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Posted to uk.d-i-y
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Question about house re-wiring
In article ,
says...
Having recently changed my CU, on a 1980 house, I found the wiring to be
in excellent condition. No insulation resistance problems, visually the
wiring looks fine. The only point to note is that the cpc on the 2.5mm
T+E is 1mm, not 1.5mm. MCBs disconnect in the event of a fault current
much faster than fuses - if the cable is protected with a fuse, then the
disconnect time and resistance is such that the cable will get too hot
in the event of a fault, possibly causing damage (as I understand it, it
*isn't* a fire risk). Since I now have a shiny new CU with MCBs, no problem.
If the total Zs of the circuit is within the specified limits, the fuse
/ MCB will go within the time required (0.4s or 5s generally). For a 30A
BS3036 fuse, the max Zs is 1.14 ohms, for an equivalent BSEN60898-B MCB
rated at 32A, it is 1.50 ohms. If your circuit is within the BS3036
figure, changing to MCBs is going to really achieve anything, certainly
not on safety grounds. It just makes it more convenient to reset.
If you have a TN-C-S supply, with theoretically maximum Ze of 0.35 ohms,
this allows you to have R1+R2 of 0.79 ohms. Working backwards:
P+CPC = 3.16 ohms (assuming a proper ring circuit)
CPC = 2.5P
Gives P resistance of about 0.90 ohms, or about 120m of cable. Should be
possible to wire the average sized house on one ring and still fall
within the limits for a fuse, let alone an MCB, to meet the 0.4s
disconection time.
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