double pole circuit: uneven current from each pole 1

[questionguy]

I work in a facility which has 240V circuits which come from double pole breakers. Both of the poles are hot and there is no neutral in the circuit. One pole is 130V and the other is 108V and together these create 240 V circuit. We had an electrician in today doing some service on our panels and he was taking current measurements and we noticed that the two poles coming from one breaker didn't always have the same current. A couple of them were off each other by 25%. I think the difference is going to ground. The electrician said he would expect them to be the same and couldn't think of why they would be different. I'm concerned that we're losing a bunch of energy somewhere.

Does anyone know what it means when the poles have different current and can suggest were we would start trouble shooting.

Thanks in advance, and let me know if there's any information I can include that would make this more clear.

 

[itsmoked]

For starters you're saying "no neutral" then announce different voltages between the lines and...what?

Measure them to the panel neutral and what do you get?

Is this a single phase panel or three phase panel?
Has the facility got three phase?


Your two lines (Hots) could be going out to somewhere or a machine and someone has tagged a neutral from elsewhere and so there is a 120V load to that neutral - hence the different line currents.

OR

Even worse, someone has added a 120V load from one of the lines to ground, an intolerable issue. Hunt down what machinery is turned off by that breaker then inspect it for dangerously added additions and modifications. If you don't find one you must then track back down the conduit pulling all the covers and hunt down the hacked-in tap that's causing the noted issue.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

I bet his 240 volts is coming from the red leg of a three phase red-leg delta supply.

https://en.wikipedia.org/wiki/High-leg_delta

 

[LionelHutz]

It's fairly simple, if the currents aren't equal then current is flowing somewhere else besides those 2 conductors. Shut it down and do a resistance to ground and/or insulation test on the circuit to see if you have leakage to ground.

 

[Mbrooke]

Sounds like an ungrounded system- I mean that would explain the voltages. Different current would mean current is coming/going from another circuit, perhaps hots tied from another circuit. Its a guess, but to be honest there is not enough detail here to say for sure.

 

[questionguy]

Thanks for the replies all here's some more info.

The voltages I quoted are wrt ground. The facility does have three phase. The power is delivered by the Utility at 480V 3 phase then we've got a step down transformer that goes from 480V to 240V. I've been told each breaker is 240V single phase, but I realize it's not quite that simple. It's a high leg delta setup (I learned that term today from the electrician) and I can correct the voltages I quoted yesterday.

-The high leg is 208V RMS
-The other two are 120V RMS
-The difference between any of the two phases is 240V RMS

Each breaker has two of these legs and the currents I'm measuring are just one of the legs at a time. The outlets all have a ground wire also.

itsmoked: I don't think their's any sketchy stuff at the end of the lines. I'm pretty well aware of everything that's plugged in.

LionelHutz: that must mean that the extra current is going to the ground wire I assume. Does that mean that current represents wasted energy?

A little more detail: Facility is a data center. Each breaker circuit ends on a power distribution unit running several servers. And a few of the circuits power cooling fans.

 

[itsmoked]

Wow, a data center with open delta and wild legs. Sounds like crazy fun.

A data center consists of a lot of switching power supplies which can confuse your meter somewhat and can couple power to ground thru their input EMI filters.

Large currents over a couple of amps are not typically 'waste' or leakage though.

Measuring the line voltages to ground on delta is typically futile and will mislead you quickly. The legs are not referenced to earth so measuring to earth gives varied results that depend on capacitive coupling, which will always vary between legs.

Also, modern DMMs have such high impedance that they don't swamp the aforementioned capacitive coupling. If you've got a drop-light with an honest-to-gawd incandescent bulb hook its two prongs between the phase and ground and see what you then measure with your DMM. It will not be 130 or 108V any longer. It may well measure zero because the 'leakage' has been swamped.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

The voltages appear to be what should be expected from a high-leg delta supply, but the unbalanced currents indicate that significant current from one of the two poles of the breaker is returning through the ground wire. I would surmise that there are some 120 volt loads being supplied by this two pole breaker and the ground wire is serving as a neutral for these loads because there is no separate neutral wire available. I guess this breaker is inside a three phase panel.

I have seen this situation in an old commercial building, with 200 amp service, where all the panels were 240 volt three-phase, high-leg delta, daisy chained together (not sub-panels). Only four cables in conduit ran to each panel, the three phase wires and a ground/neutral. Great care was required to make sure that no 120 volt loads were drawn from the high-leg phase wire (every third slot in the panels). It was a great improvement when a single phase panel was placed next to the three-phase panel and all the single phase loads were moved into the new panel, where there was no high-leg anymore. However, ground and neutral had to be connected in this panel, making it, in-effect, a main service panel.

The building was supplied from three single-phase pole mounted transformers, connected in delta. The center-tap of one transformer was the ground/neutral.