Earthing metal backboxes

[malcolmsanford]

An exposed conductive part is a metal chasis of a class I piece of equipment such a a metal toaster frame, or metal conduit, trunking etc, these have to be Earthed, as they are not normally live, but can become live if basic insulation failed.

An Extraneous conductive part is a piece of metal that has the capability of producing a potential between itself and an exposed conductive part, but is not part of an electrical installation, so it could be a water pipe, which is why it is Bonded.

If you bonded a piece of metal that is not part of the electrical installlation and is also not an extraneous conductive part, lets' say it is a metal window, then when there is a fault within the installlation the fault currents could be transfered to that frame, causing the frame to becme live

 

[stevie2011]

If you bonded a piece of metal that is not part of the electrical installlation and is also not an extraneous conductive part, lets' say it is a metal window, then when there is a fault within the installlation the fault currents could be transfered to that frame, causing the frame to becme live

so its a metal part that has no connection to earth, atleast not by a very very very high impedance, so you don't want it earthed because if a potential rises in it you could get a shock off it. so for that reason it wants to remain separate from the installation?

 

[IQ Electrical]

Just out of interest, can you give an example of a potentially dangerous situation?

exposed conductive part is something like a metal grid switch plate, or trunking part of the installation. I think, someone correct me

I cant work out what extraneous conductive parts are, even looking at definitions.

man i need to work on my theory.

An extraneous-conductive part has to meet these 3 definitions:

1 A conductive part

2 Liable to introduce a potential, generally earth potential

3 Not forming part of the electrical installation


Examples of extraneous-conductive parts:

Water service pipes
Gas service pipes
Other service pipes (oil etc.)
Air conditioning ductwork
Exposed structural steel parts of a building
A lightning protection system (bonds to be sized and installed by LPS contractor)

These parts are bonded to create an equipotential zone, reducing shock voltages in the event of a fault.

If you were to just 'bond anything metal' as advocated in post #12 then you create the scenario where the non-extraneous conductive parts can attain a considerable rise in potential during the time permitted for fault clearance by ADS.

 

[Des 56]

"Ah" the metal window enthusiasts of the local authorities :banghead:

Go back a few years and for a period, interpretation of what needed bonding sort of became the self inherited decision of council desk jockeys who's only qualification was "no electrical understanding whatsoever needed"

The metal window bonding enthusiasts were prolific in many of those council meetings
"What next" would be uttered,the cuttlery

 

[pj installation]

Hi i always thought as one of the previous people if you so long as you have a fixed lug on the back box is ok but with swa,micc or sheathed cable then put a fly lead.

 

[malcolmsanford]

"Ah" the metal window enthusiasts of the local authorities :banghead:

Go back a few years and for a period, interpretation of what needed bonding sort of became the self inherited decision of council desk jockeys who's only qualification was "no electrical understanding whatsoever needed"

The metal window bonding enthusiasts were prolific in many of those council meetings
"What next" would be uttered,the cuttlery

Yes mate seen many a pop rivet with a bit of 2.5mm G/Y cable attached to a metal window or door frame. As you say Des a throw back to the 16th where most peoples interpretation of bonding was
" If it's metal .......bond it"

Which is why I had to comment on the post as it is dangerous to do this.

 

[IQ Electrical]

"Ah" the metal window enthusiasts of the local authorities :banghead:

Go back a few years and for a period, interpretation of what needed bonding sort of became the self inherited decision of council desk jockeys who's only qualification was "no electrical understanding whatsoever needed"

The metal window bonding enthusiasts were prolific in many of those council meetings
"What next" would be uttered,the cuttlery

Too true, I came into the business right at the end of the 15th Edition when we were bonding staircase handrails, ceiling grids, hand-rails in disabled toilets etc.
Some Clerks of Works and LA building inspectors still have those ridiculous requirements permanently stuck in their brains to this day!

Like I said yesterday, I'd make Guidance Note 8 a compulsory purchase for anyone carrying out electrical installation.

 

[IQ Electrical]

Hi i always thought as one of the previous people if you so long as you have a fixed lug on the back box is ok but with swa,micc or sheathed cable then put a fly lead.

Yes, 1 fixed lug is a requirement and the fly lead must be fitted where the CPC is not directly connected to the accessory-SWA,MICC,Conduit/trunking used as CPC etc.

 

[repcen]

I Just do it without thinking .Good habit !