Isolate MCB, and then check appliance or accessory is dead.
If in doubt turn off main switch
If in doubt turn off main switch
Solve an argument: Should you turn ALL of the main switches off when working on electrics.
- Thread starter reoreborn
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If you turned off the whole board, how did you get a shock. I have however previously lived in a property with 2x 32 amp wire fuses come across crossed neutrals but the circuit showed as live until I pulled both fuses.
Ancient fuse board , only live was isolated by switch.
I'll make sure *not* to follow his advice when doing something like changing a socket in a hospital waiting room, or connecting a tea boiler in the canteen of a factory thats got a load of CNC milling machines going on the machine shop floor.....
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It’s not unheard of for the ends of rings to be in different MCBs
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Proving for dead, is not the same as isolation. I have had some arguments over proving dead, only sure way is spike the cable, but without going to that point, the 'safe' voltage is 50 volt AC unless in a swimming pool etc. So the proving unit needs to show the test device will work at 50 volt, using 400 volt to test the tester tells you nothing, it does not show the tester will work with 50 volt, so you could have a 100 volt which the tester is not showing.
Neither can we be 100% sure no borrowed neutrals unless a clamp on put around the cable. Before the clamp on became so popular the neon screwdriver was a very good second string to ones bow, yet heard electricians who should know better telling people to bin them. Yes not a good primary tester, but still good secondary tester.
My father-in-law told me the story of telling a contractor he must spike the cables before cutting them, as there were some high voltage cables around, he did is walk around to find a guy with a paddies motorcycle spike and a sledge hammer about to drive it through the cable, the machine
is set around the cable, and the operator retires to a safe distance first.
This is the problem with forums, people can read between the lines, and get it wrong. And there is no way to check the person reading is actually understanding what you have mean by your statement.
Even when proving for dead, one can still get it wrong, I sawed through a cable after proving dead, I had never considered it could be connected to a contactor which would later make it live, lucky no one injured, but it was a near miss.
Today with smart devices, this is so easy, it seems dead, but it is connected to a device which turns it on at dusk, or when the temperature drops, or even the the DNO grid supply is lost, if I had been the guy changing my meter for a smart meter, I would have been rather shocked to hear the central heating boiler start up, when I have main fuse removed, but he did not even flinch.
Just everything forward and trust to the rubber gloves.
Neither can we be 100% sure no borrowed neutrals unless a clamp on put around the cable. Before the clamp on became so popular the neon screwdriver was a very good second string to ones bow, yet heard electricians who should know better telling people to bin them. Yes not a good primary tester, but still good secondary tester.
My father-in-law told me the story of telling a contractor he must spike the cables before cutting them, as there were some high voltage cables around, he did is walk around to find a guy with a paddies motorcycle spike and a sledge hammer about to drive it through the cable, the machine
This is the problem with forums, people can read between the lines, and get it wrong. And there is no way to check the person reading is actually understanding what you have mean by your statement.
Even when proving for dead, one can still get it wrong, I sawed through a cable after proving dead, I had never considered it could be connected to a contactor which would later make it live, lucky no one injured, but it was a near miss.
Today with smart devices, this is so easy, it seems dead, but it is connected to a device which turns it on at dusk, or when the temperature drops, or even the the DNO grid supply is lost, if I had been the guy changing my meter for a smart meter, I would have been rather shocked to hear the central heating boiler start up, when I have main fuse removed, but he did not even flinch.
Just everything forward and trust to the rubber gloves.
You presumably turned off an MCB (or RCBO) but not the main switch? (Main Switches are invariably double-pole, so would work just as well with reversed polarity.
... except that, as you know, BS7671 says that single-pole isolation is OK with TN supplies.
Their argument for saying that (which they mention) seems reasonable for TN-C-S (since N & MET/CPCs are then joined within the installation), but I'm not so sure about TN-S
No MCB. Not like anything I have seen before . Must have been over 80 years old , fried my friend tv and microwave and her neighbours tv ( maisonette ) before it was discovered .Some dodgy works in the street . Don’t think any other properties were affected .
In that case, I presume that you must have turned off the 'main switch' (since it would have had fuses, hence nothing else to'turn off'), and the implication of your story is that that 'main switch' was only single pole - was that the case? If so, I'm not sure that I've ever seen that, even in ancient fuse boxes.
In terms of the OP, in the final analysis, no matter what one 'switches off' (or doesn't), what really matters is 'testing for dead' - since we've heard stories of sockets or lights which were supplied from a neighbour's electrical installation (and hence would remain live even if everything in the house was 'switched off').!
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Was always taught on TT-fed installations to isolate at the main switch, IE both poles, not just to isolate the line.
That would be consistent with what BS7671 currently says since, having initially said that both poles should be isolated, it adds that neutral does not have to be isolated in TN installations - so double pole isolation only needed with TT.
As I wrote this afternoon, I can understand their reasoning in relation to TN-C-S, but 'am not so sure' about TN-S.
In any event, as I've written more recently, the most crucial thing is the 'testing for dead', since even 'switching off the main switch' (of the installation being worked on) does nothing to protect you from a socket or light which is fed from a neighbour's installation
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I have come across this situation once as far as I recall in my working lifetime.
Whereas installations where RP has been present were far more common.
DP isolation is certainly safer in a DIY situation where the correct testing for dead procedure is less likely.
Whereas installations where RP has been present were far more common.
DP isolation is certainly safer in a DIY situation where the correct testing for dead procedure is less likely.
or from a UPS or SPS
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