Wiring Matters Article and GN3

Indeed, a nice clear article - though I suspect in a lot of cases in the field grotty old sockets and slightly tarnished leads also introduce similar random errors when trying to check if changes have been done correctly to an existing circuit, especially measuring a plug in test lead. The cross-over test will always find gross errors like an open CPC but may not find more subtle problems, like a short length of a different cable size or a short fig 8 loop - equally things that cannot be detected are also unlikely to be dangerous.

Mike.

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. 

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!

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.

mapj1 said:

though I suspect in a lot of cases in the field grotty old sockets and slightly tarnished leads also introduce similar random errors when trying to check if changes have been done correctly to an existing circuit, especially measuring a plug in test lead.

Agreed ... but not something we'd expect too much of on initial verification, and on many installations, a measurement at the rear of the socket-outlet (if you are fault-finding), unless there's a lot of detritus and corrosion, gets over the issue of dirty switches and socket-outlet contacts.

Resistance readings for Step 3 of the ring final circuit test - Electrical (theiet.org)

Steve Briggs said:

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

This had me doing some calcs. Using Graham's circuit and adding a 10m spur to each point i get

Note these are Graham's figures for the cross connection plus 0.167 for the spurs (except at CU where the ring doesn't count)

If calculating the actual parallel resistances of R1 and R2 at each point and adding the spur resistances i get

Which are a fair bit lower away from the centre than those derived from Method 3.

(I put R1+R2 in quotes in the first table, since other than at the mid point, we aren't really measuring R1+R2 of the circuit, just points on a loop)

Electricians are normally not equipped mentally or practically to deal with sub-Ohm values. Especially considering that when croc clips are squeezed they can alter their resistance values substantially.

Look...............www.youtube.com/watch

Z.

The correct calculated (R₁+R₂) values are the ones in your lower table - actual parallel resistances - because the cross-connection (or "figure 8 connection" or "double loop connection") is not present once the circuit is restored to its proper configuration.

It just so happens that, at the mid-point of the ring, the resistances for the cross-connection and parallel connection have exactly the same value of (r₁+r₂)/4 at that point.

Hence, the maximum value of (R₁+R₂) at the end of the longest spur at the mid-point of the ring is still (R₁+R₂)_(max) = (R_1spur+R_2spur) + (r₁+r₂)/4 but of course you could, theoretically, have longer spurs part way round the ring, and a starting impedance at the point of connection of the spur to the ring being much lower, but an overall higher Z_s.

All this complicated stuff to catch the unwary Sparks. It is just so difficult to visualise. It confuses apprentices no end. I hold by the old adage: If yer ends show continuity all is a goodun. It has held good for me for many years, and for my father, and for his father before 'im, well until his accident anyway.

Z.