Measuring Zs, and when an RCD gets in the wayWould you measure Zs at a ceiling rose?

Measuring Z_s is a live test at every point of the circuit. That’s why it comes near the end of the sequence: everything about the circuit has already been proved safe.

• Sockets: a plug-in adapter makes it straightforward and hands-free.
• Lighting: two probes into the live terminals of a ceiling luminaire, up steps. Not a safe situation. Calculate it.
• A periodic you can’t switch off: measuring is how you check the circuit still disconnects in time without isolating anything.

Testers have “no-trip” settings to get round it, and they work in different ways:

• Some saturate the RCD with DC so it can’t operate during the test. That works on an older Type AC RCD — but Type A and F RCDs, now the norm, are made to keep working with pulsating DC (and with smooth DC up to 6 mA for Type A, 10 mA for Type F, Reg 531.3.3), so the DC can’t reliably hold them off. The test may trip them or refuse to run. Only a Type B handles smooth DC fully.
• A low-current, three-lead no-trip Check what your own meter does.

If you can’t get a reading without tripping, calculate. That’s the preferred method anyway.