I have looked for standards without success.
Ladder type schematic diagrams are fairly standard, but some have more information than others.
Additional information not found on all drawings:
Line numbers; fairly common.
Wire numbers; optional, may be consecutive, a better scheme is wire numbers tied to line numbers.
eg: Line numbers are multiples of 10 or 100. Wire numbers on each line are consecutive to the line numbers, The first wire on line 100 will be 101, the next wire number will be 102, and so on.
Operator numbers; Each relay coil or contactor coil may have suffix numbers to the right of the right hand vertical line or L2.
The suffix numbers will indicate each line with a contact controlled by that relay or contactor.
Contact subscripts; below each contact may be a number indicating the line containing the operating coil for this contact.
Embedded terminal numbers;
By that I mean the numbers moulded or stamped on a relay adjacent to the terminals.
In many applications it is important which particular terminal or contact set is used in a particular circuit. This is to avoid inadvertent opposite polarity connections.
On many drawings these numbers are shown both to avoid opposite polarity connection and to aid in trouble shooting.
Information with more variety;
Multi contact selector switches and push buttons.
Where the normally open contact and normally closed contact of a two pole contact are on different lines the mechanical connection may be shown with a dotted line.
More complex devices may include a truth table to indicate the individual contact positions at the various switch positions.
Where to use dotted lines versus truth tables? This is a moving target. Different engineering firms and different manufacturers will do this differently.
Power wiring associated with a control schematic; A wide variety of standards.
Boolean; Often a Boolean diagram may be arranged in a similar format to a ladder diagram. I have found these to be quite easy to follow for trouble shooting.
Diagrams incorporating a Programmable Logic Controller or some type of Control Module.
The diagrams that I find the easiest to follow are often arranged with the inputs on one side and the outputs on the other side.
The panel wiring may be shown on one drawing with the PLC of CM shown as a rectangle, and the internal circuit either not shown or shown on a separate drawing.
When I had to draw a schematic diagram I tended to either try to copy the format already in use for the plant in question or to use a format that I had found to be easy to use for trouble shooting.
Anecdote alert: When I was half of the age that I am now and much more than twice as smart, I designed a control panel for an oil heater in a small refinery.
The drawing was fully annotated and the PLC Boolean logic was depicted as a conventional ladder diagram,fully annotated.
The panel was built and put into service with no issues.
Several months later, I left the refinery in Toronto for an extended vacation near Vancouver.
Then the program crashed and the plant went down.
I got a phone call out on the west coast.
The plant was willing to fly me back to Toronto if needed.
I set up another call in an hour or two. On the Toronto end were the plant engineer and the plant instrument mechanic with the prints spread out on the board room table. On my end I had a large supply of paper on the kitchen table.
With the full annotation on the prints, I was able to quite easily reproduce the section of the circuit where the bug was hiding.
Once I had the circuit copied I was able to locate the bug and instruct the instrument mechanic how to connect the programming device and insert one jumper in the Boolean logic.
The plant was back online within an hour.
The point is: Annotation is important.
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Ohm's law
Not just a good idea;
It's the LAW!
