Sounds like if the external power option was really necessary for some valid engineering reason, the setup could be made compliant by running the external power through a relay which disconnects it when the main VFD AC input is removed?
But the AC kludge versus the sanctioned DC input rules out the way it was actually done.
It was common in the old controls design to have separately sourced power that enters a control cabinet.
A typical feature in a spread-out facility when several manufacturing processes are being undertaken.
I've serviced equipment in one facility where the complex was divided in two different areas. . .one area is the what they call the EDIBLE AREA and the other the INEDIBLE AREA.
No mingling of raw materials was high priority.
One section manufacture products like detergents, hygiene, cosmetics and bar soaps.
This is the inedible area.
The other was the edible area where they manufacture pancake syrup, margarine and cooking oil to mention a few.
In both of these areas-- on some control panels-- there were equipment that have common control stations that can be operated from either manufacturing areas like air vents, vehicle movement warning signage etc.
Needless to say, maintenance personnel have to open these MCCs and have to deal with voltages coming in from different sources that are not part of the MCC being worked on.
Although highly visible signs that remind technicians of the presence of foreign voltage (CAL OSHA) requirement, the local AHJ also required provisions like interlocks to disable all power. . .local or foreign.
These interlocks could be either mechanical or electrical. (relays or solenoids )
In today's modern facilities that cover a wide area like refineries, they are using wireless technology.
They still have to adhere to: "no ifs no when" compliance.
