Is there a set of standard symbols for connectors / wires / protocols?
I have seen umpteen different schematics when i used to do elictrical installations.
The only ones that followed any guidelines are in Building electrics. 

When it comes to networking and custom installations it more important to have the plug type(any symbol really as long as it the same across the drawing and legend) clearly marked(RJ-45), the cable configuration standard(T-568A to T-568A) and a reference to all the standards near the legend. 

The protocol should not be mentioned as its not relevant to installations.

I cant find any perfect examples now- people just use lines and squiggles now a days and leave everything to assumption of plug and play.



This is a good example. But again you might want to be more specific with symbols on either side of the red line for eg to denote a shielded RJ-45 and the red line is a Shielded CAT6. other wise its all left to technicians assumptions. This diagram does not take into account spatial requirements. Like if I had to install this in a house I would just place it in the room that feels most comfortable and could result in poor wireless performance. So the best would be to over lay it onto a floor plan.





This is a good example but not perfect.





This is fair (50/50) examples. As it specifies the cable- but no standards used- The technician will have to assume. Like for the camera? What would you use RJ-45 or USB?



This is a poor example. As it describes the type of cables and plugs- but again the technician is left to assumptions of standards. Does the designer want Cat6 or Cat5E? 100mbs/ or 1Gbps.. etc. IS the patch a cross over?