PAT visual checks, or How To Wire a Plug

[Bob]

I have recently done the C&G courses for Portable Appliance Testing. The IEE Code of Practice for In-service Inspection and Testing of Electrical Equipment latest edition 2003 gives details of the visual checks to be applied to a standard plug after removing the cover. These include checking the cord anchorage, cable core terminations etc. On the practical part of the course the instructor also emphasised that the relative free lengths of conductors was very important. The Earth conductor should have the most play, followed by the Neutral, then the Live. The reason being that if a user pulled the cable, with maybe the cord anchorage a bit loose, the Live conductor would become disconnected first, followed by the Neutral, then the Earth. This would seem to be sensible, as the Earth connection would be maintained up to the last point. The CoP book (para 14.5) does not mention this, although the diagram does show the Live having the shortest amount of free play. If I am inspecting equipment, how do I treat a plug which has all conductors with the same free length of cable, or say the Earth has the shortest length? Do I fail it? Do I rewire it?

[Andrew Gabriel]

Personally, I'd pass it if they're all the same, and fail it if the live has significantly more free. I might also be influenced by how likely the cord is to pull out of the cord grip.

Whether you try to repair failures depends on what you were tasked with doing, but that's a different activity from testing.

By the way, an interesting excersise is to work out what the safest order of cable pulling out is on a trailing socket...

[Dave Plowman (News)]

Yup. I was taught this at the BBC technical operator's course (ahem) years ago. But in practice none of the wiring diagrams you get on replacement plugs ever show this.

Most of my Hi-Fi stuff is on IEC connectors to save space. With most, this means hacking off the moulded plug. But some things still come with removable ones. And I've never seen one wired with the correct 'loop' in the earth wire - they're always just cut to the exact length.

[:::Jerry::::]

But surely that is, in effect, the exact opposite to what was suggested by the course tutor, due to the placements of terminals inside the plug would mean if (the core grip failed) your suggestion would result in the positive terminal failing last.

You answered that one in your answer above !

[Bob Eager]

Lucky, then, that no mains plug these days *has* a positive terminal...!

[Joe]

In message , Andrew Gabriel writes

Err, the cord grip isn't there to stop the cable being pulled out. Sufficient force, particularly tripping over it, will pull any cable out. The grip is there to prevent lateral movement of the internal cable, and hence fatigue breakage of conductors at the pins. (BBC again...)

Tighten the grip too far and it will break the conductors under it. I've seen intermittent cables caused by this. Flex it near the plug and it comes and goes. Open the plug, the screws are tight, but pull on the live and neutral conductors and one will slide out of the sheath. Before this stage, the cable would have heated up at this point through reduction in area. Often some of the broken ends look burnt.

[:::Jerry::::]

I see you didn't offer any answer, just the usual snide remarks....

[Stefek Zaba]

I've always reasoned it's the same as for a plug: live shortest, so that a yank will cut power (and prompt investigation), neutral slightly longer, and E with a loop so that any casing etc. stays earthed "longest". I s'pose you could argue swapping the lengths of N and L, so that a "small" yank will provoke investigation (thing stops working, L still reaches but E continuity still in place).

I'm pretty sure the idea of E-longest is/was documented in something fairly widely accessible in the d-i-y/simple-trade electrics line: Reader's Digest Repair Manual (mid-70s vintage ;-)? or something reliably Regs-based wot I read not much later? Don't think anything as sensible as this figures in current pubs, since we live in the world of pre-moulded plugs by and large...

Stefek

[Jeff McGhie]

Surely having Love as the shortest on a socket wouldn't be a good idea as a 'yank' would leave a live end floating around, possibly outside the casing ? From memory on most trailing sockets, Earth is the shortest, possibly relying (nowadays) on the fault tripping the RCD ?

[Peter Parry]

It's called syllabus drift- where instructors veer away from what they should be teaching to what they "know". Invariably it's codswallop and relates to half of a quarter of what they think their foreman told them in 1953 when they were an apprentice.

Quite a few plugs these days are designed to have all conductors cut to equal length.

It wouldn't.

[nightjar]

Some plugs are designed to have the conductors all cut to the same length, to overcome the problem of people not reading the instructions or getting the relative lengths wrong. However, the way that I read Andrew's post is that he would pass the plug if the amount of play was the same for each, which does not imply equal lengths. I would certainly agree with that.

Colin Bignell

[Malc]

No I don't think so. If the live has more free then it gets pulled out last which is a fail, which is what Andrew suggested nicht war?

[MBQ]

There might not be any other fault and a missing earth alone will not trip the RCD. I would rather discover the fault through having the Live disconnect than wait to test the RCD (*if* there is one) when a real fault occurs.

MBQ

[Capitol]

At the end of the day, who cares which wire is longer? This whole thread is about as useful as a navel contemplation exercise. A properly clamped cable will not pull free, if it does, then the clamp is faulty. Which wire is disconnected is irrelevant, as the chance of actually electrocuting someone is pretty close to zero! This is another example of ISO9000 standards, which achieves nothing apart from ticking boxes.

Regards Capitol

[nightjar]

The whole idea dates back well before ISO 9000 or even BS 5750. It comes from practical experience in the electricity supply industry of what has killed people in the past. You cannot assume that the cable clamp will be fitted properly, if at all, and if the earth wire disconnects first, leaving the other two connected, there will be no reason for the average user to suspect a fault. However, a fault to earth within the appliance will make the case live, rather than tripping the circuit protection.

One reason that moulded on plugs are required on new equipment today is that, historically, amateur electricians have managed to find so many different ways to kill people with rewireable plugs. I was once bought a kettle that was not working, only to find that the person who had fitted the plug had connected the earth wire to the live terminal. Fortunately the owner had only held the metal kettle by the insulated handle.

Colin Bignell

[Dave Plowman (News)]

When but a kid, I bought a small mains radio at a house sale - roupe - which was described as not working so got it for near nothing. On getting it home and needing to change the plug anyway, discovered the wires wrapped round the cover fixing screws.

[Andy Dingley]

Or equally likely, it's a badly made plug with poor quality threads on the terminal screws. If you want to stop your wires pulling out, start by using a decent plug !

[Andrew Gabriel]

I don't believe _moulded on_ plugs are required, just that appliances are supplied with a fitted plug, unless there's a more recent update to the plugs and sockets regs which I haven't seen.

[Andy Wade]

Correct. Chapter & verse at