Confusing Fault.. Help!

[Trip Hazard]

So I've just completed (well apart from this issue) an installation for a coffee shop/cafe. It's all tubed out in galv conduit, as the client wanted the 'industrial' look. 

The circuit thats being problematic is the lighting. I have created pendants from thru boxes on the ceiling, which drop to metal E27 holders, via 0.75 3 -core flex. All holders have correct polarity and really good r2. All meggerred A:OK between all lines and conduit etc

So take the bank of 8 pendants in the window for example. Disconnect switched live from light switch over at the grid, with no lamps in, L-E is open circuit. Put your first lamp in and its 150ohms, stick another lamp in and it reduces to about 75 - you keep adding lamps until eventually the circuit has a low enough resistance to throw the breaker. 

I nit picked through it for hours today, with a rather worried client looking on as he needs to open (the woes of commercial work!)

Could it be related to the fact that at the origin of the supply, there is a reading of about 0.25 between neutral and earth. Putting a lamp in makes a pathway between L and N so therefore to earth as well?

I need to look at it with a fresh head on Monday really, but thought i'd see what others think.

Cheers

 

[roys]

When cold an incandescent lamp (60w) will have a resistance of about 100 to 150 Ohms, when 230 V is switched on to it filament heats up in the lamp rises to a much higher resistance.

When you say it throws the breaker are you only assuming this or does it actually do this, if it is only an assumation then as described above it won't happen, so worry not, if it does happen then something else is wrong. 

 

[NozSpark]

Is your circuit neutral still connected and/or your main switch energised?

Remember... neutral and earth are connected at the star point of the suppliers transformer (and at the head if its TNCS)

 

[ProDave]

As above.

Exactly what IS your problem?  Is the lighting MCB tripping? Too many lamps? have you measured the current being drawn?

 

[NozSpark]

I think that I'd be worried if I was the client

 

[Trip Hazard]

Thanks for the replies

There's 27 40W lamps on this circuit, connected to 6 independent MK Logic dimmers

The breaker is a B10 and it IS tripping.

The board is a Schneider TPN - the neutral is not switched.

I can't currently get into the plant room in the basement to verify the supply. It's a recently converted building in a city centre, it used to be HSBC.. However am I right in assuming that if supply is PME, then the 0.25ohms between earth and neutral on my board is correct

Like I say, I've meggerred the live on this circuit down to earth, checked the wiring and the lampholders and it's all good.

Would it be worth trying to swap the circuit on to another phase? Perhaps there's something going on there

 

[steptoe]

Mmmm,

Without even looking at anything else,

And in concurrence with other posters, as I've obviously read it wrong somewhere,

How on earth can you even think N E at 0.25 on a TNCS to be correct,? :|

 

[Trip Hazard]

Well, I think its correct because the neutral on the board remains connected, and the main cut outs are on the floor below, prob around 20 or 30m away. So if the earth connects to neutral there, the 0.25 sounds about right

 

[NozSpark]

You said that you've meggered L-E,,,, did you link out the dimmers???

Have you meggered L-N and N-E yet??

 

[Trip Hazard]

Definitely linked out the dimmers at £25 a pop!

L-N is ok. So is N-E (with the neutral bridge removed in the board)

 

[binky]

sounds like you 'nicked' as in sightly damaged the insulation at one of the thru-boxes on the circuit. In my experience it trips under load, but shows as ok under test.

the other possibility is a faulty lamp holder?

just re-read your post, if L-E is open circuit you have a straight short circuit.

depemdent on your teat gear adding 1 lamp may have slightly lower resistance than the fault, when you add a second lamp you are halving the result - revise resistance calc fro parallel resistors, which i would type out but I'm pissed :Y :Y :Y :Y

 

[Trip Hazard]

Thanks Binky.

I spent a lot of time making sure the galv was deburred fully, but accidents can always happen I guess.

Since the fault is common to all switches I may as well run a new line from the board.

I cant believe I'm even thinking about it at this time of night and especially when tomorrow is the first day off in 3 weeks. Just want to get this one signed off and handed over

 

[binky]

Go back to basics, take the lamps out, and re-test circuit. Then split circuit and test both ends etc etc. Chances are its a cover screw chaffing live conductor.

 

[Trip Hazard]

yeah I took all box lids off today and pulled out and separated a lot of the terminal blocks to no avail

I guess im gonna have to disconnect everything to single it out. Would be easy if I had all the time in the world, but during the the building is now starting to flood with all the staff, it's very off putting.

 

[binky]

get in early so you can think clearly, and just work through the circuit, Chances are it's something simple.

 

[Trip Hazard]

cheers mate

 

[Andy™]

Well, I think its correct because the neutral on the board remains connected, and the main cut outs are on the floor below, prob around 20 or 30m away. So if the earth connects to neutral there, the 0.25 sounds about right

you stated that reading was at origin of supply in first post, now its at the end of a submain?

but i agree with others, probably damaged insulation somewhere