Disaster - maybe

[empip]

...... If the readings are different, this will indicate the presence of a bridge or may be due to incorrect connection of the ends of the ring. If they are connected P1 to NI and P2 to N2 then readings will increase or reduce as successive measurements round the ring are taken, as is the case where a bridge exists....

http://www.tlc-direct.co.uk/Book/8.4.2.htm

 

[Softus]

Okaaayyy, but I got the impression from b-a-s' post that in a non-fault situation the readings will increase, and that it's normal to record the highest reading in the range.

Is that not what happens then?

 

[Adam_151]

Okaaayyy, but I got the impression from b-a-s' post that in a non-fault situation the readings will increase, and that it's normal to record the highest reading in the range.

Is that not what happens then?

Its too do with the fact that T&E cable has a reduced CPC

Imagine a ring of 10 units of length total (and we will say the cable has 1 unit of resistance for each unit of length on phase + neutral, and 1.5 on the earth), if we cross connect at the board and put the ohm meter across at a S/O we will be measuring the resistance of 2 ten unit long lengths of cable wired in parralel,

If we are in the middle of the ring, we are seeing 5 units of conductor A in series with 5 units of conductor B all in parralel with the same. If we are a quater way along, we see 7.5 of A, together with 2.5 of B, and thats in parralel with 2.5A + 7.5B.

As long as A and B are the same resiatnce per unit length, it doesn't matter but if they differ, and the readings will increase towards the centre of the ring, just to work it through quickly

A is the phase, B the earth, (2.5x1)+(7.5*1.5) = 2.5+11.25=13.75
(2.5x1.5)+(7.5*1) = 3.75+7.5=11.25

Pop them in parralel and we get 6.18

and for the centre

(5*1)+(5*1.5) = 5 + 7.5 = 13.5
the other half is exactly the same and so you get 6.75 in parallel

If it was phase and neutral instead of phase and earth, it'd always be 10 in parral with 10, giving a reading of 5 whereever you were.

Anyway, hope this helps, might be usful to get a pen and paper and scribble it out (I just did), I hope I haven't made any mistakes with the calulations, and sorry for using silly values just plucked out the air, it makes the calculations easier for proving the physics, oh and I couldn't be arsed to go and look them up

 

[cozycats]

On-site guide p 70....are these guys wrong then? The whole point of cross-connecting is that you will be measuring the ENTIRE length of the cable at each outlet!!!!!!!!

 

[Softus]

Adam_151, as soon as I read "reduced CPC" the realisation hit me.

Your wonderfully clear working through was icing on the cake!

Thanks for taking the time to explain - I really appreciate it.

 

[ban-all-sheds]

If a ring the values should not increase towards the mid point

That's what I thought.

If tested properly the values should be roughly the same all the way round .05 ohms variation I think is allowed. The only increases you should see are those from a spur which will be slightly higher

Did you not read the bit where I said that you see increasing readings when you've cross-connected phase and cpc?

On-site guide p 70....are these guys wrong then? The whole point of cross-connecting is that you will be measuring the ENTIRE length of the cable at each outlet!!!!!!!!

You will be measuring the same length of cable at each socket.

But when phase and cpc are cross-connected the fraction of that length which is phase, and the fraction that is cpc will change from socket to socket, hence the changing readings.