114 Electric Wiring: Domestic
Disconnect only if supply is isolated
Earth clamp with label Main protective bonding conductor
Gas
Zero/null instrument or subtract test lead resistance
FIGURE 7.2
Water
Continuity of main protective bonding conductors.
The continuity of circuit protective conductors (cpc’s) may be established in the same way, but a second method is preferable, as the results of this second test indicate the value of (R1 R2) for the circuit in question. The test is conducted in the following way (Figure 7.3): 1. Temporarily link together the line conductor and cpc of the circuit concerned in the distribution board or consumer unit. 2. Test between line and cpc at each outlet in the circuit. A reading indicates continuity. 3. Record the test result obtained at the furthest point in the circuit. This value is (R1 R2) for the circuit. There may be some difficulty in determining the (R1 R2) values of circuits in installations that comprise steel conduit and trunking and/or SWA and MIMS cables because of the parallel earth paths