Flying leads and the regulations.

[telectrix]

comment has to be " why some poor sparky is prodding about inside an accessory with the circuit energised?"

 

[malcolmsanford]

"In fact it is quite a poor example of bonding, as a fly lead in a back box is earthing"

Please explain your understanding of the differences between earthing and bonding as they apply within an installation final sub circuit as opposed to at incoming supply connection.

Electrical bonding - Wikipedia, the free encyclopedia

Think you find that Tel as just explained it rather well in a sentence, and if your looking for a more in depth explanation this may help you.

 

[hermetic]

A metallic back box for a surface-mounted socket-outlet is an exposed-conductive-part, and a metallic back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched). Therefore, such back boxes, no less than every other exposed-conductive-part, are required to be earthed in accordance with Regulations 411.4.2 (TN systems) and 411.5.1 (TT systems) of BS 7671.

 

[Deleted member 26818]

Yes we are quite aware that exposed-conductive-parts require earthing (not bonding).
However, whether a back box that is concealed in a wall, can be considered as exposed is debatable.
Irrespective of that, it is generally accepted that use of the socket plate fixing screws and a fixed lug provides the required earthing.

 

[malcolmsanford]

Well I think you will need to define exposed, a flush back box can only be touched when dismantling an installation so I would argue the point of that, as the installation should be isolated before any dismantling and so therefore technically it can not be come live, when it can be touched.

But apart from the semantics of exposed or not, you still do not bond it, you earth it, as you stated several times in your reply.............so your

"I would have thought that a fly lead to a back box was a perfect example of "earthed equipotential bonding"

Really isn't which is why I pointed it out, as it could cause confusion

 

[hermetic]

"and a metallic back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched)."

this is a direct qoute from the BS 7671

 

[hermetic]

I have checked wikipedia (lols) and it's definitions and mine are not at variance all extraneous metalwork should be bonded AND earthed. A bonded non earthed "equipotential zone" is dangerous in that should a fault occur the potential of all the bonded meatlwork can rise above that of earth

 

[telectrix]

"and a metallic back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched)."

this is a direct qoute from the BS 7671

what reg. no.is that?

 

[malcolmsanford]

Would you mind sharing where that is ?

Again even if it is an exposed conductive part you are earthing it not bonding it ........................

 

[hermetic]

A metallic back box for a surface-mounted socket-outlet is an exposed-conductive-part, and a metallic back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched). Therefore, such back boxes, no less than every other exposed-conductive-part, are required to be earthed in accordance with Regulations 411.4.2 (TN systems) and 411.5.1 (TT systems) of BS 7671.

 

[malcolmsanford]

A metallic back box for a surface-mounted socket-outlet is an exposed-conductive-part, and a metallic back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched). Therefore, such back boxes, no less than every other exposed-conductive-part, are required to be earthed in accordance with Regulations 411.4.2 (TN systems) and 411.5.1 (TT systems) of BS 7671.

Sorry you must have the newer ..................I make it up as I go along version of the amendment, where in those regulations does it state

"and a metallic back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched)."

this is a direct qoute from the BS 7671

 

[hermetic]

there is essentially no difference between earthing and bonding! this is what I have been waiting for someone to comment on, all bonding should be to "earth" or the equivalent provided by supply authority. real life fault situation, catering kitchen with stainless prep tables. aalong comes cheffy with his old school aluminium mixer, not knowing it has a detached earthwire and is faulty (its pat test hasn't been done by a 2dw) he puts it on the ss table and switches on, all bonded non earthed metalwork in the kitchen is now higher than earth potential and no fault is registering till some poor sap standing on the wet quarry tiles touches any metalwork. All extraneous metal should be bonded and earthed.

 

[hermetic]

malcolmsandford, I am quoting from the version posted earlier in this thread by someone else, I have explained above that in my installation days I used IEE regs and ammendments exclusively, go back through the thread and you will find it.

 

[telectrix]

so, in the kitchen scenario, cheffy accidentally touches a live part with his right hand. his other hand is resting on the metal worktop, which is earthed/bonded. 240V straight across the heart--dead.

wotktop not bonded/earthed. cheffy gets a shock, teaches him not to mess with electrics.

 

[malcolmsanford]

Well I've enjoyed this silliness, and I honestly feel I can take this no further.

All I can say is that I'm glad your not working on my project or training my staff. I wish you luck with your bonding of back boxes, and earthing of metal work, after all as you say when all said and done it all goes back to the same place ................

 

[hermetic]

This actually illustrates the problem very clearly There should be ONE set of rules which everyone works to. It used to be the IEE regs which had only one "current" edition which encompassed everything electrical, we now have a hopeless mish mash of guidlines ammendments and explanatary notes issued by organisationd which are actually in financial competition with one another, oh boy am I glad to be out of it after 40 years!!

 

[hermetic]

telectrix, no, the metal mixer is already in contact electrically with the work top, and if all the bonded metalwork is not earthed thn its potential rises, and anyone touching any bonded non earthed metalwork gets a belt! this is actual scenario, fault from the past that I personally attended. the bonding cable which connected the wole of the extraneous metalwork to earth had been badly fitted in an exposed position, and one of the kitchen staff had dumped a load of boxed polythene bags on top of it, and the mane earth/bonding connection was detached.

 

[hermetic]

malcolmsandford, please explain to me where and why I am wrong

 

[malcolmsanford]

No you converted me, as we speak I'm drawing up a plan to fit all my metal work with BS 951 clamps and earth everything, I think your right this is the way forward

I think I've been lax with this and thank you for your posts.

 

[hermetic]

Thank you (lols)