New CU / ring main calcs (Long)

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 <300 therefore disconnect times ok
Volt drop = 70/4 * 32 * 2*0.009 = 10.1 Volts (this is just over the allowable volt drop but assumes that the load is at the mid point of the ring. As I understand it I don't have to assume this and can distribute the load around the ring which would probably allow me to meet the regs. Are there any examples of such a calc in any text books (the library here has quite a good selection) you know of? Is it basically a case of dividing the load evenly between all the sockets?). In any case I plan to put in a separate ring for the kitchen in the not too distant future so the problem will then disappear. Do you think the leccy man will care about the voltage drop when he comes to connect to test/reconnect?
Since I don't need a lot of power (no electric heating/water/shower) I was going, as a temporary measure, to power the new CU off of a couple of 30A rewirable fuses in the existing cu until I can arrange for the leccy man to return. Is there any regulation that says I must not do this?
Many thanks
Jim
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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.
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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.
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And the RCDs?
Christian.
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On Thu, 12 Feb 2004 09:25:34 UTC, "Christian McArdle"

Don't the newer testers cope with that? (mine doesn't, but...)
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Bob Eager
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SJW A.C.S. Ltd.
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On Thu, 12 Feb 2004 23:12:01 UTC, snipped-for-privacy@telling.you (Lurch) wrote:

No, I simply meant that it is possible to test Zs with RCDs in circuit.
I agree that the RCDs can't be tested.
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