Motor CPC Continuity in a Wet Plant Room
A factory maintenance team has installed a new 7.5 kW, 380 V three-phase pump motor in a wet plant room. The motor is fed from a local sub-board by a 4-core 4 mm² PVC/SWA cable, with the armour used as the circuit protective conductor (CPC) at the motor end. The sub-board is connected to the main earthing terminal by a 4 mm² single-core CPC. The supply is declared by the utility as TN-S. A long-lead continuity test from the motor terminal box to the main earthing terminal gives 1.9 Ω; the line conductor of the same cable run gives 0.35 Ω. The cable run is 25 m. The motor frame is independently bonded to the structural steel, which is also bonded to the main earthing terminal. The plant room floor is frequently washed down, and the motor is controlled by a local DOL starter with a 20 A overload relay. The maintenance team wants to accept the installation because the structural bond gives a parallel earth path and the motor runs normally.
1. Is the 1.9 Ω CPC continuity reading acceptable? Explain your reasoning and state the immediate action.
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The reading is not acceptable. For a 25 m run, the armour CPC resistance should be in the order of a few tenths of an ohm at most, not 1.9 Ω. The line conductor reading of 0.35 Ω shows that the run is not long enough to explain the difference. Using the measured CPC value, the prospective earth-fault current on a 220 V TN-S system is only about 220 / 1.9 = 116 A. Even ignoring the 20 A overload relay for fault protection, a 20 A Type C MCB may need 100–200 A to operate magnetically in the required disconnection time, so the fault may not be cleared quickly enough. A high CPC resistance commonly indicates a poor gland or armour termination, corrosion, paint, a broken CPC or a loose sub-board earth connection. The immediate action is to isolate the circuit, remake and verify the armour/gland termination and the sub-board CPC connection, then repeat the continuity test. Do not energise until the CPC resistance is consistent with the cable data and the earth-fault loop impedance has been verified.
2. Can the structural steel bonding conductor be used as the sole CPC for the motor? Justify your answer using earthing principles.
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No. The structural steel bond is main or supplementary equipotential bonding of an extraneous conductive part; it is not a substitute for the circuit protective conductor of the motor circuit. A CPC must be part of the circuit, or a compliant metallic enclosure or armour verified for the purpose, and it must be sized, continuous and reliably connected. Structural steel may be discontinuous, painted, isolated by building movement or of unknown resistance, and its bonding conductor is selected for bonding, not for carrying the fault current of the motor circuit. The parallel path may make a continuity test look better, but it cannot be relied on for automatic disconnection. The correct approach is to restore the armour CPC, or install a separate copper CPC, and keep the structural bonding as supplementary protection. In the wet plant room, main bonding should also be verified, and additional protection by a 30 mA RCD for the final circuit should be considered where the risk assessment requires it.