If the breaker trips the moment the motor is started, there is either a real short circuit or earth leakage in the circuit, or the breaker is not the type that can handle the high current the motor draws while starting. First separate the two: a short circuit is found by measurement, a wrong selection by reading the marking on the breaker.

Possible causes

  • Wrong trip curve: Fast-acting (type B) miniature breakers used for lighting and socket circuits may treat the motor's starting current as a short circuit. Motor circuits use a motor protection breaker or a breaker with a curve suited to starting. Why starting current is high is explained in the question on high current at start-up.
  • Wrong connection: A motor rated 230/400 V connected in delta on a 400 V supply draws excessive current. The consequences are covered in the question on delta instead of star connection.
  • Winding or cable short: A burnt winding, a crushed cable or a terminal box that has taken in moisture.
  • Mechanical seizure: A motor with a collapsed bearing or a locked load cannot turn; its current stays at the starting value.
  • Star-delta changeover fault: If changeover happens too early or two contactors close at once, the circuit sees a momentary short.

Check order

  1. Isolate the power and lock the switch; have the work done by a qualified electrician.
  2. Compare the breaker type and rating with the rated current on the motor nameplate.
  3. Turn the motor shaft by hand; if it is stiff, fix the mechanical fault first.
  4. Disconnect the cable from the motor and have the insulation of the cable and the motor winding tested separately; this shows which side the fault is on.
  5. Check that the links in the terminal box match the voltage on the nameplate.

Fitting a bigger breaker hides a short circuit and puts the cable at risk; find the cause first. If the fault is in the motor itself, you can choose a new motor of the same power among our three-phase electric motors. Send the nameplate photo and a picture of the terminal box on WhatsApp and we will quickly identify the right equivalent.