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