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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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 14 April 2015 10:41 AM
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davezawadi

Posts: 3848
Joined: 26 June 2002

I think you are trying to apply magic to torque settings J, because they are not the magic thing you are saying, and whilst they can prevent accidental breakage of highly stressed threads, that is all. Torques are not used to tighten highly stressed components because they don't measure anything much. If you lubricate the bolt the stress will be about twice the unlubricated figure, and it is this stress which says if the bolt will break or not. A bolt with oxidation in the thread (a heat cycled terminal bolt, for example) will need several times the clean bolt torque to get the same bolt stress. So your car head bolts (and many other highly stressed components) are tightened to some low torque, and then rotated through a known angle to tighten, which gives a very accurate and predictable bolt stress. However you can only do this once because the bolt will yield slightly and next time the stress will be significantly higher, and these are sometimes called stretch bolts as a result. They must always be discarded if removed and replaced with new ones, and not doing this is a common reason for failure of second hand cars head gaskets and engines in general as the bolts get failure due to yield or fatigue stress cycles which are much larger than as designed. Using a torque wrench will prevent you breaking otherwise sound bolts and studs, but will not give a known contact pressure under various circumstances.
If you are stripping threads, this is due to incorrect material choice as much as you being careless, and many electrical items seem to use very low strength threads, presumably to save a few pennies. Brass bolts breaking as described are likely to be due to metal fatigue, or fretting corrosion as much as over tightening, brass has no fatigue limit so even low cyclic stresses will eventually cause failure. As the bolt should not carry much of the current it is difficult to see why they are used, perhaps it is the pretty colour (when new). Grade 8.8 steel bolts will allow adequate contact pressures without any risk of failure, although the "nut" strength may then be the limiting factor (nut threads are much stronger than bolts as the diameter of the root is greater, so the stress is less).

That is basically why I don't believe in this torque "instruction" it might work once when everything is new and shiny, but then its a lottery.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 14 April 2015 11:08 AM
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Jaymack

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Originally posted by: davezawadi
I think you are trying to apply magic to torque settings J, because they are not the magic thing you are saying, and whilst they can prevent accidental breakage of highly stressed threads, that is all.

Torque settings by manufacturers will have safety factors! What would you recommend, as the means of verifying tightness, whilst avoiding overstressing?


Regards
 14 April 2015 12:29 PM
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davezawadi

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Well J the problem is not simple but one which trained and experienced electricians have managed very well for a long time. The thing is whether the wire is firmly held and doesn't move in the terminal when you move the rest of the wire. 3Nm might do this on some days, but a few experiments with a bit of 25mm tails cable and a main switch will convince you that it is completely inadequate. This is completely untested politics being used to divert attention from an underlying serious problem.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 14 April 2015 12:59 PM
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Jaymack

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Originally posted by: davezawadi
Well J the problem is not simple but one which trained and experienced electricians have managed very well for a long time.

So you have no solution then to my question - "What would you recommend, as the means of verifying tightness, whilst avoiding overstressing?" All we had from you is a dubious lecture on the shear stress of metals! which is in itself - "politics being used to divert attention from an underlying serious problem".

Regards
 14 April 2015 03:47 PM
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aligarjon

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Originally posted by: Jaymack

Originally posted by: davezawadi

Well J the problem is not simple but one which trained and experienced electricians have managed very well for a long time.


So you have no solution then to my question - "What would you recommend, as the means of verifying tightness, whilst avoiding overstressing?" All we had from you is a dubious lecture on the shear stress of metals! which is in itself - "politics being used to divert attention from an underlying serious problem". [IMG][/IMG]



Regards




How about experience, feel and common sense.

oh and decent materials to work with.


Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 14 April 2015 04:01 PM
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Thripster

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What may have changed since the days when all terminals were tightened by hand and without the use of a torque wrench?

(i) Lower quality terminations with single screw fastenings?
(ii) Increased current demand with the advent of high power electric showers?
(iii) Fewer apprentice trained fitters?

A few to think about. Let's ban the hot water dispenser as the hot water might burn. Cold tea anyone?


Regards
 14 April 2015 04:07 PM
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Jaymack

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Originally posted by: aligarjon
How about experience, feel and common sense. oh and decent materials to work with.

Evades the issue of the need to routinely determine the required torque, other than by a subjective feel. A test should be applied at routine intervals, irrespective of the make! Temperature cycling doesn't know which make is involved!

Regards
 14 April 2015 06:59 PM
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phantom9

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No Jaymack, that's fantasy and you know it. It is all well and good saying routine checks should be carried out. This is the real world Jaymack. They aren't. There is far too much mention of what should be done but who is going to do it? I have zero confidence in EICRs. I have lost all confidence in the codes because everyone uses their own made up reasons and I have no doubt that using a decent correctly sized screwdriver type on a terminal screw allows much more judgement to be made as to whethr a termination is tight than relying blindly on a torqued termination from a device that is only as good as the user anyway. You appear to be placing faith in another requirement that is flawed from the outset. Torque settings are useless on copper.

Do you truly believe what you are saying, Jaymack, or just playing the usual forum game of refusing to admit you could be wrong? I won't be getting a torque screwdriver they are potentially dangerous. That's a C2 isn't it?

As an aside I was very naughty recently. I totally stripped down a service head! I managed to find a few turns on the incoming neutral but the line was solid tight. I also found a few turns on both the meter tails line and neutral out of the service head. How often are the DNO out 'routinely checking' their terminations? They never do. Pure fantasy. Thanks to my tenacity that is one service head in the entire UK that has been checked recently and the installation is much safer for it. I didn't have a torque screwdriver and didn't need one.

Edited: 14 April 2015 at 07:05 PM by phantom9
 14 April 2015 11:54 PM
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mapj1

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But if you have a torque guage, and bother to tighten a typical terminal to the right torque - or the wrong one for that matter, but just note the angle of the head associated with the end position. Now measure the torque to break free, and the torque it now reaches when it back at that angle again.
It is simply never twice the same. Actually its often not even within a factor of two. Torque is very unreliable as measure, and low torque in particular.
How do you prove a year later that at the time it was actually 'torqued right'.
It is almost impossible. What is really needed is a spring backed element - like the WAGOs that get away from this ramp wrapped round a stick altogether, and if that is not possible, at least a spring loaded screw.

-------------------------
regards Mike
 15 April 2015 12:43 AM
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stateit

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I found my torque driver again today in one of the dusty bags at the bottom of my my van.

My problem with using them will be solved when manufacturers stop using seemingly mismatched screws and busses where the threads don't match.Any single phase board at the moment seems to have little parity between the screw that you screw in, to the thread in the busbar receiving it.

It makes replacing well built 15+ year old boards seem an insult.

I noticed recently BG had addressed that insult in a mild way by having their screws coated in threadlock, but then I also thought that may be just to stop them rattling loose and getting lost in transit (which made a change from not being able to tighten them up at all)...

-------------------------
S George
http://www.sg-electrical.com
 15 April 2015 03:30 AM
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Jaymack

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Originally posted by: mapj1
But if you have a torque guage, and bother to tighten a typical terminal to the right torque - or the wrong one for that matter, but just note the angle of the head associated with the end position. Now measure the torque to break free, and the torque it now reaches when it back at that angle again.

Hopefully not in a working situation.

It is simply never twice the same. Actually its often not even within a factor of two. Torque is very unreliable as measure, and low torque in particular.

That's not disputed and applies to whether or not a gauge is used, or a subjective feeling is appied.

How do you prove a year later that at the time it was actually 'torqued right'.

What would you suggest?

What is really needed is a spring backed element - like the WAGOs that get away from this ramp wrapped round a stick altogether, and if that is not possible, at least a spring loaded screw.

Stealing my thunder there from another post! That's not going to happen unless the standards are changed; but wait ........ they already have, but by containment rather than prevention!.

Regards
 15 April 2015 03:48 AM
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Jaymack

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Originally posted by: phantom9
No Jaymack, that's fantasy and you know it. It is all well and good saying routine checks should be carried out. This is the real world Jaymack. They aren't.

Well now, that really is an admission!, They should be carried out routinely, after entering domestic consumer units etc. You also need to adopt a means of measuring your wrist action.

Regards
 15 April 2015 03:50 AM
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Jaymack

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Originally posted by: phantom9
As an aside I was very naughty recently. I totally stripped down a service head!

That explains a lot!

Regards
 15 April 2015 06:54 AM
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phantom9

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Originally posted by: Jaymack

Originally posted by: phantom9

As an aside I was very naughty recently. I totally stripped down a service head!


That explains a lot! [IMG][/IMG]

Regards


What does it explain then? When you can't explain your reasons satisfactorily you resort to insults
 15 April 2015 07:00 AM
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phantom9

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Originally posted by: Jaymack

Originally posted by: phantom9

No Jaymack, that's fantasy and you know it. It is all well and good saying routine checks should be carried out. This is the real world Jaymack. They aren't.


Well now, that really is an admission!, They should be carried out routinely, after entering domestic consumer units etc. You also need to adopt a means of measuring your wrist action. [IMG][/IMG]

Regards


No it isn't an admission, it's a fact. How many appointments do you have booked in this week with your customers to carry out their routine checks on the tightness of screws in their consumer units? And how much do they pay you for the privilege? Answer that big man.

As a point of fact I do check the tightness of all terminations when I have the front removed from a consumer unit. That is different to what we are all telling you but you won't accept it. There is no routine in place. Carrying out checks on the one's you do is not routine checks that just being conscientious. I would even suggest that few sparks even bother with that because its too much trouble to spend a few minutes doing it. Your world is fantasy and manufacturers recommendations. Our world is reality and what actually happens in practice. No doubt you are going to disagree again.
 15 April 2015 09:14 AM
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davezawadi

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My discussion of torque was not dubious Jaymack. You never know, it could be useful to someone. You may refer to a textbook if you wish to discuss the points raised.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 16 April 2015 09:35 AM
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phantom9

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Do any of you use the special slotted/pozidriv hybrid screwdrivers known as ADYB? I have found these to be superior to the standard pozi and Phillips head for breakers and dp switches terminals. The screw heads in consumer units have this hybrid pattern.
 16 April 2015 12:16 PM
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Angram

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How can you use a torque screwdriver on multiple solid strands inside a cage clamp?

How can you use a torque screwdriver on a badly fitting tight screeching thread on N & E strips?

Torque settings are essential for wheel nuts and buzz bars but can be a serious fire risk in consumer units.

Too any screw threads in consumer units and Henley blocks start out loose and become stuck before they bite into copper.

Copper is soft and will always relax away from a screw end over time.
You can come back and tighten into the copper for ever or until you eventually screw right though the copper.

Mechanical restraining of conductors to stop them moving about after first tightening is the best option which is being used for tails in some CUs.

Tactile feedback when tightening screws into copper is essential for every experienced spark. That way you can detect a binding thread and, with a gentle waggle of a conductor detect movement of multiple conductors under a screw which would make them loose and need re-tightening.

Don't over tighten. Do comply with the regulations. Don't cause a fire.

Angram.
 17 April 2015 12:30 AM
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mapj1

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If you mean by "special slotted/pozidriv" - don't most folk just have a set of modulo or "plus and minus" drivers for MCBs ? Or are you referring to there being yet another tool I need to buy ?

-------------------------
regards Mike
 17 April 2015 07:49 AM
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phantom9

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Yes plus and minus. No need to buy any other tools.
IET » Wiring and the regulations » correct torque settings for breakers and main switches

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