Neutral failure!

We have a 3 phase sistem with 380 V betwen phases and 220v phase to neutral. An electrician was doing a repairation and he just disconect
a neutral wire from a cabinet and conect it a few minutes later and in that time 3 computers were destroyed, even one of them started to burn. After that we measure the volts and with tne neutral wire connected we have 220v betwen phase and neutral and wen we disconect the neutral we have 500v betwen the same phase and the neutral. Can any one tell me wat is going on? Thanks in advance. Chris.
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Fizzle ------------------------------------------------- Ted Rubberford. 'The Man In The Red Latex Skintight Suit'
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Chris there is a earlier Ground to Neutral Post here about a diagram that might shed some light on your query.
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Only a moron works on electricity without turning the computers off first. If you don't know what is going on, hire a real electrician to do it. ever hear of neutral current? is your 3 phase really balanced?
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The Post is in alt.eng.~>compliance
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| We have a 3 phase sistem with 380 V betwen phases and 220v phase to | neutral. An electrician was doing a repairation and he just disconect | a neutral wire from a cabinet and conect it a few minutes later and | in that time 3 computers were destroyed, even one of them started to | burn. After that we measure the volts and with tne neutral wire | connected we have 220v betwen phase and neutral and wen we disconect | the neutral we have 500v betwen the same phase and the neutral. Can | any one tell me wat is going on?
The electrician who disconnected the neutral is seriously incompetent. You need to be talking with lawyers about this immediately.
A multiwire circuit, such as a 3 phase star system like you would have, has multiple loads connected in series across phase to phase wires. The neutral has a role in keeping the voltages in balance. Current flows on the neutral in proportion to the imbalance. If that neutral current cannot flow, then effectively one set of single phase loads is in series with another set. In perfect balance there would be no difference. But if one set is heavily loaded, the other set will see much higher voltage. The maximum would normally be expected to be 380 volts, but if some of the loads are reactive loads, such as motors, you can see backfeed voltages perhaps as high as 760 volts. If the grounding system is defective, it may even be worse.
Disconnecting the neutral before cutting off ALL power is an absolute NO-NO. You should not have even disconnected it for the purpose of testing voltage.
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Oh ! REMOVE THE JUMPER after reconnecting the Circuit Cable., or you'll just oule the system into thinking it's the way it's suppose to work and someone'd get shocked with the double trouble Grnd~Ntrl confusion Indoors again.
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CLT wrote:

That was no electrician - although some people should be grateful that he wasn't calling himself a brain surgeon.
To illustrate the effect of what he did, take two dissimilar elastic bands, join the ends together and bang a nail through the join into a handy bit of wood. Now stretch and relax each band or combination of them. Each acts independently, yes? So you can REALLY load, or pull on one band, without affecting the other.
Now remove the nail. When you pull on one band, the join moves and the thin band really stretches whilst the thick band hardly stretches at all.
He pulled the nail out. The join point moved and the voltage of the lightly loaded bit (the thin band) went way up whilst the voltage across the heavily loaded bit (the thick band) went way down. Unfortunately, the computers were on the lightly loaded bit.
The analogy is far from exact, but you maybe get the idea.
Sue the company, report it to your standards authority, write letters to your senator or whatever you do when you are really p**ed off in your Country. Never mind blowing a few computers - the guy could easily have burnt the place down and/or killed people.
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wrote:
| He pulled the nail out. The join point moved and the voltage of the | lightly loaded bit (the thin band) went way up whilst the voltage across | the heavily loaded bit (the thick band) went way down. Unfortunately, | the computers were on the lightly loaded bit.
Worse. The computers are switching power supplies, most likely, and these tend to be designed for the 100 to 240 volt range. That means they are switching on for a shorter time at 220 volts. During the time they are switched off, they are essentially zero load. If only computers are on one phase, then they will get hit with between 330 and 381 volts, depending on the balance of the other 2 phases.
Why he was getting 500 volts I don't know. But reactive loads could be doing that.
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snipped-for-privacy@ipal.net wrote:

Some capacitance in the power supplies' input circuitry may have interacted with the system's source reactive impedance and resulted in higher than normal voltages. Possibly at higher harmonics than the system fundamental frequency as well. When you combine non-linear loads with unusual RLC circuits, weird things can happen.
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This stupid idiot could have got someone electrocuted.
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