A close shave!

[HappyHippyDad]

Today I was at a customers house (domestic) to do a simple light change.

I turned the existing light on, went down stairs and turned what I thought would be the correct breaker off. I went back up the stairs and the light was off. I never take it for granted that that is the correct breaker so I tested with the meter as well. It tested as dead and I was about to start removing it.

The customer then came up the stairs and asked me why I'd switched off all the power rather than just the lights, I thought I had perhaps tripped an RCD. Went back to the CU and the RCD was still up, yet all power was off.

So... Whilst I had been walking back up the stairs after switching the breaker off they had had the power cut in the street! This has never happened to me before and I'm just wondering if I made any errors in my safe isolation procedure which could have avoided this? I can't see how I could have though. Luckily it was the right circuit breaker but that is besides the point. Bit scary!

 

[David Prosser]

My safe isolation procedure includes proving you have identified the correct circuit, motor, pump or what ever is to be worked on. This can be difficult some times but in this instance a couple trips between the MCB and the light circuit using the approved voltage tester should of been sufficient (this can be made much simpler if there are two of you).

 

[R-fur]

Even if you follow the correct procedure for isolation, a borrowed neutral can still get you once you start disconnecting wires.

 

[HappyHippyDad]

Just disconnecting the line does not isolate a circuit. To isolate a circuit all live conductors (Ls and N if used) must be disconnected from the source of energisation. A single pole MCB is a functional switch not an isolating switch. Isolation requires a minimum air gap between the energised contacts and circuit contacts dependent on the applied voltage and a secure way of locking off.

One checks for energisation on the supply side (which confirms the Approved Tester is working at the circuit voltage) and then on the circuit side of the closed isolator. Then one opens the isolator to isolate the circuit from the source of energisation and checks for energisation on the circuit side and then the supply side to demonstrate de-energisation of the circuit side. The evidence of isolation is de-energisation on the circuit side and energisation on the supply side tested in that order.

The case of de-energisation on both sides of an isolator does say anything about disconnection or de-energisation of the circuit because for multifarious reasons the source of energisation may have temporarily disappeared ie: a power cut.

What you are saying does make sense Marconi but BS7671 does state that a circuit breaker to BS 60898 (I.e a standard mcb) can be used as a form of isolation. See table 53.4 BGG.
Apologies for not quoting the BBB but I am out and only have the BGB to hand.

 

[NDG Elecs]

I have had similar. Was running in new sockets in an office and was second fixing the faceplates when suddenly it went pitch black. I stupidly assumed (as slightly panicky at the time) I tripped something despite the new circuit cable not being connected to the board yet! With the alarm blasting and in relative darkness I fumbled about trying to suss out what had happened. It was only after a couple of mins that I realised the street lights and other premises were all off too..!

The answer in my mind would be to check live, check dead, check live again and check dead again..

 

[BruceBanner]

Normally use the incomming tails on the meter side as the "known" source.

 

[HappyHippyDad]

Normally use the incomming tails on the meter side as the "known" source.

That would solve the issue that I raised at the beginning of the thread, but unfortunately I can't see that as a realistic method for everyday use.
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My safe isolation procedure includes proving you have identified the correct circuit, motor, pump or what ever is to be worked on. This can be difficult some times but in this instance a couple trips between the MCB and the light circuit using the approved voltage tester should of been sufficient (this can be made much simpler if there are two of you).

Even with 2 trips I would have got the same result.
I would have switched the MCB off. tested live on my proving unit, tested dead at the light, tested live on my proving unit. Then I would have done this again.
Also (as above), 2 trips would not really be realistic on a day by day basis.

I'm really not sure what the answer is. Luckily it is an extremely unlikely event. Statistically it will probably only happen perhaps once in a career lifetime so I am now fine

 

[Pete999]

Lots of good Sparks about

never mind HHD its shell shock for me when the customers shouting why did you turn off the power when me was watching tv.

Rule no 1 ask for permission to turn Juice off

 

[HappyHippyDad]

What you are saying does make sense Marconi but BS7671 does state that a circuit breaker to BS 60898 (I.e a standard mcb) can be used as a form of isolation. See table 53.4 BGG.
Apologies for not quoting the BBB but I am out and only have the BGB to hand.

Just checked in the BBB. Table 537.4

 

[KWCFC]

I've had the light bulbs flicker while I've been drilling into a wall... Does make the guts drop for a second

 

[davesparks]

Just disconnecting the line does not isolate a circuit. To isolate a circuit all live conductors (Ls and N if used) must be disconnected from the source of energisation. A single pole MCB is a functional switch not an isolating switch. Isolation requires a minimum air gap between the energised contacts and circuit contacts dependent on the applied voltage and a secure way of locking off.

It's very rare that I disagree with you, but this is one of those occasions.

Disconnecting the line is considered to be sufficient for isolation on a TN supply. Circuit breakers are considered to be suitable devices for single pole isolation.

Most three phase isolators (even off load devices designed purely for isolation) don't disconnect the neutral unless specifically required. Plus it is generally considered better not to isolate the neutral of a three phase TN system.

 

[MFS Electrical]

Even if you follow the correct procedure for isolation, a borrowed neutral can still get you once you start disconnecting wires.

And they hurt I’ve had a few belts through my own stupidity and by far the worst was from borrowed neutrals they don’t just fry you they burn you too

 

[dmxtothemax]

This is exactly why we don't take anything for granted,
Test, test and test again

 

[marconi]

Thank you to those who have corrected my error about the suitability according to the BBB of single pole mcbs for safe isolation of TN supplies at low voltage which should be regarded as the reference.

My personal opinion is that using a line pole mcb amounts to safe disconnection of the high potential line only which is not equivalent in degree of safety to complete physical isolation(disconnection) of all live conductors (ie current carrying) from the source of energisation.

 

[David Prosser]

Even with 2 trips I would have got the same result.
I would have switched the MCB off. tested live on my proving unit, tested dead at the light, tested live on my proving unit. Then I would have done this again.
Also (as above), 2 trips would not really be realistic on a day by day basis.

I'm really not sure what the answer is. Luckily it is an extremely unlikely event. Statistically it will probably only happen perhaps once in a career lifetime so I am now fine

From your first post "Luckily it was the right circuit breaker but that is besides the point. Bit scary!".
I'm pointing out that if you correctly identified the circuit then it's not luck it good practice. Also there are several ways of identifying the correct circuit not just using a proving unit.