Measuring Zs

[Lectrician]

Surely if you test at the switched live terminal of the rose or fitting, that takes into account the return from the switch. Also, by doing it this way, you're automatically confirming correct polarity too.

The live is longer due to the down and back, but the earth is shorter

 

[GaryMo]

Surely if you test at the switched live terminal of the rose or fitting, that takes into account the return from the switch. Also, by doing it this way, you're automatically confirming correct polarity too.

There's nothing automatic about manually turning a switch on and off whilst measuring R1+R2 (assuming you're talking about R1+R2 as polarity should have been verified before energising) at the switched live and cpc at the light fitting (test method 1).

BTW R1+R2 should be measured at every point on the circuit, switches and fittings.