New CU / ring main calcs (Long)

[Jim Ingram]

Hi,

I tried to get an isolation switch fitted between the meter and consumer unit to enable me to be able to fit a new consumer unit at my convenience. I was told by my leccy company that this could be done and they arranged a call out. However, I was informed by the chap who turned up (EDF energy I think) that it was not their policy to fit one of these or (anything else).

My first question is would a 100A CU incomer/isolation switch mounted in a 2way din rail box be acceptable if I was to purchase one?. (not sure I actually want one but might be cheaper in the long run , i.e. less than £54 (see below), if I need to get the main fuse pulled at a later date)

My intention is to replace the current CU (and maybe add isolator between meter and CU at the same time). I was told by the electrician that I would have to arrange for them to come out in the morning to disconnect and before they would re-connect I would have to produce a test certificate or they would test it and charge me £54.

I believe for this they will megger the wiring and check the earth loop impedance and earthing to water/gas pipes. Sounds like a reasonable price to me?

Whilst he was there I asked him what type of earth it was (TN-S) and he measured its resistance as 0.15ohms. Most of the existing circuits are pretty short however there is currently one ring main (covering

73m2 floor area). The cable is 2.5mm2 T+E but the cpc is undersize i.e. 1mm2. I estimated the total length of cable (excluding spurs??) in the ring (L) to be 70m. By my calculations (see below) this ought to be OK run off a 32A type B MCB, but please let me know if I'm wrong. phase 0.009 ohms/m, cpc 0.022 ohms/m, Phase + cpc 0.035 ohms/m. (btw since this circuit will be on an RCD do I need to worry about this anyway or should one still aim to meet the disconnect times if its practical to do so?)

earth conductor resistance = 70/4*.022=0.385ohms current to trip 32A b MCB in 5s = 160 Amps shock voltage after 5s = 160*0.385= 62 volts

total earth loop resistance = 0.15 + 70/4*0.035 = 0.76 ohms Fault current = 230/0.76 = 300 amps Current to disconnect in 0.4 s 160 Amps

[Christian McArdle]

Just pull the fuse yourself and change over the tails. That's what everyone else does.

Borrow, beg or steal the required test equipment and you can test the earth fault loop impedence, insulation resistance, RCD operation etc. It appears you would know what to do with them.

Yes. It's very poor practice to rely on RCD to get you out of an earth loop impedence hole, unless you're TT. Even then, you should plan your circuits so that they would have complied had they had TN-C-S earthing. Then you can swap it over when it becomes available. Same principle applies with main equipotential bonding. Do it all compliant with TN-C-S even for TT systems, as when TN-C-S becomes available, you don't want to dig up the slate floor in the kitchen to put the proper stuff in to the rising main etc.

What a load of brain dead tosspots. You need the power on to do many of the tests needed to write the certificate. Just how are you supposed to measure the earth loop impedence or fault current without power?

Christian.

[John Southern]

Bet theyd measure Ze and then add the R1+R2 values but its easier with the power on just to do a quick Zs test.

Jon.

[Christian McArdle]

And the RCDs?

Christian.

[Bob Eager]

Don't the newer testers cope with that? (mine doesn't, but...)

[Lurch]

So they can test RCD's with no power then, clever. ..

SJW A.C.S. Ltd.

[Bob Eager]

No, I simply meant that it is possible to test Zs with RCDs in circuit.

I agree that the RCDs can't be tested.

[Lurch]

Oh, that makes more sense. I was wondering! ..

SJW A.C.S. Ltd.