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Ring main problem!

Postby marsh369 » Sat Nov 13, 2010 12:47 am

my upstairs ring main keeps blowing the fuses. i tested the cables and found which cable is shorting out. was wondering if cud jst disconect this cable and turn it into 2 radial circuits or this 2 dangerous. the cable that needs to be replaced had to be the hardest one so dont really wanna change it. any help would be appreciated.
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Postby ericmark » Sun Nov 14, 2010 11:33 am

It is possible that a ring could be turned into two radial circuits with a lower automatic disconnection fuse/MCB limit but it is not just a case of splitting into two radials. The earth loop and line/neutral loop need testing to ensure it will still fail safe.

There is a special meter used by electricians that measures the amount of power available and it can be set to read in ohms or amps. If one looks at a MCB typically it says something like "B32" the B refers to the magnetic part of the trip and means that 5 times the rated current will trip the MCB magnetically which is far faster than the thermal part of the trip. So if 32 x 5 = 160A flows the MCB will trip is about 0.1 of a second. For 160A to flow using ohms law then 230/160 = 1.4375 ohms and this is the maximum value allowed. If it was a B20 then 2.3 ohms would be allowed. With C20 then 1.15 ohms as C is 10 times and with D20 then 0.575 ohms as D = 20 times.

If the removed link is central to ring then likely it would pass but often years ago the electricians did not test the ELI and so when changing the protection device one does have to be careful.

Where a RCD is used the earth loop impedance is no longer a problem as the RCD will work OK with very high readings of 200 ohms for example. However the line / neutral impedance is still important as should a cable become shorted of course you want the protective device to open. However for some reason we tend to measure the prospective short circuit amps rather than ohms.

The hire costs of the test equipment is likely higher than cost to get an electrician to do the job. And if you include the price of Part P registration then likely the cost of DIY will be 4 times the cost of an electrician.
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