40 years ago early in my career I got to design both my radio circuits and my test equipment circuits. The standard for the test equipment power was a crowbar circuit- basically a zener is used to trip a triac to short the circuit, blowing the fuse. This works for overvoltage (and most references describe this), but if you put the trigger upstream of a resistor inline with the load, you can set it to trip due overcurrent – the incremental voltage drop across that inline load sets the overcurrent limit.
These were all DC circuits. So for the OS with 24VAC, it would probably need to trigger the crowbar off the current-measurement rectifier.
I am considering inline fuses for the solenoids. Ideally I’d have one 0.3-0.5A fuse per solenoid, and a 1-2A fuse per com line. The former fuses protect the individual triacs, and the latter allow a multiple fault situation (solenoids operating in parallel) to not (usually) blow the onboard fuse, and thereby allow the control to continue to operate.
With the above, if no current flow (or the wrong current flow) is detected when a triac is closed, it means either no solenoid is connected or one of the fuses is blown. This means a simple diagnostic program could be run (as I was running to test my installation). If it shows no current flow when a zone is switched on, you know that either a wire is disconnected or the zone fuse is blown. Furthermore, the OS could log the lack of current during the program.
Ideally these could be placed in a closed fuse block I’d mount adjacent to the OS. I’m not finding one online for 5×20 glass fuses, although they are common for the blade fuses. Unfortunately, the blade fuses are only available down to 1A ratings (and only in the full-size blade package), so the margin gets a lot lower. Blade fuses are designed for DC automotive application but they are rated for 24V DC busses, which means they are actually rated to 32V. I’d want to test, but in a clean environment I think they’d be okay with the ~ 37V peak that might be across a blown fuse in a 24Vac nominal system.
For the glass fuses it looks like I’ll need to either create a bundle of inline fuse holders or mount some PCB sockets to protoboard and put it in a case. Once I put it on a circuit board, I might as well add a couple of LED indicators to each circuit – one is parallel to the fuse, the other goes from the switched port to COM. The latter will illuminate if the board fuse and triac are okay when the zone is switched on, the former will illuminate if the fuse is blown due to a short.