Today we encountered a problem and have not found a solution yet any ideas would be appreciated.
The situation is this: while working on a large pump panel with several motor starters that are controlled at 120VAC from a control Xfmr 480 – 120 we noticed a voltage of 120VAC from its derived neutral to equipment ground. Actually one of the electricians got a jolt.
My first thought was that the secondary was not grounded so we checked and it was but the OEM had only bolted a ground to the transformer case so we ran a ground to our equipment ground to make sure of a good connection. No luck still 120VAC to ground on the neutral. We double-checked the connections and it seems to be wired properly. I believe it is fed H1 & H4, H2 & H3 Connected and insulated. Then for the secondary, which can be 120 or 240, is wired X1 & X3 connected and run to ground X2 & X4 fused. Should it be reversed X1 & X3 “hot” X2 & X4 Neutral? I don’t think it matters am I wrong?
Some other observations are that this control feeds thru a GFCI to when we check the neutral to ground voltage with something like a “Wiggy” we trip the GFCI?
I have 2 questions to ask you. First what type of meter did you use , was it a meter that reads true voltage,I think they are called true rms .Second, does the voltage stay the same when you have a load on the control circuit ?
Re: Unexplained voltage#11399 07/12/0205:59 AM07/12/0205:59 AM
If the secondary is connected as you say, they should be reversed. Otherwise, there may be some confusion as to which control ckt. conductor is grounded and which is hot. Do you get 120 to ground on the grounded and the ungrounded conductor? If not, ground the X2 & X4 side and see if the trouble goes away.
Re: Unexplained voltage#11400 07/12/0208:07 AM07/12/0208:07 AM
Electrically it makes no difference wether the x1-x3 connection or the x2-x4 is. This is not the cause of your problem.
Solenoid type testers (i.e. wiggy)have been known to cause GFIs to trip, especially if connected line to ground or neutral to ground.
Now, when you say you are getting a voltage to ground reading where are you measuring it?
From the transformer to the mounting panel? From the transformer to the enclosure? From the transformer to building steel? From the transformer to earth?
It is possible that the transformer is mounted on a painted metal panel, which is mounted into a painted metal enclosure, which is mounted on painted steel channels, the result of all this paint is a poor ground connection. If this is the case then use a grinder to remove the paint at each bounding connection.
[This message has been edited by JBD (edited 07-12-2002).]
Hi thanks for the replies. Let me try to answer your questions.
Andy: We used both a “wiggy” or soloneid type meter and it trips the GFI. We then used a Fluke 89 True RMS I know that you may see a voltage with that meter even if its induced due to magentic infulences of other conductors. But as I said we found this voltage because one of the electricians got “bit” when he touched the neutaral buss. Which leads me to belive that there is some fairly decent voltage present.
Redsy: I agree we should change the neutral point if only to removing future confusion.
JBD: The voltage we are reading is from neutral to the equipment ground conductor. And we have checked all the way to the main switchgear and the ground is intact. The control enclosure is also mounted to a steel deck and welded in place this deck is also welded to the building steel which is connected to a bonding grid / ground ring of 4/0 cu running around the building and connected to water mains, building steel, rebar in the foundation, several ground rods and has been tested to have something like 2 ohms to ground. We also checked for paint as insulation and ran a ground back to the main equipment ground.
I hope this answers all the questions. If not repost or post and I’ll try to answer them.
Re: Unexplained voltage#11402 07/13/0208:07 AM07/13/0208:07 AM
It is normal for a Wiggy to trip a GFI when connected to the hot and ground but not neutral and ground. Is the GFI protection a receptacle or blank face type? Have your checked for voltage between ground and neutral on both the line and load side of the GFI? Could the line or load be connected in reverse polarity on the GFI? On most GFI receptacles the both hots connect on one side and both neutrals connect on the other but I know there is at lease one manufacture the has the terminals criss-crossed. I have seen these wired in reverse many times and think it was a very poor design choice for the manufacture. Anyway this was just a thought. Good luck!