208V on the neutral

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steved

Senior Member
Location
Oregon
I don't work with three-phase systems (I'm a motorhome electrical troubleshooter) and I've run into a situation that has got me puzzled. One of the workers at the factory plugged his drop light into a 120V 20A receptacle and the light got bright -- really, really bright. I checked the voltage at the receptacle and measured 120V Hot to Ground, 240V Hot to Neutral, and 208V Neutral to Ground. What the heck is going on? I got my boss to call the Maintenance Department and I'm not going to monkey around with the building wiring, but I'm curious as to what could cause this situation. Would this occur if the neutral was accidentally connected to the B phase of a high-leg delta? How about if there were an open neutral on a wye?

Can anyone recommend any good websites, books, etc. on three-phase theory and calculations?
 
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radar3535

Member
steved said:
I don't work with three-phase systems (I'm a motorhome electrical troubleshooter) and I've run into a situation that has got me puzzled. One of the workers at the factory plugged his drop light into a 120V 20A receptacle and the light got bright -- really, really bright. I checked the voltage at the receptacle and measured 120V Hot to Ground, 240V Hot to Neutral, and 208V Neutral to Ground. What the heck is going on? I got my boss to call the Maintenance Department and I'm not going to monkey around with the building wiring, but I'm curious as to what could cause this situation. Would this occur if the neutral was accidentally connected to the B phase of a high-leg delta? How about if there were an open neutral on a wye?

Can anyone recommend any good websites, books, etc. on three-phase theory and calculations?
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My un educated guess is you lost a neutral, on a 3 wire -2ckt shared neutral situation.
 
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gar

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Location
Ann Arbor, Michigan
Occupation
EE
080707-0529 EST

If this is a delta system with one center tapped secondary to provide 120-0-120 single phase, then there is a wild leg. The wild leg voltage to the center tap (neutral) is 240*0.866 = 207.8 V. This you derive from a vector diagram. The 0.866 is 1/2 the sq-root of 3 and it is also the sin 60 deg. The sin of 30 deg is 1/2 and thus the 120 V.

If the white neutral wire in the outlet is connected to the wild leg, then there will be a white wire in the breaker box connected to a breaker that is connected to the wild leg bus. Easy to get a single pole breaker in the wrong location, but why a white wire on a breaker? And why wasn't the work checked?

In a 208 V leg to leg Y source and thus 120 V leg to neutral there is no 240 V available without an additional transformer or severe unbalance. Your measurements do not imply this.

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