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Topic Title: Main Earthing Terminal
Topic Summary: Location Pros & Cons
Created On: 19 April 2006 10:46 AM
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 19 April 2006 10:46 AM
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khales

Posts: 1271
Joined: 16 January 2002

Domestic existing supply with TN-C-S service head and meter in external cabinet; meter tails and earth conductor (16mm/10mm) go through wall and run 4-5m to to MK Sentry consumer unit mounted on plaster wall in understairs cupboard. Main Earthing Terminal is earth bar within consumer unit; but not enough ways to terminate additional main bonding conductors required - so new earthing terminal is needed. Where to put it? Plaster-wall-mounted below the consumer unit, or outside in the meter cabinet (is that allowed - presumably the cabinet belongs to the DNO, not the householder?)? Pros and cons from experience, please?

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khales

Edited: 19 April 2006 at 10:48 AM by khales
 19 April 2006 11:48 AM
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aligarjon

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put it in the meter cabinet if you are able. Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 19 April 2006 06:24 PM
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finbahr

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the consumer unit should be mounted on wood or mdf.
If it is, fit an earth block under or beside the consumer unit.
 19 April 2006 10:52 PM
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kaichung

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Beside the consumer unit is the best place, handy for testing etc.
 20 April 2006 12:30 AM
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siall

Posts: 39
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just a quick ques on your quote the cu being mounted on wood or mdf is this reg or personal pref or from experiance i would be gretefull if you could let us know
thanks siall
 20 April 2006 07:22 AM
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ebee

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Mount on board fastened to wall via stand off collars (20 or 25mm plastic conduit is good for collars) leaving a gap behind the board.
This gives a nice flat surface for mounting and reduces moisture transmittion from wall to board

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 20 April 2006 09:33 AM
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khales

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quote:

Originally posted by: finbahr
the consumer unit should be mounted on wood or mdf.
If it is, fit an earth block under or beside the consumer unit.

No. If it had been I'd have put the MET there, as you suggested. In this case the CU is mounted directly to the plastered wall with cables entering through the wall from behind.

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khales
 20 April 2006 12:53 PM
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perspicacious

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I'd be more worried about the meter "tails" exceeding 3 m as you've not mentioned any protective devices being present.
Regards
BOD
 20 April 2006 04:59 PM
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khales

Posts: 1271
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quote:

Originally posted by: perspicacious
I'd be more worried about the meter "tails" exceeding 3 m as you've not mentioned any protective devices being present...

Indeed. That's another issue. Since it's EDF's policy not to retrofit load-side isolators for existing customers, then they'd have to give me permission to put one into their outside meter box; but it's difficult to find someone with whom to discuss it. (Now done, see below.)

-------------------------
khales

Edited: 21 April 2006 at 02:08 PM by khales
 20 April 2006 05:01 PM
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deleted_tpenman

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Hi,
Good point about tail lengths </= 3m. Although with outside cabinets I find there is usually some "limited flexibility" applied to the 3 metre rule. Distributors requirements for Meter tails CSA and length probably worth a check. Reg 313-01-01.
You mentioned 16mm / 10mm sizes in the above post;
TNC-S normally 25mm / 16mm standard arrangement. Protective device rating worth a check?
Regards,
Terry
 21 April 2006 11:46 AM
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khales

Posts: 1271
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Discussion with EDF reveals (in this particular area) that tails up to 4m are acceptible, after that they require a switch-fuse (and not just a Main Switch). This must not be in their meter box; e.g. fitted either alongside the external meter cabinet; e.g. in a cabinet of it's own, or inside the premises. Main Earth Terminal may not be fitted in their meter cabinet. Also, they require minimum 16mm² for TN_C_S earth conductor (although not, presumably, at the time this property was originally connected up).
Perhaps I'm missing something here; but I can't see the point of putting another 60A fuse in a switch-fuse unit about 1/2-meter on the load side of the supplier's main cutout.
I don't like the idea of screwing a bare brass earth terminal to a plaster wall, so I think I'll use a Henley block as the MET and screw that to the wall below the consumer unit.

-------------------------
khales

Edited: 21 April 2006 at 02:03 PM by khales
 21 April 2006 06:06 PM
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truss

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quote:

Originally posted by: khales

Perhaps I'm missing something here; but I can't see the point of putting another 60A fuse in a switch-fuse unit about 1/2-meter on the load side of the supplier's main cutout.


I think the main reason for this is so you can isolate the distribution circuit without having to pull the main fuse.

Doesn't answer why a 2nd fuse is needed though.

Regards

Trussy
 21 April 2006 07:54 PM
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perspicacious

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I think you'll find the answer in 473-01-04 (iv).
Regards
BOD
 02 April 2007 06:49 PM
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sanjaysharma

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Been trawling for info on what to use as a MET if the CU not suitable.

Khales suggests Henley block - I assume this is rated at 100A which would be inadequate under fault conditions.

Any suggestions?

-------------------------
Regards,

Sanjay.
 02 April 2007 06:59 PM
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alancapon

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If a Henley block was only rated at 100A under fault conditions, it would be unsuitable for splitting meter tails as well.


Regards,

Alan.
 02 April 2007 07:07 PM
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dbullard

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RE- Henlt blocks,

i have seen the rec use henly blocks for an earth terminal when rectifying a tn-s head replacement.

Infact some of the older properties that i have done inc shops etc had a 60a henly type block.

Regards Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 02 April 2007 07:21 PM
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alancapon

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Originally posted by: dbullard

. . . . had a 60a henly type block . . . .


Yes, but that is continuously rated current, not fault current.


Regards,

Alan.
 02 April 2007 07:26 PM
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sanjaysharma

Posts: 150
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Alan,

Thanks, I get it finally, the 100A relates to normal service not fault conditions presumably because these are not likely to be long in duration).

So a 100A Henley block would be OK as a MET?

-------------------------
Regards,

Sanjay.
 02 April 2007 10:09 PM
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windypick

Posts: 278
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sanjaysharma,

compare your henley block to the earth terminal in the CU then sleep well tonight.


windy
 02 April 2007 10:25 PM
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FizzleBang

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Originally posted by: sanjaysharma

Been trawling for info on what to use as a MET if the CU not suitable.



Khales suggests Henley block - I assume this is rated at 100A which would be inadequate under fault conditions.



Any suggestions?


I use these, made for the job.

http://www.tlc-direct.co.uk/Products/TLEB8.html

My local suppliers (small independant) have the exact same item for £2-50 inc VAT!!

It has two large terminals for your 16mm Main and either 4 or 8 terminals for your 10mm bonding.

Very sexy

Paul

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"I learned very early the difference between knowing
the name of something and knowing something". - Richard P. Feynman
IET » Wiring and the regulations » Main Earthing Terminal

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