Broken Ringmain Fixing Query After Drilling Through Wire


Postby MrBeee » Fri Apr 29, 2016 9:55 am

Hi all

Had a failure of my live wire detector which resulted in a drill bit through my 32A ringmain cable in a wall which feeds sockets.

DOH.

Now in an ideal world I would just stick a junction box at the problem point and fix it, but there's 1.5cm of depth to fix this (cable is under plaster on a brick wall - good old Victorian houses) - and it's in the middle of a living room wall so a big sticky out junction box is a no no.

I cleaned up and checked the cables, then crimped them together to fix them - not a problem done and dusted (I actually crimped and soldered the crimp too, just for good measure).

However, I want advice on how to insulate things before any wiring is put back.

This second I have put heat shrink around fix points in the wire + 3 layers of insulation tape on top of the heat shrink - this is per wire. The thickness of the insulation tape alone helps separate the wires too. Then around all of this, 2 layers of heat shrink over everything. Then a metal shield over the top of the finished cable.

Everything works, checks out as electrically fine.

However is this enough for me - insulating tape isn't necessarily the best thing in the world in my brain. Friend recommended taking some spare 32A ringmain cable - taking off the gray outer plastic shielding and sticking this around individual heatshrunk wires to give a `more solid` insulation. Also want to make sure when plastering over everything there wouldn't be problems (moisture when drying). I could of course be going for complete overkill.

Unfortunately replacing the whole wire from socket just above the break to the next point of access would involve removing 1/2 a wall of plaster and going all the way back to the consumer unit. Not an option I want to go with.

Any ideas/comments?

Thanks!
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Postby ericmark » Fri Apr 29, 2016 12:32 pm

To begin with your repair was OK then you messed it up.

Any metal in the wall needs earthing so wrapping metal around the repair makes it worse not better as it is not earthed.

Shrink is good, also self amalgamating tape does a good job, Solder in a problem if used for mechanical connection, not a problem for electrical connection.

Main point is once complete it needs testing, Make sure the ring final is still a ring final not two radials.
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Postby MrBeee » Fri Apr 29, 2016 11:30 pm

The metal isn't wrapped around, its a metal shield that fits over the top (like a cover) - its slid under the existing shielding that was there (that I had to cut a hole in to get to the wires) with all sharp edges removed - so I believe that's ok and there's a huge amount of old to new shielding contact - it's not a floating piece of metal on its own :)

So you think the insulation side of things sounds ok?

Testing the ring to see if its still a ring isn't something I've looked at before, not sure how to go about that - any guidance?

Thanks!
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Postby ericmark » Sat Apr 30, 2016 4:05 pm

In theroy you should be able to open any socket on the ring final and take the two like cores for line, neutral and earth and measure with an ohm meter and line and neutral will be 1.44 ohms or lower earth may reach 1.6 ohms being slightly thinner but all cables should be under 3 ohms between like cores.

However the problem arises replacing the socket as it would be easy to dislodge a wire when replacing a socket. With an earth loop impedance tester checking before you remove socket and then after replacing double checks the readings. If you get 0.8 before you remove socket and when replaced you get 1.5 then clearly you have dislodged a cable.

At the consumer unit you don't disturb cables as you remove and replace the cover so testing at the consumer unit it is less likely that your testing could disturb any cables, but you can't completely isolate the consumer unit so there is more danger involved while testing.

Where the cables have been tested before and readings recorded then very easy just plug in the meter and press the test button and compare readings. In theroy the meter should test line ~ earth when set to earth loop impedance (ELI) and line ~ neutral when set to prospective short circuit current (PSCC) but with some meters you have to make up your own leads to test PSCC as default is always ELI.

It is the inspection and testing where most DIY people fail, think my set of meters cost around £750 three meters. One does PSCC, and ELI. The next does both low ohms and installation testing first has to use 200 mA and second has to be at 500 volt. And the last meter tests the RCD. The three meters often called a 17th Edition test set.

To get my exam results to allow me to do inspection and testing was a 12 week night class cause for electricians at 3 hours per night, so 36 hours extra training and the exam was called the C&G2391. Clearly going through the whole test procedure is beyond a forum reply.

The ring final is a special case. With most supplies it is easy, but with the ring final you have two 20 amp cables supplied from a single 32A MCB so unless the ring is sound overloading is a very real problem.
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Postby MrBeee » Sun May 01, 2016 12:00 am

That's fab info, thank you, it's much appreciated :)
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