Talking of plugs

I think people are more careful about tightening the live. In plugs without an earth wire, people often forget to make sure the screw is tight, and vibration can undo the screw completely. This is usually the reason for a plug which rattles.

Strain relief ?

Had some work done recently, and the contractors left a 'failed' heavy duty extension cable on a reel. As part of that failure it had melted one of my sockets, and apparently taken out an industrial vacuum.

Some months later I got round to having a look before binning it. Inspection revealed both nuetral and live cable screws were rattling about inside the plug. Screwed them back, checked for continuity, and the extension seems to work fine. Plug otherwise unaffected.

Well, the live usually has a bit of wiggleability being attached to the 'loose' fuse holder, but the neutral is more rigid.

Yes. If you buy a new plug, the packaging or label usually shows the required lengths to ensure that they pull out in the right order and are not too long to fit comfortably.

Of course, many times we a re-using old plugs and just have to estimate the lengths.

Is no one using the strain relief ??

Yes, but that's not to stop the cable pulling out of the plug. If somebody heavy, moving quickly, trips on the cable, it *will* come out no matter how hard the strain relief is tightened. Indeed, the cable coming out may save the person from a fall, if the cable goes slack they may be able to recover.

The strain relief device is there to prevent movement of the cable becoming movement of the wires at the terminals. You tighten it until the wires don't move when you waggle the cable fairly hard. Movement of the wires will cause work-hardening of the copper strands, and breakage.

Over-tightening the strain relief is also a fire risk. Too much pressure on the wire strands inside the cable will also cause breakage. If an appliance dies, and the plug fuse is OK, pull on the live and neutral wires. If they stretch, or the copper comes out completely, the strain relief was too tight. Measure the distance back with the detached wires and you will see the break happened under the strain relief. This isn't theoretical, I've seen this actually happen three or four times. If the appliance is a high-current one, then before the last strands break the cable will overheat.

Yes, I also worked for the BBC and was taught this. And Clive didn't mention it, but you cut the live wire to have no slack inside the plug, the neutral to have a bit of slack, and leave the earth wire as long as will physically fit. That way, they break in the order live, neutral and earth when the cable is pulled out.

This. This a million times over.

Or power cables with pre-fitted moulded on plugs ?

There are two types of strain relief that I have seen in UK mains plugs. Either two tabs of plastic arranged in a V that points towards the live pin and so resists the cable (inserted between the tabs) from pulling out of the plug. Or a flat piece of fibreboard, with a slight upward kink, which is tightened against the cable with a screw either side.

I presume you are describing the second type since the first type has no adjustment. I've found that the fibreboard is usually so flexible and already kinked to fit around the cable that even with the screws fully tightened it still allows movement if the cable is tugged really hard. I suppose that meets the need of providing a bit of slack to prevent someone tripping over.

I have vague memories of once seeing a rather unusual plug where the three screw terminals were arranged in a line (with a copper strip from each terminal to its pin) so all three wires were cut to the same length/ This didn't allow for live to pull out first, then neutral, then earth. It made it easier to wire up the plug at the expense of not causing earth to be the last wire to pull away from it pin.

Something harder than fibreboard, IME.

Are they not compulsary on new UK equipment?

Many are actually solid plastic and not the old "fibre board" these days.

Plugs do get damaged, cables get damaged, cables need to be extended, plugs need to be removed to be fed through the back of the cupboard or through a worktop. All require a re-wireable plug to be fitted.

Fitted plugs (not necessarily moulded-on) are compulsory on domestic equipment.

But that is satisfied if e.g. the supplied lead has a europlug, which has been fitted with one of those screw-down 13A adapters, ignore the word shaver in the URL

Some suppliers think it's OK to just chuck a travel adapter in the box, but I think that's not deemed sufficient? Still happens though ...

Ideal application for your 3D printer. Print new clamps

Pre-fitted yes, moulded on no... .. which is why I get peeved when someone says:-

Q. "why doesn't any one know how to fit a 13amp plug" A. Because they have never needed to do it...

Dave

It's what my father taught me over 50 years ago. No idea where he got it from.

Andy

I've never seen an appliance with a pre-fitted mains plug that can be opened and removed/rewired after feeding the cable through small hole in a wall etc. In my experience, pre-fitted is always moulded.