When the residual current device (RCD) trips as the motor starts, current is leaking from the motor circuit to earth. The most common causes are winding insulation weakened by moisture or age, water or dust in the terminal box, a damaged cable and, on inverter circuits, leakage current flowing through the filter capacitors.

Where to look

  • Damp winding: Motors that have stood idle in cold, humid places take moisture into their insulation. The RCD usually trips at the first start; if the problem eases once the motor warms up, that is a warning, not a fix. Precautions are in the question on motors running in damp places.
  • Terminal box: If the gasket has failed or the cable entry leaks, a moisture bridge forms between terminals.
  • Cable: A crushed, gnawed or water-soaked cable leaks to earth.
  • Inverter: The input filter of a frequency converter and a long motor cable pass leakage current to earth even without a fault. In that case the RCD type must match the inverter maker's recommendation; for wiring see the question on shielded cable and earthing.
  • Shared device: If many loads sit behind the same RCD, small leakages add up and the threshold is exceeded at motor start.

Check order

  1. Isolate and lock the power; have measurements done by a qualified electrician.
  2. Disconnect the motor from its cable and have the winding-to-frame insulation tested with a megger.
  3. Have the cable tested separately; the fault may be in the cable too.
  4. Open the terminal box and look for moisture, rust and dark marks.
  5. If there is an inverter, have the RCD type and the motor cable length checked.

Bridging or disabling the RCD is not a solution; a motor leaking to its frame is a danger to life. If the measurement shows weak winding insulation, you can review our new motor options, including inverter-compatible electric motors. Photograph the motor's nameplate and terminal box and send them on WhatsApp; we will pick a suitable replacement together.