Sometimes I find with questions like this, it can help me to think though the maths & the electrical science calculation part of the problem.
As generally most problems are looking at series or parrallel circuits & load arrangments. I try and consider a practical example..
When I do R1-R2 reading I take the phase & CPC out of the CU and join them with a bit of connector block..
so R1 & R2 are not connected to the CU at any point.
then take my reading at furthest point of circuit.
The only parrallel path when testing this way is if two or more bulbs are switched on...
so to work some figures out lets say..
a 1.0mm T&E lighting circuit 30m long.
from the tables (9a pg158 OSG) R1&R2 should be
(30m x 36.2)/1000 =
1.086ohms.
now lets say two 100watt light bulbs are in circuit switched on.
one at the very end of the circuit..
the other 10m from CU (20m from end).
{40w bulb approx 100ohm (cold), 60w bulb approx 70ohm, 100w bulb approx 50ohm}
R2 = 30m @ 18.1 = 0.543
R1= (10m & 18.1) plus (20m @ 18.1) in parrallel with (20m @ 18.1 + two bulbs)
assume 100w 50ohm bulbs
0.181+(0.362 in parralel with 100.362)
= 0.181+0.361
R1 = 0.542
R1+R2 = 0.542+0.543 =
1.085ohms
0.001 difference with bulbs in..
you would probably get this error by how firmly the meter probes are used?
