MMI install


Senior Member
Installing a remote MMI that controls a PLC via a Din plug class 2 cable, the remote MMI is being powered by a 24V DC 60 watt class 2 plug in wall transformer (wall wart) the box for the MMI is a steel hinged junction box. is it ok not to have a egc for the box?


Staff member
250(A) Alternating-Current Systems of Less Than 50 Volts.Alternating-current systems of less than 50 volts shall be grounded
under any of the following conditions:
(1) Where supplied by transformers, if the transformer
supply system exceeds 150 volts to ground
(2) Where supplied by transformers, if the transformer
supply system is ungrounded
(3) Where installed outside as overhead conductors.


EC, Westchester, New York NEC: 2014
Hawthorne, New York NEC: 2014
If all you have is Class 2 wiring and devices in that box, no, it doesn't have to be grounded- unless you think it may somehow become energized by some external source.



Senior Member
190503-2144 EDT


What is "MMI"?

From Wiki ---

So from that list Man Machine Interface seems the most logical.

From your comment you said the power supply was 24 VDC output, but that is not your input.

If your enclosure is metal I would ground it independent of any rules. You said metal box. So I would connect that box to adjacent metal objects. Note: an EGC can have up to or possibly more than the source voltage on it under short circuit conditions. Somewhere in the past I have indicated how this can happen.

If whatever this device is totally insulated, and it is designed such that anything an operator could touch is non-conductive, then I would see no need for grounding.

In general I would not assume any power supply from 120 to 24 V could not develop an internal short. I can design one that has an extremely low probability of a shorting failure, but I can not generally assume this for any commercial product.