240volt Delta non grounded

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LarrywdH

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Menomonee Falls, WI, US
I have a system 480v 3ph to 240v 3ph delta to delta transformer which has a voltage detector device on the secondary, which is detecting voltage on the grounded enclosure, which has people walking by concerned, but the grounded cabinet is grounded back to another voltage source, not the 240v source because it is not grounded at the transformer.

The voltage between the grounded cabinet and the ground wire on the voltage detection device is 17volts, which is what is blinking the lights.
The voltage phase to ground is 130v on 2 phases, 158 on the 3rd. But since there is no path back to the source, I don't know what that really means.
I am told that it was installed that way on purpose, because grounding the circuit interfered with some testing of equipment.

I have another identical system which is not blinking the ground indicator lights, but I can't tell what the difference is.
I would prefer a grounded system, but that is not what I have.

Thanks for any insight.
 
LarrywdH said:
I have a system 480v 3ph to 240v 3ph delta to delta transformer which has a voltage detector device on the secondary, which is detecting voltage on the grounded enclosure, which has people walking by concerned, but the grounded cabinet is grounded back to another voltage source, not the 240v source because it is not grounded at the transformer.

The voltage between the grounded cabinet and the ground wire on the voltage detection device is 17volts, which is what is blinking the lights.
The voltage phase to ground is 130v on 2 phases, 158 on the 3rd. But since there is no path back to the source, I don't know what that really means.
I am told that it was installed that way on purpose, because grounding the circuit interfered with some testing of equipment.

I have another identical system which is not blinking the ground indicator lights, but I can't tell what the difference is.
I would prefer a grounded system, but that is not what I have.

Thanks for any insight.
Click to expand...
The enclosure should not have any connection except capacitive to either the 480 source wires or the 240 output wires. The ungrounded 240 delta should be floating, tied by load capacitance only to a ground reference. If everything is balanced it should float with about 137 to ground on all three phase wires. There are only two ways that the enclosure could be raised to 17V above remote ground is the combination of two things:
1. There is some unbalanced leakage from the output delta to the enclosure and at the same time the EGC connecting the enclosure ground to the remote ground source is no longer a solid connection, or
2. There is some current (source unknown) being injected into the local earth so that the local earth ground electrode is at a 17V potential difference from the remote ground point.
Either of these two situations needs to be corrected!
 
Golddigger

Golddigger

"The enclosure should not have any connection except capacitive to either the 480 source wires"
The cabinet is grounded through the supply conduit, and equipment grounds for other grounded circuits in the same enclosure, from a separate supply.

"If everything is balanced it should float with about 137 to ground on all three phase wires"
This is true on the other system I mentioned but not the one in question, I have 158v to ground on one phase.
The 17volts I am getting is from the grounded enclosure to the ground wire on the voltage indicating device.


https://www.graceport.com/cms/wp-content/uploads/2014/04/Voltage_Indicator_How_does_it_work.pdf

Thanks for the comment
 
LarrywdH said:
https://www.graceport.com/cms/wp-content/uploads/2014/04/Voltage_Indicator_How_does_it_work.pdf
Thanks for the comment
Click to expand...
Can you test for bonding integrity, and remote injection current, or try a new sensor, then post your results.

GoldDigger said:
1. There is some unbalanced leakage from the output delta to the enclosure and at the same time the EGC connecting the enclosure ground to the remote ground source is no longer a solid connection, or
2. There is some current (source unknown) being injected into the local earth so that the local earth ground electrode is at a 17V potential difference from the remote ground point.
Either of these two situations needs to be corrected!
Click to expand...
 
Looking at the not quite complete information in the product brochure, it appears that the unit is sensing current from the floating wye point inside the device and its ground wire, not to any hypothetical connection to the enclosure.
1. Measure from the actual enclosure (connected via conduit to remote ground) and the local ground terminal of the device. (What exactly is your ground reference?
2. Measure between the local ground reference (if there is one) and the enclosure.

It is not clear from your description so far whether there is indeed a voltage on the enclosure or just an offset of the ungrounded delta from the EGC connection. If the former, it needs to be fixed. If the latter, it suggests that either the transformer or the connected load equipment is not balanced in impedance between each phase and the enclosure. That may or may not indicate a more serious problem, but it does not indicate any immediate shock hazard.
JMO
 
"Can you test for bonding integrity, and remote injection current, or try a new sensor, then post your results."
The cabinet is supplied with 3 different voltage sources all with egc's and metal conduit that is installed and bonded.
The 480v supply is right beside the 240v, and has an identical voltage detector device, and reads properly with no flickering ground indicators, when connected to the 480v supply. I swapped the connections for the 2 identical indicator devices between the 240v and 480v supplies and the problem followed the 240v supply not the device.
I'm not familiar with "remote injection current".

The supply transformer for the 240v system reads 240v phase to phase, and reads 128, 158, 131volts to ground at the transformer secondary with the main circuit breaker on the 240v panel open.

"1. Measure from the actual enclosure (connected via conduit to remote ground) and the local ground terminal of the device. (What exactly is your ground reference?
2. Measure between the local ground reference (if there is one) and the enclosure. "
The ground wire from the voltage indicating device is normally connected to the grounding bus in the enclosure, when connected the ground lights flicker on the device.
I disconnected this conductor and measured voltage from the conductor back to it's connection point on the grounding bus.
I will measure from the enclosure to the local ground reference.
 
Just to make sure I'm clear this is a 240v 3 phase ungrounded system and there is another 240v 3 phase ungrounded system in the same room, from the same enclosure, fed from an identical 480v to 240v transformer, and the panels that are fed from the other 240v system are not causing the flickering ground indicator lights.
Thanks for your time and expertise.
 
Seems to me you effectively eliminated remote injection currents, or a faulty indicator as causes.

You may have also eliminated System bonding integrity as a cause.

Turning off (isolating) each connected load may be next, but first consulting the indicator MFG, Customer Tech support might determine if 17volts really is problematic.
 
"Turning off (isolating) each connected load may be next"
The 240v supply feeds numerous test points in a lab, all of those were not in use with nothing connected at the time when I was taking readings.
The voltage to ground readings are what I think is causing the issue, but I don't know what is causing the readings to begin with.
I did take readings at the transformer, with everything else isolated and still got the odd readings, 128, 158, 131v to ground.
As stated earlier the readings should be 137v to ground on all three phases, and I have an identical set up right beside the one in question, that the readings are 137v to ground.
Thanks
 
171010-1416 EDT

LarrywdH:

Without going back thru the various posts my suggestions are:

1. Assume that your cabinet is what you call ground (your voltage reference measuring point), and this has negligible difference in voltage relative to remote earth.

2. Disconnect all wiring from the secondary of the problem transformer. Hopefully this can be done with a disconnect switch, breaker, or fuses.

3. Measure your three voltages from the secondary to the cabinet. Should be moderately balanced. If not balanced, then is the unbalance about what it was with the wiring connected? This includes meaning that the same phases show about the same results.

Report back.

.
 
gar

gar

If I understand you correctly, the transformer feeds a 225amp main breaker, I opened that breaker, so the transformer is energized but only feeding about 5feet into the top of an open breaker and goes no where else.
The readings phase to phase are perfect 240v, the readings phase to ground are 131, 158, 128volts.
I think this answers your question.
Thanks
 
171010-1457 EDT

LarrywdH:

Your problem is within the transformer, or the short wiing from the transformer to the breaker, or within the breaker on the input side.

.
 
gar

gar

Thanks,
I haven't seen this before, and I can't imagine what would cause what I am seeing.
One conductor, winding, etc. has a different capacitance to ground?
Anyone with an insight?
I'll keep thinking/investigating.
 
171010-1604 EDT

LarrywdH:

Assuming you can now definitely say the problem is in or associated with the transformer, then it would be interesting to see if you can estimate the magnitude of the leakage impedance.

I assumed you used a high impedance meter for your measurements, 10 megohm meter. Your readings 128, 158, 131 imply an unbalanced leakage in the transformer. Assuming an average of 139 V (calculated from 128, 158, 131) a 47,000 ohm resistor would dissipate 0.41 W. At 158 V 47k dissipates 0.53 W.

Create a wye resistive load of three 47,000 ohm 1/2 W each resistors. Balance this load within 1%. Put this load on the transformer. This should not change any of your readings. Measure the voltage from your ground reference to the resistor midpoint.

Next ground the resistor network midpoint to your ground reference. Measure the three leg to ground voltages. The change in the readings from 128, 158, 131 to the new readings will indicate the relative transformer leakage compared to the 47k value.

If the readings come close to 139 V, then your leakage is much greater than 47k. If the readings don't change much 128, 158, 131, then leakage is an impedance much less than 47k.

The capacitive reactance of 0.06 ufd at 60 Hz is around 47k.

.
 
Is there a safe way to disconnect transformer power, and Megger test transformer coils to ground?
 
171011-1130 EDT

LarrywdH:

Do the megger test first and see what that tells you.

If you have not disconnected the wires at the transformer yet, and made the output voltage test, then do that before anything else to make sure the problem is not outside of the transformer.

.
 
171011-1200 EDT

LarrywdH:

By any chance is there something like an MOV or other transient suppressor hung across the input side of the breaker, and therefore output of the transformer?

.
 
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