What cable from head to CU.

ericmark · 4 Jan 2014

TT supply moving head outside so cable will be behind stud wall so needs some protection. This must be standard thing with flats so what cable is used?

RF Lighting · 4 Jan 2014

It depends on how it's going to be run.

Is the cable going to be over 50mm deep due to the stud wall?

Would you be happy with a 100mA time delayed RCD at the incommer?

ericmark · 4 Jan 2014

Cable will not be 50mm deep this is the problem. I suggested SWA but radius would not allow use of SWA other idea was earthed conduit but this must be common problem with flats where meter at the entrance and CU in each flat so there must be a standard way around the problem.

No point re-inventing wheel what is standard method? What my son does not want to do is use some method which when the house is sold another electrician does not understand.

We talked about things like Al-tube but to open the circuit it will need 100A and even if the tube was not blown clear with something like a nail through it to get an earth good enough to blow a 100A fuse is not going to happen. The earth loop impedance needs to be better than 2.3Ω and it's not that low on the DNO side so even if enough rods to get zero still would not get it low enough.

So (iv) be mechanically protected against damage sufficient to prevent penetration of the cable by nails, screws and the like seems only option so how thick it required to resist a Hilti gun?

Adam_151 · 4 Jan 2014

The standard arrangement with flats tends to be a metal clad 80A switchfuse in the meter room, 16mm 3C SWA to the flat con/unit where it is run up the stud wall and into the back of the unit with the amour just cut back (earthed at the other end). With a TT supply you would need time delay RCD as well.

With a house its normally arranged so that the tails pass through the wall and straight into con/unit or surface trunked up, tails plastered into the wall are considered undesireable, even in 16th edition days

RF Lighting · 4 Jan 2014

What ever you run is going to have to be contained in earthed metal to keep if free from 30mA RCDs, and allow proper division of the installation at the CU, and as you have a TT incommer, you're going to have to install a 100mA time delayed RCD at the mains along with some sort of OCPD.

Can you not use SWA if you drill through the wall at a steep angle, so the bend in the SWA is much reduced?

For something like this, I would run a SWA and gland it off into a conduit end box with some extention rings fitted, and screw the CU to that so the cores remain fully contained.

If SWA really isn't viable, then maybe you can use ali tube. Remember you only need enough current to flow to operate a 100mA RCD, not full operating current of the OCPD.

Other options are pyro, providing you can still find anywhere that sells it in big enough CSA for this and you are used to working with it, steel conduit all the way between the incommer and the sub mains board, but again this is tough if you're not used to working with it, or maybe you could look at a departure from the regs and use split concentric cable?

ericmark · 4 Jan 2014

I think the RCD in the meter box is only option. As to what size I don't know could fit much bigger than 100ma even with adjustable settings while my son is living there as like me he is a commercial electrician.

Pyro would be neat and likely he can get some working in a glass works likely he can buy from same supplies who supply his firm.

Problem is when he sells how would other electricians look at 16mm² pyro? I would assume no need for 25mm if in pyro so would be a lot neater.

RF Lighting · 4 Jan 2014

I can't see any electrician other than a very inexperienced domestic installer having any issues with pyro.

As far as I undersatnd the regulations, you can use what ever size RCD you like, so anything up to 500mA as it is there purely to provide fault protection to the cable, but 100mA will be easier to get hold of, and offers no real disadvantage from a higher rated device. The key is making sure it is time delayed and all upstream RCDs are double pole switching devices to provide proper discrimination.

As for cable size, it depends on how well loaded you plan for the sub mains to be. You have to install a switch fuse at the incommer end to protect your submains cable any way, so if a 60A supply will be big enough, you could probably get away with 10mm² providing the volts drop calculations work out ok.

flyingsparks · 4 Jan 2014

RF Lighting said:
I can't see any electrician other than a very inexperienced domestic installer having any issues with pyro.

I agree, but unfortunately there are more and more "5 day wonder 'electricians'" around now that as time goes on your more likely to come across one who has never seen pyro before, and rips it out because it's not SWA.

RF Lighting · 4 Jan 2014

According to the apprentices I work with who are doing the 'full' two and a half year apprenticeship

they are not taught pyro at college any more

flameport · 5 Jan 2014

ericmark said:
This must be standard thing with flats so what cable is used?

In older blocks of flats (1960s build) in this area, the usual arrangement is a Ryefield board on the ground floor, pyro cables from there to each individual flat, gland into a metal box, and short single wires to the meters etc.

Heating is all electric so usually 2 separate meters, and either a switch/timer for each flat, or individual contactors operated by a single timer on the ground floor.

Adam_151 · 5 Jan 2014

ericmark said:
Problem is when he sells how would other electricians look at 16mm² pyro? I would assume no need for 25mm if in pyro so would be a lot neater.

I would expect a lot would be as RF says ok in realising it is compliant, but if the DB needed changing 20 years down the line and it needed re-termining, a lot would look at the 16mm size and rip it out, not being able to terminate anything which wont fit in the joist stripper

Adam_151 · 5 Jan 2014

RF Lighting said:
According to the apprentices I work with who are doing the 'full' two and a half year apprenticeship they are not taught pyro at college any more

I assume you are teaching them on site when the chance comes up?

I was lucky in the fact that I was taught how to strip it back with cutters and slotted bar at college by a old school workshop tutor who insisted that we should know how to do it. Very usful when I was handed a joist-stripper with a blunt blade.

The problem I find is that I come across it very rarely to become good it it, yes I can make an end off in anger, but I'm no means quick at it and last time I did it, I messed it up, I was struggling with an end under a church pew, must have twisted the cores slightly. Made off the other end into an adapt box, meggered it, all fine, tightened the gland into the adpt box fully and P clipped it nearby, suddently down L-N, stripped down the end near where I had been disturbing it, and tested with no pot on, all clear, re-built it, down again, pulled it appart again, still down. Put it back together carefully, and stripped the other end down, Somehow I'd managed to twist it enough that it only went down if there was a disturbance along the cable

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What cable from head to CU.

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