Thoughts on R1 + R2 on lighting circuits

[davetheglitz]

Just a thought - generally we do R1 + R2 (or Zs) at the furthest light. It strikes me that with the loop in method this gives the longest length of line conductor - so max R1 - but we miss out the cpc to the switch - which may have a break in it.

Would we better off doing R1+R2 at the furthest switch - gets the maximum cpc length and as cpc has less csa the worst case result at an accessory.

You would already have a good idea that there isn't a problem with R1 as the light works!

 

[sellers]

I use my R2 lead to test the cpc everywhere, then by common sense and high readings you can work out where you end is most likely and get your zs from there.

 

[ProDave]

Well I (nearly) always loop at the switch, so R1+R2 at the light DOES test ALL the run of the cpc.

Your point is valid though for loop at the light fitting wiring.

 

[springcrocus]

On a CU change, or a report, I generally only do a long-lead R2 test. It's easy to drop a rose cover for R1R2 or Zs but a pain if the end of the circuit is a chandelier, multi-lamp fitting of floury light. Generally the metalwork of the light is earthed (unless it's SELV) so no need to dismantle. The main thing is to ensure there is an adequate earth path to handle any fault current. Then I do the same for the screws holding the switches on and open up if I don't get a reading. Some of the back boxes from the seventies/eighties have insulated lugs.

 

[wicksy]

I can see your point Dave, i take my supply's to and from the switches so if you are teating R1 & R2 at the furthest point then it will still pick up the R2 as all the earths are terminated together in the switch and sent to the last light all in one feed

 

[Manator]

I have not done this for a while now but I did used to incorporate a test socket for lighting circuits saved me loads of time.

 

[Doc Hudson]

R1+R 2 is one of the methods to verify R2 continuity throughout the final circuit. Assuming the light switch is in the same room as the light fitting, then I think your point is only really valid if the reading at the fitting will result in a max Zs very close to the max permissible values. As long as it is reasonable well under max values then the additional resistance (if any) to the switch would have negligible impact.

Doc H.

 

[steptoe]

R1+R 2 is one of the methods to verify R2 continuity throughout the final circuit. Assuming the light switch is in the same room as the light fitting, then I think your point is only really valid if the reading at the fitting will result in a max Zs very close to the max permissible values. As long as it is reasonable well under max values then the additional resistance (if any) to the switch would have negligible impact.Doc H.

but you havent actually proved cpc to its furthest point if you dont takew the reading at the switch,

in that instance then we should be doing a Z or R2 at every switch really to confirm continuity.......

dear o dear, yet another problem answered by more problems.....

 

[Stator_Electrical]

The guidance in this instance is to "test for continuity AT EVERY EXTREME END" within the circuit. Therefore just in case you couldn't prove the method of wiring used (if you hadn't wired it yourself) you would prove continuity of the cpc to every point and also verify the highest value of continuity to note the R1+R2 value.

 

[JTI sparks]

What if there's no earth at the light itself? Do you code it as something/ how would you record a correct Zs for that light circuit? From the furtherest switch??