zs and r1 + r2 querey

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chrisgjudd

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hi newbie doing first major noticed a sparky on a forum stating he removed lights while measuring r1+r2 and zs, is this necessary?? I have been doing this but wonder if it is neccessary as phase earth (cpc) shouldn't involve neutral or am i missing something?? :^O

chrisgjudd

 

The Godfather

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Depends in what manner (existing installation), the test is conducted, in particular if the main switch is still on (circuit MCB off, live removed).

The Neutral and cpc would be common at the HED (PME), so with the load in place, there is a path from live through the load to the Neutral, effectively providing a high (the lamp) resistance across the R1 + R2 measurement.

Don

 

extension15

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I suppose the resistance of the lamp is only going a to make a small % difference...

(Unless it's a 500W bulb)

:D

 
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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?

:D

 
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