Protection equipment manufacturer NewElec reports that it is common practice to utilise earth leakage protection relays having a sensitivity of 375 mA or greater for use on three-phase induction motors over 75 kW, while the tendency is to fit 250 mA sensitivity relays on smaller motors. The logical conclusion that can be drawn from this is that engineers appear to be less concerned about earth leakage faults occurring on the larger motors than on the smaller ones! This does not make for good engineering. Luc Dutrieux of NewElec says: "Most design engineers will tell you that it has always been like this in the past - they really do not know the reason."
It emerges that design specifications have been written in this manner to resolve the problem of nuisance trips during motor starts. These trips are caused by short duration harmonics or sustained harmonics and are experienced when starting large motors by means of direct on-line methods, and more lately, variable speed applications. However, elevating the sensitivity trip threshold in this way to overcome shortcomings in the earth leakage protection devices being used diminishes the protection to the motors.
NewElec's GA range of earth leakage modules obviates the need to elevate sensitivity trip thresholds because it includes harmonic filtering as a standard feature. In addition the tripping curve characteristics are user selectable between IDMT or instantaneous so that systems grading can also be achieved when this is required.
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