When electric fan motors are connected to the public supply, protective devices are required for two main purposes. In the first place it is necessary to ensure that a breakdown in the insulation of the motor, its control gear or connecting wiring, shall not cause overheating of the supply cables or interruption of the supply to the whole premises. Fuses perform this function effectively and economically for small and moderate power circuits, while circuit breakers are employed for high power applications. These devices must be kept for their proper function of interrupting instantaneously the heavy rush of current which flows into an earth or short-circuit before it has time to open the main breakers further back; otherwise the power interruption will spread beyond the particular motor or controller which is faulty.
In the second place it is desirable to limit the amount of damage, which may be done to a fan motor by accidental overloads or minor faults. This is largely an economic matter, and it would be clearly unsound to load a small fan motor of low first cost with the comparatively heavy cost of fully protective control gear, when the chance of breakdown is in any case small. Moreover, fan motors are inherently unlikely to encounter overloads, except with the forward curved centrifugal fan. Nevertheless it is sound practice to instal starters with overload protection when the power exceeds about 0.33 kW.
Posted in Fans Ventilation A Practical Guide