3-Phase Ungrounded 480 Delta, over 1,000V to Ground

[RyanMP1011]

Hey all,

I'm having this weird concern with a 480V Ungrounded Delta Secondary. With all loads removed, I read about 500V line-to-line, but the line-to-ground readings were:

A to Ground — ~1000V
B to Ground — ~1000V
C to Ground — ~700V
I tested these voltages with a Fluke T6 and a Fluke 87

Once the main breaker is turned on, there are 3 ground-fault indicator lamps that come on. A and C are brightly lit, while B is on, but very dim. Taking line-to-ground readings at this point shows:

A to Ground — ~350V
B to Ground — ~170V
C to Ground — ~350V
I tested these with a Fluke T6 and 87, as well as a solenoid meter.

Unknown primary voltage, but I believe it is no less than 12kV and no more than 15kV

Is this something to be concerned about?
If any more information is needed, let me know.

Thanks in advance.

 

[jim dungar]

If your system is ungrounded line - ground voltage readings are all but meaningless. You are measuring the capacitive coupling which can be highly variable.

 

[don_resqcapt19]

Try reading the voltages with some type of low impedance meter. It appears there is partial ground fault on B.

 

[jim dungar]

Do you know the wiring diagram of your ground detector lamps? Is it just incandescent lamps or does it include transformers or resistors?

Do the lights go back to normal when any specific feeder circuit is removed?

 

[RyanMP1011]

Do you know the wiring diagram of your ground detector lamps? Is it just incandescent lamps or does it include transformers or resistors?

Do the lights go back to normal when any specific feeder circuit is removed?

Indicator lamps are all wired to ground on one side, and then A, B, and C phase to the other side. It's a transformer that steps it down for use with 6.3V lamps.

Optimally, when a particular phase becomes grounded, that indicator will turn off, and the other two will get brighter. It becomes a corner-ground and feeds 480V to the little transformers, except for the corner-grounded phase, which will be at 0V.

All feeder circuits off, main breaker on (which just feeds the bus and the indicator lamps and their transformers)

 

[don_resqcapt19]

So the transformer type of ground detector gets rid of my suggestion to use a low impedance meter as the transformers serve that purpose.

 

[__dan]

Yes you should be concerned.

If the site does not have intentional contiguous equipment ground connections to everything metal, the fault to ground or the leakage (probably fault more likely) can impose a floating Voltage on the exposed, metal not connected to the equipment ground.

So if it goes to another metal building but the equipment ground is not also carried, that fault can show up as a Voltage above ground on that exposed metal.

One way to test for this would be to bang a pipe or rod into the ground at a remote point for a remote Earth Voltage reference. then, with a very long lead, see if you can detect a voltage raise on the exposed metal, supposed to be grounded, in the area of the suspected fault.

In order of priority, finding the fault and finding if everything on the site is solidly equipment grounded making it all one system, a break or high resistance in the site / equipment grounding system is the larger hazard, (a risk of fatality hazard).

 

[__dan]

That B to ground 170 V, that ground could be raised above the Earth 0 V reference. That ground could be trying to connect back to the 0 V Earth by a path through the Earth, in the absence of an equipment ground.

You would want to make sure that ground reference point, is not itself at a Voltage above the Earth, from the combination of a fault plus a break in the site contiguous equipment grounding system. If it were solidly (equipment) grounded, the B phase fault to ground would be expected to be 0, 480, 480.

If the equipment grounding is good, then probably the leakage current is pretty small to not pull it all the way to 0, 480, 480.

 

[synchro]

Indicator lamps are all wired to ground on one side, and then A, B, and C phase to the other side. It's a transformer that steps it down for use with 6.3V lamps.

Optimally, when a particular phase becomes grounded, that indicator will turn off, and the other two will get brighter. It becomes a corner-ground and feeds 480V to the little transformers, except for the corner-grounded phase, which will be at 0V.

All feeder circuits off, main breaker on (which just feeds the bus and the indicator lamps and their transformers)

How easy is it to exchange the input to the transformer that's now on the B phase with one that's on either the A or B phases? That might tell whether there's actually a low current ground fault on the B phase, or if there's an issue with the ground fault detection apparatus itself.

 

[RyanMP1011]

How easy is it to exchange the input to the transformer that's now on the B phase with one that's on either the A or B phases? That might tell whether there's actually a low current ground fault on the B phase, or if there's an issue with the ground fault detection apparatus itself.

I wondered the same thing myself, and should have tested that while I was there. Unfortunately I have left the site and won't be back any time soon, but I could possibly have someone else check. This particular site has over 30 different ungrounded systems within it, this particular system just happened to be one of the few that has ground detecting lights. That's the only reason I decided to check line-to-ground voltage in the first place.

I was thinking maybe the stress cones on the primary of the HV transformers could be leaking voltage through the shielding.

 

[jim dungar]

A fault to ground on the HV/primary side would not be seen as a liner ground fault on the delta LV/secondary side.

You need to make sure there is not a high impedance line to ground fault on your intentionally ungrounded 480V.

 

[RyanMP1011]

Thank you all, I appreciate the responses! I will relay this information

 

[RyanMP1011]

Yes you should be concerned.

If the site does not have intentional contiguous equipment ground connections to everything metal, the fault to ground or the leakage (probably fault more likely) can impose a floating Voltage on the exposed, metal not connected to the equipment ground.

So if it goes to another metal building but the equipment ground is not also carried, that fault can show up as a Voltage above ground on that exposed metal.

One way to test for this would be to bang a pipe or rod into the ground at a remote point for a remote Earth Voltage reference. then, with a very long lead, see if you can detect a voltage raise on the exposed metal, supposed to be grounded, in the area of the suspected fault.

In order of priority, finding the fault and finding if everything on the site is solidly equipment grounded making it all one system, a break or high resistance in the site / equipment grounding system is the larger hazard, (a risk of fatality hazard).

We had just performed motor-ground continuity for this site, as it falls under MSHA rule. This MCC feeds only three motors, all of which show a solid equipment bond, back to the fence which contains the HV transformers. The switchgear and feeder disconnect are all solidly bonded with the fence and transformer cases. There is only about 20ft of wire from transformer secondary to the switchgear, the transformer cage is directly adjacent to the MCC building.

Should also mention that these transformers are privately owned.

With all this being said, I did not verify that the ground-fault indicator lamps were properly wired in the first place. Assuming they are wired correctly, it does seem like there is a high-impedence ground-fault on B phase.

 

[zbang]

Another SWAG, look for a fault through the dirt around the MCC and transformers. Or wait for it to rain and see what changes .

 

[winnie]

Instead of a ground fault, you might also be seeing very slight leakage across one of the transformers from HV to LV.

With the main open, this gives the > 700V readings. With the main closed you now have the ground detection impedance shunting some of the voltage to ground, and the voltages drop.

Jon

 

[GoldDigger]

Instead of a ground fault, you might also be seeing very slight leakage across one of the transformers from HV to LV.

With the main open, this gives the > 700V readings. With the main closed you now have the ground detection impedance shunting some of the voltage to ground, and the voltages drop.

Jon

And it could even be capacitive leakage which would not show up on a megger or DC test.
An offset from ground which is greater than the peak line-to-line voltage and is not coming from the step-down isolaton transformer and might carry significant current could result from a restriking arc fault from one of the lines to ground. This could turn the simple capactive coupling to ground into a voltage doubler rectifier circuit.