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Wiring Matters Article and GN3

Steve Briggs

Good article by Graham Kenyon in the latest Wiring Matters.

Interestingly (to me) I wrote this same thing about four years ago to try and explain to our instructors why they do not see the promised variation at the sockets in Step 3, a supposition which has been around since the early days of the [proper] 2391.

The article suggests changes afoot to GN3 and I just wondered if anybody had yet proposed that Step 3 really ought to be the same as for a radial circuit, Method 1. The current method returns artificially high values of (R1 + R2) for any spurs not at the midpoint and misleads students into expecting near identical values of Zs at each point.

And while I'm on it, is there any chance that the numerous recurring oddities/errors/inconsistencies in GN3 might be addressed in the next edition, or will we just have to continue explaining them to trainees as we go along? Wink

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  • I find it easier to explain by stretching the ends of the phase conductor into a straight line with the socket terminals marked along it and then do the same with the cpc but the opposite way round. The figure eight links are then connected at each end. It then becomes clear that the test is made with two distinct resistances in series in a dual parallel branch.

    In GKs example the test at the origin would comprise 60m of 4mm in parallel with 60m of 1.5mm. At the mid point it would be 30m of 4mm in series with 30m of 1.5mm and both in parallel with the same values also in series. At position 6 in GKs example, it would be 10m of 4mm in series with 50m of 1.5mm both in parallel with 50m of 4mm and 10m of 1.5mm in series. 
    As to the OP’s point about irritating mistakes in technical documents like GN3, it was ever thus but it seems to be more prevalent recently. There is even a mistake in GKs article where 2.5mm2 copper was mistakenly given a resistance of 4.61mohms per m. 

  • in reply to lyledunn
    lyledunn said:
    I find it easier to explain by stretching the ends of the phase conductor into a straight line with the socket terminals marked along it and then do the same with the cpc but the opposite way round. The figure eight links are then connected at each end. It then becomes clear that the test is made with two distinct resistances in series in a dual parallel branch.

    Definitely. I think the problem is, that some of us can see that more easily than others.

    lyledunn said:
    There is even a mistake in GKs article where 2.5mm2 copper was mistakenly given a resistance of 4.61mohms per m. 

    Thanks Lyle, this is, of course, a situation where the '2.5' should be '4.0' as the example uses 4.0/1.5. I've asked for this to be corrected.

    lyledunn said:
    As to the OP’s point about irritating mistakes in technical documents like GN3, it was ever thus but it seems to be more prevalent recently.

    A lot goes on behind the scenes to try and address this, and not just by authors. Try as you might, it's really difficult to get them all.

    A similar situation also exists with standards themselves - you see occasionally even those that are a few pages long (well,of real technical content) have Corrigenda issued.
    I'm looking at one issue with a standard at the moment, that's been published for about 10 years, where there is an unintended error in a formula, which alters the result by a factor of 1000!

  • in reply to gkenyon

    I sent in a few pages of observations long ago, and then again, four years ago, via Mark Coles, ahead of publication of the 18th. Not a single point was addressed, even in the form of a rebuttal. It's a shame, really, as it would be nice to strive to make GN3 a better publication all round.

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  • in reply to gkenyon

    I sent in a few pages of observations long ago, and then again, four years ago, via Mark Coles, ahead of publication of the 18th. Not a single point was addressed, even in the form of a rebuttal. It's a shame, really, as it would be nice to strive to make GN3 a better publication all round.

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