Lighting Circuit - Part 2

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bnicholson

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Hi All, I am still workimg on tracking down a dodgy light fitting / cable in a house which has 10 plus rooms upstairs - 20 plus light fittings.

After a tough day in an attic trying to resolve the problem, I still have some questions that I do not know the answers:-

1. Apparently if I just Megger the cct I will find the short - I know how to carry out the IR test but I cannot understand how to use the findings to find the location of the short?

2. If you use a 100W bulb as a series limiter, it will help in the fault finding process - I seemed to have a problem in the Loop feed - why would the 100Watt bulb not blow as well?

3. If you disconnect the lighting cct half way round, this will help to isolate the fault. I struggled today to find the feed cable from the CU - it came up through a partition wall, nowhere near where expected!

4. If it had been downstairs lighting - how do you find out which light/rose/fitting is first in the radial?

I hope your feedback will also help others when trying to fault find in extremely large houses, or maybe it's just me struggling!

Regards

Bob N.

 

cheggers

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I would first find out what fault I have example L to E

then I would split the circuit where I thought was middle but it doesn't matter if it isn't

recheck at cu to see if fault is still there, if not energise the circuit

see which light works, they have no faults, then split down again the other half that is not working

then turn off circuit and check at the first light you split and megger there to see if the fault is still present if not reconnect and re-energise circuit. keep doing this until you find it

 
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Hi Bob , The thing with the 100w lamp across the fuseway is you wont keep blowing fuses. So , in theory anyway, if you had all the switches off and the lamp is off , then the fault is not there at this point. If there is a fault between SW -N or SW - earth say, instead of the fuse blowing, the 100w lamp lights up because it has a circuit to flow round , either to N or to Earth. If the lamp is on all the time then, presuming the wiring is 3 plate , if you start to disc. the loop feeds to split it up and narrow it down.

The bulb wont blow because of its resistance, the filament will just light up because there is a return path down either the N or the E.

I think you are stuck with a process of elimination, you'll only find the first fitting by dissing all the others. Obviously if you dissed half the circuit you've narrowed it down to ,say 5 fittings,.

Re : Meggering tells you theres a fault but not where, same as the lamp, you still need to narrow it down.

DON'T FORGET THE NEUTRAL WILL READ TO EARTH IF YOU DON'T DISSC. IT IN THE BOARD!!

Deke

 
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Why would the 100Watt bulb not blow as well?

Bob N.
a bulb is a "LOAD".

a fuse is a weak bit of circuit cable.

the load draws only as much current as it consumes!

every bulb you wire is DIRECTLY across Live & Neutral!

they don't blow...

until they are old and worn-out...

unless of course its a 110v bulb or a 12v bulb across 230v!

NOW that would blow! ;) :|

 

steptoe

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would it not be a better idea to wire a bulb in series with the fuse at the board?

that way there is a load present to hopefully prevent the fuse blowing, but also a fuse to protect the circuit in case of a major fault?

only a thought mind.

 
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Hi All, I am still workimg on tracking down a dodgy light fitting / cable in a house which has 10 plus rooms upstairs - 20 plus light fittings.After a tough day in an attic trying to resolve the problem, I still have some questions that I do not know the answers:-

1. Apparently if I just Megger the cct I will find the short - I know how to carry out the IR test but I cannot understand how to use the findings to find the location of the short?
I don't think you have read all the answerers on your other thread?

If you check back...

NO one said it will find the location of a fault...

But it WILL identify which pair of conductors are breaking down... (if any)

e.g. short Live-neutral OR short Live-Earth???

On your previous thread you hadn't actually proved with any readings there was a short! you said you assumed there was because the fuse was blowing?

you stated values of >99.9K across all conductors...

(with a resistance test), which is NOT a short..

it was pointed out that cables could be breaking down when 230V applied.... which an IR test will highlight?

What are your IR readings?

2. If you use a 100W bulb as a series limiter' date=' it will help in the fault finding process - I seemed to have a problem in the Loop feed - why would the 100Watt bulb not blow as well?

[/quote']

see my earlier post! :)

3. If you disconnect the lighting cct half way round' date=' this will help to isolate the fault. I struggled today to find the feed cable from the CU - it came up through a partition wall, nowhere near where expected!

[/quote']

This is not a question??

just a statement! ?:| :|

4. If it had been downstairs lighting - how do you find out which light/rose/fitting is first in the radial?
Disconnect the circuit at the fuse box.

Join Live neutral & earth ALL together.

pick a pair of conductors.. say Live-Neutral OR Live-Earth..

then go and read the resistance at each accessory/switch across these conductors.

It will increase at a rate of 0.0362ohms per meter for 1.0mm copper conductors. or 0.0242ohms per meter for 1.5mm copper conductors.

Pg 166 On site guide.

This will tell you the length of the cable run from the CU.

If it is wired as one long radial circuit it will tell you the order of connection...

If it is branched in a spurred off or spider arrangement.. it will just tell you which accessory's have longer total runs from the CU.

In many cases if it is looped at ceiling roses or switches' date=' it is not to hard to break a circuit and work out an approximate order.

if boxes concealled... that can be a bit more of a "child without a parent"!

I hope your feedback will also help others when trying to fault find in extremely large houses, or maybe it's just me struggling!

Regards

Bob N.
Size of the property is not an issue...

basic fault finding is the same however big the property...

say like tracing a disconnected or shorting cable though a factory?
 
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would it not be a better idea to wire a bulb in series with the fuse at the board?that way there is a load present to hopefully prevent the fuse blowing, but also a fuse to protect the circuit in case of a major fault?

only a thought mind.
Applaud SmileyApplaud Smiley

concur with that thought Step's

don't like taking fuses out of circuit!

have a feeling it may even go against good safe working practice such as required by your fave book (EAWR)! ;) :x

 
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