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#146373 - 12/05/06 09:16 AM Shared main bond & sub-panel earth
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
As related to BS7671.

If a sub-panel happens to be part way along the same route that the main equipotential bonding cable must go to the incoming water supply, is there anything to prevent that condcutor being shared as the earth to that sub-panel, assuming that it can be fed through and connected to the earth bar in the panel without being cut?

I can't see anything against doing this, but it's not something I ever recall coming across before.

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#146374 - 12/06/06 04:31 PM Re: Shared main bond & sub-panel earth
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Paul,
I can't personally speak for BS 7671.
What sort of length of run are we talking?.

But this argument came up here a few years back in a new house I was to hook up at work.
The Inspector called me to delay me turning up to hook up the pillar box end.
What had happened was the Electrician had used the Earthing Lead (or Main ECC) to earth the meter enclosure and the inspector had other ideas.
What the Electrician had done was stripped the ECC and wrapped it around the braised stud in the meter enclosure, without actually cutting the copper wires themselves.
All nice and tight once the washer and two nuts were added to the stud, what's the problem?.
Under the 1992 Regulations, I'm told, you have to run a seperate bonding conductor from the Earth busbar in the Main Switchboard, to the meter enclosure (6mm2 too + tags at both ends).
I really don't see the point in it to be honest.
Lets face it, how often in a safe installation, will that conductor actually carry any real fault current?.
If you wanted to be really sure that the connection was safe, you could cut the Earthing Lead and terminate the ends at the stud with two properly sized eyelet crimp lugs.
I have a sneaking suspiscion(sp?), that at one time, somebody did a poor connection at a board or the like with a Main ECC and someone got killed or injured.
That seems to be the way we make Regulations these days (ie: knee-jerk reaction).
I don't have a problem with using the Earthing Lead as long as the connection is made properly and is tested after the connections are all made.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#146375 - 12/07/06 06:33 AM Re: Shared main bond & sub-panel earth
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Actually the question is somewhat academic now as I've decided not to go with the sub-panel option. Nevertheless, it poses an interesting question for any similar scenario in the future.

So purely for sake of argument.......

 Quote:
What sort of length of run are we talking?.


Rough estimate: Total run from main panel to water-supply bonding point about 100 ft.; sub-panel about 10 ft. away from the water end.

I was planning a 50A sub-feeder on 10mm cable, which under our Regs. also happens to be the minimum size needed for the main bonding on a TN-C-S/PME system.

Given the cost of cable now and the fact that the sub-panel is right on the way, doubling up could have saved 90 ft. of 10mm earth cable, not to mention easing the conduit fill.

Edited for typo.

[This message has been edited by pauluk (edited 12-07-2006).]

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#146376 - 12/07/06 07:35 AM Re: Shared main bond & sub-panel earth
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Oddly enough Paul, 10mm2 cable here uses a 6mm2 Earth conductor in it.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#146377 - 12/07/06 07:55 AM Re: Shared main bond & sub-panel earth
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Our "T&E" has a reduced size earth conductor as well, but this is (or would have been) a PVC conduit job.

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#146378 - 12/11/06 12:51 AM Re: Shared main bond & sub-panel earth
kiwi Offline
Member

Registered: 12/04/04
Posts: 347
Loc: christchurch new zealand
Pauluk, the main earth conductor is better in one piece than with a join in it. Even if the join is really safe. Baring a cable and terminating without chopping it still constitutes a "Join".

This may be impractical but its a fact.

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#146379 - 12/11/06 03:01 AM Re: Shared main bond & sub-panel earth
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Lets throw this idea into the waters.
I saw a job done a few years back where a guy drove a 9 foot rod sideways in a trench.
3ft wide trench.
He was told to rip it up.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#146380 - 12/11/06 09:58 AM Re: Shared main bond & sub-panel earth
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
We can use the same cable to bond water and gas by looping from one to the other, so long as the cable passes uncut through the first clamp.

BS7671 seems to consider a the main "through" cable to still be one single length so long as the join is made without cutting it.

By the way, just in case there is any confusion with my original scenario, the cable in question is not the main earthing leading lead as such but the main bonding lead. The main earthing lead here would be considered to be the one running from the panel's earth bar to the supplier's incoming neutral on a PME/TN-C-S system.

I did think about the shared cable situation a little more carefully though, and there is one thing which might need some consideration. If a bolted fault happened to occur at the sub-panel (unlikely, but possible), then until the fault was cleared by the 50A MCB at the main panel you'd have a series connection of two 10mm (phase and earth) across the supply. As the sub-panel is almost at the same end at the bond to the pipework, for the split second until the MCB tripped the potential at the bonding point could rise to almost half the supply voltage.

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