PAT visual checks, or How To Wire a Plug
 

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?

On Thu, 4 Nov 2004 19:46:52 UTC, ":::Jerry::::" wrote:

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.

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


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"Bob Eager" wrote in message
...
On Thu, 4 Nov 2004 19:46:52 UTC, ":::Jerry::::" wrote:

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.

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


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

Andrew Gabriel wrote:

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

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

"Stefek Zaba" wrote in message
...
Andrew Gabriel wrote:

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

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).

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 ?

On Thu, 4 Nov 2004 18:35:24 -0000, "Bob" wrote:

On the practical part of
the course the instructor also emphasised that the relative free lengths of
conductors was very important.

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.

The CoP book (para 14.5) does not mention this,

It wouldn't.

--
Peter Parry.
http://www.wpp.ltd.uk/

":::Jerry::::" wrote in message
...

"Andrew Gabriel" wrote in message
...
.....
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.

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.

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

"Jeff McGhie" wrote in message .. .
"Stefek Zaba" wrote in message
...
Andrew Gabriel wrote:

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

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).

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 ?

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

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

"Capitol" wrote in message
...

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.

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

In article ,
nightjar wrote:
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.

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.

--
*Why isn't there a special name for the back of your knee?

Dave Plowman London SW
To e-mail, change noise into sound.

On Fri, 05 Nov 2004 23:09:58 +0000, Capitol
wrote:

This whole thread is about as useful as a navel contemplation exercise.

it's Usenet !

properly clamped cable will not pull free, if it does, then the clamp is faulty.

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 !

In article ,
"nightjar" writes:

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

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.

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.

--
Andrew Gabriel

Andrew Gabriel wrote:

I don't believe _moulded on_ plugs are required, just that appliances
are supplied with a fitted plug, [...]

Correct. Chapter & verse at
http://www.legislation.hmso.gov.uk/s...41768_en_1.htm

--
Andy