Star configuration is to avoid ground loops.
– Source: Omron MX2 User’s Manual
Connection of the spindle to the frequency converter
Lines 1, 2 and 3 are to be connected to terminals U, V and W on the frequency converter. The protective conductor connection must be placed on the PE terminal of the frequency converter. On the frequency converter side, the cable shield must be laid as large as possible (using a suitable cable clamp) on the grounded plate (usually the base plate of the control cabinet) on which the frequency converter is mounted. If the screen is not connected, there may be potential differences, which can lead to EMC problems or even electric shock!
– Mechatron HFS Series High Frequency Motor Spindle’s Operator’s Handbook
If your spindle has a PE connector, the instructions say that the cable shield should be connected at both ends. Unlike flimsy no-name spindles, my Mechatron spindle has the fourth “PE” pin connected to housing which must be connected to cable shield. However if the spindle housing has contact to the 80 mm mount which has electrical contact to Z Slider which has contact to X Axis which has contact to Y Axis which has contact to machine’s feet which could have been grounded, it would introduce a ground loop. Is the machine grounded at the feet?