Transformer with wierd test results

[ogbuehi]

We tested a small, 3-phase, dry type, 20kVA transformer that is delta to wye and 1000V/416/240V. We took a multimeter and measured from from Phase A to ground and phase B to ground on the low side and measured 416V. When we measured from phase C to ground we measured 0V. This told me that phase C and ground were at the same potential. If the transformer was energized shouldn't it have been damaged if this was occuring? I'm wondering if anybody else has ever observed something like this before.

I only ask because someone wants to hook this XFMR up to a small distribution panel. There are no 3 phase loads in the distribution panel but I'm almost positive that something will get damaged if we do.

 

[waross]

You should check from each phase to the wye point. If you get 240 volts on each phase to the wye point, ground the wye point and proceed. If you DON'T get 240 volts from each phase to the wye point, replace the transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

One phase could be grounded without damage, but not if the neutral is grounded.

 

[ogbuehi]

Hmmm, interesting because the neutral is grounded. And I'm almost sure that we checked from A to neutral and B to neutral as well. I will double tap though and verify that. It's just a really long story on why it is such a cumbersome task to gain access to this small transformer. Thanks for the help guys.

 

[racobb]

Your original post indicated you got 416 volts from A and B to ground. Sounds like the neutral point is not grounded but phase C is. If you get 416 phase to phase on all, AND 416 from A and B to ground, AND 0 from C to ground, then C must be grounded AND the transformer neutral floating (with respect to earth ground). The first ground on a system does not cause a fault. It is the second ground that does.

As Bill said, also be sure you are checking your phase to (ground) voltages to the neutral point, not to ground because it appears your neutral is not tied to ground.

If the neutral point is tied to the transformer case (because you said it was grounded) and the case is not tied to ground, you need to be sure you clear the ground on phase C FIRST, and then tie the transformer case firmly to ground......All to be done with the transformer de-energized! Very dangerous to have the neutral tied to the case and not also to ground.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[racobb]

If the neutral is floating and designed to be that way, then the only reason to check voltage to ground is to know if you have developed a ground on one phase, the first ground. Then you can hopefully find it and fix it before the second ground occurs. The advantage of a true three wire system.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[ogbuehi]

This system is definitely not designed for that. It's supposed to just provide three different phases of 240V to ground via a wye connection with the neutral tied to ground. Very simple system. The transformer was put there as a means to reduce the amount of copper needed to make a very long run.

 

[davidbeach]

If your measurements are correct you don't have the neutral tied to ground, but do have C phase tied to ground. Simple as that.

If the neutral were grounded and you had a C-G fault, your A-G and B-G voltages would be much closer to 240V than to 416V. If the neutral were floating, you would have had something close to the same value from phase-ground; generally around 240V each, but could be more or less depending on couplings and meter impedance.

 

[racobb]

Is it a newly commissioned installation? If not, have your line to neutral loads been working properly? When you checked the line to ground voltages, were you checking to neutral or ground? Should be the same, but sounds like they are not tied together. Could be serious touch potential between different metal surfaces. I would suggest that you check the voltage between neutral and ground ASAP and if there is a difference, the transformer should be de-energized immediately.

Were the original voltage tests routine or was there a suspected problem?

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin