Discuss A close shave! in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

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.
[automerge]1579719192[/automerge]
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 :D
 
Last edited:
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
 
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.
 
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 :coldsweat:
 
Last edited by a moderator:
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.
 
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 :D

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.
 

Reply to A close shave! in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock