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Topic Title: correct torque settings for breakers and main switches
Topic Summary: a bit of help please
Created On: 12 April 2015 05:15 PM
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 12 April 2015 05:15 PM
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aligarjon

Posts: 3854
Joined: 09 September 2005

Hi guys. i have my annual assessment tomorrow and have gone a bit blank re CU torque settings for screws on circuit breakers and main switches. i can't find any paperwork for any of the CU's i have fitted and can't remember what i have to do them back up to. does anyone know off the top of their heads. i have a denmans designa board and an MK on the 2 jobs i am looking at. i think both boards are the same but not 100%.

thanks guys

Gary

i have 2.2 for breakers and 2.5 for mainswitches in my head but not sure.

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 12 April 2015 05:35 PM
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phantom9

Posts: 1757
Joined: 16 December 2002

No torque screwdrivers in my tool box Gary.
 12 April 2015 05:41 PM
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colinhaggett

Posts: 465
Joined: 08 July 2004

MK reckon max torque of 3Nm which seems rather low to me.

https://www.mkelectric.com/Documents/English/EN%20MK%20Technical%20Specifications/T27%20SENTRY+SOCKET%20Tech%20661-689.pdf
 12 April 2015 05:44 PM
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colinhaggett

Posts: 465
Joined: 08 July 2004

I also have never used a torque srcewdriver on a consumer unit. I often wonder how we managed before, perhaps this is the cause of consumer unit fires!
 12 April 2015 07:05 PM
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John Peckham

Posts: 8783
Joined: 23 April 2005

Different manufactures have different torgue settings for their various components as specified in their instructions.

For those of you without a torque screwdriver how are you complying with Regulation 134.1.1?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 12 April 2015 07:19 PM
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phantom9

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Joined: 16 December 2002

The same way as we did before.
 12 April 2015 09:28 PM
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Jaymack

Posts: 5359
Joined: 07 April 2004

Originally posted by: phantom9
The same way as we did before.

Yours will be the wrist action of a Barclays then!

Regards
 13 April 2015 07:51 AM
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phantom9

Posts: 1757
Joined: 16 December 2002

LoL J Mc. Spot on
 13 April 2015 08:42 AM
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JZN

Posts: 787
Joined: 16 November 2006

Probably is too low Colin. Any more and you'll strip the threads on their rubbish quality terminals. The conductor will still not be tight enough though and the house will eventually burn down taking the entire street and its residents with it. Oh! I know! lets just forget about that and make the units out of metal. Problem solved. I should be a Prof back at university me and sit on panels of learned bodies like the IET.
 13 April 2015 11:11 AM
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davezawadi

Posts: 3848
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134.1.1, I have no faith in the manufacturers instructions at all, since if one tightens the tails to the specified torque they fall out of the terminals with minimal movement! Its funny that this reg is different in amdt 3, perhaps the manufacturers want to take responsibility for these fires? I have had no difficulty with stripped threads and I certainly use more than 3Nm. My manufacturers instruction say check that all screws are "tight" and that is what I do.

When you are checking all the screws for tightness John, what torque do you use as the manufacturers instructions are long gone on an EICR? I hope that it is "tight"! Checking torque would require that all connections are loosened and then re-torqued to the manufacturers figure, which I consider to be asking for trouble as something will break at some point.

The recommended torque for M4 bolts of steel (grade 4.6, the weakest normal grade) is 0.95Nm and for M5 2.28Nm, which is much more than the "instructions" and there is no reason that more clamp pressure is a bad thing, in fact the opposite. So again I doubt the competence being expressed in these bits of paper. Because the contact area of a cage clamp is large, a high clamp force would be expected, thus a big screw size, fine thread and plenty of torque.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 13 April 2015 12:38 PM
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potential

Posts: 1632
Joined: 01 February 2007

Perhaps we need to wrap a spiral of solder (with flux) around the wire in the termination so if the CU catches fire the wire gets soldered into place.

So we can say:
Fire? What fire? Nothing to do with me mate.

(joke)
 13 April 2015 02:21 PM
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aligarjon

Posts: 3854
Joined: 09 September 2005

For the record the 2.2 and 2.5n/m i mentioned at the top were actually correct so it was obviously in there somewhere. so not as tight as some do already then. i appreciate different boards say different things. on a side, i am working on an ongoing job at the moment where i installed the CU last week, metal clad(light commercial unit). i tightened everything that was in there last week. i added a circuit this morning and was able to turn every screw that i did up last week, 1 of them half a turn.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 13 April 2015 03:32 PM
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phantom9

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I don't believe that there is sufficient information about this torque requirement. I come from an engineering background (steel bolts and all that). From an engineering point of view torque settings were to ensure that something was very tight indeed, a lot tighter than one could estimate as being tight enough. But, I don't know the purpose of the torque requirement for consumer units. Is it to ensure that a minimum tightness is achieved? Or is it to ensure that a maximum tightness is achieved? The engineering background suggests to me that it is a maximum. If you torque something tight it ensures it is enough for that particular requirement. But torque settings are best on metals that won't yield. High tensile steel bolts are very tough indeed and can be torqued to very high settings. Copper, on the other hand, is a relatively soft metal that is both malleable and ductile. If you work it it will compress and any torque setting will be quickly released as the copper squashes. I wonder if the people insisting on giving us these requirements really do know what they are doing. I question the validity of using a torque screwdriver on copper at all.
 13 April 2015 03:50 PM
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potential

Posts: 1632
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I question the validity of using a torque screwdriver on copper at all.

Absolutely right.
IMO torque screwdrivers and their settings are primarily to stop assemblers snapping off nuts, bolts or screws and has very little to do with a secure copper/brass/steel connection.
 13 April 2015 04:23 PM
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whjohnson

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Joined: 24 January 2009

Well said - I'm pleased to witness some voices of sanity here at long last.

I think we'd all agree that the numpties who came up with this requirement need to be sent on a course to study Materials Technology - This was a compulsory part of my old 1st Year Full-Time EITB course back in the mid-70's, not to mention the fact that we studied stuff like this at school - the theory part in Physics, and the Practical part in welding & metalwork.

Oh but of course, everything is dumbed down now isn't it?

For example - ICT courses are only teaching kids how to insert their SIMM cards the correct way round!

So much for so-called 'Progress' eh?

That said, if you really are interested in torque settings etc, the Dormer rep used to hand out small diary-sized books which contained drill/tap sizes, steel and other metals hardness grades, torque settings for different-sized nuts & bolts etc which was really handy to stick in the top pocket of your overalls. I wonder if such things are now posted online somewhere instead.

-------------------------
Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
 13 April 2015 05:46 PM
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Zoro

Posts: 300
Joined: 31 July 2011

Well if the majority of CU Manufacturers did not bother to be CE compliant with the plastic enclosure, why should we believe that the terminations are?

.
 13 April 2015 07:58 PM
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Jaymack

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Joined: 07 April 2004

Torque settings by manufacturers are useful, in that they can attempt to achieve uniformity in removing the physical disparity between differing users. As I've stated before, a screw thread simply relies on what holds your woolly jumper from falling apart ....... Friction! It is also quite simply a long wedge; this relaxes with expansion and contraction caused by temperature cycling, and possibly the residue from manufacturing lubrication. This relaxation can be halted by methods such as locknuts, but this not a viable option for such electrical screws.

A periodic retightening of screws in distribution boards and accessories, is recommended by routine inspections, particularly with those carrying higher currents. This would direct the solution to the root cause of the problem, and not to the expensive remedy of fire containment. "Too late the man cried"

Regards
 13 April 2015 09:36 PM
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whjohnson

Posts: 934
Joined: 24 January 2009

OK - I'll bite!

If we are going to capitulate to the manufacturers of cheap n nasty kit by way of agreeing to periodically re tighten their crap screws, how frequently should this be required?

And, how much should we charge for a screw-tightening session?

£50 a visit?

Say, every 7 days just to be on the safe side?

This way, the manufacturers and bean counters can continue to design their products down to the lowest possible standard they can get away with.

It strikes me that demanding that we all rush out and buy a torque screwdriver is akin to just adding another lifeboat to a leaky hull instead of mending the hole the ship's side.

-------------------------
Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.

Edited: 13 April 2015 at 09:47 PM by whjohnson
 14 April 2015 07:38 AM
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BillI

Posts: 94
Joined: 26 August 2006

I recently bought a set of torque screwdrivers. Why? I'm not an electrician.
Well long ago as a trainee I was on the roof of the Royal Mint Refinery with a fitter servicing a regulator. We had put the regulator back in its tank and I was left to refit the cables. Tightening up the last stud it came off in my hand. Fortunately the fitter was able to switch things about a bit and repair the situation. Back at the factory I was laughing stock of the week. Not long afterwards I broke off another stud on a transformer I was testing. More laughter! This time I was presented with a specially cut half-length spanner. I never did that again at work. However at home I was still breaking things from time to time - things like cylinder head bolts. So when I discovered torque wrenches I bought one - not for tightening up correctly so much as to make sure I didn't overtighten. The screwdrivers are for the same reason.

As for these dreadful CUs it seems to me they were made to suit Continental situations and, perhaps, the simple answer is to replace the tails with continental more flexible ones. If JP's advice about squaring off the tails is to be followed instead then I think the CU manufacturers should provide electricians with suitable swaging pliers FOC.

Of course really there should be a recall.

I wonder what will happen if/when this gets into the mainstream media and people start to realise what it will cost them.

Bill
 14 April 2015 09:16 AM
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Jaymack

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Originally posted by: BillI
Tightening up the last stud it came off in my hand. Fortunately the fitter was able to switch things about a bit and repair the situation.

When I was the electrical engineer for 2 steel rolling mills, I organised a maintenance schedule for the whole site; this included MV motors and verifying the incoming connections etc. An assistant engineer, was called out by the shift electrician in the early hours for a motor that wouldn't start, and just humming; he diagnosed the problem as a faulty main bearing on the rolling stand for the cross country mill.

I arrived in the morning and went to the mill, the fitters (some on callout), had spent 6 hours and still hadn't finished removing the bearing. Since the symptom could also mean a faulty motor, I asked the electrician to remove the cover on the connection box. One of the main connections to the motor was obviously sheared off, whether at the time of the recent inspection or not, it was a case of checking these large brass stud connections using a spanner and a hammer. We are probably all familiar with the use of torque meters for tightening car cylinder heads etc., but rarely think of other uses, but the manufacturer's torque instructions should be rigorously followed.

Regards
Statistics

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