Safe isolation "locking off"

[RMS]

How many of us actually lock off MCBs when working on electrical circuits?

I suspect some will lock off depending on the location but many will be happy to work on an isolated circuit by means of switching off the MCB only. Does everyone always prove dead?

Interested in hearing people's answers and any problems this has caused.

A old work collegue of mine once installed a flood light and decided to wire it up using a plug top. He fitted the plug top first then went outside to wire the light and blew his snips up when cutting the cable. The customer had plugged it in thinking it was for something else. He was working up a ladder when this happened.

He ran in the house and expressed his frustration to the customer for some strange reason

 

[vibrobullit407]

He ran in the house and expressed his frustration to the customer for some strange reason

What an absolute idiot! How could he possibly blame the customer for his stupidity! Hope the customer told him where to go! And to never darken his door again.

I made the mistake when I was training to believe that my colleague had isolated a cooker cable on a kitchen rewire as it was just hanging from the ceiling. I went to snip off the excess and bang went my snips. If I'm working in a occupied property or with another spark I will always test for dead and lock of the circuit!

 

[JohnW2]

How many of us actually lock off MCBs when working on electrical circuits? I suspect some will lock off depending on the location but many will be happy to work on an isolated circuit by means of switching off the MCB only. Does everyone always prove dead? Interested in hearing people's answers ...

As you say, an interesting question, but I fear that you will probably find yourself fairly short on 'fully frank' answers! (that's why an 'anonymous polling' facility on this forum would sometimes be useful). What I think one probably could say is that if anyone says that they always lock off MCBs (is that even possible with all MCBs?) and/or that they have never failed to test for dead, then there is a high probability that they are being less that totally honest!

... and any problems this has caused.

The difficulty, of course, is that 99.9%+ of the time, there would be no 'problems', so that even someone who engaged in 'gambling' for a whole career might never have experienced any downside!

Kind Regards, John.

 

[Spark123]

It is possible with almost every MCB I have seen, if not then popping the wire out into a terminal block is another way to avoid accidental energization.
I think all of us have worked on a non-locked off system before, also failing to test for dead mainly through laziness.
Have been caught out before too, luckily last time it was the old flick with the back of my hand on a live wire!

 

[JohnW2]

It is possible with almost every MCB I have seen...

Fair enough; my exposure to a wide range of makes/models is limited.

... if not then popping the wire out into a terminal block is another way to avoid accidental energization.

Indeed - but I wonder how many people would do that?

I think all of us have worked on a non-locked off system before, also failing to test for dead mainly through laziness.

Quite - as I said, I think that anyone who denies that is probably either being 'economical with the truth or else is surprisingly forgetful!

Kind Regards, John

 

[RF Lighting]

I do lock off on a lot of my work. I will always lock off on commercial jobs, and domestics if I can't see the CU, and there is someone else in the house.

I have a wide range of lockoffs ond I don't think I've met a breaker yet that I haven't been able to lock off.

As for testing for dead, I am very much in the habit of proving dead before working. I do have a lot of trouble getting our apprentice to prove for dead for some unknown reason. I think it's just young complacency.

I got caught out some years ago. I was working in a school, and the job was to simply replace a single 5' fluorescent with a double.

The wiring in the building was very old. It had ceramic fuses in big old cast iron fuse boxes. I pulled some fuses until the light went out, and then went to swap the fitting.

Upon removing the fitting it went with an almighty bang. I nearly soiled my self. It turns out I'd pulled the neutral fuse.

It was the wakeup call I needed and I always prove dead before working.

I have also done a job where the meter tails were crossed at the main switch, so turning off the MCB puts the light out, but it remains live.

 

[kai]

One feasible action, is to disconnect the phase core from the breaker, and place it into a spare hole in the earthing bar in the CU before starting. (You need to do this anyway to perform the R1+R2 earth continuity test at the end anyway).

 

[streetlighter]

I have a warning notice to place at the point of isolation it clearly states Danger men working on circuits do not re-energise supply please conatact my name and mobile number.
Often i will remove the fuse if posable and keep it in my posession at all times.

 

[OwainDIYer]

I have also done a job where the meter tails were crossed at the main switch, so turning off the MCB puts the light out, but it remains live.

Single-pole disconnection isn't isolation, so should be treated as working live for exactly this reason.

And if you're relying on the MCB off to provide isolation because you want to leave other power on, touching the N to the E may trip the RCD anyway, leaving you up a ladder without any lights, etc., so you might as well plan a safe system of work assuming no power is available anyway.

 

[ericmark]

Where I worked it was required that we always locked off. However we only had the multi-lock clamp and a single personal lock. We did however have plenty of electrical locks for which only electricians had the key.

However my working life ended when an electrician removed the electrical lock and turned the power back on.

Locking off alone is not good enough. There has to be a laid down method as at some time some one will go home without removing the lock first or remove a lock when it should not be removed.

A foreman of mine was not in favour when he sent two electricians to work independently on the same circuit. One of them was using a insulation tester and was getting odd readings and when he investigated found the other one inspecting the lock and scratching his head wondering why he got a shock.

The problem with all the safety procedures is it has to be a full procedure not just a part.

The regulations to consider when isolating.

537.1.2 Combined protective and neutral (PEN) conductors shall not be isolated or switched except as permitted by Regulation 543.3.4.
Except as required by Regulation 537.1.4, in a TN-S or TN-C-S system the neutral conductor need not be isolated or switched where it can be regarded as being reliably connected to Earth by a suitably low impedance. For supplies which are provided in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, the supply neutral conductor (PEN or N) is considered to be connected to Earth by a suitably low impedance.

This regulation I would say allows one to isolate a circuit using the MCB on a TN system but not of course with a TT system.

There are some exceptions boats and caravans need all pole isolation. There does not seem to be any regulation to say all pole RCD's must be used except for specials like boats and caravans. I have questioned this a few times as with a TT system I would like to see all pole isolation on all RCD's but it seems this is not required.

I have also talked about the simple tie rap. To me some one can remove a lock or any other device if they really want to. All we have to do is make it so it requires a tool (Key is a tool) before the supply can be reconnected. So putting a simple tie rap on an isolator which requires snips to remove it would be enough. However tape on the MCB would not comply.

So plug a plug into a old socket and tie rap them together would effectively stop it being plugged in without use of a tool.

I will admit in for example a house with no one but me and my son in it I would switch off without using a lock. But that's only with my son don't think I would trust anyone else after my mauled hand. I have heard people say "If I am on my own not one else in the house" well if I was on my own with no one else in the house I would not be playing with electrics.

 

[ban-all-sheds]

Let's not forget the DIYer in his own home. We should all strive to work to the same standards as a professional electrician, but in this respect our environment is different.

Only yesterday I replaced a luminaire.

• I KNOW that MCB #0 is for the downstairs lights.
  
• I KNOW that before I turned it off I had 5 working lights in the room and now I have none. Yes, I will trust my life to that and not use a tester in case the MCB went faulty at the same time 5 lamps failed when not under power or several conductors just fell out of several terminals at the same time.
  
• I KNOW that the only other person in the house is Mrs Sheds, and she is upstairs and that there is no way she can get down them without me noticing and that even if she does she is not going to go into the cupboard under the stairs and flick MCB #0 back on while I'm clearly visible on some steps, luminaire and screwdriver in hand. Unless there's some new insurance policy of which I am unaware.
  
  So I'm not going to lock anything off.
  
• Nor am I going to turn off at the main switch, rendering the entire house dead, when the worst that will happen is I clip N & E and render the entire house dead...

 

[OwainDIYer]

537.1.2 Combined protective and neutral (PEN) conductors shall not be isolated or switched except as permitted by Regulation 543.3.4.
.....

This regulation I would say allows one to isolate a circuit using the MCB on a TN system but not of course with a TT system.

But the PEN conductors won't be *combined* after the service intake, or the installation would be TN-C not TN-S or TN-C-S.

TN-C (usually implemented as MICC earthed concentric wiring) is I think not permitted now, and was formerly permitted only on private generator supplies?

 

[ricicle]

I always lock off when the DB is remote as some DBs at our site are accessible to everybody. When the DB is in a locked, single entry cupboard, within eyesight I sometimes don't.
I never lock off at home as I will only do electrical work when no one else is there so I don't get the kids asking when the telly will be back on.

 

[JohnW2]

I always lock off when the DB is remote as some DBs at our site are accessible to everybody. When the DB is in a locked, single entry cupboard, within eyesight I sometimes don't. I never lock off at home as I will only do electrical work when no one else is there so I don't get the kids asking when the telly will be back on.

Exactly ... what is known in the trade is common sense!

Not testing for dead is a somewhat different matter. I confess that, in the situation described by BAS, I would probably do the same as him (not test), but one has to accept that there are some (extremely small) residual risks. Transposed meter tails is incredibly unlikely and, in an installation one 'knows', will presumably have been excluded by prior testing. The main reason for considering 'testing for dead' in such a situation is really simply 'belt and braces', just in case (incredibly unlikely in BAS's scenario) one has been silly or overlooked something.

Dare I admit that, if I did fail to formally test for dead in BAS's scenario, I very probably would 'test' with an unmentionable 'neon test screwdriver'. Sure, that is flawed (and could give false confidence) but it's better than not 'testing' at all.

Kind Regards, John

 

[JohnW2]

537.1.2 Combined protective and neutral (PEN) conductors shall not be isolated or switched except as permitted by Regulation 543.3.4.
..... This regulation I would say allows one to isolate a circuit using the MCB on a TN system but not of course with a TT system.

But the PEN conductors won't be *combined* after the service intake, or the installation would be TN-C not TN-S or TN-C-S.

That's very true, but it was the rest of 537.1.2 (i.e. all of the bits which you did not quote) which Eric was referring to - and they do, indeed, say that SP 'isolation' is OK for TN, but not for TT (for the reasons they state).

However, this regulation is just about the requirement for installed isolators. It does not necessarily mean that working on a circuit 'isolated' by a SP isolator would be considered to be a safe (or necessarily even acceptable) working practice - which is, I think, probably a matter beyond the remit of BS7671.

Kind Regards, John