Pier has excellent suggestions if you can influence the design of the connector. If that’s not possible, and this problem must be solved with instructive art, you’ll need to look at available symbol standards. Ask your regulatory department to get you a copy of:
IEC TR 60878 “Graphical symbols for electrical equipment in medical practice.”
ISO 15223 “Medical devices – symbols to be used with medical device labels, labelling and information to be supplied.”
IEC 60601: Medical Device Marking and Labeling
I’m guessing that you’ll be using a Padlock symbol plus an AC plug symbol together. Since you also need to show two different states of action, which are linked to the locking, I would suggest combining the padlock and plug symbols with instructive art that depicts the sequence and actions. Actions are frequently depicted by arrows or by including the source of the action, like a hand.
1: Unlock then plug
2: Plug then lock
If the twist action isn’t intuitive, or if the twisting isn’t intuitive as a lock, you’ll want to indicate the twist action in the graphic. ie.
1: Twist left then plug-in
2: Twist right
In this case, the fact that it’s a lock may not even be important–it’s the actions that are important that get you to a correct state.