480/480-277v xfrm

[evan06]

This 480/480-277v, delta-wye xfrm was wired backwards and nobody caught it until start up. After energizing, all the voltages were reading ok.(primary and secondary voltages) At the lighting panel 476v phase to phase. 268/270 phase to ground. After we energized a lighting load the lights were blinking. The plant electrician checked voltage at the breaker and saw 480v phase to neutral. We turned the circuit off and checked voltage. Now with no load we read 476 phase to phase, 166 phase to ground. Why wouldn't we have seen this before turning on a lighting load.

 

[don_resqcapt19]

A delta transformer without any connected load will often have the same voltage readings to ground as a wye transformer as a result of capacitance coupling.
Don

 

[Pierre C Belarge]

A delta transformer without any connected load will often have the same voltage readings to ground as a wye transformer as a result of capacitance coupling.
Don

Don
Is there a procedure that can be performed during the testing that can nulify/block the capacitance during the testing?

 

[dtiller]

Is there a procedure that can be performed during the testing that can nulify/block the capacitance during the testing?

I'm not an electrician, but I would assume some sort of load that could withstand the miswired voltage and was of a magnitude to offset the capacitive coupling would bring the voltage down. My guess is that the meter you used was high impedance and was not capable of causing enough current flow to negate the very high source impedance voltage that was being capacitively coupled into the winding(s). A small 480V primary transformer with a light bulb across the 120V secondary would work up to 480V, I think. Alternatively string 4 same-wattage 120V light bulbs in series. You could also build a load center using relatively high power dissipation resistors. With 3 resistors rated at 2500ohms, >100W and > 480WVAC, like this hum-dinger, for instance, you can connect all 3 legs for testing at the same time. Note that these resistors will get _hot_ after being connected for more than a little while.

 

[don_resqcapt19]

You shouldn't have to test to find this problem. Just see if the conductors that serve the load originate at the side of the transformer with the XO termination. That being said a load will do the job. It would be connected line to ground and you should see the voltage go to zero on that leg and the other two legs would then read 480 to ground.
Don

 

[micromind]

When I energize a xfmr for the first time, (480 or lower) I make sure no loads are connected (if possible), then using a solenoid-type wiggy, check voltage phase to phase, phase to neut, and phase to ground.

This is not an exact voltage test, but a solenoid wiggy 'loads' the circuit enough to nullify any capacitive coupling. A DMM has around 11 megohms of impedance, and will read 'phantom' voltages. A solenoid wiggy has much less impedance, and will not read phantoms.

The same holds true with multiple wires in a conduit where some are carrying current, and you need to find the one that's open.

Rob.

 

[don_resqcapt19]

Rob,

This is not an exact voltage test, but a solenoid wiggy 'loads' the circuit enough to nullify any capacitive coupling.

That has not been my experience. When checking an ungrounded delta with a wiggy, it often showed ~277 volts to ground.
Don

 

[jim dungar]

When I energize a xfmr for the first time, (480 or lower) I make sure no loads are connected (if possible), then using a solenoid-type wiggy, check voltage phase to phase, phase to neut, and phase to ground.

Don't forget to check the N-G voltage also.
I don't know how many times I have had to ask the electirican to go back and get me all 10 voltage readings.

 

[frenchelectrican]

Jim now that is very true with Delta system I do see sometime get funky voltage if not reading right but for Wye system it should be zero if set up right [ basied on N-G voltage reading ]

Merci, Marc

 

[jim dungar]

Jim now that is very true with Delta system I do see sometime get funky voltage if not reading right but for Wye system it should be zero if set up right [ basied on N-G voltage reading ].

Marc,
You are correct, so if the reading is not "0" then a problem may be present. It just amazes me how few electricians routinely check and/or report the N-G voltage when they are experiencing "weird" voltage problems.