Three phase 480V with odd voltage

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Jdmseven

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Winchester, VA
I hooked up a temporary air handler. It's three phase 480V. The feed is coming from a mcc. After the unit was started the first time, it wouldn't restart. I checked voltage in the 100A disconnect, and found that the C phase is only 103V to ground. The other two phases are 270V to ground. How is this possible?
 
Jdmseven said:
I hooked up a temporary air handler. It's three phase 480V. The feed is coming from a mcc. After the unit was started the first time, it wouldn't restart. I checked voltage in the 100A disconnect, and found that the C phase is only 103V to ground. The other two phases are 270V to ground. How is this possible?
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Most likely the feed is 480Y/270 and one of the phase conductors (C) is open between the POCO supply and your MCC.
If you have a motor or a delta transformer primary connected (or even any tiny three phase load) the C phase conductor will be at the average of the A and B lines with respect to ground. If you do the vector math that might come out to about 103V.
Or else the C line is open and you are just reading a phantom voltage on the open wire. Much more likely IMHO.
 
Thanks. I checked voltage on the bus ahead of the bucket, and had 270 for each phase to ground. I think the breaker might not be making proper contact. It's a very old mcc
 
Jdmseven said:
Thanks. I checked voltage on the bus ahead of the bucket, and had 270 for each phase to ground. I think the breaker might not be making proper contact. It's a very old mcc
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As a general rule, and especially when working with known defective energized switchgear, I hope you are using proper PPE for the energy available from your 480V bus.
 
GoldDigger said:
As a general rule, and especially when working with known defective energized switchgear, I hope you are using proper PPE for the energy available from your 480V bus.
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There you go again... chasing off new members, trying to impose those ridiculous safety rules.

:blink: ^intended to be completely facetious^ :blink:
 
GoldDigger said:
Most likely the feed is 480Y/270 and one of the phase conductors (C) is open between the POCO supply and your MCC.
If you have a motor or a delta transformer primary connected (or even any tiny three phase load) the C phase conductor will be at the average of the A and B lines with respect to ground. If you do the vector math that might come out to about 103V.
Or else the C line is open and you are just reading a phantom voltage on the open wire. Much more likely IMHO.
Click to expand...

480/270? Did they change things without telling us again?:)
 
Jdmseven said:
Thanks. I checked voltage on the bus ahead of the bucket, and had 270 for each phase to ground. I think the breaker might not be making proper contact. It's a very old mcc
Click to expand...
Did you exercise the breaker? If it was unused for a long time, an oxidized film can build up on the contacts, increasing the resistance. Then when you put the load on the first time, that contact got hotter and maybe arced a little too much when you opened it, making the resistance worse. Try exercising it now by turning it on and off with no load connected a half dozen times, then check it again.
 
Jraef said:
Did you exercise the breaker? If it was unused for a long time, an oxidized film can build up on the contacts, increasing the resistance. Then when you put the load on the first time, that contact got hotter and maybe arced a little too much when you opened it, making the resistance worse. Try exercising it now by turning it on and off with no load connected a half dozen times, then check it again.
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If you have to do this to make it work
Then replace it !
it's obviously faulty
otherwise it's just gunna come back and bite you.

do it once ! Do it well !
 
dmxtothemax said:
If you have to do this to make it work
Then replace it !
it's obviously faulty
otherwise it's just gunna come back and bite you.

do it once ! Do it well !
Click to expand...
depending on how critical the supplied load is I'd say it is worth "exercising" the breaker at least the first time it fails. The contacts move with a slight "wiping" action which may help if there is something on the contact surfaces.
 
Jraef said:
Did you exercise the breaker? If it was unused for a long time, an oxidized film can build up on the contacts...
Click to expand...

dmxtothemax said:
If you have to do this to make it work
Then replace it !
it's obviously faulty
otherwise it's just gunna come back and bite you.

do it once ! Do it well !
Click to expand...

kwired said:
depending on how critical the supplied load is I'd say it is worth "exercising" the breaker at least the first time it fails. The contacts move with a slight "wiping" action which may help if there is something on the contact surfaces.
Click to expand...
I agree.

Base replacement on value and conditions of use, abuse, or lack thereof. :D
 
Smart $ said:
I agree.

Base replacement on value and conditions of use, abuse, or lack thereof. :D
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Common problem I seem to run into at grain storage bins in the fall is flies. It would probably help if the panelboards were in NEMA 12 cabinets instead of NEMA 3R as most usually are, but as the weather starts to get colder at night they are looking for a warm place to go, and these panels are often full of them. They also find their way into the breakers and occasionally you will get one that won't make contact. I have many times taken compressed air hose and blow into the "arc relief chutes" on the breaker and usually get several flies out of them after that they work again.
 
ptonsparky said:
A smashed green bug is one hell of an insulator.
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In a related story...

A few yers ago my central A/C quit working. The relay that supplies power to the compressor had failed. I watched while the repairman took the system apart, and when he opened up the enclosure where the relay was it was packed solid with dead fire ants.
 
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