Lost Utility Phase

[wirebender]

We were called out to an auto dealer's shop this afternoon that had lost power to a large portion of their equipment.

The problem turned out to be a fuse on one of the utility pole mounted transformers (208Y/120V service) had dropped out, a dead buzzard was found nearby. POCO came out and remedied the situation.

They called us back and the air compressor had tripped the breaker and when they closed it, it went boom.

Air compressor is 480V. There is a transformer mounted above it fed with 125A 208Y/120V.



I opened the transformer and the neutral was melted.



Took all the feed wires loose and had direct shorts between A, B and N. C was clear.

Took the FMC off and the wires were all melted together. 50' away the feeder conduit goes through a wall into an LB. I pulled the wire out some and cut it which cleared the shorts back to the panel. Megged them and they are okay. So we replace the LB with a j-box and refeed the transformer and everything works fine.

Any ideas on the cause of this?
Could the single phasing from the POCO had done this?
Why was the neutral melted?

The transformer is bonded with an EGC.
There is no GEC. Does it not need one without a derived neutral? I'm a little unsure about proper procedure when you are hooking up a transformer backward.

None of the derived phases are grounded and no fault detection equipment is used.
Should we ground one of these phases and wouldn't that require a GEC?



While we were there they started noticing other things not working properly. Some F bays, some equip in the car wash, etc.

Could the single phasing from the POCO have caused unseen damage. What should I look for and what tests should be run to check out the system?

Sorry, lots of questions, just don't want to leave an unsafe situation.

 

[augie47]

Answer your 1stv question by asking a question
How was the neutral terminated ? On a 208 to 480 transformer there should be no neutral connections.
The 480 delta secondary could remain ungrounded but 250.21(B) should be observed,
You could also ground one phase and make it a corner grounded secondary.
Is there a drive involved ?

 

[texie]

We were called out to an auto dealer's shop this afternoon that had lost power to a large portion of their equipment.

The problem turned out to be a fuse on one of the utility pole mounted transformers (208Y/120V service) had dropped out, a dead buzzard was found nearby. POCO came out and remedied the situation.

They called us back and the air compressor had tripped the breaker and when they closed it, it went boom.

Air compressor is 480V. There is a transformer mounted above it fed with 125A 208Y/120V.

View attachment 9454

I opened the transformer and the neutral was melted.

View attachment 9455

Took all the feed wires loose and had direct shorts between A, B and N. C was clear.

Took the FMC off and the wires were all melted together. 50' away the feeder conduit goes through a wall into an LB. I pulled the wire out some and cut it which cleared the shorts back to the panel. Megged them and they are okay. So we replace the LB with a j-box and refeed the transformer and everything works fine.

Any ideas on the cause of this?
Could the single phasing from the POCO had done this?
Why was the neutral melted?

The transformer is bonded with an EGC.
There is no GEC. Does it not need one without a derived neutral? I'm a little unsure about proper procedure when you are hooking up a transformer backward.

None of the derived phases are grounded and no fault detection equipment is used.
Should we ground one of these phases and wouldn't that require a GEC?

View attachment 9456

While we were there they started noticing other things not working properly. Some F bays, some equip in the car wash, etc.

Could the single phasing from the POCO have caused unseen damage. What should I look for and what tests should be run to check out the system?

Sorry, lots of questions, just don't want to leave an unsafe situation.

If I had to guess from you description, I would say you have a 208Y/480delta transformer wired in reverse and the neutral was connected on what is now the primary side. As augie said, there should be no neutral connection when a transformer is used in this fashion. Loss of 1 phase from the the POCO can lead to exactly as happened to your install and I have seen this a number of times over the years.
I have never been a fan of using a delta/Y in reverse as there are to many negative issues that come along with it. A much better choice is to use a 208 delta X 480Y transformer and solves all the downsides when stepping voltage up.
I think it is worth noting that the 2014 NEC is stepping in on this issue by requiring that a transformer used in reverse as in your case will have to be listed for this along with instructions. It is amazing to me how many transformers I come across like this that are just accidents waiting to happen.

 

[wirebender]

Answer your 1stv question by asking a question
How was the neutral terminated ?

Which first question? "Any ideas on the cause of this?"

Neutral was terminated on XO.
I realize it wasn't necessary, but could it have been the cause of this?
I hate to second guess original installers, trying to see if there were any reasons this was connected as it was with a neutral and no GEC.

On a 208 to 480 transformer there should be no neutral connections.
The 480 delta secondary could remain ungrounded but 250.21(B) should be observed,
You could also ground one phase and make it a corner grounded secondary.
Is there a drive involved ?

I guess there could be a drive built in to the compressor, there is not an external drive. It's an air compressor, looks like a big box instead of the cylinder looking compressors. Not sure if it would qualify for 250.21(A)(2) but thanks for pointing that out.
There would be no problem grounding a phase, building steel is right there.

If I had to guess from you description, I would say you have a 208Y/480delta transformer wired in reverse and the neutral was connected on what is now the primary side.

Yes, that is exactly what we have.

As augie said, there should be no neutral connection when a transformer is used in this fashion. Loss of 1 phase from the the POCO can lead to exactly as happened to your install and I have seen this a number of times over the years.

What are the drawbacks of connecting the neutral?
How does the phase loss cause the insulation to melt? Is this only because the neutral was connected or would it happen even if the neutral was not there?
What do I need to look for in other equipment for possible damage?

Keep in mind that we don't know what the buzzard shorted to open the fuse, phase to phase, or phase to ground. He was found some distance from the tx bank, so I don't really even see how he could have shorted the transmission lines. It was drizzling rain all day.

I have never been a fan of using a delta/Y in reverse as there are to many negative issues that come along with it.

What are these negative issues?
Primary side or secondary? Both?

A much better choice is to use a 208 delta X 480Y transformer and solves all the downsides when stepping voltage up.
I think it is worth noting that the 2014 NEC is stepping in on this issue by requiring that a transformer used in reverse as in your case will have to be listed for this along with instructions. It is amazing to me how many transformers I come across like this that are just accidents waiting to happen.

I agree it would be better to use the proper transformer, but is it a serious enough problem that it needs to be replaced?

And what about the GEC. Doesn't it require a GEC even if there are no grounded circuit conductors?
Why did the neutral melt?

 

[augie47]

To address part of your post:

I can only tell you that connecting XO creates some current nightmares. Some of the sharper guys here can elaborate.
I mentioned a drive as there have been comments on the Forum about some drives not being
compatible with corner grounded or ungrounded systems. (I sent an IM to Jraef for input).

IMO, there should be a GE system on the transformer secondary.

 

[kwired]

Which first question? "Any ideas on the cause of this?"

Neutral was terminated on XO.
I realize it wasn't necessary, but could it have been the cause of this?
I hate to second guess original installers, trying to see if there were any reasons this was connected as it was with a neutral and no GEC.

Go ahead and second guess them. This seems to be confusing for many people, even some with a fair amount of trade experience. Grounding the XO in this kind of situation has come up several times in this forum, never with any desirable results, usually there is problems even without a phase loss condition, so it may have already been working on damage and the loss of phase just finished it off.

I guess there could be a drive built in to the compressor, there is not an external drive. It's an air compressor, looks like a big box instead of the cylinder looking compressors. Not sure if it would qualify for 250.21(A)(2) but thanks for pointing that out.
There would be no problem grounding a phase, building steel is right there.

Maybe a "screw" type of compressor instead of a reciprocating compressor. They are more efficient and less moving parts to maintain.

Yes, that is exactly what we have.



What are the drawbacks of connecting the neutral?
How does the phase loss cause the insulation to melt? Is this only because the neutral was connected or would it happen even if the neutral was not there?
What do I need to look for in other equipment for possible damage?

Keep in mind that we don't know what the buzzard shorted to open the fuse, phase to phase, or phase to ground. He was found some distance from the tx bank, so I don't really even see how he could have shorted the transmission lines. It was drizzling rain all day.



What are these negative issues?
Primary side or secondary? Both?



I agree it would be better to use the proper transformer, but is it a serious enough problem that it needs to be replaced?

And what about the GEC. Doesn't it require a GEC even if there are no grounded circuit conductors?
Why did the neutral melt?

There is no overcurrent device to interrupt the neutral, so if a condition develops where it is overloaded it is never removed from the circuit. You have two phases trying to deliver and a neutral as a path to try to help out. You very well may have had enough voltage on each of the secondary conductors for this compressor to try to run, though probably somewhat severely unbalanced, you did not say whether or not the neutral supplied was same size as the ungrounded conductors or not, but if it were reduced in size would be even more reason for it to be overheating. It will likely carry at least as much as the ungrounded conductors when in this condition.

If the buzzard tripped the primary fuse it would have had to made contact with the protected conductor, which usually only is 2-5 feet of conductor between the fuse and the transformer bushing and another potential - either another phase or a grounded object.

 

[texie]

Which first question? "Any ideas on the cause of this?"

Neutral was terminated on XO.
I realize it wasn't necessary, but could it have been the cause of this?
I hate to second guess original installers, trying to see if there were any reasons this was connected as it was with a neutral and no GEC.



I guess there could be a drive built in to the compressor, there is not an external drive. It's an air compressor, looks like a big box instead of the cylinder looking compressors. Not sure if it would qualify for 250.21(A)(2) but thanks for pointing that out.
There would be no problem grounding a phase, building steel is right there.




Yes, that is exactly what we have.



What are the drawbacks of connecting the neutral?
How does the phase loss cause the insulation to melt? Is this only because the neutral was connected or would it happen even if the neutral was not there?
What do I need to look for in other equipment for possible damage?

Keep in mind that we don't know what the buzzard shorted to open the fuse, phase to phase, or phase to ground. He was found some distance from the tx bank, so I don't really even see how he could have shorted the transmission lines. It was drizzling rain all day.



What are these negative issues?
Primary side or secondary? Both?



I agree it would be better to use the proper transformer, but is it a serious enough problem that it needs to be replaced?

And what about the GEC. Doesn't it require a GEC even if there are no grounded circuit conductors?
Why did the neutral melt?

It is imperative that you remove the neutral connection or you will have a repeat at some point of the situation, guaranteed. There should be nothing connected to the X0 terminal
Yes, the transformer needs a compliant GES, even it you operate the secondary ungrounded.
One big downside is that your only option to ground the 480 side is corner grounding or operate it ungrounded, both of which have draw backs. If corner grounded there are very specific rules, including equipment rated for this application. If ungrounded you will need to comply with 250.21(B). Except in very rare cases, in this day and age it is considered by most that ungrounded or corner grounded systems are not advisable. Also, as pointed out by augie, some equipment today is not approved for connection to a corner grounded or ungrounded systems.
I've heard it a million times, "this thing has been working 25 years, it must right. You don't know what you are talking about". There are many things done in our trade by misinformed people that work but are just accidents waiting to happen.
If it were my call, I would put in an appropriate transformer. After all this is not a very big one.

 

[wirebender]

Thanks for the information, everyone.

XO is no longer terminated and a GEC is established.

 

[Jljohnson]

Check out 450.11 in the 2014 code. Transformers will no longer be allowed to be reverse fed like that without instructions from the manufacturer. good luck getting those documents.....lol. I've seen what you described enough times in my career that I am happy to see that change in the NEC.

 

[meternerd]

I understand why the 480 was fried, but you mentioned other equipment. Was it fed off of the 480 or the 120/208? Just because only one primary fuse blew, that doesn't mean you didn't have faults that produced voltage spikes on the other two phases. A phase to phase primary fault may have caused secondary voltage problems. Did the utility check the primary and secondary neutrals for a good connection? Is the primary neutral grounded? Is the bank wired delta/wye? Is the utility primary 3 or 4 wire? Overhead or underground? Just some thoughts from a utility perspective.

 

[wirebender]

I understand why the 480 was fried, but you mentioned other equipment. Was it fed off of the 480 or the 120/208? Just because only one primary fuse blew, that doesn't mean you didn't have faults that produced voltage spikes on the other two phases. A phase to phase primary fault may have caused secondary voltage problems. Did the utility check the primary and secondary neutrals for a good connection? Is the primary neutral grounded? Is the bank wired delta/wye? Is the utility primary 3 or 4 wire? Overhead or underground? Just some thoughts from a utility perspective.

Primary is overhead but i don't know what the utility did. If i remember, I will check the primary next time I am there.

The other equipment I found was in the automatic car was and the problem was in a control box where 2 small ocp devices (same phase, 277) were tripped. No other problems have been reported, yet.

 

[meternerd]

Primary is overhead but i don't know what the utility did. If i remember, I will check the primary next time I am there.

The other equipment I found was in the automatic car was and the problem was in a control box where 2 small ocp devices (same phase, 277) were tripped. No other problems have been reported, yet.

Good....the problems were associated with the 480 side or you wouldn't have any 277. Sounds like you have the problem solved.

 

[GoldDigger]

Just a very brief note on why you cannot connect the X0 terminal of the wye primary to a ground or neutral:

If the secondary winding is delta, the three phase voltage windings form a closed loop. That means that they MUST add to zero as vectors or else there will be circulating currents large enough that the resistive voltage drop in the windings makes the math come out right.

If the primary is a delta, then the input voltage must already meet that zero sum condition, or else it would indicate a tap or equipment problem on the POCO side and you would get large circulating currents even with no load on the secondary.

Now, if instead of a delta primary you have a wye primary AND the POCO source is also wye, then there is no mechanism that forces the three wye to center point voltages to be equal in magnitude. As a result, the delta output voltages may not match properly. Result: overheating and likely damage.

Now if you take that same wye primary and let the X0 float, you now have a natural mechanism that allows the center point of the wye to float in voltage to the point where the three phase to common point voltages add to zero as vectors. The eliminates the potential problem at the delta secondary.


Note that if you have a wye-wye transformer, there is no requirement for the voltages to add to zero, and you will not have any problems unless you try to connect a delta primary downstream somewhere.

Losing a phase is the ultimate wye voltage imbalance, and combined with the load of the temporarily single phased motor caused high currents just about everywhere.

 

[texie]

Good....the problems were associated with the 480 side or you wouldn't have any 277. Sounds like you have the problem solved.

I still think there is some confusion here. The OP indicated that this was a derived 480 delta system that had design issues with the step up transformer. Based on his description of the transformer there is no 277 possible.

 

[wirebender]

My bad. The car wash was 120V.


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