Definitely switch the AC feeding the transformer OR the AC on the secondary of the transformer, before the rectifier, if the transformer has multiple windings feeding other stuff that you won't like to turn off in an E-stop condition.
You can use an auxillary contact on the relay/contactor to switch in a discharge resistor across the capacitors to dump the charge quickly.
As Al says, switching the DC stresses the contacts in the relay but it also stresses the drives, when the contacts closes the drive "sees" the voltage "instantly" which may cause a very high inrush current. (Beacuse its internal capacitor is discharged). The VSD-A is a solidly built drive so I'm sure it can withstand it but it IS hard on them.
Another issue with switching the DC is that the returned energy from the motors (if they were spinning when the power was cut) has no way to go except the relatively small capacitor in the drives. This will pump the voltage across said capacitors up and it may go above the rated voltage of the drive depending on your particular set-up.
So, in short, switch the AC and dump the charge if you want it stop ASAP - that's my €0.02
/Henrik.  |