On 20/05/10 10:30, Dave Plowman (News) wrote:
In ,
Tim wrote:

In my personal experience, once in the last 2 years. Broken neutral in a
ring. Would never have noticed if I hadn't followed the standard testing
regime.

You'd have found that with any continuity testing device - like a two quid
DVM.

I did say that. I also said you cannot test an RCD properly with a DVM
too (as an aside on testing, not to do with pyro).


I'm very much *not* against testing. Just pointing out that for a small
DIY job you don't need to rush out and get a megger as suggested.

There are many classes of DIY job. The ones that could be tested with
nothing more than a DVM could include small scale PVC internal works.

Ones I think are best tested with the correct gear (at least R1+R2,
end-end ring and RCD checks) would include new circuits with any
complexity, including rings and lighting (the impedance tests *are*
useful for discovering bad terminations), *anything* outside and pyro.


IMHO the vast majority of likely faults on a simple installation will be
shown up with a DVM.

Except RCD tests. They do go wrong and it isn't totally impossible that
a faulty one has made it onto the shop shelves (note recent protective
device recall by well know manufacturer).

And relying on expensive test equipment to guard
against poor workmanship is the wrong way round.

It is still a very good idea.

For example, if you
changed a plug, would you feel it necessary to have that device PAT
(tested)? It's the same sort of thing.

No, because it is *one* set of terminations that I can inspect manually,
and putting the back of the plug on doesn't disturn them in the same way
that putting a socket back into the wall does - that is where I had my
broken neutral. All 6 conductors survived a pull test, the wires are
neatly looped so as to collapse in a helical fashion when the socket was
returned into its 35mm backbox. And the bloody wire still fractured. I
suspect the screw had bitten it too much or something.

A clear case where the most careful workmanship in the world still
resulted in a fault.

Re the plug, also I can visually inspect the lead. I still do a rough
and ready appliance test on some of my stuff, eg external cement mixer,
extension leads, Class 1 second hand stuff: (500V IR with Megger and
earth loop impedance test, and R1+R2 as well for extension leads).



But, what if the RCD was out of spec? That does happen and it's rather
harder to test those. Pyro, especially if it is terminated for the first
time by an inexperienced person, AFAIC *must* be tested, if if the test
is gerry rigged. Pyro has a known failure case (mositure) that is
considerably more likely to occur compared to leakage in a T+E
installation.

Damp pyro will show up on a DVM. I've checked this.

You should have said before. That is worth knowing, in which case the OP
should at least do that.

Have you?

No, because as yet I have had no reason to use pyro - but I am well
aware of its failure modes and *I* wouldn't be happy unless I'd checked
them. You're happy because you've done loads before.

However,
it's nothing like as common as some suggest. If bought from a reputable
seller, just terminating it will remove any ingress of moisture. Stored
outside or in damp conditions for a considerable time would be a different
matter.

Which is entirely possible with cable of unknown origin (ebay).

And what would be the result of leaking pyro? A fire? Explosion?

Compound fault on non RCD circuit (entirely possible):

Earth fails + L-E leaky.
Pyro sheath and connected metalwork now live.

Of course, the earth failing is always bad, and compound faults are very
rare, but it has been known for people to die due to one in a million
compound faults.

Why take the risk - eliminate that which is known to be reasonably possible.

Do you
have your own installation regularly tested for faults that may develop
after installation?

Yes - I do it myself...

Seems to me these are equally as likely as
installation ones.

With respect, I don't care if *you've* done thousands of pyro
terminations - that means you have a feel for what's right. Newbies
don't. I wouldn't either, so they and I ought to be testing the stuff.
And IMHO *we* (uk.d-i-y) ought to be promoting good practises Not
saying "********" to someone who is doing so...

I dislike someone saying 'you mustn't do this' unless there is a very good
safety reason. With pyro in particular there isn't.

Basically, I'm saying don't let the pros who are terrified by pyro put you
off having a go DIY wise if it's what you want. Just practice first and
take care. As it says in the FAQ.



--
Tim Watts

Hung parliament? Rather have a hanged parliament.