Nuisance RCD tripping on a generator-supplied compound
A 500 kVA diesel generator supplies a site compound through a 400 A distribution board. The alternator star point is bonded to the generator frame and to a local earth electrode (3.2 Ω). The distribution board also has a neutral-to-earth link and a 300 mA RCD. Standing earth leakage with cabins only is 80 mA. When a 400 A engine-driven welder starts, the RCD trips intermittently; clamp readings show 1.8 A on the main earth conductor between generator and board and 0.9 A on the bonding conductor at the board. The welder is fed from a 63 A outlet on the board and its own earth electrode measures 4.5 Ω. The site electrician proposes increasing the RCD to 500 mA. The compound includes sleeping cabins and a canteen. Evaluate the earthing and tripping problem.
1. Explain why the RCD trips and why raising the setting is not the correct fix.
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The RCD trips because there are two neutral-to-earth bonds: one at the generator star point and one at the distribution board. Neutral current can return through the earth conductor and electrode, so the RCD does not see all the outgoing and returning current and interprets the difference as earth leakage. The clamp readings support this: 1.8 A on the generator-to-board earth conductor and 0.9 A at the board bonding conductor are circulating neutral current, not fault current. Starting the welder adds a transient and makes the imbalance exceed the 300 mA setting. Raising the RCD to 500 mA would hide the symptom but leave the parallel path and reduce protection for the sleeping cabins and canteen. It would not remove the circulating current, and a real earth fault would be less likely to trip. The correct fix is to remove the duplicate bond, not to desensitise the protection.
2. Recommend the correct earthing arrangement and immediate actions.
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Create a single neutral-to-earth bond and keep it at one declared point. For a generator-supplied compound, bond at the generator star point or at the main board, not both; remove the link at the other location and separate the neutral and earth bars downstream so the system is TN-S. Keep the 300 mA RCD for the distribution board and use 30 mA RCDs for final circuits, especially sleeping cabins and canteen sockets. Bond the welder frame to the same earth system; its separate 4.5 Ω electrode must not create a second path. Before re-energising, verify with a clamp that the earth conductor carries no standing neutral current, then test the RCD with a ramp tester. Do not re-energise the sleeping cabins until the duplicate bond is removed and the tests are recorded.
3. What tests and records are needed before re-energising?
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Test and record: continuity of protective conductors and bonding; insulation resistance of the installation; earth electrode resistance; earth loop impedance at the distribution board and at the welder outlet; RCD trip time and current at 1× and 5×; neutral-earth voltage; and clamp current on the main earth with the welder starting. Confirm that the generator and board have only one neutral-earth link and label it. Record the welder start current and any residual imbalance. Issue a short method statement and a certificate for the changed earthing arrangement. Re-check after the first shift because welder transients can reveal a loose bond. If the RCD still trips with no parallel path, investigate the welder for an actual insulation fault rather than increasing the setting.