Which phase is out

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arnettda

Senior Member
I have a three phase service. Only three phase are the a/c unit. I have a blown fuse in the mAin disconnect that feeds 2 single phase panels and 6 a/c disonects. I have not been able to set up a time to kill the power to change the fuse. The phasing to the A/C disconnect is not kept straight and all wires are red. Here are my voltages at the a/c unit. Can you tell which phase is bad.
A-B 240v
B-C 240 volts
A-C. O volts.
All phases 120 volts to ground.
I am thinking the A phase is the bad one?
Thanks
 

kwired

Electron manager
Location
NE Nebraska
I have a three phase service. Only three phase are the a/c unit. I have a blown fuse in the mAin disconnect that feeds 2 single phase panels and 6 a/c disonects. I have not been able to set up a time to kill the power to change the fuse. The phasing to the A/C disconnect is not kept straight and all wires are red. Here are my voltages at the a/c unit. Can you tell which phase is bad.
A-B 240v
B-C 240 volts
A-C. O volts.
All phases 120 volts to ground.
I am thinking the A phase is the bad one?
Thanks

since you have three phases, and you have both 120 and 240 volts - it about has to be a high leg delta system. The fact you don't have any 208 to ground readings tells me you lost the high leg somewhere.

Why not place meter across each fuse to narrow down to which one is blown? No voltage means low resistance across the fuse, a voltage reading will mean high resistance or open circuit. It can trick you some if there is no load currently connected when testing, but any voltage reading across any fuse at any time will still mean that fuse is bad.
 

mwm1752

Senior Member
Location
Aspen, Colo
An open fusable disconnect wired properly will have no voltage passing thru the fuse -- take out the fuse & ohm the ends -- for bad phase check each phase to grd when switch is open -- your words "I have a blown fuse in the mAin disconnect" my assessment would be B as A-C is 0 volts --
 

kwired

Electron manager
Location
NE Nebraska
I have a three phase service. Only three phase are the a/c unit. I have a blown fuse in the mAin disconnect that feeds 2 single phase panels and 6 a/c disonects. I have not been able to set up a time to kill the power to change the fuse. The phasing to the A/C disconnect is not kept straight and all wires are red. Here are my voltages at the a/c unit. Can you tell which phase is bad.
A-B 240v
B-C 240 volts
A-C. O volts.
All phases 120 volts to ground.
I am thinking the A phase is the bad one?
Thanks
I read this carefully again - how do you know it is a main fuse? You said you measured at AC disconnect, you said there are 6 AC disconnects - is only one AC unit down? Are all of them three phase and all are down? Apparently the single phase panels are seeing proper voltage?

Otherwise you have lost either A or C, and if this is a system with a high leg it is the high leg that was lost because you did not have a 208 to ground reading on anything. If it is not a high leg system you can't have three phase with 240 line to line and 120 to ground on all three lines - not possible, one has to be high leg or if a 240 volt wye the line to neutral would be 139 volts.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
Sometimes, especially where there are running 3 phase motors, that doesn't work very well.
Why? You simply go to the disconnect open it check power on the line side of the fuse and if it's not there then that's the fuse that's bad or you could simply pull the fuses and do a continuity check. Everyone is making it awful difficult.
 

growler

Senior Member
Location
Atlanta,GA
I have a three phase service. Only three phase are the a/c unit. I have a blown fuse in the mAin disconnect that feeds 2 single phase panels and 6 a/c disonects. I have not been able to set up a time to kill the power to change the fuse. The phasing to the A/C disconnect is not kept straight and all wires are red. Here are my voltages at the a/c unit. Can you tell which phase is bad.
A-B 240v
B-C 240 volts
A-C. O volts.
All phases 120 volts to ground.
I am thinking the A phase is the bad one?
Thanks

Why? You simply go to the disconnect open it check power on the line side of the fuse and if it's not there then that's the fuse that's bad or you could simply pull the fuses and do a continuity check. Everyone is making it awful difficult.


The OP ask if you can tell which phase is out without shutting power down ( useing the process of elimination ).

I don't see it because you could end up with feed back from other loads than just the three phase motors if there is a three phase panel ( water heater elelments ).

It's either A or C phase. That doesn't help much.
 

Smart $

Esteemed Member
Location
Ohio
Why? You simply go to the disconnect open it check power on the line side of the fuse and if it's not there then that's the fuse that's bad or you could simply pull the fuses and do a continuity check. Everyone is making it awful difficult.
Line side of fuse?

As already stated, check voltage across each fuse. With loads connected, any fuse showing above minimal voltage across it is bad.
 

kwired

Electron manager
Location
NE Nebraska
Why? You simply go to the disconnect open it check power on the line side of the fuse and if it's not there then that's the fuse that's bad or you could simply pull the fuses and do a continuity check. Everyone is making it awful difficult.

Sorry meant load side.
You sure that is what you meant?

If you open the disconnect an then check the load side - you have a problem with the disconnect if you read any voltage, or something is backfeeding it that probably shouldn't be.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
Well it could be they dropped a phase at the dam where everything originates too, but I'm guessing that's probably not the problem.

When I said open the disconnect I meant open the cover.
 

kwired

Electron manager
Location
NE Nebraska
Well it could be they dropped a phase at the dam where everything originates too, but I'm guessing that's probably not the problem.

When I said open the disconnect I meant open the cover.

Oh, but you can't do that without opening the switch or operating some cover interlock bypass on most of them;)
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
Oh, but you can't do that without opening the switch or operating some cover interlock bypass on most of them;)
What ever it take. Taking pi and dividing it by the number of phases and then adding the voltage less the size of the wire, just to find out which phase is out, just seems like a little bit of over kill.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
since you have three phases, and you have both 120 and 240 volts - it about has to be a high leg delta system. The fact you don't have any 208 to ground readings tells me you lost the high leg somewhere.

Why not place meter across each fuse to narrow down to which one is blown? No voltage means low resistance across the fuse, a voltage reading will mean high resistance or open circuit. It can trick you some if there is no load currently connected when testing, but any voltage reading across any fuse at any time will still mean that fuse is bad.


I am having trouble wrapping my head around this. I drew it out and I don't see a way to get 120 volts on all three phases to ground. It seems to me you would have to have some weird multiple fault, open situation to get this certainly not just a blown fuse. Can you please expound? I am thinking the initial information is not quite right.
 

arnettda

Senior Member
I am having trouble wrapping my head around this. I drew it out and I don't see a way to get 120 volts on all three phases to ground. It seems to me you would have to have some weird multiple fault, open situation to get this certainly not just a blown fuse. Can you please expound? I am thinking the initial information is not quite right.

The actual problem was not in the disconnect. It was the power company's problem out on the pole. In the main disconnect there was a capacitor looking thing with 4 wires coming out of it. One to each phase and one to the grounded conductor. I am assuming that is what was back feeding and giving me readings to ground on all phases. My original post was from my phone so the less typing I needed to do was easier.
With the capacitor looking thing installed I do not believe there was any way to tell exactly what my problem was until after I could open up the disconnect. Which I could not do until after my original post.
 
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