It makes sense that could happen. Sorry I may have confused things by talking about 2 different things in the same post. The gate motor and flooded industrial 415V equipment were separate events.
In this case the gate motor was a small 270W motor and not direct on line, but fed through a controller that limits power into the motor in order to limit the torque the motor can generate (this is a requirement for electric gates in case someone gets trapped by the gate). Part of the commissioning procedure is to block the gate, stalling the motor and measure the force the gate exerts. So the controller is happy with a stalled motor in this situation.
As JohnW assumed correctly, the motor itself wasn't flooded, just the IP67 connector to the motor which was in the underground box next to the motor. The FCU was inside the house and dry. I went through about five 13A fuses before finding the connector was the problem. Examining rhe connector showed it was full of carbonised material, presumably where an arc had been forming between the terminals.
To me it seems odd the controller could cope with a stalled motor, but not the low resistance of the arcing connector. It’s just a guess but as impedance of an inductor increases with frequency, perhaps the controller runs the motor with a much higher frequency as the motor starts to stall (opposite to what you would normally do, but in this case the point is to reduce torque to stop people being squashed) and perhaps that allows the motor to survive a stall condition without too high a current draw?
