C phase-173 v to ground

[Dave123]

I ran across something I had never seen before today. I found a receptacle with 97 volts to ground. Upon further investigation I found A phase had 97 volts to ground, B phase had 131 volts to ground and C phase had 173 volts to ground. Recently added in the building were 2 induction cooktops. When I kill power to the cooktops the problem goes away. Any ideas?

 

[ptonsparky]

What type of service are you supplied with? 3Ph 208/120? 3Ph 240 Delta? Grounded/ungrounded. 3Ph with a hi leg. Typically 240 line to line with 120 line to neutral on 2 phases and about 212 on the third.

 

[masterinbama]

Sounds like a lost neutral on a 120/240 3 phase Delta where the high leg was landed at C rather than B

 

[kwired]

I have to lean toward the bad neutral idea also. But this means the induction cooktops have to have line to neutral loads otherwise the neutral wouldn't be a player in the issue where the problem goes away when cooktops are not in use.

 

[ptonsparky]

Sounds like a lost neutral on a 120/240 3 phase Delta where the high leg was landed at C rather than B

We come across the C phase hi leg often enough. The combination of all, is one I have not encountered. Sounds right to me.

 

[Dave123]

What type of service are you supplied with? 3Ph 208/120? 3Ph 240 Delta? Grounded/ungrounded. 3Ph with a hi leg. Typically 240 line to line with 120 line to neutral on 2 phases and about 212 on the third.

Its a 277/480 wye. Through a step down transformer to a 120/208 400 amp panel. All phases read 121 toneutral. 4 volts between ground and neut.

 

[don_resqcapt19]

Its a 277/480 wye. Through a step down transformer to a 120/208 400 amp panel. All phases read 121 toneutral. 4 volts between ground and neut.

Where in the system did you read the 4 volts between the grounding system and the neutral? That voltage reflects the voltage drop in the neutral and seems to be on the high side.

 

[GoldDigger]

Its a 277/480 wye. Through a step down transformer to a 120/208 400 amp panel. All phases read 121 toneutral. 4 volts between ground and neut.

If the phase voltages to neutral are constant and consistent at the supply transformer, then there would have to be a high resistance neutral somewhere in the wiring that feeds both the induction units and the receptacles. And as kwired said, there would have to be line to neutral loads in the cooktops. Maybe even loads which are still connected when the units are not in use, such as transformer primaries.

However, the question now is how you measured the line to ground voltages at the receptacles. If you used a high impedance meter, you may be seeing phantom voltages on the ground caused by the ground itself being open at the receptacle. In that case the power feed itself to the cooktops may be inducing a voltage in the ungrounded ground terminal at the receptacle(s).

 

[Dennis Alwon]

I want to know if you tested the voltage to neutral when the cooktops were running or not. If there is not any load or minimal loads on then the voltages will be normal. Once a load is introduced then the voltages may change. This is classic bad neutral. I would want further info.

 

[kwired]

If the phase voltages to neutral are constant and consistent at the supply transformer, then there would have to be a high resistance neutral somewhere in the wiring that feeds both the induction units and the receptacles. And as kwired said, there would have to be line to neutral loads in the cooktops. Maybe even loads which are still connected when the units are not in use, such as transformer primaries.

However, the question now is how you measured the line to ground voltages at the receptacles. If you used a high impedance meter, you may be seeing phantom voltages on the ground caused by the ground itself being open at the receptacle. In that case the power feed itself to the cooktops may be inducing a voltage in the ungrounded ground terminal at the receptacle(s).

Or maybe there is no system bonding jumper leaving the equipment ground "floating" and like you said a high impedance meter will see a large range of voltages to this floating ground depending on loading conditions.

 

[jeremysterling]

Or maybe there is no system bonding jumper leaving the equipment ground "floating" and like you said a high impedance meter will see a large range of voltages to this floating ground depending on loading conditions.

I agree. The OP should check if X0 is bonded with a SBJ.