Long vs Short Earth Connection for Single Phase Motor vs 3 Phase Motor Frame

Question

What is the correct way of grounding single phase motor frame if I have to choose between two options:

(1) Short to earth connection: from the frame to nearest grounding rod. See below the schematic. (2) Long to earth connection: from the frame to the ground bus power panel that the motor is supplied by (which could be just far away. See below.

I think (1) is not enough since it does not have a fault current path with the neutral if the hot conductor (s) energizes the frame.

I see the fault current path as a way to trip the breaker or the fuse

However, I just do not understand why this is not the case with 3 phase motors

In 3 phase motors, you don't usually have a neutral, and if the frame is energized, then how will the trip happen ?

^{simulate this circuit – Schematic created using CircuitLab}

^{simulate this circuit}

Answer 1

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 – 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.