High Line to Ground Voltage Readings

[Smart $]

Hmmm...

 

[Rick Christopherson]

Well, I guess on the other hand, you have put me on notice that you have absolutely no sense of humor, and are so insecure in your own knowledge that you cannot take even the lightest of ribbing either.

So Noted.

The next time I comment on your electrical faux pas, I suppose I should be more succinct, and not assume it was merely a late night excursion. Silly me for thinking otherwise.

 

[philly]

Assuming .040A of leakage current and 10,000ohms of impedance, your meter could read 400V.
.

So are you saying that this coupling voltage may depend on the impedence of the meter although we are reading in parallel across the L-G? So for a lower impedence meter we would see less volage?

In my previous post I should have clarified my statement to be "floating L-N" versus ungrounded measurements. Yes, a direct connection to the neutral point of the transformer will 'lock' that conductor/ground at 120V relative to a line. The point I was trying to make, was to perform all of the L-G, L-N, L-L, and N-G measurements at the source and at the 'problem area'.

In this case we had a broken or floating neutral since the neutal connection from the loads back to the transformer was severed. However in this case the neutral voltage didn't appear to be determined by the load impedences (in series) but rather was affected by the grounding.

When troubleshooing the secondary of a transormer like this we usually check with either a megger or meter to see if any lines are going to ground. When checking either L1, or L2 to ground should we read an open circuit with either a meter or megger? I would think that you should read an open circuit of 0meg however if you look at the schematic, with the center point grounded you have a path through the transformer coil between the L1, or L2 conductor to this ground point. This would lead me to believe that this would not show an open circuit?

 

[jim dungar]

So are you saying that this coupling voltage may depend on the impedence of the meter although we are reading in parallel across the L-G? So for a lower impedence meter we would see less volage?

Yes.

In this case we had a broken or floating neutral since the neutal connection from the loads back to the transformer was severed. However in this case the neutral voltage didn't appear to be determined by the load impedences (in series) but rather was affected by the grounding.

I didn't try to follow your resolution completely. If the TX neutral to the load neutral was severed then you had a floating neutral problem. If the TX neutral to ground connection was severed you had an ungrounded problem. I thought you had both.

When troubleshooing the secondary of a transormer like this we usually check with either a megger or meter to see if any lines are going to ground. When checking either L1, or L2 to ground should we read an open circuit with either a meter or megger? I would think that you should read an open circuit of 0meg however if you look at the schematic, with the center point grounded you have a path through the transformer coil between the L1, or L2 conductor to this ground point. This would lead me to believe that this would not show an open circuit?

A megger requires the neutral to ground bond to be opened. An open circuit is infinite resistance not 0meg. My suggestion for all of the voltage measurements was to help in troubleshooting an energized system. Over the years I have seen many people simply measure voltage L-G and then assume they have a bonded/grounded neutral.

 

[philly]

A megger requires the neutral to ground bond to be opened. An open circuit is infinite resistance not 0meg. My suggestion for all of the voltage measurements was to help in troubleshooting an energized system. Over the years I have seen many people simply measure voltage L-G and then assume they have a bonded/grounded neutral.

I assume then that the same applies to a 3-phase wye secondary transformer. In order to get a propper megger reading and an expected result of infinite resistance then the neutral to ground bond needs to be removed before megging all of the phases. If the neutral to ground bond is not removed then you will see low resistance to ground.

Good point about measuring all voltages when troubleshooting.