Long vs Short Earth Connection for Single Phase Motor vs 3 Phase Motor Frame
If the motor is fed from a star (wye) connected transformer then there should be a neutral to earth link at the transformer. This is standard in Europe. (You have omitted your location from your user profile so I can't be more specific.) This neutral link references all three phases to earth so that 230 V RMS appears on each phase.
Now if a phase to frame fault occurs then the protective device on that phase should trip.

The single-phase motor you have described has the neutral wire but the fault current (in the event of a phase to frame fault) does not return through the neutral so the protection behaves the same as in the three-phase situation.

If the motor were fed from a delta-connected secondary supply then the first phase to frame fault would be tolerated and the fusing would not trip. 







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) A star (wye) connected 3-phase supply system with three voltmeters or lamps will pull the star/wye point to neutral potential. (b) With delta-connected supply the situation will be the same even though there is no star point in the transformer or generator output.







simulate this circuit

Figure 2. With a single earth fault the system can still run but a second phase fault will cause a trip.

Note that earth fault detection circuits can be designed using the arrangement of Figure 2 and can be used to signal a fault (where reliability is most important) or trip immediately.