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Topic Title: Guidance on terminal torque
Topic Summary: Is there any?
Created On: 14 February 2013 06:03 PM
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 14 February 2013 06:03 PM
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DR2366

Posts: 705
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Was looking to see if I can find a definitive answer on this but no joy yet.

There is an article in this months PE P64 but its just an advert on torque wrenches.

Has there been an article in wiring matters or similar? Can anyone point me in the direction of some reference material?

Many thanks.
 14 February 2013 06:18 PM
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DR2366

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I just found this.

Torque settings List

yet more info

Edited: 14 February 2013 at 06:26 PM by DR2366
 14 February 2013 06:25 PM
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Legh

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2.3 Nm for main switch contacts and 1.7Nm for everything else.....
Seems pretty straight forward to me

Legh

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 14 February 2013 06:27 PM
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DR2366

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So you have a torque wrench then?
 14 February 2013 07:04 PM
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alanblaby

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ELECSA did a download of different manufacturers torque settings. It wasnt simple, or clear. I've still got a copy if you want it.

On the same subject, I fitted a MK CU today, on the instructions were the torque settings for the CBs and RCDs, but it said see manufacturers data for the tails. That's really helpful!
 14 February 2013 07:19 PM
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ebee

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If we can`t tell by skill & experience (and waggling sufficiently) then we need a torque wrench.

Norman,
I bet you don`t use one.

I remember being with a group of skilled engineers in the seventies, when it was announced they could not measure to within a tenth of a thou accurately without the ratchet on their micrometers being calibrated and doing three clicks they walked away, none of them even used the ratchets and their work was more than accurate.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 14 February 2013 07:49 PM
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MrOther

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1. Select correct tool.
2. Tighten as much as possible, if possible leaning some wieght into it.
3. Wiggle cables/gently but generously stress connections.
4. If all okay: sound as a pound.
 14 February 2013 07:56 PM
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OMS

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2. Tighten as much as possible, if possible leaning some wieght into it.


Nope - we are electricians - a good wire man can feel the bite of the terminal against the copper - it's all in the wrist

Over-tightning terminals cause more damage than under-tightening - try a google for "creep" and "plastic deformation" and "Cold flow" or "elastic modulus" wrt to metals (particularly copper) for a bit of background.

Regards

OMS

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 14 February 2013 08:00 PM
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dickllewellyn

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The royal "we" (ECA) have been trying to get manufacturers to answer. The regulation effected is manufacturers instructions at present. Ie. if they say you must tighten to a specific torque, then that is what you must do. In that respect, it is hoped an answer can be obtained from manufacturers as to what torque settings apply, and for the information to be printed in all literature, and on the products themselves. Some manufacturers are responding and doing so gradually, others are slow on the uptake.

The outgoing chairman of our branch has a son in the insurance industry who has given examples of claims where a consumer unit has been taken away in one piece having cut all cables entering in order that it may be looked at under lab conditions.

Other insurance companies have also been contacted to give an opinion, and the general consensus seem to be that a claim may not pay out if it can't be shown that work has been carried out to manufacturers instructions (see above). Where it would be difficult for anybody to prove terminals were not tightened to the correct torque, the theory is that it is down to the installer to demonstrate how they would ensure the correct torque during instalation. That means owning and using a torque setting tool, and ensuring said tool is calibrated.

There are still many questions within the ECA with it bouncing back and forth from branches, through regional and council etc, and it is thought that the next run of ECA instalation certificates will include a column to specify torque settings.

All that said, it is now starting to creep into other areas too. Lutron dimmers specify a terminal torque, as do some contractors, relays, time clocks etc.

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Richard (Dick)

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 14 February 2013 10:50 PM
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MrOther

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Originally posted by: OMS

2. Tighten as much as possible, if possible leaning some wieght into it.




Nope - we are electricians - a good wire man can feel the bite of the terminal against the copper - it's all in the wrist



Over-tightning terminals cause more damage than under-tightening - try a google for "creep" and "plastic deformation" and "Cold flow" or "elastic modulus" wrt to metals (particularly copper) for a bit of background.



Regards



OMS


You missed out the most important part - we are "professionals" - so we make that choice based on understanding and knowledge, sometimes just knowing, as you say that "it feels right, that's tight enough."

So if you do a terminal nice and tightly (okay, little cables you need to be careful of, especially say, T&E is a fave to flake, but generally I work with 4mm2+) you leave a little slack on the cores, dressed in appropiately so that heat and all the other stress tangible and in tangible don't take too much adverse of an affect (as they will, naturally, always have some affect.)

I usually use Merlin Gerin, a 8 pozi-driver usually fits (I think) do it up tightly, then lean in slightly just for that last turn, that double up cable would need an elephant tied to it to pull it off.

Which inveriably what happens on building sites when I turn up to find a 100A panel has been removed from the wall and sub circuits (SWA) pulled from their glands. Then they wonder why I throw a hissy.
 14 February 2013 10:51 PM
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MrOther

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Originally posted by: OMS

2. Tighten as much as possible, if possible leaning some wieght into it.




Nope - we are electricians - a good wire man can feel the bite of the terminal against the copper - it's all in the wrist



Over-tightning terminals cause more damage than under-tightening - try a google for "creep" and "plastic deformation" and "Cold flow" or "elastic modulus" wrt to metals (particularly copper) for a bit of background.



Regards



OMS


I'll have a searh now OMS - thanks.
 14 February 2013 10:52 PM
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MrOther

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Cannae find anything for cold flow.
 14 February 2013 11:35 PM
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Jaymack

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I don't see a need for torquing to limits, I think it will cause more problems than it will solve. Thermal cycling will always be present, this is probably the real reason for relaxation of the screw friction.

One guy I knew in South Africa bought a new Volkswagen combi, months later he had a puncture and couldn't budge the wheel nuts, because they had been overtorqued on assembly in the RSA factory!

Regards
 14 February 2013 11:47 PM
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DR2366

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I think that once the insurance companies get involved then it won't be long before we all will be having to record the fact that things were tightened correctly, and if required prove it in court just like complying to BS7671.

If insurance companies can shift the blame to the installer then I think they will.

Buy shares in tamper proof seals!
 15 February 2013 06:07 AM
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ebee

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The old Wylex consumer units with 16.0 tails, two screws per terminal each screw end a dome used to be pretty easy to tighten correctly.

Thicker tails 25.0 & up, flat terminals one screw each, you gotta tighten them then waggle waggle waggle then re-tighten and re-waggle .
Use a torque wrench if you must but if you do not waggle then forget it.

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 15 February 2013 07:53 AM
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stateit

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OK then, for those that have them / have used them:

The Wiha set or the Scneider set?

They seem to use a different system. What's the most 'user friendly'?

Be nice to have some guidance before shelling out a 100 cockles on a set

TIA

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S George
http://www.sg-electrical.com
 15 February 2013 08:01 AM
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dickllewellyn

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The Schneider one is made by wiha. The actual torque handle is identical. Where the Schneider one has a long shaft on the bits to go into the torque handle, the wiha has an adapter to replace the long shaft and uses shorter bits. This allows a second non torque handle to be used with the wiha bits. On that note though, I along with a few others I know have had the adapter or the non torque handle fail with the mechanism that holds the bits in. In that respect I think I prefer the one they let Schneider license.

I had the Schneider one first (albeit before Schneider branded it so it was an early wiha kit), and then bough a non torque wiha set (slim vario I think it's called), and managed to get the rep to throw in an adapter so I had the best of both worlds.

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Richard (Dick)

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 15 February 2013 08:11 AM
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stateit

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OK... Thanks for the info.

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S George
http://www.sg-electrical.com
 15 February 2013 09:58 AM
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OMS

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Originally posted by: MrOther

Cannae find anything for cold flow.


It's akin to "creep" - which essentially the rate of change of the material dimensions over time when exposed to a force at a given temperature.

Cold flow is a similar idea but it's permanent deformation due to the force being appied and doesn't vary with time - it's a function of the "hardness" or elastic modulus of the material.

So as you tighten a teminal you cause a cold flow - ie you deform the wire - and then that deformation continues due to the effect of the force and heating effect over time. How hard or soft the conductor is (ie the elastic modulus) depends on how much you deform it.

For cable conductors, we also have a thermal expansion rate.

Aluminium has 35% more thermal expansion than copper, it also has significantly more creep (and at much lower temperatures) - you would need to have the copper running at about 150C to experience the same rate of creep as aluminium at 20C

Aluminium also has a plastic modulus approx half that of copper - ie it's soft

Based on all that, is it any wonder BS 7671 allows copper cables down to 1.0mm2 CSA but won't accept aluminium conductors less than 16.0mm2

Of course, as I said, an experienced wire man knows all this - he just doesn't know the theory - but he most certainly understands the physical implications of it, instinctively usually.

So the rule is - tight enough, not as tight as you can make it.

Regards

OMS

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 15 February 2013 09:58 AM
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tomgunn

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Quite frankly I really do think that the world has gone even madder! I have never heard of such nonsense - theres no way that anyone could use a torque wrench and get the same results each time - for small screws its simply impossible and anyone who does this must leave feeling that they have completed a good job but we all know that the screws will never be 100% tight and that any good sparks would always automatically tighten up all the associated screws inside a CCU when altering / upgrading it - its just common sense for us! Oh well.... wot next - perhaps you'll have to supply the client / contractor with a cert for each screw as to what tightness was achieved? Duh - dont tell me, theres already a law for this and the punishment would be 12 years hard labour watching telly in a max security prison? Or worse still - you would have to complete 10 hours of watching, and supervised you understand, a DVD while strapped into a sturdy chair, of Michael McIntyre !

Tom - dunno wot else to say - seems like my bloods draining away each time I read things that some wally has come up with!

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