Observation

The expected answers were;

(a)       (i)        Predictive maintenance: it is time bound and specified by the manufacturer
           (ii)       Preventive maintenance: it is a routine day to day maintenance to prevent
                       breakdown                                                                                   
(b)       Effects of dust on electric fan:
            -  It can cause corrosion/rusting.
            -  Reduces the speed of the motor.
           -  It increases resistance.
           -  Reduces efficiency

Many candidates who attempted this question did the second part better.