What's the best way of wiring a screw-terminal plug with flex?

[unopened]

Hi all, it's not meant to be a silly question.

What's the best way of wiring a screw-terminal plug with flex?

I don't think solid-core flex exists - in any case all the flex I've come across has conductors composed of multiple small strands of copper wire. How best should they be used in a plug with the standard hole in the pin with a grub screw to screw down?

Some commercially supplied equipment has plugs where the end of the conductors is tinned, which makes it easy to insert in the hole in the pins, then seems* to provide a good connection when the grub screw is tightened up.

(* I say seems, as there is some debate about the dissimilar metal interface providing a good, long-lasting contact)

If I don't tin the conductors, then what is the best way?

1) Stuff the stranded core into the hole in the pin, and tighten up the grubscrew? 2) Twist the stranded core, put in the hole and tighten up the grubscrew? 3) Twist the stranded core, fold back on itself and re-twist, place in hole and tighten up grubscrew? 4) Use some other method?

I ask because I'll be wiring up a lot of screw terminals in the near future and I'd like to do a good job that I don't have to come back to.

I did know an American at university, now a high power lawyer, who approached the unfamiliar English electrical system in an interesting way. Having bared the conductors, the flex was stuffed into the plug and wiggled about until 'it worked', then left alone. I was speechless, but then spent a brief period rewiring all the plugs in the said lawyer's rooms.

Many thanks,

Sid

[Adrian]

I'd advise against tinning the flex ends.

One company I worked for did this on some equipment that was drawing reasonably high currents at 110V - and then had to send an engineer to the States (all the way from Washington to the Mexican border) to replace these leads - as the tinned conductors had a tendency to deform over time. As they deformed, they heated up - which made them deform further.... and so on....

Narrowly avoided getting sued over it !

So - don't tin the flex ends...

I've always twisted the ends and then tightened the screw down - doubling it over on itself if it's particularly thin flex...

Not sure what the 'official' way might be though

Regards Adrian

[Andrew Gabriel]

You mustn't do this. Back in the days when appliances were supplied like this, the instructions always said to cut the tinned part off. It was there for easy connection for testing on the production line. Using it in a screw terminal results in the solder creeping, loss of contact pressure, and a poor contact forms over time.

The right way to do this is to crimp a bootlace ferrule onto the strands -- if that's already been done, leave it on if the cable has been stripped the right length for the plug.

2 or 3, depending on the size of the conductor relative to the hole.

Consider buying bootlace ferrules and an crimping tool for it (sadly the tool is very expensive).

Can you change the lead? You can buy leads with molded plugs very cheaply in quantities.

[unopened]

Thanks all for the replies.

The strip, twist, fold over (if necessary), insert in hole and screw looks like the way to go. As you say, the bootlace ferrule crimp tools are expensive, which is a pity. The cheapest are the plier style ones, which I know from experience don't give a good crimp - I can't justify a proper tool for the one-off occasion. Unfortunately, it's not just a case of buying another set of IEC flexes with moulded plugs, which I'd prefer.

Some of the terminal connections are likely to be the screw with washer type as well, where you loop the conductor round a screw under a washer, then screw down tight. I hate doing those. At most it's 54 of them, so I guess I'll be a dab hand by the end of it. I'd be tempted (if I had the appropriate tools) to use an uninsulated ring crimped onto the conductors for those because they are so fiddly otherwise, but I suspect the crimped connection wouldn't be secure enough unless it were made with the proper (horrifically expensive) tools.

Cheers,

Sid

[Lurch]

On Mon, 23 Jul 2007 10:19:24 -0700, [e-mail] mused:

What is the lead you're making going to be then OOI?

[unopened]

It's a host of BS 546 2A plugs and a smaller number of standard (BS

1363) plugs that are hard-wired (not IEC connectors/sockets) to bits of lighting and kit that are being moved in a friend's room rearrangement exercise. I just wanted to make sure we do things properly. I suppose that in an ideal world, every corner of the room would have enough sockets for the hi-fi, video, dvd, satellite, X-box, and latest toy-du-jour, but his house was built before the explosion in consumer electronics.

Cheers,

Sid

[Graham]

You will probably cringe at this confession Andrew, but sometimes, if I am making a connection to something like an in-line switch or connector I fold the conductor strands over the PVC insulation and tighten the screw hard down on the insulation. My thinking is it gives more mechanical strength than just the copper alone should the outer sheath slip through the main clamp.

I suppose I developed this technique years ago when there was a lot of imported crap fittings with fibre cable clamps that were not up to the job.

[Lurch]

On Mon, 23 Jul 2007 21:22:53 +0100, "Graham" mused:

I have found a few CE marked items with terminals wired like this.

[Dave Plowman (News)]

They are tinned to allow quick and easy testing at the factory. The tinned ends should be cut off before fitting a screw connection plug - the tinned end has no 'spring' to it and is more likely to work loose.

Twist the strands reasonable tightly with your fingers. If the hole is large enough double over.

[Ragnar Bartuska]

Not really - I found a nice crimp tool along with a set of different ferrules for less than 10 Euros. I don't think screwfix or any other UK store doesn't carry those for a similar reasonable price.

Tinning was done for ages in many European countries... and was the origin of several fires as I was told.

greetings Ranger

[Graham]

Best example is lampholders that dangle from a flex.