Big OOPS!

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don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
The way that I understand it is that there are two different scenarios, which are a floating neutral and and open neutral.

With a floating neutral all loads have a neutral wire connecting the load back to the neutral point of the transformer secondary, however the netural point is not bonded to ground. In this case, depending on the load balance, the neutral voltage with reference to ground is free to float to any voltage determinded by the current impbalance. (Ia + Ib + Ic = In). It is possible in this case to then have a neutral reference with 100V like someone had mentioned, and therefore have an higher than rated voltage across L-N loads. Bonding this neutral point to ground ensures that the neutral point has a 0V reference and all L-N loads are correct voltge.
The floating neutral does not result in a higher than rated voltage across line to neutral loads. Yes, you will not see the voltages to ground that you expect, but the load is not seeing the voltage to ground...it is only seeing the line to neutral voltage.
 

mull982

Senior Member
The floating neutral does not result in a higher than rated voltage across line to neutral loads. Yes, you will not see the voltages to ground that you expect, but the load is not seeing the voltage to ground...it is only seeing the line to neutral voltage.

So say this neutral was floating, and lets say that it measured 100V with respet to ground. What would then be the L-N voltage across the lights. Would it add to give 377V or subract to give 177V, or do you need the phase angle of the neutral voltage vector to determine calculation?
 

Smart $

Esteemed Member
Location
Ohio
So in this case the lights would see less than rated L-N voltage and would not necessarily be damaged due to overvoltage.

Depends on which side of the "equation" they were on :grin:

We do not have a breakdown of what loads were supplied by the 3 pole breaker, or to which lines the L-N loads were connected. If some L-N loads saw less than nominal voltage then it is quite likely the others saw greater than nominal voltage. Some of the temporary loads may have been switched on and off, so the common-point voltage would have shifted every time this occurred.
 

mull982

Senior Member
Depends on which side of the "equation" they were on :grin:

We do not have a breakdown of what loads were supplied by the 3 pole breaker, or to which lines the L-N loads were connected. If some L-N loads saw less than nominal voltage then it is quite likely the others saw greater than nominal voltage. Some of the temporary loads may have been switched on and off, so the common-point voltage would have shifted every time this occurred.

Not following you here. Wouldn't all loads reference 177V since the neutral was floating at 100V?

I understand that this neutral may shift depending on loads turning on and off.
 

Smart $

Esteemed Member
Location
Ohio
Not following you here. Wouldn't all loads reference 177V since the neutral was floating at 100V?
No. Depends on how the loads balance out. In the following depiction, the convergence of the load vectors (solid line) can take place anywhere on the 100V circle (since you locked that parameter in at 100V to Ground).

balanceact.gif
 

don_resqcapt19

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Location
Illinois
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retired electrician
So say this neutral was floating, and lets say that it measured 100V with respet to ground. What would then be the L-N voltage across the lights. Would it add to give 377V or subract to give 177V, or do you need the phase angle of the neutral voltage vector to determine calculation?
The fact that the neutral is not bonded to earth does not change the line to neutral voltage. Even if it is at 100 volts to earth, the line to neutral voltage is still 277. A floating neutral is not the same as an open neutral. An open neutral will cause line to neutral voltage problems.
 

mull982

Senior Member
The fact that the neutral is not bonded to earth does not change the line to neutral voltage. Even if it is at 100 volts to earth, the line to neutral voltage is still 277. A floating neutral is not the same as an open neutral. An open neutral will cause line to neutral voltage problems.

Ok I see. The L-N voltage would still be 277V no matter is bonded or not. It would only be the N-G voltage that would be different in this case 100V.

If the L-N voltages are the same then how could this damage equipment, or couldn't it?

Why have others said that the L-N voltage would vary depending on what side of the equation you were on. In one example is was 177V.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Ok I see. The L-N voltage would still be 277V no matter is bonded or not. It would only be the N-G voltage that would be different in this case 100V.

If the L-N voltages are the same then how could this damage equipment, or couldn't it?

Why have others said that the L-N voltage would vary depending on what side of the equation you were on. In one example is was 177V.
Because a floating neutral is not the same as an open neutral. It appears that in the case in this thread there was an open neutral or one with a high impedance path. In other words there was not a solid path back to XO at the source transformer. An open neutral does cause voltage changes that will damage equipment.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If the L-N voltages are the same then how could this damage equipment, or couldn't it?
It could, because if that 100v were to jump up into the 1000's, like during an accidental primary-to-secondary contact, or a nearby lightning strike, insulation values can be easily exceeded.

It can also matter if one line faults to ground. Nobody would know*, the nuetrals whould now be fixed at 277v to ground, and only a second line's faulting would cause a short circuit.

*Unless a fault detector was used, as with ungrounded Deltas.
 
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