3 Phase panel problems

[squaredan]

Need some help on this problem i got with a 3phase wye panel..It is a house panel to a small shopping center about five stores and offices on second floor.. I am reading 120 volts phase to neutral on A and B phase but reading about 23 volts on C phase..When I turn of Main Breaker and all branch circuit breaker to panel I read 120V on all three and 208v between all..When I start turning on with single pole breakers first, phase C starts bottoming out and the more breakers on it will bottom to about 23volts..they are two three phase loads on this panel, the elevator and trash compactor.. Thinking it is a Transformer problem(transformer is about 150 feet away) I asked around to all the stores and offices and no one is having any problems..I checked a couple of meters to stores and they all read normal..Back at the panel in question I check the Main breaker line side to the load side to all phase and read zero volts, so no problem with the Main breaker..Now that I am home I just was thinking why didnt I check meter line and load side,Dam.. Peco was call but never showed.. Could this be a meter problem..No loose connection anywhere..If anyone has any ideas that would help..thanks Dan

 

[kwired]

Need some help on this problem i got with a 3phase wye panel..It is a house panel to a small shopping center about five stores and offices on second floor.. I am reading 120 volts phase to neutral on A and B phase but reading about 23 volts on C phase..When I turn of Main Breaker and all branch circuit breaker to panel I read 120V on all three and 208v between all..When I start turning on with single pole breakers first, phase C starts bottoming out and the more breakers on it will bottom to about 23volts..they are two three phase loads on this panel, the elevator and trash compactor.. Thinking it is a Transformer problem(transformer is about 150 feet away) I asked around to all the stores and offices and no one is having any problems..I checked a couple of meters to stores and they all read normal..Back at the panel in question I check the Main breaker line side to the load side to all phase and read zero volts, so no problem with the Main breaker..Now that I am home I just was thinking why didnt I check meter line and load side,Dam.. Peco was call but never showed.. Could this be a meter problem..No loose connection anywhere..If anyone has any ideas that would help..thanks Dan

You lost a phase someplace. You didn't say that you checked voltage from phase to phase while loads were connected just phase to neutral. You can/will get backfeed through 2 and 3 pole loads on the missing phase and voltage to neutral will appear to be normal but if you check phase to phase you will likely see one of them is not right.

another suggestion, disconnect loads and apply a 120 test load from each phase to neutral something like a heat gun. If you have a weak connection that reads good voltage with no load this test load will make it go to little or no voltage fast.

 

[squaredan]

hey kwired, yes i did measure voltage between phase under load forgot to mention that part, it was I think A to B like 206V; B to C was I think 167V.. I am sure Peco never showed so i will check again..I was told by someone long time ago that if you read voltage from phase to phase under load and equals up 208 the problem is on your end..Ex. AB phase reads 208V, AC phase read 165v, BC phase reads 43V.. Was this Guy wrong?

Dan

 

[kwired]

hey kwired, yes i did measure voltage between phase under load forgot to mention that part, it was I think A to B like 206V; B to C was I think 167V.. I am sure Peco never showed so i will check again..I was told by someone long time ago that if you read voltage from phase to phase under load and equals up 208 the problem is on your end..Ex. AB phase reads 208V, AC phase read 165v, BC phase reads 43V.. Was this Guy wrong?

Dan

He is right. If you find a point where voltage is normal while loaded you are upstream from the problem. Voltage will not drop without an impedance in the circuit. Conductors themselves have some impedance when loaded but we are only talking about a very small voltage drop compared to what you have.

 

[gar]

110706-1315 EDT

squaredan:

hey kwired, yes i did measure voltage between phase under load forgot to mention that part, it was I think A to B like 206V; B to C was I think 167V.. I am sure Peco never showed so i will check again..I was told by someone long time ago that if you read voltage from phase to phase under load and equals up 208 the problem is on your end..Ex. AB phase reads 208V, AC phase read 165v, BC phase reads 43V.. Was this Guy wrong?

I do not understand this paragraph.

I was told by someone long time ago that if you read voltage from phase to phase under load and equals up 208 the problem is on your end

What does "equals up 208" mean.

If you have a balanced 3 phase source and at some point in the circuit, for example the main lugs in a panel, you read approximately equal voltage for each phase, with or without a load, then the source can be classified as good at that point.

If one phase voltage is about correct, in your example 208, but the other two voltages are not correct, and the sum of the two incorrect voltages is approximately equal the one correct voltage, then one of the supply lines is open or of a high impedance relative to the load. You might describe this as a lost phase.

Find a point in the system where the 3 phase voltages are about correct under loaded and no-load conditions. From this point toward the source the circuit is probably OK.

Find the first point beyond the good point where the problem is displayed. Then the problem is between the last good point and the first bad point.

.

 

[squaredan]

110706-1315 EDT

If one phase voltage is about correct, in your example 208, but the other two voltages are not correct, and the sum of the two incorrect voltages is approximately equal the one correct voltage, then one of the supply lines is open or of a high impedance relative to the load. You might describe this as a lost phase.

.

Hey Gar, I was saying what you wrote in this paragraph.. But the guy who told me this said that if this situation would occur then the problem would be on your end not the Power Co. end... So if you got Phase AB -208v, BC- 120v, AC- 120v then the problem would be on the Power Co. end...Lost Phase

 

[gar]

110706-1852 EDT

So the guy that gave you the information had his head on backwards.

This guy needs to do a little thinking on the meaning of the measurements he was making, or he has no understanding of 3 phase circuits.

If you still have contact with him you might try to get him to think through the logic of the circuit.

.

 

[cadpoint]

...
If you still have contact with him you might try to get him to think through the logic of the circuit.

.

As an electrician one should address a (or) the service as it needs to be addressed!

Logic of what a circuit or service is doing comes into play real quickly, I frankly couldn't agree more!

 

[the blur]

a house panel is 3 phase ?

you have a feed problem to that panel, plain and simple. if unloaded, you may read 120v to ground because there is a hair of a bad connection remaining. once you start loading that bad leg, the voltage drops right down.

sounds like a sub panel feed, and the breaker at the feed is probably bad or melted. or a fused disconnect for that sub panel is bad.

I just had a similar problem, where the internals of a main breaker was melted, but externally it looked fine. 1 phase was dropping out, then coming back.

 

[kwired]

a house panel is 3 phase ?

House panel in shopping center or any multiple occupancy building means common area items are supplied from it.

Not a house as in a dwelling (if that is your question)

 

[Hv&Lv]

Would this happen to be an underground fed meterbase?