Preventable risks in Ex “N” and Ex “e” explosion protected motors

As a result of a number of explosions involving large induction motors, warnings have been issued with regard to the safe use of such machines. Investigation has shown that there are a number of possible causes of such ignitions, some are fully understood and others are clearly still the subject of speculation, as seen from the fact that the safety warnings specifically refer to high voltage motors. The possible incendive conditions may be divided into four different causes, and are as follows: (1) incendive sparking within the rotor slots, caused by currents passing between the rotor bars and the rotor core; (2) a corona discharge between the rotor and stator end windings during starting; (3) point discharges on the surface of stator high voltage windings, normally associated with contamination of the surface; and (4) incendive sparking caused by the interruption of currents flowing in the cladding of machines, normally associated with large 2 or 4 pole machines only. The author discusses these incendive conditions in detail and discusses the action that can be taken to minimise the risks