Neutral

[Ronnie Barham]

Two 20 amp circuits share One neutral that supply work stations at a commerical building. All the computer's were off but when the neutral was disconnected with the breaker on it fried all the power strips and one computer. There is no ground wire in the conduit, they are useing the conduit as ground. Can you explain why this happened.

Thanks

 

[jwelectric]

When the grounded (neutral) was opened the multiwire circuit became a series 240 volt circuit. The difference in voltage drop across each piece of equipment was proportionate to the impedance of the equipment.

 

[dsteves]

Additionally, modern ATX power supplies in computers are never really "Off" - They are always making +5 volt standby power to allow for features like soft power buttons and wakeup events to function. jwelectric's explanation of the MWBC is spot-on.

Dan

 

[Ronnie Barham]

Thanks, so should a power strip protect a computer against that because one computer was fried.

 

[dsteves]

If the outlet strip has surge protection that could hold together long enough to clear the breaker, perhaps.

Dan

 

[charlie b]

A surge protector (i.e., on the outlet strip) would never protect you against this circumstance. That 240 volts (or somewhere between 120 and 240) would stay there forever, or until something fries or the breaker trips. This isn't a "surge." It's a continuous overvoltage condition.

 

[hardworkingstiff]

....when the neutral was disconnected with the breaker on ....

Thanks

Curious, how did this happen?

 

[dsteves]

A surge protector (i.e., on the outlet strip) would never protect you against this circumstance. That 240 volts (or somewhere between 120 and 240) would stay there forever, or until something fries or the breaker trips. This isn't a "surge." It's a continuous overvoltage condition.

I was a little vague in my reply, which was not unintentional

The MOVs in a surge suppressor outlet strip avalanche at around 150V, but I doubt they'd absorb the i^2*t of the OCPD. Also, as you say, the neutral legs of the loads would be floating back and forth between the lines, limited only by the MOVs, which likely wouldn't hold up to clear the BC OCPDs.

Sounds like the wrong neutral got lifted off the bar in the panel.
Dan

 

[Ronnie Barham]

Yes it was but there was very little load.

 

[dsteves]

Load magnitude doesn't matter in that case. As I mentioned, the computers aren't isolated from the line like they used to be when you had the big red switch on the back side or top front of the cases. Charlie and jwelectric are right.

[edited to include]
By the way, welcome to the forum, Ronnie.

Dan

 

[infinity]

Was the neutral intentionally disconnected? In a commercial building I would guess that the voltage is most likely 208Y/120.

 

[DaveTap]

I've seen it happen and the surge supressor in the strip got smoked but it did kill the power and the pc survived. Had to replace the outlet strip though and it never tripped a breaker.

 

[icesoft]

Had it happen to us once also when I was working for a PC repair shop. PoCo cut an old ground from the transformer into the building, fried all of our surge protectors (Tripp Lite IsoBar branded) which served their purpose in protecting the PCs (none of which were damaged in the incident).

I later opened up one of the IsoBar's and dumped the charcoal out to inspect the damage and design that had saved our customers' equipment. The design has a series of MOVs, a fast-acting fuse, and a thermal cutout. Designed with the thermal cutout sandwiched between two MOVs, and the fuse in series between the MOVs and the line cord.

MOVs shunt the excess voltage until either the heat from the MOVs opens up the thermal fuse, or the excessive current opens the fast-acting fuse.