Terminal Fittings - How tight should they be ?

[WhatNext]

Evening

When I was removing some MK double sockets ( new house only 3 years old ) I found that the terminal screws to the live, neutral and earth cables were very tight ?

In some instances the terminal screw on the live connection stripped and so I had to cut the cable away so that I could removed the socket.

I understand that the latest edition of the regulations recommend that a torque screwdriver is used, however I also learned that many of the socket manufacturers do not specify a torque figure ?

Could anyone please advise is it possible to overtighten the terminal screws to the point where you can potentially cause a problem ?

In my case the live terminal always seemed to be the one which stripped - does the flowing current cause the fittings to tighten up through time ?

Many thanks.

 

[EFLImpudence]

They could indeed have been corroded or burnt stuck rather than too tight on the wire.
It is possible to overtighten connections and damage the screws - some aren't very well made.

I think it's just down to experience and/or common sense how tight you do it.
Some of the recommended torque values do seem a bit loose.

 

[JohnW2]

In my case the live terminal always seemed to be the one which stripped - does the flowing current cause the fittings to tighten up through time ?

I suspect that's down to psychology, with some people's minds thinking (incorrectly) that it is more important to tighten the line/live terminal - which is also probably the reason why, as often discussed here, thermal damage due to terminals not being tightened enough seems to much more commonly affect neutral terminals!

Exactly the same current goes through the line/live terminal and the neutral one, so (if they were both initially tightened to the same torque) any effects of current-related heating would be the same in both.

For components such as MCBs/RCDs etc., for which specified tightening torques are specified by manufacturers some (but not all) of us (including myself) are a bit concerned about how loose the terminals are when tightened to the specified torque. I am aware of no regulation which explicitly requires use of a torque screwdriver, although there is a requirement to 'taken into consideration' (without any compulsion to comply) with any instructions issued by a manufacturer.

Kind Regards, John

 

[flameport]

Could anyone please advise is it possible to overtighten the terminal screws to the point where you can potentially cause a problem ?

Unlikely, doing so would result in the copper wire being cut through and therefore falling out of the terminal.

If the screws are damaged when removing, the screwdriver being used is probably the wrong size.

 

[333rocky333]

Only ever use MK, I am quite heavy handed, with mk I would have thought your more likely to break half the head off than sheer the thread putting them on, next time i remove a broken one I will try.
must admit the brass quality lately seems poor and screwdrivers dont seem to fit snug these days it seems a struggle to fit them lately.

I cant see how you could sheer thread taking one off, unless it was originally forced in, which is unlikely as they use captive screws with them already started, or the terminals have somehow overheated.
I have never encountered such a problem

 

[333rocky333]

Unlikely, doing so would result in the copper wire being cut through and therefore falling out of the terminal.
.

Im sure i read overtightening was just as bad as them being loose

 

[JohnW2]

Im sure i read overtightening was just as bad as them being loose

I must say that I would think it unlikely that would be the case unless, as flameport said, one tightened so much as to break the conductor.

As far as I am aware, copper is not significantly 'compressible', although it is easily distorted by pressure ('squashed'). I therefore doubt that overtightening would significantly decrease the CSA of the conductor at the point of termination (which is another potential problem that some might think about).

Kind Regards, John

 

[WhatNext]

Hi

Many thanks for all your answers - much appreciated.

What I found was that I could not move the live terminal screw at all which eventually resulted in the head of the screw becoming stripped, having never moved !

 

[333rocky333]

Best I could find though its american.

http://r.search.yahoo.com/_ylt=A9mS..._145.pdf/RK=0/RS=5wYZI80WPGrqAtYZHInTycVThDo-

"The wording of the Code is usually taken to imply that
terminations are to be tightened to the value (if any) that is
marked on the equipment. But that is implicit rather than
explicit. The practice has become increasingly common.
Back in the spring of 1985, a leading electrical trade publication
ran this headline over its lead editorial: “Torquing
electrical terminals—soon, it will be mandatory!” For five
years, the magazine had been advocating the practice for
“all current-carrying electrical terminals and joints,” to
achieve “a precisely determined value of contact pressure.”
Either “inadequate” or “excessive” tightening was said to
account for most electrical failures."

 

[Iggifer]

MK terminal screws are rubbish now. They're not particularly strong, you need to use a bigger driver than will fit through the plastic to get a good grip on them

 

[JohnW2]

Best I could find though its american....

Interesting, although I'm not sure how much weight should be put on opinions expressed in a "leading electrical trade magazine", in any country!

Kind Regards, John

 

[333rocky333]

Interesting, although I'm not sure how much weight should be put on opinions expressed in a "leading electrical trade magazine", in any country!

Kind Regards, John

I will do some research
maybe it in the sales gumph with the torque drivers in the wholesalers

 

[JohnW2]

I will do some research maybe it in the sales gumph with the torque drivers in the wholesalers

One certainly has to consider such questions as who are the 'sponsors' of a magazine.

I recall that it was several decades ago that I was first disillusioned about such matters. A very well known and respected monthly publication had a regular section called "Jane Doe Recommends", or something like that (can't remember her name, even if I wanted to), and each month she would 'recommend' a number of products - I think mainly health-related, and perhaps also cosmetics and various household goods. In my naive youth, I had taken that all at face value. However, I was then exposed to the situation from the commercial side of the fence, and found myself looking at a 'price list' of what it cost to get Jane to 'recommend' one of one's products in various ways!

Kind Regards, John