see how the two sets of measurements are consistent, thus my guess of some sort of high impedance fault so that the act of connecting the meter changes the voltages being measured.

-Jon