High Line to Ground Voltage Readings

[philly]

I am getting 400V and 600V on L-G and L-N readings on the secondary of a 240V/120V transformer.

We have an outdoor 4.16kV stand alone breaker cubicle. Inside this cubicle there is a 4.16kV to 240/120V transformer to supply control power to the breaker relays and heaters that are in the cubicle. We powered everything down today to put high temp wire on heaters and when we turned power back on we noticed a relay smoking. This is a realy which we just had repaired sometime back and was damaged for some reason.

We took voltage measurements and saw that we has 240V on the two hot wires coming of the secondary of the transformer however we read 400V from one leg to ground and 600V from the other leg to ground. The tranformer neutral is center tapped and tied to ground. These readings do not make sense to me. Even if something was grounded on the secondary of the transformer I would not expect to see more than 240V for a L-G reading. How in the world would I be reading 600V?

X2 and X3 of the transformer are tied together and then brought out and bonded to the cubicle casing as a ground reference. This same point is used to tie all of the neutrals in from the various relays and heaters. This is also giving us L-N voltages of 400V and 600V. I am afraid that with these L-N voltages across these electronics that I wil fry the relays.

Does anyone know what could be happening?

 

[jim dungar]

How do you know the center of the 120/240V was actually connected to ground? Your voltage readings could be explained by an ungrounded, floating, neutral point.

 

[philly]

How do you know the center of the 120/240V was actually connected to ground? Your voltage readings could be explained by an ungrounded, floating, neutral point.

We can trace the center of the 120/240 to a lug on the enclosure frame that is being used as a ground. When you ask the question above, are you referring to an actual earth ground? The enclosure frame is also bonded to the EGC which is coming into the enclosure with the 4.16kV feeders.

I dont understand how we could have such high voltages as high as 600V. I would expect that with a floating neutral the most voltage we would have for L-G would be 240V as a result of capacitance coupling. However in this case I would expect to see 240V L-G on one leg and 0V L-G on the other. I dont see where we would get 600V.

Even if the neutral was floating I would still expect to see 120V for our L-N readings. This is not the case since we see 400V and 600V L-N. We even see this when we believe we unground the neutral and just strictly read L-N voltage without center grounded. Am I correct in thinking that we should still read only 120V here no matter what is going on with grounding?

Thanks for the help

 

[iaov]

What are you seeing L to L voltage wise?

 

[LarryFine]

Phil, try reading from neutral to ground. Try it both at the transformer's neutral connection and at the panel. Also, try reading from each line and the neutral, to a known independent ground if you can.

 

[philly]

What are you seeing L to L voltage wise?

We are seeing 240V as expected.

Phil, try reading from neutral to ground. Try it both at the transformer's neutral connection and at the panel. Also, try reading from each line and the neutral, to a known independent ground if you can.

This transformer is hard to troubleshoot due to the fact that it is located in a 4.16kV compartment and therefore cant make direct live measurements on the transformer itself.

However I belive that I isolated the wire coming from the neutral and removed it from the frame ground. I then measured the neutral to ground (frame) and only saw somewhere in the neighorbood of 20V or so. Nothing to get excited about. I can try again to ensure that I am indeed isolating the neutral and measuring it to ground.

There is not panel in this application. The two hot legs go directly to two breaker and the neutrals come directly of the transformer tied into the frame.

Would another EGC or ground bus in a nearby MCC constitute the known independ ground that you suggested? I will try it. What if measurements stay the same?

 

[iaov]

Your readings certainly are strange. Would I be correct in assuming these are no load(open breaker) readings. I'm still curious about what your line to line is and though it might sound strange, you may have an open line here. I've seen some stange readings from one leg opening before. Especially when you are looking at the voltrages with a high impedence meter.

 

[philly]

Your readings certainly are strange. Would I be correct in assuming these are no load(open breaker) readings. I'm still curious about what your line to line is and though it might sound strange, you may have an open line here. I've seen some stange readings from one leg opening before. Especially when you are looking at the voltrages with a high impedence meter.

Yes you are correct, these are open breaker readings, on the line side of the secondary breakers. Here we get 240V.

 

[zog]

I am getting 400V and 600V on L-G and L-N readings on the secondary of a 240V/120V transformer.

We have an outdoor 4.16kV stand alone breaker cubicle. Inside this cubicle there is a 4.16kV to 240/120V transformer to supply control power to the breaker relays and heaters that are in the cubicle. We powered everything down today to put high temp wire on heaters and when we turned power back on we noticed a relay smoking. This is a realy which we just had repaired sometime back and was damaged for some reason.

We took voltage measurements and saw that we has 240V on the two hot wires coming of the secondary of the transformer however we read 400V from one leg to ground and 600V from the other leg to ground. The tranformer neutral is center tapped and tied to ground. These readings do not make sense to me. Even if something was grounded on the secondary of the transformer I would not expect to see more than 240V for a L-G reading. How in the world would I be reading 600V?

X2 and X3 of the transformer are tied together and then brought out and bonded to the cubicle casing as a ground reference. This same point is used to tie all of the neutrals in from the various relays and heaters. This is also giving us L-N voltages of 400V and 600V. I am afraid that with these L-N voltages across these electronics that I wil fry the relays.

Does anyone know what could be happening?

Do a TTR test yet?

 

[jim dungar]

We can trace the center of the 120/240 to a lug on the enclosure frame that is being used as a ground. When you ask the question above, are you referring to an actual earth ground? The enclosure frame is also bonded to the ECG which is coming into the enclosure with the 4.120V feeders.

No, I do not care about any connection to dirt. I am interested in knowing where you actually put your voltage leads on the transformer, a terminal block or the enclosure. Also was this done with a high input impedance meter?

Regardless where you make your measurements, your possible combinations are:
L1-L2
L1-N
L2-N
L1-G
L2-G
N-G

Have you made all of these measurements at different locations?

In your OP you did not mention that you read 120V L-N. If your system does not have a solid connection to what you are measuring (your reference point) then it becomes hard to predict what your voltage will really be.

 

[Rick Christopherson]

Your readings certainly are strange. Would I be correct in assuming these are no load(open breaker) readings. I'm still curious about what your line to line is and though it might sound strange, you may have an open line here. I've seen some stange readings from one leg opening before. Especially when you are looking at the voltrages with a high impedence meter.

Actually these readings are not that strange. The transformer has nothing anchoring it to a reference point, so the whole thing is floating up to a large voltage. One leg is 600 volts above his reference point (assuming this was to ground) and the second one was about 200 volts less, putting it also above the reference point by 400 volts. (If he took simultaneous measurements, I am sure he would find these to be 240 volts apart).

The reason why the voltages might seem odd is because you are normally expecting one to be above the reference point and one below the reference point because the neutral is between them. In this case, they are both above (relatively speaking since you don't know above from below).

After looking at your original posting again, it appears that you are getting the same readings for both neutral and ground, so the neutral and ground are bonded, but your neutral wire is broken somewhere between the point you are measuring and the transformer itself (possibly internal to the transformer, like a broken tap connection).

 

[Smart $]

Actually these readings are not that strange. The transformer has nothing anchoring it to a reference point, so the whole thing is floating up to a large voltage. One leg is 600 volts above his reference point (assuming this was to ground) and the second one was about 200 volts less, putting it also above the reference point by 400 volts. (If he took simultaneous measurements, I am sure he would find these to be 240 volts apart).

The reason why the voltages might seem odd is because you are normally expecting one to be above the reference point and one below the reference point because the neutral is between them. In this case, they are both above (relatively speaking since you don't know above from below).

After looking at your original posting again, it appears that you are getting the same readings for both neutral and ground, so the neutral and ground are bonded, but your neutral wire is broken somewhere between the point you are measuring and the transformer itself (possibly internal to the transformer, like a broken tap connection).

I don't see how there could be an internal [open] short... it's not a center-tapped secondary... it's a dual voltage secondary, i.e. two windings... X2 connected to X3... and if both leads had a short you wouldn't get 240V across the hots.

The only way I see it happening is if both windings shorted open to terminal X2 and X3... and the winding side of the open shorts shorted closed to each other. What's the odds of that happening

It appears there is an open short between X2/X3 and Ground... simply got to find and correct it.

 

[Rick Christopherson]

I don't see how there could be an internal [open] short... it's not a center-tapped secondary... it's a dual voltage secondary, i.e. two windings... X2 connected to X3... and if both leads had a short you wouldn't get 240V across the hots.

The only way I see it happening is if both windings shorted open to terminal X2 and X3... and the winding side of the open shorts shorted closed to each other. What's the odds of that happening

It appears there is an open short between X2/X3 and Ground... simply got to find and correct it.

No, it is not that much of a stretch. X2 and X3 are connected together and then brought out as the neutral point. Somewhere between this connection and his measuring point, the neutral is broken.

===============================
I wouldn't comment on this, except I know you are a pretty sharp guy (hence Smart $)

and I am sure that you know I am just poking fun, but if you are going to be in "the business" you really do need to know the difference between a short circuit and open circuit.

 

[philly]

We went through this whole circuit yesterday and eneded up finding that indeed the transformer was ungrounded. The ground from the transformer went to the neutral terminal of a heater before it went to the case ground, and when the electrician went to change out a heater he forgot to attach this ground wire coming from the transformer thus leaving it ungrounded or floating.

Even when the transformer is not grounded I should still always read 120V from L-N right? In other words if I had two wires coming out from the hots of each side and then a wire coming out from the ungrounded center of the transformer (X2 & X3 tied together) the I should always have 120V L-N between each of the hots and this neutral regardless of what the ground is. Only if for some reason the neutral becomes broken, then the L-N voltage will read what ever the voltage drop is across coils depending on loading. This all is independed of ground however. The only time that ground comes into play is when we are measuring L-G readings.

Actually these readings are not that strange. The transformer has nothing anchoring it to a reference point, so the whole thing is floating up to a large voltage. One leg is 600 volts above his reference point (assuming this was to ground) and the second one was about 200 volts less, putting it also above the reference point by 400 volts. (If he took simultaneous measurements, I am sure he would find these to be 240 volts apart).

The reason why the voltages might seem odd is because you are normally expecting one to be above the reference point and one below the reference point because the neutral is between them. In this case, they are both above (relatively speaking since you don't know above from below).

I have seen cases where the neutral is ungrounded and I saw 180V L-G on one side and 60V L-G on the other. I guess this is a case where on is above the reverence and the other is below, however there is still 240V between them. I rounded off the voltages in my OP (my mistake) so I'm not sure exactly what they were buy it looks like the 400V and 600V we were seeing were both above the reference however still would have almost 240V between them (in this example 200V) representing the 240V across the transformer. Does this mean that both of these sin waves would be in phase instead of 180deg offset. What would cause these referecne voltages to both be above the reference and be soo high?

 

[winnie]

What would cause these referecne voltages to both be above the reference and be soo high?

Remember what the _primary_ voltage of this particular transformer is.

Depending upon the design of the transformer, you might be seeing capacitive coupling of primary voltage to secondary, you might be seeing actual leakage across the insulation, or some other coupling between primary and secondary circuits via another component.

This transformer might be on its way out, but I have no experience at these voltages or with leakage in this sort of hardware.

The 'phase' of the line-ground voltage would entirely depend upon what the 'boosting' voltage looked like. I can imagine situations where an oscillograph of the two signals looked 'in phase' or 'out of phase'.

-Jon

 

[jim dungar]

I have seen cases where the neutral is ungrounded and I saw 180V L-G on one side and 60V L-G on the other. I guess this is a case where on is above the reverence and the other is below, however there is still 240V between them. I rounded off the voltages in my OP (my mistake) so I'm not sure exactly what they were buy it looks like the 400V and 600V we were seeing were both above the reference however still would have almost 240V between them (in this example 200V) representing the 240V across the transformer. Does this mean that both of these sin waves would be in phase instead of 180deg offset. What would cause these referecne voltages to both be above the reference and be soo high?

Do not confuse L-N readings with 'unreferenced' readings. With a 120/240V connection, the L-N voltage readings can swing based on the unbalanced loading of the 120V windings, but they will 'add up to' 240V

In an ungrounded system, you do not have a completed circuit so your readings can be almost any thing. I just took my voltmeter, set on VAC, connected one lead to a metal switch plate screw and the other lead to an AA cell battery - should I trust the 1.6VAC that it displayed, why or why not?

Well actually in an ungrounded 120/240VAC system you do have a completed circuit due to 'coupling capacitance' as well as some other leakage paths, so a high impedance voltmeter will read a 'phantom' voltage when connected L-G.

 

[Smart $]

No, it is not that much of a stretch. X2 and X3 are connected together and then brought out as the neutral point. Somewhere between this connection and his measuring point, the neutral is broken.

X2 and X3 are terminal points and I do not consider them internal to the transformer itself. I'm certain others agree (as well as disagree ).

===============================
I wouldn't comment on this, except I know you are a pretty sharp guy (hence Smart $)

and I am sure that you know I am just poking fun, but if you are going to be in "the business" you really do need to know the difference between a short circuit and open circuit.

My apologies for using slang

As an added note, a short circuit is defined as my emotional response to your comment, while an open circuit is this written response:

 

[philly]

Remember what the _primary_ voltage of this particular transformer is.

Depending upon the design of the transformer, you might be seeing capacitive coupling of primary voltage to secondary, you might be seeing actual leakage across the insulation, or some other coupling between primary and secondary circuits via another component.

If there is this coupling between the primary and secondary as mentioned, then why would these voltages not show up on any L-L voltage readings? I am assuming that this 600V L-G reading through coupling would be just as dangerous as an intentional 600V L-G circuit.

The 'phase' of the line-ground voltage would entirely depend upon what the 'boosting' voltage looked like. I can imagine situations where an oscillograph of the two signals looked 'in phase' or 'out of phase'.

What is the boosting voltage that you referenced?

Do not confuse L-N readings with 'unreferenced' readings. With a 120/240V connection, the L-N voltage readings can swing based on the unbalanced loading of the 120V windings, but they will 'add up to' 240V

I was always under the impression that a transformer with a two winding secondary would always have 120V across each winding and therefore 120V L-N due to a voltage divider with two equal impedences. Unless the actual impedences of each each of the loads connected across each side of the transformer alter the impedences of the voltage divider and thus change the voltage across each coil?

In my case we found that the neutral for the loads was not connected to the neutral therefore we had a broken neutral. With this broken neutral the L-N voltages would be strictly what was created by the voltage divider with the two loads in series?

All this should be independent of grounding is this correct? However becasue the neutrals at the loads were tied to ground but not to the transformer we saw L-G voltages on the open neutral readings?

I didn't get a chance to check before we fixed but what do you suspect we would have seen for a N-G reading reading the neutral from the transformer?

Well actually in an ungrounded 120/240VAC system you do have a completed circuit due to 'coupling capacitance' as well as some other leakage paths, so a high impedance voltmeter will read a 'phantom' voltage when connected L-G.

We took measurements with a fluke 787 which I belive is a high impedence meter with a 10kohm input impedence. How would a high impedence meter make a difference if measuring across the capacitance coupling?

 

[jim dungar]

We took measurements with a fluke 787 which I belive is a high impedence meter with a 10kohm input impedence. How would a high impedence meter make a difference if measuring across the capacitance coupling?

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

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'.

 

[Rick Christopherson]

As an added note, a short circuit is defined as my emotional response to your comment, while an open circuit is this written response:

Hmmm, if your system can develop a short circuit this easily, you might want to check your wiring and beef it up a bit. It sounds as though your insulation might be overly susceptible to light chaffing, abrading, or ribbing.

It is unfortunate, because if you weren't distracted by your own short circuit concerns, you would have been able to provide more assistance to the OP, since my original diagnosis did in fact point to the underlying problem the OP described. Granted, a short circuit condition can result in such a rapid in-rush of current that specific details, conditions, and words are quickly overlooked. I have heard that hydraulic-magnetic breakers are less susceptible to pre-existing thermal tripping, so they might be worth looking into, if you are experiencing unrelated thermal issues.

Huh? What did he just say????