Lighting Circuit Blowing........

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bnicholson

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Hi All,

a 5A wired fuse. I first took out every light bulb - 10 plus upstairs rooms(would like to know an easier way to fault find), disconnected the the lighting cable from the CU and measured the resistance between Line & Neutral and to Earth and found the readings to be >99.9K ohms in all instances.

Believing that one of the many bulbs had been causing the problem, I re-inserted the Fuse only for it to blow extremly quickly!

I am now baffled (it doesn't take much!), I have no idea why a circuit which appears to be open is causing a "deadish" short.

Appreciate any pearls of wisdom into any aspect of my fault finding procedure please.

Many thanks

Bob N.

 

DonkeyDong

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Hi there,

Have you IR'ed the circuit (with the switches in all possible switched positions)?

D.

 

bnicholson

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Hi,

No I didn't carry out a Insulation Resistance (is this IR?) test on the cct, is it possible to have an insulation breakdown but which a Resistance test will not pick up on?

Bob

 
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Make a lampholder up with two bits of flex , put a 100w lamp in it and put it across the the fuseway, that is instead of the fuse. If it glows at half power you have left a lamp in, turn switches on one at a time, if the lamp lights up there is a short circuit in that room. If the lamp lights all the time , the short is on the feed, disconnect the loop feed about halfway round and keep trying till you narrow it down .

Deke

 

Tazmanracer

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The first thing i'd do is ask, what happened just before the MCB tripped?

Lamp Blew, putting up my new light

of course that relies on the circuit working originally and the customer being truthful

like DD says have you tested all switch lives?

 

bnicholson

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Thanks Deke for your fault finding process.

A couple of questions:-

1. Will carrying out the IR test help me to find the fault any quicker?

2. Is there any need to remove all the bulbs or should I have left them in and carried out the test as you described?

Thanks

Bob N.

 
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Hi,No I didn't carry out a Insulation Resistance (is this IR?) test on the cct, is it possible to have an insulation breakdown but which a Resistance test will not pick up on?

Bob
Your basic resistance ("continuity type") test is only testing using a low voltage.

an IR test sticks a higher voltage up the circuit which can identify breakdowns in cable that the resistance test wouldn't pick up.

But need to be aware of any dimmers or PIR etc.. on circuit.. IR can Zap them!

Do an IR with L&N together to earth?

Symptoms sound like could be:-

1/ customer replaced a light fitting & wired a switch back directly across L & N!

Or 2/

maybe a metal fitting with a lamp holder that can twist round too much when replacing a bulb..

and as the bulb is removed / replaced wires twist out of back of connections?

so appears intermitant at times...

:| ?:|

 
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You can leave them in , lamp may be glowing dim but once the short appears the lamp will go to full power . Hopefully every time you turn the (say) kitchen light on.

Deke

Good point from Tazmanracer ... always ask the question , what happened ... especially with machine breakdowns.. can save you hours of heartache.

deke

 

Theorysparky

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Dimmers

I hate dimmers,

one of the major causes of excessive bulbs blowing and lighting faults

if there are any discionnect them

 

bnicholson

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Thanks for the helpful feedback so far.

There is an extremely frail old lady, who effectively lives in one room downstairs.

A neighbour has asked me to investigate the problem, there is no obvious reason for the onset of the problem apart from the age of the wiring and the possibility of rodent damage.

Just to hark on about the IR test, apart from proving that the lighting cct has a "shorting" problem (I know this already because it keeps blowing the fuse), is there any real benefit in carrying out the IR test?

Bob N.

 

Admin1

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I'd say a metal light fitting has had it's bulb changed, but has been inserted at an angle. Therefore bending the prongs and causing a short.

I had one the other week.

If you do an IR test with the bulbs in, make sure each switch is in the "Off" position. :)

 
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Thanks for the helpful feedback so far.There is an extremely frail old lady, who effectively lives in one room downstairs.

A neighbour has asked me to investigate the problem, there is no obvious reason for the onset of the problem apart from the age of the wiring and the possibility of rodent damage.

Just to hark on about the IR test, apart from proving that the lighting cct has a "shorting" problem (I know this already because it keeps blowing the fuse), is there any real benefit in carrying out the IR test?

Bob N.
The IR test proves the integrity of the cable insulation to contain the electricity it has to pass... It is not proving a short.

you could have something that is breaking down to say around 10ohms L-N.. at 230v (but wont show on a low voltage dc resistance test.)

Which would be no problem on a socket or cooker or shower circuit!

But on a 5A lighting circuit could be letting nearly 5x is rating pass through.. then as the cable gets warmer..

lets even more current thorough.. but when it cools back down... show no problem with a low 9v or 12v resistance test...

Do a 250v then 500v IR test... find out if you are shorting L-E or L-N??

Really this should jut be a mater of checking the basic dead tests.. Continuity, Ins Res & polarity.. and tracing through till you find the offending item?

going back to your first post you said you had reading greater then 99.9Kohms.. with resistance..

do you mean they were off the scale of your meter?

so you don't actually know what readings they were?

Or was that the actual value?

there is obviously something missing in what you have tested or the reading you have got?

An IR test should be greater then 2MegOhm... so anything in the K's is too low anyway!

to exceed 5amps you should have a reading 46ohm or lower?

 

bnicholson

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Hi,

I have been thinking about Deke's use of the 100 W bulb to isolate the Lighting cct problem.

What I am having difficulty in understanding, is if the problem causes a 5 Amp fuse wire to melt, won't the bulb just blow as well, if connected across the fuse way?

I must be missing something really important about this test!

Bob

 
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Hi,I have been thinking about Deke's use of the 100 W bulb to isolate the Lighting cct problem.

What I am having difficulty in understanding, is if the problem causes a 5 Amp fuse wire to melt, won't the bulb just blow as well, if connected across the fuse way?

I must be missing something really important about this test!

Bob
A 5A fuse wire wouldn't melt in a container filled with argon - no oxygen.

 
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Hi,I have been thinking about Deke's use of the 100 W bulb to isolate the Lighting cct problem.

What I am having difficulty in understanding, is if the problem causes a 5 Amp fuse wire to melt, won't the bulb just blow as well, if connected across the fuse way?

I must be missing something really important about this test!

Bob
Assuming you have a dead short..

the live wire connected at the CU could be considered as at neutral potential.

In a normally wired circuit..

a bulb is places directly between the live & neutral! ;)

otherwise.. it don't work! :) ;)

The only difference is..

normally the bulb is protected by a 5A or 6A fuse / MCB

bypassing the CU fuse is now a circuit without any protection other than the property incoming fuse!

Personally its a method I wouldn't recommend

strictly speaking there is a bit of flex going to your lamp holder without proper overload protection. :eek:

spose you could wire it via a plug with 3A or 5A fuse? to make it a bit safer? ;) :)

 
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Assuming you have a dead short..the live wire connected at the CU could be considered as at neutral potential.

In a normally wired circuit..

a bulb is places directly between the live & neutral! ;)

otherwise.. it don't work! :) ;)

The only difference is..

normally the bulb is protected by a 5A or 6A fuse / MCB

bypassing the CU fuse is now a circuit without any protection other than the property incoming fuse!

Personally its a method I wouldn't recommend

strictly speaking there is a bit of flex going to your lamp holder without proper overload protection. :eek:

spose you could wire it via a plug with 3A or 5A fuse? to make it a bit safer? ;) :)
I have never heard of testing this way. Megger the circuit and find where the short is job done.

Batty

 

bnicholson

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Megger the cct and find out where the short is - job done!

I know how to use my test kit to carry out the Insulation Resistance test, what I do not know is how I can use the results to find the exact location of the short.

Regards

Bob

 
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Megger the cct and find out where the short is - job done!I know how to use my test kit to carry out the Insulation Resistance test, what I do not know is how I can use the results to find the exact location of the short.

Regards

Bob
What are the results of the insulation test is it shorting l-e or l-n. Is it three plate wiring or on joint boxes or to switches and then to lights. More info is needed. What is wiring made of lead, rubber or pvc. Their is no quick way to test if you have a fault. I had a fault on a socket circuit the other day. Every socket was reverse polarity apart from one. It took me all day to find faulty piece of cable. My clamp meter said circuit was ok but on insulation resistance at 250v it was shorting phase to earth. The circuit was in use but not blowing fuse. I still do not understand why I was getting reverse polarity but if I had meggered earlier I would have saved a lot of time.

Batty

 
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