Question 13 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is A. ### Explanation for Option A (Correct Answer) **Option A: an open overload "OL" heater** An overload (OL) heater is part of the protection circuit and is wired in series with the motor windings. If one of the three OL heaters opens (burns out or is broken), it creates an open circuit (single-phasing) in one phase supplying power to the motor. When the start button is pushed, the contactor (M) pulls in, applying power to the motor through the two intact phases and the one open phase. A three-phase motor operating on only two phases (single-phasing) will typically fail to start under load, draw excessive current on the two remaining phases, and produce a characteristic loud humming noise (because the magnetic field is no longer rotating uniformly). This perfectly matches the described symptoms: "fails to start and gives a loud hum." ### Explanation for Incorrect Options **Option B: an open main contactor "M" coil** If the main contactor (M) coil is open, it means the coil will not energize when the start button is pushed. If the coil does not energize, the main contacts will not close, and no power will be delivered to the motor. Therefore, the motor would remain completely silent, not produce a "loud hum." **Option C: the disconnect switch "DS" is open** If the disconnect switch (DS) is open, no voltage is available anywhere downstream, including to the motor power circuit and typically to the control circuit (unless the control transformer is wired ahead of the DS). If power is completely disconnected, the motor cannot receive power when the start button is pushed, and it will remain silent, not produce a "loud hum." **Option D: an open overload "OL" relay contact** The overload (OL) relay contacts are normally closed (NC) and are wired in series with the control circuit (usually between the stop button and the M coil). If an overload occurs, these contacts open, de-energizing the M coil and shutting down the motor. If these contacts are already open (due to tripping or being faulty), the M coil will not energize when the start button is pushed. As explained in Option B, if the M coil does not energize, the motor receives no power and will remain silent, not produce a "loud hum."