Strange voltage reading

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dreamsville

dreamsville

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Michigan
I checked a 600A transfer switch being fed from a 480v ungrounded delta service and these are the voltages that I recorded on the feed terminals. It has an equipment grounding conductor hooked up to ground rods.

A=330v to EGC
B=260v to EGC
C=260v to EGC

I have 480v between all phases. Not sure what to make of this? Any help.
Thanks
 
Not so strange at all, it is an ungrounded system (Automotive plant I am guessing) and there is no reference to ground so when you take a voltage reading to ground the voltage will depend on system capacitance.

If you were to get a ground on one phase there would now be a referance point and the grouded phase would read 0V to ground and the other 2 phases would read 480V to ground.

Looks to me like you have a partial ground on a load connected between B&C phase.
 
zog said:
Not so strange at all, it is an ungrounded system (Automotive plant I am guessing) and there is no reference to ground so when you take a voltage reading to ground the voltage will depend on system capacitance.

If you were to get a ground on one phase there would now be a referance point and the grouded phase would read 0V to ground and the other 2 phases would read 480V to ground.

Looks to me like you have a partial ground on a load connected between B&C phase.
Click to expand...


Could it be that the capacitance of the load is also different, creating the difference in the readings?
 
Yep, could be but I have spent the last 10 years in automotive plants and I am sticking to my guess, I am suprised he didnt get 0,480,480
 
Not an auto plant

Not an auto plant

This is a service feeding a pumping station for a community sewer system. So I understand the ungrounded system as it's a critical load. Pierre's capacitance of the load theory sounds like a plausible answer, but would I get the same voltage reading on (2) of the legs? That does sound like a partial ground between (2) of the phases.
 
On an ungrounded system any reading is possible, do not try to rationalize a "snapshot" reading. If you measure the individual phase coupling capacitances then you would stand a better chance of explaining your readings.
 
The readings mean nothing. What you are seeing is capacitance coupling to ground. Trouble would be 0 to ground or all three measuring 277.
 
The readings may not mean anything, but wouldn't it be a good idea to make sure there isn't a ground fault?
 
steve66 said:
The readings may not mean anything, but wouldn't it be a good idea to make sure there isn't a ground fault?
Click to expand...
It should already have a ground fault detector installed. The simplest and most effective is light bulbs.
 
The IEC 61010 is the new standard for low voltage ?test, measurement and control equipment?. The IEC 61010 provides much improved protection against over voltage impulse transients (voltage spikes) and is the basis for:

? ANSI/ISA-S82.01-94 (US)
? CAN C22.2 No. 1010.1-92 (CAN)
? EN61010-1:1993 (EUR)

IEC 61010 defines four locations or categories:

 CAT I - Protected electronic circuits
 CAT II - Receptacle outlet circuit; plug-in loads.
 CAT III - Distribution wiring, including ?mains? bus, feeders
and branch circuits; permanently installed loads.
 CAT IV - ?Origin of installation? Utility level and any outside cable run
 
Thanks

Thanks

I knew I could get the answer from the braintrust. I don't believe there is a ground fault detector on this service. I better look in to that. Can anyone expand on the lightbulb ground fault detector.
 
Basically 3 lights connected phase to ground via PT's. One you get a ground, the grounded phase light goes out and the other 2 get brighter.
 
dreamsville said:
I knew I could get the answer from the braintrust. I don't believe there is a ground fault detector on this service. I better look in to that. Can anyone expand on the lightbulb ground fault detector.
Click to expand...

The light bulbs (or transfromer primaries) are connected in a wye configuration. This means each light will only see 57.7% of its rated voltage during normal operation and so the will be dim. By connecting the wye point of these lights to ground (through a NC pushbutton contact), when a phase goes to ground that corresponding light will have 0V across it and go out, the remaining two lights will have full votlage across them and be at full brilliance. The pushbutton is used to test the lights, if there is a ground 9indicated by one light out) opening the pushbutton contact will remove the ground reference causing all three lights to be on and dim.

Be aware this is the concept behind ground indicating lights. Before these are employed there are many other application considerations including: system chagring capacitance, voltage level for each light, and presence of any existing ground fault.

I used to recommend a pre-built indicating system built by Erickson electrical Equipment http://www.ericksonelectric.com/catalog/accessories/sec6catalog-main.htm
 
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