3 phase high leg

[green2012]

I have a 480 3 phase system. I wanted a little information about 3 phase high leg versus standard 3 phase. how can you tell which one is which with a voltage tester? I took reading from phase to ground and got 277.
A to ground, B to ground, C to ground all were 277. The tester I had with me went overload if I attemted phase to phase readings. What should phase to phase rating be in a 3phase with and without a highleg.

3 phase with high leg:

A to ground=?
B to ground=?
C to ground=?

A to B=?
B to C=?
C to A=?

3 phase WITHOUT a high leg

A to ground=?
B to ground=?
C to ground=?

A to B=?
B to C=?
C to A=?

 

[david luchini]

3 phase with high leg:

A to ground=240
B to ground=416
C to ground=240

A to B=480
B to C=480
C to A=480

3 phase WITHOUT a high leg (assuming a grounded wye)

A to ground=277
B to ground=277
C to ground=277

A to B=480
B to C=480
C to A=480

3 phase WITHOUT a high leg (assuming a corner grounded delta)

A to ground=0
B to ground=480
C to ground=480

A to B=480
B to C=480
C to A=480

 

[augie47]

As David showed, if you are reading 277v from all three phases to ground you have a grounded wye system of some type. Your phases would be 480 (any) phase to (any) phase. You would not have a "high leg".

 

[jim dungar]

As David showed, if you are reading 277v from all three phases to ground you have a grounded wye system of some type. Your phases would be 480 (any) phase to (any) phase. You would not have a "high leg".

Also, an ungrounded system frequently looks like a wye system when high impedance voltmeters are used.

Augie is correct, a 'high-leg' system will not have all three phases to ground be equal.

 

[Jljohnson]

If your phase to ground voltage in a 3 phase system is 1/2 of the phase-phase voltage (as in 120/240), you are working in a delta connected system. If the phase to ground voltage x 1.732 is equal to the phase-phase voltage (as in 120/208 or 277/480), you are in a Wye connected system. Delta connected systems will have a high leg, Wye systems will not. The only exception I can think of would be the corner grounded Delta system, which would read voltages as shown a couple posts above.

 

[GoldDigger]

If your phase to ground voltage in a 3 phase system is 1/2 of the phase-phase voltage (as in 120/240), you are working in a delta connected system. If the phase to ground voltage x 1.732 is equal to the phase-phase voltage (as in 120/208 or 277/480), you are in a Wye connected system. Delta connected systems will have a high leg, Wye systems will not. The only exception I can think of would be the corner grounded Delta system, which would read voltages as shown a couple posts above.

That is a rather large generalization, which is more likely to get the OP into trouble than to help him. In a high leg delta, only two of his three phase to ground voltages will be 1/2 of the phase-to-phase voltage. The third will be the high leg, at a higher voltage to ground or neutral.

In a delta system there will not be a neutral unless it is a high-leg system with a "neutral" which is only the neutral for one of the three phases, the one which produces 120-0-120.
The other likely form of delta would indeed be corner grounded delta, with ungrounded coming in a distant third.
In a wye system there will be a neutral (whether it is used by anything or not, and by definition all three phase to neutral voltages will be identical. With very few exceptions the neutral in a wye system must be grounded.

PS: The standard notation in the code and elsewhere is 208Y/120 and 480Y/277, even though the "Y" appears to be in the wrong place.

 

[ActionDave]

I took reading from phase to ground and got 277.
A to ground, B to ground, C to ground all were 277. The tester I had with me went overload if I attemted phase to phase readings.

It should be noted that if your tester goes out of limit when trying to read phase to phase on a 480V system you really need to get a new one. Bad things can happen to meters and those that are holding them when they are used above their rating.

 

[kwired]

It should be noted that if your tester goes out of limit when trying to read phase to phase on a 480V system you really need to get a new one. Bad things can happen to meters and those that are holding them when they are used above their rating.

Good point, especially when you consider it is a little hard to find a tester that can not display a reading up to at least 600 volts that also is not a cat III rated instrument. If it can not go more than 300 volts, it is much more likely to be a cheap meter that does not have the safety rating to take much for overvoltage.