Lighting cct help

[Set s]

Hi all, I'm new to this but I'm after a bit of advice.

I have been asked to look at a lighting cct which has a 6a mcb that keeps tripping even hen no load is connected.

I have checked all fittings and switches for loose connections allof which are ok. I have then conducted a ir test on the circuit, all are acceptable until it came to the stair 2 way cct Which had a reading of 120Mohm L-N then when both switches are operated it drops to 30Mohm. Is this the probable cause?

Any help is welcome

S

 

[Zeespark]

Your IR readings on the face of it seem fine, have you checked the continuity of your conductors?

AndyGuinness

 

[Andy™]

possibly a faulty MCB not holding on?

 

[Set s]

Thanks for the reply. No I haven't checked continuity yet. This is a job I've inherited from a friend. I'm led to believe I'm not the first to have look. Im lead to believe the owner had a prob last year which resulted in the upstairs cct and down cct being combined to avoid a rewire!

I haven't come across a fault like this before so the more advice the better thanks.

S

---------- Post Auto-Merged at 18:15 ---------- Previous post was made at 18:07 ----------

Ive changed the mcb as it had been operated excessively, and may be fatigued.

---------- Post Auto-Merged at 18:23 ---------- Previous post was made at 18:15 ----------

Guys prob missed a bit out. When the cct is energised the mcb holds and does not trip. Then after a period of time (and if no lights are on) it will randomly trip!

 

[PC Electrics]

Thanks for the reply. No I haven't checked continuity yet. This is a job I've inherited from a friend. I'm led to believe I'm not the first to have look. Im lead to believe the owner had a prob last year which resulted in the upstairs cct and down cct being combined to avoid a rewire!I haven't come across a fault like this before so the more advice the better thanks.

S

---------- Post Auto-Merged at 18:15 ---------- Previous post was made at 18:07 ----------

Ive changed the mcb as it had been operated excessively, and may be fatigued.

---------- Post Auto-Merged at 18:23 ---------- Previous post was made at 18:15 ----------

Guys prob missed a bit out. When the cct is energised the mcb holds and does not trip. Then after a period of time (and if no lights are on) it will randomly trip![/QUOTE]

even with the new MCB??

 

[ProDave]

Does it happen when someone walks about?

Last "mystery tripping even when lights are off" job I sorted was caused by a cable pinched between a floorboard and a joist. When you walked over that board, it pinched the cable, caused the short and tripped the MCB.

 

[Set s]

Mcb has only tripped the once. Owner has said none of the lights were on at the time.

 

[martinxxxxxx]

Hi

I would definitely look for the pinched cable.

First I would close all of the light switches so that they are all on. As we want every piece of wire to be tested. Then I would remove all lamps and link out any dimmers. Then I would test IR at 1000v with the 2 way switches operated in both directions. There is a lot of current to cause the trip so if this did not reveal any useful data, I would reenergise the circuits and have a good stomp around.

These are my thoughts on it anyway

Good luck

Martin

 

[ProDave]

Another much easier one I had a while back was the MCB tripping every few minutes, but not always.

That was quickly traced to the cable feeding an outside light. Whenever that was turned on, the MCB tripped almost straight away. That faulty cable showed up very clearly on an IR test.

Upon enquiry, it turned out the house owner had the flat roof replaced recently, and that was when the tripping started. No doubt a pinched cable or nail through a cable.

 

[Set s]

Gents, thanks for all your advice. I will be calling to see the owner, asking a few questions and have a good stomp about!

Cheers

S

 

[batty]

HiI would definitely look for the pinched cable.

First I would close all of the light switches so that they are all on. As we want every piece of wire to be tested. Then I would remove all lamps and link out any dimmers. Then I would test IR at 1000v with the 2 way switches operated in both directions. There is a lot of current to cause the trip so if this did not reveal any useful data, I would reenergise the circuits and have a good stomp around.

These are my thoughts on it anyway

Good luck

Martin

Any particular reason to test at 1000 volts?

 

[Sidewinder]

Martin,

You need to be careful testing @ 1kV, as you can cause pin point breakdown in even sound insulation, remember phase to earth the cables are not required to withstand such voltages, also the capacitive affect could be significant.

I would strongly discourage 1kV testing on domestics.

If the breaker is a 6A unit and is tripping on short circuit or overload, then you should easily find this with 500V or even 250V.

If there is no rcd on the cct, then it is likely not a L-E fault as the rcd would be more sensitive than the mcb.

So you would then be looking for a L-N fault...

 

[Andy™]

If there is no rcd on the cct, then it is likely not a L-E fault as the rcd would be more sensitive than the mcb.

if there is no RCD on the circuit, there then would be no RCD to be more sensitive to L-E faults than the MCB....

 

[Sidewinder]

Quite Andy, typo there!

In my defence I was eating pizza at the time!

 

[martinxxxxxx]

Any particular reason to test at 1000 volts?

If you are trying to test the strength of a dialectric, then to me using the highest test voltage available seems to me to be good sense.

But I do concede that there may only be a marginal improvement over using 500Vs, but any thing I can get going my way in these situations I do.

I am guessing that your thinking is that if there is a 3mm gap when the wire is not being trodden on then 500 extra volts is not going to find it, again I agree.

In any case the IR test will only tell you what you already know - that we have a pinched cable- it is finding it that is the trick. Two people are better than one for this (one for stomping and the other by the consumer unit.)

If you could arrange for an in line fuse holder to be fitted the fuseboard and fit a 250mA quick blow fuse then you might have a sharper tool to find the fault and at much lower energy.

Good luck

Martin

 

[martinxxxxxx]

My research leads me to believe that generally the specification for testing can be

BS6004 6242y Twin & Earth Installation Cable

Conductor: Plain annealed copper solid (1.0 - 2.5mm

 

[Sidewinder]

Martin,

You should NEVER test to product standards in use.

I can understand what you have turned up in research, and I have often worked with and to product standards.

HOWEVER, once the cable is installed you must test to the requirements in BS7671.

Look at the IET CoP for PAT and the product standards that apply to the equipment you may be testing, would you want to put 5kV, 7.5kV, 10kV or perhaps even more through a clients product, when if it fails you are going to be paying for the replacement as "it worked before you tested it"!

Product standards may also only be applied to a sample, and may be designed to test to destruction, not to be a test that sees the product fit for long term use.

So in summary I stand by my earlier comment, do not test at 1kV unless it is specified in the "in service" testing standard, which in this case it is not.

Unless you can guarantee that the test will not damage the product under test, which for an installed cable in a domestic premises, I defy anyone to be able to prove that no damage has been done without destructive microscopic inspection and testing which is above and beyond that which any of us are kitted out for unless we have an ISO approved testing laboratory as part of our organisation with the necessary equipment.

BTW where do you get the 707VDC from, my tester IIRC puts out 1005V d.c. when testing at 1kV as it has a voltage readout, and this has been checked during its calibration and my ongoing verification checks against my other calibrated reference meters.

(I have a meter that will read up to 3kV)

So in short whilst product standard testing is one thing, the product after testing does not necessarily have to see 20 years in reliable service.

I don't think that you should test at anything above 500V for in service testing.

I love the slug story!

We had a slug today on the garden wall about 6" long, damn big thing it was too!

As you say, interrogate the customers, machine operators, clients etc. as much as you can as they can be an invaluable source of information.