Repairing nick in 2.5mm cable

[Nathan Critchlow-Watton]

After re-wiring the upstairs ring main I tested one of the sockets with the first appliance to hand; unfortunately this was an angle grinder.

Inevitably once started ("oh good, the sockets work") it then wrested itself from me, scored a line down my leg and writhed across the floor then through a lifted floorboard. By the time I'd switched the socket off it had stripped the insulation off a section of my new cable! It's taken off the outer sheath and also the inner sheath of just one of the wires.

Can I get away with binding this in insulation tape, then putting the whole thing into a junction box, but without actually connecting it to the terminals, i.e. using it to mechanically protect the cable?

Yours, Nathan

[DMac]

You'd be best advised to replace this cable rather than "bodging" it I'm sure your insurers wouldn't be best impressed if they found this was the cause of a fire.

(Cue the Part P police...)

[Bob Watkinson]

A fire in an electrical circuit is usually caused by a loose connection. In this case there is no connection at all just some exposed copper so there is no chance whatsoever of a fire. If the OP does as described there will be no breach of regulations and the result would be no better or worse than an undamaged cable. As for part P then if the OP is in Scotland as I am then part P is irrelevant.

[The3rd Earl Of Derby]

Heh! replace he says huh! a junction box is quite sufficient...but not taping it.

-- Sir Benjamin Middlethwaite

[Nathan Critchlow-Watton]

I'm in Scotland, so free of the Part P menace.

I can just strip the extra insulation and connect it into a junction box, but I can't see the difference in terms of fire risk. As it is, the cable has only half of one sheath stripped, so no other wire to touch to short it. Once taped and "trapped" inside an empty junction box nothing else can touch it.

If I cut the cable and use a junction box to re-attach it, doesn't that make the connection potentially worse, and also the terminals themselves are exposed, so there's still the same "amount" of exposed conducting material?

I'm willing to do either (though obviously prefer the option with less work!) but I can't see the difference in safety or improved connectivity between the options.

Thanks, Nathan

[Mike Harrison]

If the conductors are undamaged, taping would be better than cutting and joining as the latter introduces more potential failure points. This assumes the use of good quality tape. I'd be tempted to put a small blob of hot-melt on the exposed conductors & then taping while it's still soft to make a 'moulded' joint that is not dependent on the integrity of the tape over time.

[Frank Erskine]

There's a good chance that the wire itself will have been nicked, therefore a hot spot could occur and cause a fire.

Replace the length of cable whilst the floorboards are lifted.

[Bob Watkinson]

"Nathan Critchlow-Watton" wrote in message news: [e-mail]...

Good innit :-)

If you leave the cable intact you will have no fire risk at all if you put it in a junction box.

Best just leave it as it is and clamp the junction box around it. No need to go chopping more bits out of it.

Yes you then introduce a potential fire risk, you'll make things worse doing that and as I say, no need.

[Bob Watkinson]

No it can't. Let me explain. With a loose connection you have two surfaces rubbing together at 50hz. The friction created causes the heat which in turn can cause fire. A nicked cable is still one surface so as long as it is intact there is no chance of heat. Having said that the OP should check that there is no damage to the conductor which could result in either a future break or a reduction in CSA.

[Rob Morley]

That's a joke, right?

[Bob Watkinson]

No it's a fact mate.

[mike]

I don't understand this either, why are they rubbing together?

Mike

[Bob Watkinson]

If a joint is slack there is a miniscule vibration. IE with minute amplitude but with a frequency of 50hz in the uk. This is one of the main factors causing heat build up in a loose connection and the major cause of fires in installations.

[Owain]

And there was me thinking it was the heat generated by a high resistance connection.

Live and learn, eh.

(Unless get burned to death first, of course.)

Owain

[mike]

I still don't get this, why does the 50Hz mains frequency cause vibration in the wires. And how do such miniscule movements between the wries cause so much heat build up.

I always thought that loose joints got hot because in a loose joint there is very little contact area so you end up with a lot of current flowing through a very small area.

Mike

[Nathan Critchlow-Watton]

Thank you all very much.

I'm going to check that the copper itself isn't compromised (pretty certain it isn't) then wrap in insulation tape, followed by encasing the weak area in a junction box, with both it and the cable entering it secured to a block of wood.

Thanks, Nathan

[Bob Watkinson]

No. You see the opposite is true. Think of the joint as a load, a resistor if you like similar to an electric fire element. Heat generated by electricity is the result of low resistance and goverened by the power formula V*A. As I = V/R you can see that the higher the resistance the lower the current and so the lower the power (manifested as heat) in the load (joint inthis case)

[Bob Watkinson]

It's a product of the chnging electromagnetic effect on the conductors. Reversing 50 times per second

And how do such miniscule movements between

not at all. It doesn't have to be a smaller area to get hot. The surface area of a grub screw in a 30A junction box is quite large compared to the a

1mm conductor. Nevertheless I have seen one burn out in a lighting circuit due to slackness.

[Bob Watkinson]

"Nathan Critchlow-Watton" wrote in message news: [e-mail]...

No probs, that'll do the job perfectly

[Mike Harrison]

But is it significant...?

I can't believe frictional heating is of any significance compared to other factors. I suppose EM vibration could concievably have some initial loosening effect, although I doubt the field strengths would be sufficient to have any real effect. I would think that much more movement would occur due to thermal cycling effects, both environmental and resistive, plus any normal vibration etc., e.g. due to people walking on the floor. If they are loose enough for any movement, resistance will be increased, which will cause a temperature rise, which will cause further movement due to expansion. Any copper contact areas that get exposed to air through movement will oxidise over time, which will further increase resistance, and if this continues progressively across the contact surface, resistance will rise enough for the temp rise to accellerate due to ohmic losses, and could then progress to the point where arcing starts, which is what is most likely to start a fire.