The order tests are carried out in, and why that order matters — dead tests before energising, live tests after.
Before energising: continuity of protective conductors (643.2), insulation resistance (643.3), protection by SELV/PELV/separation where relevant (643.4), and polarity (643.6).
BS 7671 Reg 643.1
After the dead tests and safe energising: earth fault loop impedance (643.7), prospective fault current (643.7.3.201), RCD operation (643.8), phase sequence for polyphase circuits (643.9), and functional testing (643.10).
BS 7671 Reg 643.1
A basic measurement accuracy of 5 % is usually adequate; for analogue instruments, 2 % of full-scale deflection gives the required accuracy over a useful proportion of the scale. Operating accuracy in the field is always worse than basic accuracy — the BS EN IEC 61557 series requires a maximum operating error of no more than ±30 % of reading over the stated measurement range, taking worst-case environmental, supply and battery conditions into account. A low-resistance ohmmeter should derive its test current from a source with a no-load voltage between 4 V and 24 V and a short-circuit current of not less than 200 mA, and should cover 0.2 Ω to 2 Ω with a resolution of at least 0.01 Ω on digital instruments.
| Instrument | Standard | Requirement |
|---|---|---|
| Any test instrument | BS EN IEC 61557 | Maximum operating error ±30 % of reading over the stated measurement range |
| Low-resistance ohmmeter | BS EN IEC 61557-4 | No-load voltage 4–24 V; short-circuit current not less than 200 mA; range 0.2–2 Ω; resolution at least 0.01 Ω (digital) |
| Insulation resistance tester | BS EN 61557-2 | Capable of developing the required test voltage across the load |
IET GN3 10th Ed §4.2–4.4 (pp.134–136)
