high leg

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ejbred45

Member
Location
33160
Occupation
Electrician
readings
A to ground 120
B to " "
c to ground " (high leg)
A to C 0
A to B 240
B TO C 240
lost high leg?
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I would say lost high leg. If there was a high leg it should measure higher than 120 to ground, My bets guess would be high leg lost and feed back from A phase is giviong you the 120v reading on C.
 
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hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Had the very same call from one of my techs last week, the machine he was working on made appear he had 120 to ground on the high leg. Turns out, the panel that one of our new construction guys put the breaker in, was missing the middle bussbar half way down. Don’t know why he didn’t catch that when putting in the breaker.
 
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ejbred45

Member
Location
33160
Occupation
Electrician
ejbred45 said:
readings
A to ground 120
B to " "
c to ground " (high leg)
A to C 0
A to B 240
B TO C 240
lost high leg?
Click to expand...
That was my assessment A and C together but what i find puzzling is this is a apartment building the only 3phase is fire pump moters and elevator
elevator still working with the same readings
 
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ejbred45

Member
Location
33160
Occupation
Electrician
ejbred45 said:
That was my assessment A and C together but what i find puzzling is this is a apartment building the only 3phase is fire pump moters and elevator
elevator still working with the same readings
Click to expand...
any way waiting on power company
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
ejbred45 said:
readings
A to ground 120
B to " "
c to ground " (high leg)
A to C 0
A to B 240
B TO C 240
lost high leg?
Click to expand...
There might be an open delta to create the high leg, but in this case there's zero volts across the primary of the transformer supplying the high leg. This could be caused by a ground fault on the primary or the feeder to it, or by some other problem on the utility side. With zero volts across the primary there will be zero volts across the secondary, and so A will look like it's shorted to C.
 
O

oldsparky52

Senior Member
Location
Wilmington, NC USA
synchro said:
There might be an open delta to create the high leg, but in this case there's zero volts across the primary of the transformer supplying the high leg. This could be caused by a ground fault on the primary or the feeder to it, or by some other problem on the utility side. With zero volts across the primary there will be zero volts across the secondary, and so A will look like it's shorted to C.
Click to expand...
A and C would be coming from the same transformer, right? The 2nd transformer would supply the high leg. If the second transformer lost the primary, wouldn't A and C still be working properly? I must not be understanding what you are saying.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
ejbred45 said:
readings
A to ground 120
B to " "
c to ground " (high leg)
A to C 0
A to B 240
B TO C 240
lost high leg?
Click to expand...

oldsparky52 said:
A and C would be coming from the same transformer, right? The 2nd transformer would supply the high leg. If the second transformer lost the primary, wouldn't A and C still be working properly? I must not be understanding what you are saying.
Click to expand...

The OP is calling C the high leg in the post above, and so the first transformer of an open delta would be A-N-B and the second transformer would be A-C. Therefore if the second transformer primary voltage is zero then there'd also be zero voltage from A to C.
 
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