Washing Machine Trips the 30 mA RCD Only During Spin
A two-year-old front-load washing machine with a 2 kW heater trips a 30 mA RCD at the distribution board during the spin cycle, not during wash or drain. The RCD also protects a fridge and a freezer. Insulation resistance tests at the plug with a 500 V tester and the machine switched off give: heater 200 MΩ, motor 50 MΩ, drain pump 20 MΩ, but these are cold and dry readings. There is visible carbon dust around the motor brushes, and the repair history includes a replaced door seal. A leakage clamp meter reads 4 mA at standstill, 8 mA during wash, and 26 mA briefly at spin start, then 12 mA. The customer says the trip is worse on humid days. You have a leakage clamp, insulation tester, RCD tester and a spare motor. The customer wants the RCD replaced with a 100 mA unit.
1. Is the RCD faulty? Interpret the leakage readings and the insulation results.
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The readings do not prove the RCD is faulty. A 30 mA RCD is not a fixed trip at exactly 30 mA; it must not trip below 15 mA and should trip at 30 mA, with a maximum trip time at rated current. The washer alone shows 26 mA briefly at spin start, which is already close to the threshold, and the fridge and freezer add their own leakage, perhaps 1–3 mA each. On humid days, moisture in the motor or harness raises leakage further. The 4 mA at standstill and 8 mA during wash also indicate a real leakage path, not just an RCD nuisance.
The cold insulation readings of 200 MΩ, 50 MΩ and 20 MΩ are healthy and do not reproduce the warm, wet, vibrating condition. Therefore the first hypothesis is accumulated leakage plus a spin-related fault in the washer, not a defective RCD.
2. Give a diagnostic sequence to isolate the leakage source without masking it.
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Use a leakage clamp around the live and neutral together at the machine plug, or around the protective conductor, and record leakage during a full cycle. Disconnect the fridge and freezer temporarily to establish the base leakage. Then isolate the washer's heater, motor and drain pump one at a time, using safe insulated connections, and repeat the spin test. Because the fault is intermittent, repeat with the machine warm and with the door seal wetted if needed.
Perform insulation resistance tests at 500 V DC on each load when warm, and inspect the motor brush area for carbon dust tracking and the harness for chafing. Check the RCD with an RCD tester at half, one and five times rated current to confirm it is within specification, but only after the appliance leakage is understood. Do not use a 100 mA RCD to mask the fault.
3. The customer insists on fitting a 100 mA RCD. Explain your decision and what you would offer instead.
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Refusing to fit a 100 mA RCD is the correct safety decision unless the installation is under supervision and the protective device is part of a documented design. A 100 mA RCD does not protect people against lethal shock in the same way as 30 mA and may not meet the wiring regulations for socket outlets. The machine already shows 26 mA at spin start, so a 100 mA device would merely hide a developing insulation fault that could become a fire or shock hazard.
Explain that the RCD is doing its job. Offer to repair the motor, brush gear, harness or seal as indicated by the tests, and to verify leakage after repair. If the customer refuses repair, disconnect the machine and label it unsafe rather than changing the protective device. Record the leakage readings and the advice given.