Positive Isolation

[securespark]

however, I would always also test to ensure the circuit is DEAD before working. How do you know you have isolated the correct circuit at the CU?

By the sequence:

PROVE

ISOLATE

TEST

PROVE

LOCK OFF


If you prove your two-pole device OK then isolate then test again and it does not light up, but does with the proving unit, surely you have proven dead?

 

[plugwash]

One thing I would add is that it is best to do the first prove on the circuit you are about to isolate. That way you know it was live before and YOU MADE IT DEAD. Knowing that you made the circuit dead gives you greater confidence (but still isn't a guarantee since a circuit could be fed from multiple sources) that you are indeed properly locking off the circuit in question.

You should also test wires for dead after disconnecting them because borrowed.neutrals can cause wires to become live on disconnection.

Some electricians adopt a policy of "work like it's live even if it's dead" because unless you are 100% sure the plans are accurate and everyone on site is trustworthy enough not to bypass a locked out breaker there is really no way to ensure that wiring is dead and will remain dead.

 

[Spark123]

How do you mean set wrong mate??

If you set the multimeter to volts DC accidentally and try to measure volts AC it will read zero volts.

err are the leads fused to protect the multi meter or to protect the user ?

thanks again

Both, but more to do with user safety.
If you accidentally had the meter set to amps and measured across L-N on a supply with a high fault current the multimeter could explode and take your hand off, end up with arcing and burns etc etc etc.
With adequate fusing you'll probably only need a new pair of underpants.

 

[onlyfitidealboilers]

thanks pal, can you use a multi meter on ohms on a switch ie like a overheat stat, say if its broke 240v to one side & no volts on the other side, use the meter to pass voltage from one side to the other, I have done this by accident once and it worked but i thought it would blow my meter but it never...

thanks for reply so far

 

[ricicle]

thanks pal, can you use a multi meter on ohms on a switch ie like a overheat stat, say if its broke 240v to one side & no volts on the other side, use the meter to pass voltage from one side to the other, I have done this by accident once and it worked but i thought it would blow my meter but it never...

thanks for reply so far

Not recommended !

 

[ericmark]

I followed the procedure of testing with a tester which had no switches so could not be set on wrong range and testing the tester to prove tester was OK.

By enquiry I was told the cables were from an old installation and were no longer used. The JB had no cover and whole lot covered in concrete so hard to physically trace.

So after proving dead started to remove the cables. However while I was doing this one cable became live. It seemed it supplied a motor which was not in use when I tested the cables.

Lucky no one hurt. But taught me a valuable lesson. Where the purpose of the cable can't be with certainty worked out then there is only one safe way and that is to switch off whole supply to building.

Even in a house cables supplying central heating for example may not be live when tested but may become live while working on them.

Even with main isolator off one can still fall foul I had this when trying to remove a 13A socket. Again lucky I was using a neon screwdriver which lit. Seems connected to neighbours supply. It was that which made me start to prove every circuit dead even when whole house isolated.

Cards and locks are also not fool proof. One would hope where one sees a circuit locked off it would not be energised. However seen an electrician get a belt where another electrician unaware anyone else was working on same circuit used an insulation tester.

So even when locked off, and proved dead I still treat circuits as if they may be live.

I know my father-in-law worried that there may be an error in identifying a circuit to be removed ordered that the cable was to be spiked first. The spiking is done with a special machine and is set up around the cable then one retires to safe distance and presses the remote which fires the spike through the cable.

However he went to inspect only to find a guy with a band jack spike and a sledge hammer about to hammer it into an old 11kv cable. Had it been live I don't put much to his chances of survival.

Although with 230v to earth we often don't bother really the idea of earthing cables before working on the does make sense. It would have worked in the first example.

I also question some of the proving units. To prove a voltage tester will show voltage in excess of 50v to supply the tester with 500v seems flawed. The proving unit should start at lowest then slowly go up the range and then return to lowest setting. The 25v or 50v Neon may have a bad connection and although it will light with 500v it may not light with 50v. Some proving units do test all voltages but many only give one voltage output. And oddly that is highest voltage not lowest. I could not give a dam if the 500v lamp works it's the lowest voltage lamp I am worried about when proving dead.

And yes I know I made mistakes. However I hope what I have said will stop others making same mistakes.

Biggest mistake I made was to use a lock which had more than one key. It was removed by another electrician. I have not worked since.

 

[Risteard]

Biggest mistake I made was to use a lock which had more than one key. It was removed by another electrician. I have not worked since.

Which is why it should have a unique key with no duplicates and should be kept in your possession at all times. Add a clasp for multiple locks (still with non-duplicated keys).

In fact the key for my lock-off has engraved on it "DO NOT DUPLICATE".

 

[flameport]

http://www.tynaghnetworks.ie/pdf/best-practices/Five Golden Rules 28 05 09.pdf

 

[ericmark]

How many locks do you carry. I had one with single key but needed four. Used electrician's locks for other three which one would hope would not be removed without checking. Wrong.

 

[Risteard]

How many locks do you carry. I had one with single key but needed four. Used electrician's locks for other three which one would hope would not be removed without checking. Wrong.

I have a few. Don't keep duplicates of any key on keyring.

 

[SimonH2]

Cards and locks are also not fool proof. One would hope where one sees a circuit locked off it would not be energised. However seen an electrician get a belt where another electrician unaware anyone else was working on same circuit used an insulation tester.

A friend told me this story from a few years ago. He had to work on a machine in a mill - think cotton mills around Bolton area. The installation was old and the switches didn't have any lockoff facility - so switched off and hung up the warning notice. Thinking back there must have been fuses, so they should have been in his pocket - but given the period and his age, I suspect this was when he was still a young lad and learning - quite quickly in this case.

Back at the machine, he found it live - went back to the switchroom and found the switch back on and the card on the floor. Switched off and hung the card up again.

Repeat same thing !

This time he switched off, hung up the card, and stood round the corner out of sight. Sure enough, in a very short time one of the machine operators came in, threw the card on the floor and switched the supply back on. At this point my mate put him out on the floor.

After this, the machine operator was sacked, and my mate was escorted off site for his own safety !

It turns out there were two machines on the circuit, and the operators were on piece work. Being foreigners with a lesser attitude to safety than even the worst British workers, they really couldn't see any reason to take any notice of safety notices.

 

[securespark]

The installation was old and the switches didn't have any lockoff facility

Find this hard to believe. My two lock offs fits all the mcb's/ switches/ RCD's I've come across so far.

One is the Wylex MCBLDX, the other I cannot locate the part number, but a red plastic body with metal thumbscrew.

We are told if it cannot be isolated, do not work on it!!

 

[SimonH2]

The installation was old and the switches didn't have any lockoff facility

Find this hard to believe. My two lock offs fits all the mcb's/ switches/ RCD's I've come across so far.

This would have been dark ages electrical stuff - before MCBs were even dreamt of, and when an RCD would have been considered science fiction. Reading that back, it does sound like some terrible sterotype - but as it was described to me, it did sound like it was ancient stuff. It would have been a 1/4 century ago - at which point I suddenly feel a bit older.

I don't know, I wasn't there, but I'm fairly certain I've come across stuff without any lockoff facility - and my experience is very limited. When you mention thumbscrews, I'm guessing this is something designed to clamp on where no lockoff facility was thought of. Like this one perhaps ?

 

[bernardgreen]

one of the machine operators came in, threw the card on the floor and switched the supply back on

I recall being told a similar story where the electrician eventually got the message across with a length of thin wire between live and neutral in the switch box which created a small explosion when the switch was turned back on.

 

[SimonH2]

I recall being told a similar story where the electrician eventually got the message across with a length of thin wire between live and neutral in the switch box which created a small explosion when the switch was turned back on.

Or perhaps a very large bit of wire, so it takes out the upstream fuses and then the guy really gets it for shutting down part of the factory

Someone else I know told me a story about when he was working on a supply in a tractor factory. They had this UPS which was 4off 1/4MVA units in parallel and the manufacturer claimed they could do pretty much anything to one module and the rest would keep going. While crouching down taking measurements, something went wrong. After picking himself up off the floor, he found what was left of his AVO smouldering away (one lead completely missing) - and everything had gone quiet. Apparently it took four days to get everything back up and running smoothly. I guess the system couldn't cope with whatever he shorted out

But that's getting a little off topic.