The user needs to insert a plug into a socket, then lock the socket with a dial.
So the lock needs to have graphics that indicate that its OPEN and CLOSED, or UNLOCKED and UNLOCKED. This machine will be used internationally, so no words.
What are the best symbols for this? Green and Red? Padlock symbols? The problem is that, when its GREEN, what does that mean? GOOD TO GO on what, the inserting of the plug, or ready to turn on the machine? The problem is that one position has two “phases.” Before plugging and after plugging. And they change indications of thumbs up and thumbs down, so to speak.
Before plugging, you want the user to know that the unlocked position is GOOD, but after plugging, you want the closed position to be GOOD. So thumbs up and down, red and green, check and X, are out.
I have faced similar situations on numerous projects. I can’t comment too specifically as every situation has idiosyncracies, but what I have learned include:
Always think of this from the users point of view. “Two phases” - what happens at phase 1, what happens if “phase 2” is not done, what indications are provided for the user? Will the user, trained or not, fundamentally know that there are two phases? This is a design and engineering problem as it involves instrument conditions.
Often, this is a false problem brought on by an engineering decision of connector choice. Can a simpler, single engagement, connector actually do the job satisfactorily and/or can something else be done in the design, engineering, software, firmware, etc. to enable this simpler connection. This is a question for both marketing and whatever safety certification people responsible. Again, from the users point of view…
From numerous projects this has most often been the solution and often cost less too.
Lights and audible signal are the best indicators. Flashing to solid indicator light is best, colour change indicator light is guaranteed to be misunderstood. Audible signals work only if environment is suitable.
Geometry detail on the connector should point to or otherwise positively indicate which status. Common interface design mistake is to selectively cover up indicated positions (i.e. airplane bathroom door occupied/vacant) as its not definite to all people which state is being indicated - the airplane bathroom door example has inherent backup solution in case of mistake.
Spring load the connector so it won’t stay engaged without turning the dial. These are commercially available.
If your situation is fairly simple sometimes a clearly labeled English non-jargon indicator word is universally acceptable. I was surprised when informed of this, but it came to me from Danish, Chinese and Japanese marketing people.
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.