fitting a new light in the bathroom. isolated the supply to the loop at the mcb (pressed the button to trip it, didn't actually remove it), tested it with my voltmeter, something like 0.9 volts so obviously a tiny trickle is coming from somewhere.
once i've tested and i'm certain that the line is dead i quickly touch the wires (not to be advocated i know!) but just as one final check... better to get a shock when expecting it!
so i cut into the T&E wire and the elcb trips. this has also happened on other circuits. i know that the wires are copper and my cutters are steel and when two different metals come into contact a minute current can flow.
so if my steel cutters are creating enough current when bridging the earth and neutral as i cut into the T&E is that enough to trip a 30mA elcb? or is it to do with the 0.9 volts still flowing when isolated? when i get my meter back on the wires after the elcb has tripped there is 0.0 volts.
i guess that i need to fully isolate the supply at the main rather than the mcb just curious as to why this happens.
An earth leakage trip looks for an imbalance in the live conductors. In domestic we have two live conductors called Line and Neutral. When we operate a MCB this only switches one of the Live conductors and Neutral is still connected so when you cut through the cable you will create a direct short between the Live Neutral cable and Earth so the earth leakage trip will operate. If we work to the rule book you should isolate at the Isolator which is two pole.
I think I would prefer a notch burnt out of my snips than getting a shock if I have not fully isolated i.e. neutral connected I work as though cable is live.
As to 0.9 volts from line to where? If line to earth then this would make sense and any current being drawn will cause a volt drop in the neutral cable and what you are measuring is the volt drop going all the way through the neutrals through what ever is on the circuit so you would be measuring the neutral voltage.
Also with high impedance voltmeters where cables run close together capacitance and inductance can induce a voltage into a cable which is why you should earth any unused cores.
As to different metals although some special wires can produce a voltage when welded together i.e. thermo couples in the main you need an electrolyte to produce a voltage between the dissimulator metals the voltage would be dictated with reference to the electrochemical series most common being copper and zinc at 1.5 volts.
Here endeth the lesson.
All best Eric
thanks for that ericmark, makes sense about the neutral, however i check the circuit before i cut into the cable. let me give you an example, today i was fitting the switch live from the loop already. the circuit was not live (i tripped the mcb) and was handling the cables so i know that there was no current. i cut into the T&E to shorten the cable then the elcb tripped.
thats the funny thing - it ONLY happens when i cut the T&E, not when i touch the N and E or L and E or all three cores together. and as i already had the ends exposed and handling them i know that i was not cutting into a live cable. what am i missing?
why would it only happen when cutting a whole cable?
and my voltmeter actually registered 0.0 volts this time. but interestingly when i touched the red cable of my meter to the live 0.8 registered with the black cable of my meter in mid air! it dropped to 0.0 as soon as i touch the black to the neutral. i know that current can travel through air but why does this happen?
one other question, my mcb at the consumer unit is the wylex type. if i were to remove the cartridge from the cu would that kill the neutral as well?
just want to isolate the upstairs loop only not the whole house. reason being is that the wiring is in the attic (light up there is on upstairs loop so clearly won't work when i am doing the alteration) and i have a plug-in worklight but i need the sockets on to be able to use it. so i don't want to isolate the whole house. unless of course its the only really safe way. i have a head torch for such eventualities!
It is common for us to only switch off the line and to leave the neutral connected when we work on things. That donâ€™t make it right but it is normal. And removing the cartridge will still leave the neutral connected.
So onto why it trips with direct connection and not through body. The body is quite a high resistance and you need approx 20ma to flow to make the RCD trip. So 0.9 volt at less than 25 ohm will cause over 30ma to flow so it trips but your body is more like 2500 ohm or more so it does not trip. Simple ohmâ€™s law.
There is a problem with sites like this. We all brake the rules and take chances and if you like we do an unofficial risk assessment using our experience to minimise the risk. But to tell someone else to take the risk is wrong as we have no idea of their skill and knowledge and since this is a DIY site we must assume itâ€™s on low side.
There are two main risks one is to the person doing the work the other to other people in the house. The latter is a really big problem as most DIYâ€™ers do not have the expensive meters to check their work with so even if they were better than an electrician at doing the job they are still more likely to leave something in a dangerous condition.
Most accidents do not happen because of one fault but due to a number of faults which combined caused the accident. And in the main electricians will see existing faults more readily than the DIYâ€™er and so avoid the dangers.
For example they will realise cutting the cable may trip the RCD so will try to cut each core independently and will keep a torch switched on as well in case it should trip. Remote from the job it would be very easy to forget to warn someone to do this but because we have advised the DIYâ€™er to turn off the power they are safe anyway which is a good example of how stranding someone in the loft in the dark would need two wrongs not just one.
I hope you can see the problem and use your own judgement to assess the risks and decide how to minimise them. Remember it workman outside were to hit the mains cable they could make the neutral become line voltage which is why we treat them as live cables. Again two things going wrong would be needed to cause the problem and it is unlikely to happen but not impossible.
[quote]So onto why it trips with direct connection and not through body. The body is quite a high resistance and you need approx 20ma to flow to make the RCD trip. So 0.9 volt at less than 25 ohm will cause over 30ma to flow so it trips but your body is more like 2500 ohm or more so it does not trip. Simple ohmâ€™s law. [/quote]
sorry ericmark i don't think i was very clear. i didn't mean that the elcb doesn't trip when i touch the exposed cores. i meant that when i touch the exposed earth to the neutral or indeed touch any combos together the elcb never trips. only ever when i cut into the T&E on an isolated circuit.
i did cut the T&E as you suggested, by cutting through individual cores. it did the trick and there were no trips.
and in future i shall always isolate by the two pole mains switch. thanks for the advice.
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