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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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  • 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

    Point "R1+R2"
    1 (CU) 0.167
    2 0.395
    3 0.412
    4 0.418
    5 0.412
    6 0.395

    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

    point R1+R2
    1 (CU) 0.167
    2 0.306
    3 0.390
    4 0.418
    5 0.390
    6 0.306

    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)

  • in reply to OlympusMons

    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.

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  • in reply to OlympusMons

    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.

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