Live testing: earth fault loop impedance, prospective fault current, RCD and functional testing
Live testing
Live tests confirm that protective devices will operate within the required disconnection time and that residual current devices work.
Earth fault loop impedance. Ze is measured at the origin with the installation isolated and the means of earthing disconnected, or obtained by enquiry. Zs is measured at the furthest point of each circuit, or calculated as Ze + R1+R2. Compare Zs with the maximum in BS 7671 Table 41.3 or 41.4, or with manufacturer's data. Because those tables assume conductors at maximum operating temperature, a healthy circuit typically measures around 80% of the tabulated maximum; a reading at or near the maximum warrants investigation.
Prospective fault current. Measure at the origin and at circuit ends, record the higher of prospective short-circuit current and prospective earth fault current, and compare with the breaking capacity of the devices.
RCD testing. Use the test button as a functional check, then an instrument: half rated residual current for no trip, rated current for operation within 300 ms, and five times rated current for operation within 40 ms. Test at the device and at a socket outlet so the circuit is included.
Functional testing also covers isolators, switches, interlocks and, where within scope, emergency lighting.
Traps. Recording only PSCC when PEFC is higher, which is typical of TT systems; testing RCDs without checking supply polarity; and taking a Zs on a circuit with an open CPC.
Worked example. A 32 A Type B MCB on a ring final measures Zs 0.58 ohm against a tabulated maximum of 1.37 ohm - acceptable and well below the limit.