Soldered cable ends?

[patrick j]

Hello

Over the Christmas period I replaced all the pendant lights in my house. They all had shades I didn't like and those shades were screwed in to a lamp holder which wouldn't take the replacement shades, so I changed the lamp holders of course. These new lamp holders are pretty standard "British" ones which have been used here in the UK for many years.

In the past when I've been putting in any new electrical fitting I've always put some solder on the exposed cable which I'm going to screw into the fitting. I've done this when wiring plugs for example.

However with these MK lamp holders it says in the instructions:

Carefully strip the inner cable insulation to expose 10mm of conductor. If using an existing cable with soldered ends, they should be trimmed back and re-stripped to expose new clean conductor. On no account should soldered ends enter the terminals.

So it looks like MK believe that soldered cable ends are a bad thing and of course I did not put solder on the cable ends.

I am sure MK are quite right in their advice but I would be grateful if someone would explain to me why soldered cable ends are a bad thing.

[Andrew Chesters]

The theory goes that the solder will 'flow' over time and the joint will loosen. Once the joint starts to loosen, there will be an increase in heat, softening the solder further and a vicious circle ensues.

Personally, I strip slightly more insulation, twist and double back the conductor. Andrew

[OG]

"British" ones

At a guess it's because with soldered ends don't 'give' when you tighten the screw.

1 - you could end up with contact just along the two opposite sides of the solid cable, rather than letting the cable compress to give a larger area of contact. 2- Also if the soldered end was roughly elliptical in section the same lack of 'give' could allow you to tighten the screw onto the wider axis and it would seem to be firm; but if the cable rotates it could come loose giving arcing.

[Colin Wilson]

Perhaps some sort of reaction between dissimilar metals ? (can`t remember the phrase)

[a]

this is the explanation I have heard, and I twist and double back too :o)

[[news]]

yup

that's the reason I snip the tinned ends off any electrical appliance supplied without a plug. I only tin copper wires when I'm soldering, i.e audio cables.

RT

[Andy Burns]

I seem to remember it only being usual to 'tin' stranded cable, not solid core, also that tightening a tinned cable in a screw-connector could tend to make the solder dry or crystalline, possibly giving high resistance (and therefore heating) or even sometimes forming a partial diode?

[paul]

Don't know if it true but someone once told me that a soldered connection has a much higher resistance than a crimped connection.

[Roger]

On occation I've found the soldered part under the terminal screw can fracture entirely.

IMO it would be ok to solder just the tip of the loose ends to stop them splaying out, but making sure the part under the screw terminal was free of solder. I've seen this or the ends crimped into a brass ferrule on some manufactured retail products, presumably to speed assembly and ensure no odd whiskers cause shorts.

Roger

[Andy Wade]

Because soft solder creeps under the compressive load of the screw, resulting in a loose connection after a period of a few months or so.

Re-visit some of the ones you did earlier and the chances are that you'll find that they're all loose.

MK are right; tinning the ends is very bad practice. If you want tidy wire ends use crimped bootlace ferrules, or crimped ring terminals as appropriate.

[Stefek Zaba]

Dead right. The reason some wires come pre-tinned (and plugless in some cases, equipment construction regs notwithstanding) is that it makes final test easier for the manufacturer. Tinning the ends uses kit already in the factory, at minimal consumables cost too compared to a bootlace or similar. Popping the non-straggling tinned ends into a spring-loaded source of 240VAC/110VAC is quick'n'easy (no whiskers of cable to cause shorts). Cutting off the tinned ends is an extra process which the volume contract manufacturers won't undertake unless explicitly told (and paid) to do so...

Stefek

[Mike Harrison]

Solder is very soft, and has no springiness to maintain contact pressure over changes in things like temperature. This will be a particular issue for lampholders, which undergo significant temperature cycling.

[fredalan]

What do you mean by pretty standard British?

Andrew Chesters wrote:

"British" ones

grateful if

thing.

[Andrew Chesters]

I don't mean anything by it! The OP probably means a standard plastic BC pendant holder. Why?

Andrew

[Steven Briggs]

In message , patrick j writes

Yes MK are correct. As others say, the solder is soft and creeps over time leaving a loose joint. If you've been in the habit of soldering your wires, then I suggest you go round every item in your house and redo the plugs etc without applying solder. I suspect you'll find several loose ones if you do this!

[patrick j]

On Wed, 5 Jan 2005 16:57:15 +0000, Andrew Chesters wrote (in message ):

Hi

As the OP I will confirm that is indeed what I meant :)

I don't know what the correct terminology is for that particular item.

[patrick j]

On Wed, 5 Jan 2005 17:56:03 +0000, Steven Briggs wrote (in message ):

Thank you and for all the other replies to my query. Fortunately I have done hardly any plugs or any electrical work in the house I now own so it is good that I now know putting solder on cable ends is a bad idea :)

From here on in I will strip slightly more insulation, twist and double back the conductor.

[Clint Sharp]

In message , patrick j writes

One reason I remember reading was that it's because the solder 'flows' under pressure and you end up with a loose connection. Soldering a flex connection is always a bad idea where there may be movement anyway because the solder can contribute to a stress fracture in the cable, it's quite common to see cables fail on cars where a 'technician' has soldered a Lucar connector on.

[Dave Plowman (News)]

It's also common on old cars to get a high resistance crimped Lucar connector.

[Andy Luckman (AJL Electronics)]