Electrical stuff

Couple of wiki articles for your delectation and comments:

This one came out of discussions we have had in the past about wiring failures on rings and radials, and how each of them cope with the faults. I have added stuff on detecting, repair and test as well:

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some point in that article, I found need to talk about crimping cables. So thought that probably deserved a section on its own. So I did a pictorial tutorial:

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here on on the wiki discussion page as you prefer.

Reply to
John Rumm
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An excellent piece of work. Thanks.

One comment would be a slight expansion about soldering, mentioned only in passing, but of interest.

Reply to
EricP

stuff.

David

Reply to
Lobster

"John Rumm" wrote

Excellent stuff John - thanks for your efforts. One minor comment which may or may not be covered in the reference info is the fact that your "shorted plug" will knock out all ciruits protected by a common RCD. Or maybe it's taken as read that all ciruits will be isolated not just the subject one.

Phil

Reply to
TheScullster

Looks good. A few minor points/thoughts...

I would put a picture of the right tool somewhere near the beginning or the article, not just the wrong tool, and perhaps a pile of different size crimps. (You could add pliers, mole wrench, and hammers to the picture of wrong tools.)

"[screwed terminal] method is only acceptable where the joint will remain accessible for future inspection and maintenance, since screwed terminal connections can become loose over time." Please provide a citation for such a justification. (I don't believe that accurately describes the reason, and Wiki's need to be accurate when giving facts like this, or make it clear this is speculation of the author.)

In the sample damage you've shown, I would probably not have cut and rejoined an undamaged conductor.

A blue crimp is the wrong size for the earth conductor. You could use one if you folded the conductor double, or the crimp has no central barrier such that you can push the conductors through to overlap and crimp both conductors together. Whilst in practice a blue crimp probably would work OK as you have done, an article explaining how to crimp probably should get this right.

It's probably worth explaining crimp sizes and colour codes. Otherwise someone unfamiliar with them could well think they need a red one on the live, a blue one on the neutral and a yellow one on the earth ;-0

This is getting to the really minor nit level, but when I join T&E, I usually stagger the joins even more than you did, such that none of the crimps overlap at all. Then the cable thickness doesn't increase as much at the join, and it can if necessary be pulled through any holes with the cable intact. This is something which might happen many years later without gaining access the repair site (or even knowing about it).

This, like pipesoldering demos, is the bench demo. In real life (TM) you'll be doing it in some corner where you can't both feel and see what you're at the same time, you can't swing the crimp tool into the right position and get any grip on the handles, and there's only 2mm of conductor exposed from a 3' thick bomb shelter wall ;-)

Reply to
Andrew Gabriel

Oh and one other thing, practice on some cable off-cuts first, and try pulling your practice joins apart.

Reply to
Andrew Gabriel

very helpful. It left me with two small questions:

(i) wd it be helpful/possible to give a steer as to what size heatshrink is suitable for common applications (eg 1.0mm and 2.5 twin & earth)? I ask as TLC list 6mm etc heatshrink which left me scratching around to find out if this is diameter or radius or what; and then off to work out the circumference of "flat" cable. I think I got there in the end (eg

6242Y 2.5mm is approximately 9.5 x 5.25 so circumference rounds up to 30mm; heat shrink appears to be quoted by diameter so 12mm starts off at 38mm which should slide on and shrink nicely). But (a) I could be wrong and (b) even if I am right in principle, in practice........?

(ii) when doing a few bits of heatshrinking for car and other low voltage electrics over the years I found (in the days when I smoked) a cheap, disposable cigarette lighter a handy tool. Would one of them be ineffective (or otherwise unacceptable)?

Sorry if I am looking to be spoon-fed too much.

Reply to
Robin

One comment only - you say to use a purpose made heat gun to shrink the heat shrink sleeving. These are very expensive and an ordinary hot air paint stripping type can safely be used - just don't hold it stationary too close for a long period. A dual heat type on low is the ideal. Hold about 3 inches from the work and move from end to end until it shrinks.

Reply to
Dave Plowman (News)

I'd rather not use a naked flame for H&S reasons, although it can work ok.

I actually have the correct hot air gun designed for heat shrink but never use it as it's too slow. They are also pretty expensive. My preferred device is a cheap dual heat hot air paint stripper gun bought from a shed used on the low setting. It included a wrap round nozzle shield designed to get the hot air to the back of a pipe etc so ideal for awkward cables. I reserve this tool purely for this job and have another for general work.

Reply to
Dave Plowman (News)

Yup I too can push a button! ;-)

(although it would have helped to have three hands at times - next time I will take the tripod)

Reply to
John Rumm

That is a good point. I was trying to find a more "consumer friendly" way of testing that did not involve opening the CU and disconnecting wires there, and so came up with the plug based solution. Originally I linked just L to N, but later when writing the bit on finding CPC related faults the link to Earth came in handy. However that does connect N to E indirectly.

I shall spell that out a little more.

Reply to
John Rumm

I did (sort of) by linking to the TLC site. But yup, a piccie can be arranged.

Hacksaw ;-)

Which bit are you querying: that they come loose in the first place, or that being the reason they need to remain accessible?

I may lose the "before" picture since it adds nothing to the story I am trying to tell really.

I did think about this for a while, given that red is a better size match as you say. However if you look at the spec I listed for blue, it does show the minimum wire CSA as 1.04mm^2 which is acceptable on the

1.5mm^2 CPC in the 2.5mm^2 T&E I was illustrating.

Good idea sir, I shall nick that!

I think a table etc is a very good idea. (might swipe some piccies to go with it)

;-) Yup so do I normally!

Only having cut them a tad short and already taken a few photo's I CBA to start over.

BTSTGTTS.

Reply to
John Rumm

Yup, that's probably worth adding. (that was 12.7mm/ half inch tube I was using)

That sounds plausible. However each manufacturer may use a different way of describing it. Diameter does seem to be common though.

No, it will work ok. Its a bit harder to direct than the proper hot air tool, but I usually use a gas flame of some sort (a mini blow torch in most cases)

Not at all, not much use as a tutorial if it leave important questions unanswered.

Reply to
John Rumm

I did describe both (and I use a flame as well usually - or the catalytic blow head on my micro torch). I thought I ought to include the "approved" way as the first option, but you are right they are silly money.

Reply to
John Rumm

be circuit resistances (i.e. resistance of the phase-neutral loop) at 70 deg. C, as you would use for calculating voltage drop, or fault current calculations. In the context of resistance measurements made "cold" you need the figures at 20 deg. C, which are all in Table 9A of the OSG (p.

158). Also note spelling of "metre" :-)

You could refine the table by adding two more columns to give resistances for a single conductor, for the P-N loop (R1+Rn) and for the P-E loop (R1+R2) of twin & earth cables.

  1. Resistance measurements: the method with a 3-pole shorting plug is invalid unless you disconnect the relevant neutral at the CU. Remember that N & E are also shorted at the supply end, in the DNO's cut-out for PME and further back for TN-S, so there are parallel paths which will invalidate your resistance calculation. I see you do say "one should always ensure power is turned off at the main switch" but many will disregard that, taking it only as a safety warning and not realising the subtle effect it will have on the measurements.
  2. Disconnected CPC, ring circuit "circuit safety mostly unaffected": that's a rather sweeping statement and there will be cases where a dangerous situation exists, due to inadequate thermal protection for a CPC. The worst case is probably a broken CPC at one end of a long ring, with the old-type 2.5 T&E which only has a 1.0 mm^2 CPC and with a 30 A BS 3036 (rewireable) fuse. With an earth fault near the broken far end you might find the CPC protects the fuse and the cable catches fire; even if it doesn't, the disconnection time is likely to exceed 5 s, let alone 0.4 s, and the touch voltage other metalwork separately earthed or bonded to the MET will be over 70% of full mains voltage, so protection against indirect contact is not achieved.
  3. Ring continuity tests: the cross-connection method (per OSG, GN3 and our FAQ) should be mentioned. It's very convenient, having cross-connected the ends, to go round and measure resistance between L & N at all sockets. If all is well you should measure the same resistance value at all points. Ditto for cross-connection of L & E, except that the resistance will rise slightly toward the point furthest from the cross-connection, due to the higher resistance of the 1.5 mm^2 CPC.
Reply to
Andy Wade

Well spotted that man!

Indeed I did lift them from the wrong table (6E2 in OSG).

Is it just me, or do you spend ages looking for tables in that book that you know are there, but can never seem to find when you need them?

Probably worth doing a simplified version of 9A since the original resistance table I originally only planed to use on L/N round trip, since it does add other diagnostic options.

Yup, I had already reworded that a bit, but I agree it is still not clear about all the implications of not switching off.

Yes good point. I did some checks for modern cables and typical circuit lengths, and it usually hangs together. However older installs are going to be more vulnerable.

Yup, personally I favour that method, although for a simple test here it may get overly complex since we would need a fairly full discussion on the variations in readings one will get on spurs etc. (There comes a point where one is just duplicating swathes of the OSG!)

Reply to
John Rumm

I have had a hack about. See if you think that strikes a better balance with respect to safety on CPC faults. Also I have beefed up the resistance table (and used the right source data this time!)

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Reply to
John Rumm

Reply to
Andy Wade

John, a nice article on testing thanks. A few points:

  1. Since wiki is available world-wide should you say that you are referring to UK standards?

  1. Are you specifically targetting testing of sockets? If lighting circuits are involved then there are other issues like switches being on for testing (all combinations for 2 way switching), bulbs out.

  2. Views on other tools? Neon testers (love or hate them?) Electronic induction things (ditto)?

  1. Suggest you add that conductors should be checked before you touch them (Faulty switch? Wrong switch? Is it up or down for off?). That you should ensure that others cannot alter switches. That all circuits are isolated before you disconnect any wiring - even neutrals (in case of cross connected circuits).

  2. Is there a mismatch between the sort of meter you indicate and the sub ohm values which you refer to in your tables on continuity testing? To be honest you're probably only looking for an indicated value the same as shorted leads or for no needle movement at all and I can't think of many circumstances where that will not be adequate though it will be an issue if you are completing test reports. Maybe you need to acknowledge that such testing is on the limits of inexpensive meters?

  1. How do you test RCD? Is the test button adequate? A plug with a big fat resistance of 8K ohm between L and E? Mega expensive test meters which time the disconnect?

  2. How can you test that the earth to the house is okay? Continuity between N and E?

  1. A short glossary? CPC? RCD? MCB? T&E?

thanks again

Phil

Reply to
Phil B

I had taken that as kind of implicit since it does start off "Welcome to the uk.d-i-y wiki!", but having said, that there is no harm in spelling it out.

Power circuits were the main focus of the article, but lighting could certainly be added. (might be worth restricting it to power circuits at the moment)

I agree that stuff should be in the wiki someplace, not sure this particular article needs cluttering with that info though.

I have tried to spell out that the circuit needs to be off at the CU main switch before getting into any testing.

Yes that is worth adding.

Yes there is. The details on resistance checks grew a little beyond the scope of the basic continuity type tests that I was anticipating at the start of writing it. Hence the meter I liked to is not appropriate for some of the tests. (I specifically linked to an analogue type meter to avoid getting sucked into any problems with false positives you can get with voltage indications on very high impedance DMMs etc - however those issues have not really become relevant in the article as it stands)

I still want to try and keep it at a "useful to the end user" level rather than let it get too technical, but I think a bit more guidance on test gear is worth including. Also links to a reasonable DMM might be appropriate as well as a couple of pointers to real test gear.

In reality the resistance info for anything much beyond 2.5mm^2 circuits is not that relevant for most circumstances. However a reasonable DMM will usually give you enough resolution (down to the tenth of an ohm) to make reasonable use of the tables in more than a go/nogo way.

Slightly outside the scope of this article, although I have in mind a big article on RCD issues in general that would include testing, and tracking down sources of trips etc since that seems to be a frequent topic of discussion here.

The tests using a socket tester would probably highlight a gross failure of the external earth. However I feel that is getting a bit outside the scope again. (I want to try and avoid having one article growing and expanding into a universal document on electrical installations and becoming so dense that it is impossible to get basic information out of. I would rather have lots of smaller articles that focus on particular topics where this is possible).

Yup certainly, that is already noted in the discussion page as needed.

Thanks for that Phil, good points which I will try and incorporate.

Reply to
John Rumm

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