Today i was fitting new lighting in the shed, so turned off breaker and began removing old lights. this was fine. When fitting the new light, i stripped back the negative wire and touched the earth wire with the strippers, this caused the wires to spark. When i checked with a volt meter, it read 9v's across the negative and earth wire, not across the positive wire. i have read in other posts that over loading the breaker can cause it to not switch off properly could this be the problem as we run some very powerful tools and lots of lights in there.
Although the voltage is unlikely to do you personally any harm due to shocks it could cause wires to get far too hot is wrong ones touch and turning off the power with the twin pole main breaker rather than single pole MCB should remove any chance of injury due to burns while working on the lights. Read what I have said under "Electrics to feed new shed?" this may also help you if you can find document by IET
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