Extraneous bonds values

[Engineer54]

How did the metal flooring grid become extraneous in the first place??

I think there is some confusion here between earthing and bonding. Earthing is generally required with metalwork that forms part of an electrical installation such as metal containment systems (conduit, trunking, tray etc) or where part of an electrical installation is integrated into a metal structure etc (such as in this case, these floor sockets in the metal flooring system)... Other building metalwork that brings an extraneous potential into a building is required to be main bonded. Other building metal work that is not extraneous (proven by testing >22k ohms) needs neither bonding or earthing in fact it could well be dangerous to do so!!

 

[Guest 004]

Thanks Engineer54. I knew I could rely on you.

The main bonds were indeed to the main steelwork, and ductwork.
A couple of low noise earth's were installed to the server room as would be expected.
It's just that the consultant was insisting on this low continuity reading between points.
Thus the original question.

 

[Risteard]

Can anybody point to a regulation where 0.05 ohms is the maximum allowed measurement between points.

There isn't one.

 

[Risteard]

As nobody can pull any regulation out, just guidance notes

A misunderstanding of the Guidance Notes at that. It is suggesting that would confirm continuity between two points. It is NOT suggesting that the resistance of the main protective bonding conductor should not exceed this.

 

[Risteard]

Oh sorry, after reading on I have realised that this is not what was being talked of!

 

[Engineer54]

A misunderstanding of the Guidance Notes at that. It is suggesting that would confirm continuity between two points. It is NOT suggesting that the resistance of the main protective bonding conductor should not exceed this.

Unless of course the contract electrical specification calls for this as a maximum value. Otherwise you are correct...

 

[Risteard]

Unless of course the contract electrical specification calls for this as a maximum value.

Granted, but that would be a contractual issue.

 

[Engineer54]

Granted, but that would be a contractual issue.

Exactly, but then a contract specification that both parties have signed in agreement overrides the minimum ''guidance'' requirements of BS7671.

 

[tony mc]

This post from the past may be a good read with regards the 0.05 figure that is often mentioned re MPB !

http://www.electriciansforums.co.uk...llations/31857-general-testing-questions.html

 

[Leckie]

The 0.05 figure is just the value that is acceptable for a connection. So if you fit an earth clamp to a copper pipe, the value between the conductor connected to the clamp terminal and the pipe it's connected to should be no more than 0.05ohms. It's to confirm the connection is good. If you had a 70m run of 10sq.mm, it's not going to measure 0.05 ohms end to end is it.

 

[Pete999]

The 0.05 figure is just the value that is acceptable for a connection. So if you fit an earth clamp to a copper pipe, the value between the conductor connected to the clamp terminal and the pipe it's connected to should be no more than 0.05ohms. It's to confirm the connection is good. If you had a 70m run of 10sq.mm, it's not going to measure 0.05 ohms end to end is it.

https://www.youtube.com/watch?v=rnvNfI__kYU

There is a thought/recommendation that if you earth conductor is more than 25meters in length the you could up the csa from 20mm to 16mm (see link)

 

[Geoffsd]

It's strange that he doesn't say why.


Also, the extraneous-c-p is the pipe which comes out of the ground, not where it is bonded, but that's another story.

 

[Pete999]

It's strange that he doesn't say why.


Also, the extraneous-c-p is the pipe which comes out of the ground, not where it is bonded, but that's another story.

He is testing from the pipe as opposed to the clamp to prove he has a decent connection, your 1st point imo he is saying that increasing the csa of the cable will reduce the resistance, don't think there is anything written down, I believe its just a recomendation