Rubber Insulated Cable

Sorry to labour this, but I think it's important:

Last weekend we had a mysterious fault in the house electrics.

Although none of the circuit breakers were tripping, the main RCD on the Consumer Unit kept cutting the supply.

I traced the source of the problem to the downstairs Ring Main ... but could not identify any faulty device until I unplugged an extension lead. The extension only had two small 'double insulated' power supply units plugged in; yet we obviously had an Earth Leakage problem.

The pictures tell the story:

Although there was no visible contact between the wires, there must have been leakage, caused by the pressure of that Nylon 'strain relief' in the plug.

Edited By Michael Gilligan on 04/04/2015 19:36:19

Edited By Michael Gilligan on 04/04/2015 19:38:45

Edited By Michael Gilligan on 04/04/2015 19:40:28

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Edited By Michael Gilligan on 04/04/2015 19:42:54

Michael

Do you know for certain that the problem is at the plug end of your extension lead?

There are probably millions of that particular make and design of 13A plug tops in use and I'm surprised that the pressure from the nylon tongues would be enough to displace all the insulation between conductors.

I would measure the resistance between conductors and then cut the crushed section off and measure again to confirm its not failed at some other point.

Ian P

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Ian,

[living dangerously, and having already removed the sockets] ... I did cut off the end of the cable, and re-wire the plug to the fresh end ... No problem, until I pushed the cable into the strain relief.

The problem is simply that the rubber insulation has gone soft and 'leaky' and the pressure puts the conductors very close together.

I did, of couse, replace the cable with good modern stuff; and all is well.

Thanks for your thoughts but, I checked thoroughly and I am absolutely certain that I found the problem ... which is why I published this warning.

MichaelG.

Edited By Michael Gilligan on 04/04/2015 21:00:01

I understand now, I did not twig that you were blaming the cable.

Ian P

Not surprised, I have found old-ish rubber cable goes 'sticky' after 20+ years. Maybe the insulation starts to flow and voila - the wires contact. Very old rubber cable just seems to go crumbly and dry out. Maybe something to do with the plasticiser.

Making something that lasts a very long time is a tricky business.

.

Quite so, Roger

The particularly interesting aspect of this was that; although the point of failure was evident, there was no actual 'short' between the wires ... and the circuit breaker for the ring did not trip ... it was the RCD that worked [exactly as it should].

MichaelG.

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P.S. ... Thanks to Wylex 

Edited By Michael Gilligan on 05/04/2015 08:42:32

The thing that surprised me was the such old rubber cable had the modern blue/brown/green-yellow colour code rather than the old red/black/green coding.

Les.

Rubber sheathed/ insulated cable does not have to be old. I used it about 10 yrs ago and it still appears to be available from the same supplier by the looks of it.

**LINK**

From very vague memory it can tolerate a sharper bend radius and maybe lower temp. than PVC but I'm not certain.

Don't care now anyway ....

D.

Edited By Dave Daniels on 05/04/2015 09:58:26