Zs test furthest point ?

[tank]

hello all, could somebody clarify something for me, please, as there seems to be some disagreement among sparkies as to where the furthest point electrically in a lighting circuit actually is. this for a Zs test.

is it at the ceiling rose, as i thought, with switch closed, and probing switch live and cpc? this would appear to be the furthest point for the line conductor but not for the cpc which carries on the the switch plate.

the ceiling rose camp seems to be quite large.

but after staring at the Megger testing poster for the Zs test, ( they probe cpc and switch side of the switch), i now think this is correct. as we are testing for earth fault loop impedance anyway, surely it's proper that we test at the switch plate, as this would test a loop that incorporates all of the cpc. ( not just all of the line conductor , as with a ceiling rose test )

i'm new to testing, so i may be talking out of my hole, any thoughts?

 

[Baconhead]

Who to say that you've got an CPC at the ceiling rose.
You may have it at the switch but what does that prove.

 

[jumpin jax]

highest Zs could be at either point depending how it is wired, especially if its a 2 way, the highest value is recorded anyway so test each point until you read highest reading. if you have an RCD on the circuit you would most likely need a neutral for triplock/ no trip depending on meter so would be tied again on the way the installation was wired ie loop in at rose or switch

 

[Octopus]

Anyone got the revelvant Guidance Note on testing to hand?

edit - Ok, just been to the office and its guidance note 3. A quick flick through shows pictures of the test being done at the switch.

I'm sure others will wade in with a better more comprehensive description!!