Intermittent Earth Fault on a Marina Feeder
A marina feeder protected by a 100 mA RCD at the origin nuisance-trips at random, mostly at night and in wet weather. The feeder supplies eight pedestals, each with a 30 mA RCD, and has a total leakage that reads 18 mA with all loads off but pedestal isolators closed. A clamp meter on the main earth shows 9 mA. Insulation resistance tests on the feeder are above 200 Mohm, but the tests were done with surge protection devices connected and with the pedestals energised. The owner wants to replace the 100 mA RCD with a 300 mA unit to stop the trips.
1. Evaluate the proposed replacement of the 100 mA RCD with a 300 mA unit.
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Uprating the RCD to 300 mA is not an acceptable fix. The 100 mA device is providing back-up protection for the installation and for cumulative leakage; raising the threshold reduces protection and may not stop the trips if the fault current is intermittent and above 300 mA at times. The correct approach is to find and remove the leakage. The measured 18 mA with loads off and 9 mA on the main earth indicates real leakage, not just meter noise. The tests were invalid because surge protection devices can leak and because testing with pedestals energised does not isolate the installation. The RCD should be tested for correct operation and trip time, and the circuit should be tested in sections with SPDs disconnected and loads isolated. Replacing the device without diagnosis risks shock and fire.
2. Design a sectional test plan to locate the leakage, given eight pedestals and the measured values.
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Isolate the feeder and disconnect the surge protection devices, then measure insulation resistance of the feeder alone. Reconnect and test each pedestal individually, with its 30 mA RCD and loads isolated, recording leakage and insulation resistance. Use a clamp meter on each pedestal's protective conductor to find which one carries current. Because the fault is intermittent and weather-related, repeat tests in wet conditions and, if necessary, leave a recording clamp meter in place overnight. Check outdoor luminaires, heater tapes, and pump circuits on the pedestals, since these are common wet-weather culprits. Compare the sum of individual leakages with the 18 mA total; a discrepancy suggests a shared path or a neutral-earth fault. Test the 30 mA RCDs for trip time and confirm discrimination with the 100 mA origin device.
3. State the criteria for deciding whether the origin RCD remains adequate after repairs, and what records to keep.
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After repairs, the total standing leakage should be well below the origin RCD rating, with margin for additional loads and for leakage that increases in wet weather. The 100 mA device should be verified for correct operation, trip time, and discrimination with the 30 mA pedestal devices so a downstream fault trips locally first. If the cumulative leakage remains high because of many electronic loads, the solution is to reduce leakage or split the feeder, not to raise the threshold. Records should include insulation resistance readings per section, leakage current measurements with conditions noted, RCD trip times, SPD disconnection and reconnection, and a description of the fault found. Note the weather at the time of testing, because the fault is wet-related, and schedule a repeat test after the next significant rain.