Lights out but flicker - sometimes!

[lightsout]

This snag has got me foxed. Upstairs lighting is all out. But occasionally it comes back on, then flickers briefly once or twice before going out as soon as another light switch (same circuit) is made. After 20 mins the lights sometimes come back on, before going out immediately when another light switch is made! The MCCB never trips. The lighting circuit is a mixture of radial and loop-in, with an extractor fan linked to the loo light. I have checked the security of the pendant light wiring. Is the quickest way of investigating this to conduct a continuity check from the consumer unnit? Any ideas before I call in an expert? Thanks.

 

[Big_Spark]

This snag has got me foxed. Upstairs lighting is all out. But occasionally it comes back on, then flickers briefly once or twice before going out as soon as another light switch (same circuit) is made. After 20 mins the lights sometimes come back on, before going out immediately when another light switch is made! The MCCB never trips. The lighting circuit is a mixture of radial and loop-in, with an extractor fan linked to the loo light. I have checked the security of the pendant light wiring. Is the quickest way of investigating this to conduct a continuity check from the consumer unnit? Any ideas before I call in an expert? Thanks.

Couple things..firstly I very much doubt the protective device is a MCCB..(Moulded Case Circuit Breaker) but rather an MCB (Miniature Circuit Breaker)

What have you done on the circuit that resulted in this situation..I presume the lights worked perfectly OK until you did something?

Personally it sounds like a Neutral fault, probably the lights have some how become connected in series instead of parallel and this is likely only in one place, causing the observed behaviour.

You need to VERY CAREFULLY inspect the work you last did, and ensure that everything is connected correctly.

For your information, all lighting circuits are Radials, the loop in in simply a method of connecting in parallel so that all lights operate correctly.

 

[Lectrician]

Neutral fault?

Sounds like a classic loose connection to me - in the live or neutral!

When you load it by turning a switch on, the loose connection sparks, and breaks contact. Every so often it manages to make again.

Look at every connection CAREFULLY - and tug them!

Will be near to the CU end of the cct - otherwise half the lights would still work.

 

[lightsout]

Thanks for your prompt responses. I can confirm that I have done no work on this circuit other than change a couple of blown bulbs. I rather think Lectrician has put his finger on the likely cause of the problem. So I will check these fittings first in case I disturbed anything. This may not be relevant, but we have recently had a small furry visitor in the loft - could a mouse be responsible?

 

[Big_Spark]

Neutral fault?

Sounds like a classic loose connection to me - in the live or neutral!

When you load it by turning a switch on, the loose connection sparks, and breaks contact. Every so often it manages to make again.

Look at every connection CAREFULLY - and tug them!

Will be near to the CU end of the cct - otherwise half the lights would still work.

Good Point Lec..I was assuming he had done some work on the circuit...I have seen this type of problem in the past and blinkered my view...

 

[lightsout]

Lights on! In the loft, at the first junction box after the CU, I found a neutral terminal block with a load of green sticky goo on the insulation and inside coating the wires. After a rub down with emery paper and a replacement block, all now works as it should.

The green gunge is obviously associated with the the copper, but any ideas what it is, how it got there and how it may be prevented?

 

[kai]

It's a sign of decomposing cable, you need to have the affected circuit re-wired to be on the safe side.

 

[Big_Spark]

It's a sign of decomposing cable, you need to have the affected circuit re-wired to be on the safe side.

That may not be necessary Kai..although I would certainly advocate getting it checked over properly to ensure there are no more nasty surprises in store and further damage to the cable insulation. The green gunge could equally be from the terminal block decomposing as the neutral had a loose connection causing heat to be generated...

 

[Lectrician]

Umm - the green isn't a good sign.

A batch of cable was used during the late 60's which had a new type of plastiser used. This caused a reaction with the copper, and a goo to be released.

This circuit should be tested for insualtion resistance, and checked through out (including other circuits actually!).

BAS posted a link about this at come point, and I have a print out somewhere......

 

[Lectrician]

From this thread: //www.diynot.com/forums/viewtopic.php?p=220163&highlight=green+extruding#220163



Thanks to BAS's Google'ing!



It's di-octyl phthalate.

Green exudate from PVC

Draft BCA statement (April 2001)


1) PVC

PVC comes in two main grades, plasticised and unplasticised PVC.

Unplasticised PVC (UPVC) is used for example in double glazing window frames where a rigid material is required.

The PVC used for manufacture of cables is a plasticised PVC that conforms to the relevant British Standard for the cable type in question.

2) Ageing effects

As a cable ages (at temperatures above normal ambient) the elongation to break decreases (also the Insulation Resistance increases). The life expectancy of a cable is arbitrarily considered to be when the elongation to break of the PVC is 50%.
A lower elongation to break value could be considered suitable especially for a fixed wiring cable. Therefore, providing the cable is not subject to movement or when moved due to inspection of socket outlet or the like, the PVC does not crack, a much lower value of elongation to break is considered by some as suitable.

3) Greening

Greening is the appearance of a wet green substance that is a product of an adverse reaction between certain types of plasticiser and the copper conductors. This greening, which is a rare occurrence, can happen either after a long period of time for some cables, or if the cable has been severely overheated.
The plasticiser itself is a clear oily liquid that is non conductive. The green substance is a combination of copper oxide and plasticiser which may become conductive under certain adverse conditions.

4) Action if Greening is found

Therefore whenever this green substance is found at socket outlets etc. initially it should be removed and the terminations cleaned (gloves should be used) otherwise it is possible that tracking/overheating may occur. It is strongly recommended that rewiring should be carried out as soon as possible.

If further information is required, the original cable manufacturer should be contacted.

 

[lightsout]

Thanks Lec' and BAS for identifying the root cause. I was wondering if I might have to call Ghost Busters. Fortunately, the house was mostly rewired in 1993, apart from one upstairs lighting circuit which shouldn't be too painful to replace. Surprise, surprise it was from this circuit that the green goo appeared and it was installed in the 60s, which correlates exactly with your diagnosis.

Incidentally, I did some more research on the Internet and here a couple of links with more info if anyone is interested.


http://ptcl.chem.ox.ac.uk/MSDS/DI/dioctyl_phthalate.html

http://www.phthalates.com/index.asp?page=7

Thanks everyone for helping out.