by ericmark »
Sun Aug 24, 2008 8:29 pm
The 2.5mm² cable used in a ring main is normally considered good for 20 amp but the ring main has a 32 amp fuse/MCB so if split one could draw 32 amp through a cable designed for 20amp quite a heavy overload.
There are a number of ways to get around the problem of high Zs simple is smaller MCB feeding cable. But this assumes Ze is OK and where Zs is low because of Ze then an earth leakage trip may cure the problem. Improving the earth may help for example linking into the supplementary bonding but if Zs is high often the R1 + Rn reading is also high and although you may improve the earth loop impedance the prospective short circuit current may still be too low to ensure disconnection within proscribed time. Also you may also have problems with volt drop.
It is normally considered as 1 meg ohm but in real terms one must consider what is on the circuit and under 10 meg ohm one should really find out why it’s so low. Also considering a final circuit and where not the final circuit a lower reading would be accepted. Also what is on the final circuit and where mineral insulated heating cables are used a much lower reading would be accepted.
