Motor Earthing at Transformer Supply

Having the motor star point earthed won't have any effect on protecting the motor windings during a fault condition.

A power system must be referenced to earth at a single point, usually at the supply transformer or generator star point. Referencing the system to earth at multiple points on a distribution system, for example, at the star point on your motor (in addition to the supply transformer star point), would be bad practice as it could lead to the undesirable condition of creating an additional (second) earth fault return path for the system via your motor windings for earth faults on other equipment connected to the network. This could result in the E/F protection on your motor operating in the event of an earth fault occurring somewhere else on the network. It's also likely that the motor windings will not be identically matched/balanced, which could lead to a small amount of 'unbalanced current' being present at the star point. With this in mind, connection of the star point to earth would provide a path for the unbalance current to return back to the supply transformer, though the earthing system, during normal operation of the motor i.e. not during a fault condition. A system with several motors connected in this configuration would result in the sum of all the out of balance currents from all the motors flowing in the earthing network, which would be a highly undesirable condition.

With the talking of a 'flashover condition' I'm thinking that the motor may be operating from an HV supply system (6.6/11 kV), in which case the motor would normally be protected by a dedicated Thermal O/C & E/F protection relay system. This would provide protection of the motor in the event of an E/F condition, by measurement of the algebraic sum of currents in the line conductors, which with a balanced load (induction motor), would normally be zero or very close to zero. An unbalanced sum of these currents would indicate current flow back to the source by means other than the three line conductors, i.e. via the earth conductor connected to the stator of the motor.

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