Hi guys,

Can somebody help me please....

Working on a church changed isolators and 2 3ph boards and 1 single phase... When packing up on the night time... A fault occurred on another single phase board elsewhere in the church.. We had not touched any circuits on this board...

1 A fault occurred on 1 lighting circuit and blow out all bulbs the fault is still there... Neutral and earth
2 a computer blew on this board ring main circuit
3 the church organ blew..
4 the rcd did not trip
5 there was no loose connections to mains
6 there were other lights on board that were on and never blew?

Is it possible that the lighting circuit breaker never operated and the fault current went to ring main?

Another posibility that was mentioned was a loose neutral but we checked and there wasn't? Surely if that was the case it would of blown all bulbs and appliances in that board especially as they were all on?

I would be looking for a loose neutral in one of the 3 phase boards (assuming they feed the single phase) on the incoming side


Stu

The single phase board is fed from an isolator. The isolator was checked for loose connections feeding the mentioned fuse board.

There is a definite fault on the lighting circuit.

Could there have been a loose neutral connection on the lighting circuit? That would of caused faults?

Sorry assuming breaker never operated?

Are you sure that all the Neutral links in the 3 phase boards & isolators were connected & tightened?

Did you check/ test the installation prior to and after alterations?

Did you check the voltage(s) at each board prior to powering up?

Sure seems like a neutral fault on the supply side of the affected board.

Regards

Mike

Hi mike,

Voltage was 247 volts. Before and after.

If there was a loose neutral in the board it's self would that then create a fault a lighting circuit and blow all bulbs?

And how would the lighting fault appear? Surely all bulbs on the board as they are the same lamps have blown?

What was the fault in "another single phase board elsewhere in the church"?
If a poor neutral connection exists that momentary fault may have drastically altered the voltages on all 3 phases.

A simple fault on a single phase circuit will cause a huge voltage drop - but that of itself is unlikely to damage any other equipment (the fault current would didn't go through the other equipment after all - even if it had the equipments' only internal resistance means it should be absolutely fine when exposed to full mains voltage) - most equipment certainly won't be damaged by a lower voltage.

Many blown lamps, damaged PC, destroyed organ all suggest a very high voltage. While there might be a small increase in mains voltage when fault protection opens, again I doubt that it would be enough to damage a lot of equipment.

What can produce a large voltage across single phase equipment is an open circuit N on a three-phase supply, especially if the overall 3-phase load is unbalanced. Without a N you in effect have two single phase loads connected in series across 400V. If both offer the same resistance, then each sees about 200V - so no problem. But if one has a low power (high resistance) load connected and the other a large load (low resistance), the system acts as a potential divider and the low load side gets a larger proportion of the 400V - possibly near all of it - and that can cause a lot of damage very quickly.

- Andy.

Greenstxr

I think that Andy Jewburys' explanation would pretty well match what I would have written if I had been at my desk during the day.

Is there any chance that someone got a black (phase) and the blue (neutral) mixed up when reconnecting? Thus inadvertently applying 400V to some loads and not others. Although saying that, phase-sequence & voltage tests might have shown this up - grasping at straws

Just trying to understand what happened with your unfortunate incident.

Regards

Mike

Hi guys,

Thanks for feed back.

I have since found the following faults...

Loose neutral on the single phase side of the fuse board with evidence of burning..
Also some had added a copper cable possibly 1.5mm to the black 3ph meb incoming fuse. (So maybe it was a common problem) ?

So I'm assuming that's would of been the problem? Any feed back would be most welcome?

Just a thought

Could the lights that blew have an old borrowed neutral to another phase ?

Ady

-------------------------
Resistance is futile.

Hi ady1

The lights were all fed from a single phase board.

On the black phase which coincidently had the main incoming fuse wrapped with a 1.5mm cable. I tested the fuse and it had definitely blown hence that's why someone had just used copper cable rather than replace the fuse.

I'd have expected a loose N where it's common to two or more phases to cause an over-voltage. Or a dodgy N contact if there's a 4-pole isolator somewhere.

A loose connection within a single phase circuit would account for local burning, but not overvoltage & associated lamps blowing and equipment damage.

- Andy.

Hi Andy, thanks for input.

Is it possible overvoltage could of occurred from the meb incomer? As the fuse had been tampered with so in the event of a fault it would never of blown anyway?

All neutral at common were checked.

A bit late for my maths, but I think a 1.5mm² wire has a radius of about 0.7mm - so a diameter of of about 1.4mm - so according to table 53.1 in the regs would act as fusewire rated a bit below 60A - so that's maybe a bit of a red herring.


Sure - physics doesn't care who owns the faulty cable. A dodgy supply N should show on a L-N loop test (e.g. PFC) though.

- Andy.

Yes very true it is all very puzzling...

Just another theory I'm throwing out there...

What if the isolator never made correct connection? As it is 4 pole..
Or because there was a fault on the circuit and the mcb may have failed to operate? would fault current on a single phase board blow the mentioned circuits bulbs?