Intermittent RCD Tripping in a Bamenda Boutique
You are a CAP electrical installation trainee working in a boutique in Bamenda. The shop has a 220 V single-phase supply, a 32 A main switch, a 30 mA residual current device (RCD), four twin socket outlets wired in 2.5 mm² copper, six LED lamps on 1.5 mm², and a refrigerator. The RCD trips intermittently, usually a few seconds after the refrigerator compressor starts; it does not trip when the fridge is unplugged. A socket outlet beside a metal sink has no cover and is within 300 mm of the tap. The earth electrode resistance, measured with a fall-of-potential tester, is 80 Ω. The shop owner wants you to 'just replace the RCD with a bigger one' because the 30 mA unit 'is too sensitive.' You have a multimeter, insulation tester, clamp meter, screwdrivers, and a test lamp. The shop must reopen in two hours.
1. Explain why replacing the 30 mA RCD with a larger one is not a safe or correct solution. Identify the most likely causes of the intermittent tripping and describe how you would isolate the fault.
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The 30 mA RCD is there to provide additional protection against electric shock, especially on socket circuits. Replacing it with a larger rating, such as 100 mA or 300 mA, would raise the trip threshold and reduce protection; it would not remove the fault. The RCD is not an overload device, so the fact that it trips when the compressor starts points to earth leakage rather than overcurrent. Likely causes include moisture in the refrigerator compressor or its plug, a damaged flexible cord, a leaking heating element or suppressor, a shared neutral between circuits, or cumulative leakage from several appliances. I would isolate the installation, unplug the refrigerator and other appliances, and test the RCD with an RCD tester. Then I would insulation-test each circuit line-to-neutral, line-to-earth and neutral-to-earth with the insulation tester, and use a clamp meter to look for leakage current. If the fridge trips a known-good RCD on a separate test socket, the fridge is faulty and must be repaired or replaced. I would not bypass or uprate the RCD to make the shop open on time.
2. Assess the socket beside the sink and the 80 Ω earth resistance. What corrective actions do you take?
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A socket within 300 mm of a metal sink and tap is in a wet zone and needs at least a shuttered, splash-resistant enclosure, normally IP44 or better, and it must be protected by the 30 mA RCD. If the socket is damaged or has no cover, I would isolate the circuit, replace it with an appropriate IP-rated socket, and route the cable so water cannot run into it. If the layout allows, I would move the socket farther from the sink. For the earth resistance, 80 Ω is below the 100 Ω figure often used as a practical target, and it is far below the 1667 Ω maximum that a 30 mA RCD can tolerate at 50 V touch voltage (50 V ÷ 0.03 A). So the RCD can still operate, but 80 Ω is not a good earth and may not satisfy the local utility's requirement. I would check the electrode connection, improve it if practical, and record the reading. I would not treat the 80 Ω reading as permission to leave the wet-zone socket unprotected.
3. Describe the safe isolation and testing procedure you would follow before touching the circuit.
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First, I would tell the shop owner what I am doing and agree a safe shutdown. I would switch off the main switch and the relevant circuit breaker, then lock off or tag the isolation point and keep the key with me. I would prove the tester on a known live source, test the circuit at the socket and at the consumer unit to confirm dead, and then re-prove the tester on the known source to show it still works. I would check for backfeeds, especially from the refrigerator or any uninterruptible supply, and discharge any stored energy. Only then would I remove covers and work on the circuit. Before re-energising, I would inspect connections, replace damaged accessories, test insulation resistance and RCD trip time, and record the results. The shop's two-hour deadline does not justify skipping isolation or working live.