GE MM3 Instruction Manual page 208

D.1 ASYMMETRICAL CURRENT APPENDIX D
A motor or a transformer is never a perfect inductor, therefore, the value of 1.73 will never be reached. The DC
offset will die away as a function of the X/R ratio (typically a few cycles). The following figure represents an
exaggeration of the three phase current of a motor starting.
D
Figure D–3: MOTOR START THREE PHASE CURRENT
When is this 'asymmetrical current' a concern?
When setting instantaneous relays, care must be taken to ensure that the instantaneous element does not
operate during normal operating conditions such as a motor start. Symptoms of an instantaneous element that
is set too sensitive are nuisance or intermittent tripping of the relay during energizing of the system.
Furthermore, CTs do not react predictably when a DC current is applied. The waveform that is shown above is
not necessarily the waveform that each of three phase CTs would output. If there is a residual connection for
ground fault detection, that element could operate when asymmetrical currents are present.
D-2 MM3 Motor Manager 3 GE Power Management