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Topic Title: To tin or not to tin
Topic Summary: Whether to tin stranded conductors in screwed terminals
Created On: 27 February 2006 05:58 PM
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 27 February 2006 05:58 PM
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biomed

Posts: 4
Joined: 06 October 2002

This all started when at work I examined a 13A plug as part of an electrical safety check on a new piece of medical equipment. As the equipment was imported, the supplier had cut off a Euro plug and fitted a new MK 13A plug. I noticed they had tinned the stranded conductors of the three wires before inserting them into the screwed terminals.

I was taught this is bad practice for two reasons. One, because solder is soft, it is too easy to over-tighten the connection and sever the conductor. Two, although tight when first done, over time the solder displaces away from the screw by a cold-flow process. Both these problems can lead to an intermittent connection, possibly on the earth connection, leading to a safety hazard.

When I contacted the supplier, their technical manager said he had been taught at college to do this. This set me wondering are there any regulations or standards which state the correct technique? I did contact MK Electric who although they agreed with me that stranded wires in plugtops should not be soldered, they could not quote any reference. Anyone any ideas?
 27 February 2006 06:05 PM
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lbelectrics

Posts: 1035
Joined: 26 October 2004

I'd agree. Tinning is counter-productive if screw terminals are used.

Soldering/tinning of stranded conductors is normally done to stop fraying before the cables are terminated or to avoid untrained persons being injured by stray strands.

Best wishes

Lee



-------------------------
Regards

Lee
 27 February 2006 07:24 PM
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biomed

Posts: 4
Joined: 06 October 2002

Thanks Lee, I'm pleased you agree but can anyone give me a standard to quote?
 27 February 2006 11:00 PM
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mazza29

Posts: 48
Joined: 12 October 2005

Have to admit that when I studied Electrical & Electronic servicing ( C&G 6958) - I was also taught to tin stranded cables in a 13A plug! Talked to my present tutor (now studying 2330) he says that although not against ant regs it is good practice!
 27 February 2006 11:00 PM
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Jimoldham

Posts: 1754
Joined: 29 November 2004

can i ask you to think about what would happen to stranded cores in a plug top teminal without Tinning or an oversleeve ferrule?

-------------------------
Regards

Jim Oldham

Edited: 27 February 2006 at 11:42 PM by Jimoldham
 27 February 2006 11:34 PM
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Platypus

Posts: 465
Joined: 20 January 2005

My father (electrical engineer) was adamant that stranded wire should not be tinned for the second of the reasons mentioned by biomed - namely the solder is soft and will flow, possibly leading to the screw working loose.

IME, loose 0.2mm stands don't actually clamp down under a screw that perfectly either, some dive off to the side or fracture (Jim's point, unless I've missed the point of his thought provoking question completely).

The theories seem to go both ways on this issue. I'm not that certain which one I think is better as both seem to have theoretical downsides, but I never tin stranded wire because I've never had a problem not doing so.

Ferrules would be better than tinning - strand containment, no fracturing - but not made of tin/lead either so avoiding the flow problem.

Personally I'd much rather the screw teminals used clamping plates (as occasionally seen on some DIN rail terminal blocks I've used in electronics projects) - much better idea.

I quite liked the MK plugs with the mushroom screws - seemed to make a much more convincing mechanical connection. Having said that, I've put hundreds of normal plugs with simple screw terminals on appliances (most of them when I was a trainee technician yonks ago for the EE Dept at Reading Uni - "here's the new trainee! 'ere son - see that big pile of 'scopes and that large box of plugs...") and none of them ever caused me any problems, so despite their shortcomings, they do seem to be "good enough".

There is aanother definate issue with tinning, though it doesn't apply to using them in plugs so much. If there is any regular flexing of the wire,
a lot of stresses are set up in the copper strands immediately adjacent to the tinned section - and the wire is more liable to break at this point.

Found that out the hard way soldering wires into PCBs without tying them down to a fixed point, especially if the circuit board wasn't enclosed. Like the mains direct-charged Xenon flash circuit I made when I was 15 or so. And tested on the dining room table. My poor mother, bless her soul...

Cheers

Tim
MGenPlebs (ie don't take my advice for your 3 phase genny installation)

[Edited for awful typos due to lack of sleep caused by daughter honking her guts out most of last night]

Edited: 27 February 2006 at 11:46 PM by Platypus
 28 February 2006 09:28 AM
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deleted_sludkin

Posts: 7
Joined: 30 September 2005

Ooops.

I've just replied to the other thread.
 28 February 2006 08:30 PM
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biomed

Posts: 4
Joined: 06 October 2002

Thanks for all your replies. Most seem to agree with me that tinning the strands can cause more harm than good.

I agree loose strands can be a problem, but if the strands are twisted together and bent over that stops them straying. The last part of the process should be to carefully check everything is OK. Ferrules would be the best answer but they add another task and most householders wouldn't have the tools. I too used to like the mushroom style terminals MK used to use, but alas, they have now gone, probably as a cost cutting exercise.

If no-one can come up with a standard the next step may be to ask the IEE, after all I am a member!
 01 March 2006 12:19 AM
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Jimoldham

Posts: 1754
Joined: 29 November 2004

Whether you tin or not will not give any protection to the screw crushing or cutting any strands of the core. I would offewr that by tinning you increse the conductivity of the CSA and by tinning you increase the CSA therefore lowering the resistivity of this ection of the conductor. the connection tightening down and cutting through the core conductors would/would not happen anyway. tinning stranded conductors has alway been a belt and braces way of ensuring a decent electrical connection on any size core or busbar connection. I have done this sort of connection on anything up to 3"X 1/4 copper bar down to 1.0mm starnded core.

-------------------------
Regards

Jim Oldham
 01 March 2006 10:27 AM
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deleted_mikestaunton

Posts: 84
Joined: 10 May 2005

Hi,

I've got an electronics background and in general the problems I have observed is that far to much solder has been applied. The amount of solder should be such that you can still see the strands of the conductor!

Mike
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