Cumulative Leakage Tripping a Domestic RCBO
A 15-year-old domestic flat has a 30 mA RCBO protecting the kitchen socket circuit. The owner has added a washing machine, dishwasher, fridge-freezer, microwave and small counter appliances. The RCBO now trips at random, usually when the washing machine and dishwasher are running together, but not when either runs alone. The electrician tests the fixed wiring: insulation resistance line-to-neutral and line-to-earth are both greater than 200 MΩ at 500 V. The RCBO trips at 24 mA on the 1× test and at 150 mA within 28 ms on the 5× test. Earth leakage clamp measurements give: washing machine 8 mA, dishwasher 7 mA, fridge-freezer 5 mA, microwave 2 mA and other electronics 3 mA. Appliance insulation tests are all greater than 100 MΩ. The owner wants the RCBO replaced with a 100 mA device to stop nuisance tripping.
1. Diagnose the likely cause of the random tripping. Is the RCBO faulty?
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The RCBO is probably not faulty. The individual leakage currents add up to 8 + 7 + 5 + 2 + 3 = 25 mA. A 30 mA RCD may operate at any residual current between 15 mA and 30 mA, so a standing leakage of 25 mA is enough to trip, especially when the washing machine and dishwasher are running together and their filters and heating elements are energised. The insulation resistance tests are high because they measure DC resistance, not AC leakage through filters, surge protection or capacitive components. The 1× test trip at 24 mA is within the permitted range, and the 5× test at 28 ms is also acceptable. The random tripping is therefore cumulative protective-conductor current, not an insulation failure or a defective RCBO.
2. The owner proposes replacing the 30 mA RCBO with a 100 mA device. Explain why this is unacceptable and give a practical remedy.
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Replacing the 30 mA RCBO with a 100 mA device is not acceptable for socket outlets. A 30 mA RCD is required as additional protection for socket outlets, and a 100 mA device would not provide the same level of protection. The correct remedy is to reduce the standing leakage or split the load. Give the washing machine and dishwasher separate 30 mA RCBOs or separate circuits, or remove one appliance from the protected circuit. Use an earth leakage clamp to confirm each appliance's leakage and replace any appliance that exceeds its manufacturer's limit. Check that the RCBO is not shared with outdoor or bathroom circuits. If the total leakage cannot be reduced below about 15 mA, use additional RCBOs so that each 30 mA device carries less standing leakage. The RCBO should also be functionally tested after any change.