Loose Netutral

[sohaila7]

The following incidence has happened:

It was reproted that few split airconditioners (single phase) and some other (single phase) connected load got their power supply modules and control cards burnt.

These loads were fed from a three phase supply (3 phase + Neutral).

The items that burnt were distributed on all the three phases.

Investingations led to one of the two possibilites:

1. Few days ago, there was a problem reported with the Earth Fault module of the incoming breaker. The electrican the removed that earth fault module. For doing so, he had to disconnect all the three phases and the neutral wire.

2. The neutral was found loose in the four pole circuit breaker.

3. One assumption is that the electician maight have "accidently" connected a phase wire to the neutral supply! and after realizing the damage, had quietly connected the wires to the right places. And in doing so, he had left the neutral loose.

Now, to my understanding, if we assume that assumption # 3 is wrong, loose neutral is enough to introduce spikes in the system and thus blowing off power supplies.

What do you suggest?

 

[waross]

A loose neutral will produce more than 'spikes".
It is easier to explain with single phase so I'll give a single phase example.
Consider a 100:200 volt system.
From line 1 to the neutral is connected a 1 ohm load.
From line 2 to the neutral is connected a 10 ohm load.
Now the neutral is disconnected.
You now have 11 ohms across 200 volts.
The current will be 18.2 amps.
There will be 18.2 volts across the 1 ohm resistor.
There will be 181.8 volts across the 10 ohm resistor.
The calcuations are a little more involved with three phase but you get the idea.
respectfully

 

[Scramode]

I am not familiar with the term earth fault. Is that a ground fault circuit interupter (GFCI) circut breaker?

 

[itsmoked]

Yes Scramode; one-in-the-same.

Waross is right on.

Originally:
The 1 ohm load was running 100A or V²/R = 10,000W
The 10 ohm load was running 10A or V²/R = 1,000W

Pull the neutral...

Notice now the 1 ohm load V²/R = 331W

And the 10 ohm load V²/R = [red]3,305W[/red]

The normally smaller loads are the ones that take the hit.

I had an electrician buddy who's apprentice dropped the neutral at a trailer park. It cost him 100 refrigerators.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com