I have a bunch of devices that will sit on a two-wire bidirectional RS485 bus (DMX-like, but not quite DMX).
I'm trying to protect against an edge case, where a user mis-wires a ballast, and inadvertently connects mains voltage to the bus, causing the it to become live and potentially injuring anyone else working on it.
I will be installing an 'isolating box' between the bus and the user, essentially behind a wallplate, such that the bus itself can never be directly accessed. The connection to the bus itself would be through opto-isolators and associated circuitry (with a separate isolated supply), however I'd like to 'protect the protection circuitry' if possible, such that it won't catch fire and can gracefully recover.
Would a couple of PTCs, and a couple of TVSs (for redundancy) as below, be enough to reliably save any low voltage circuitry to the left of the circuit, when IO1 and IO2 are connected directly to 240V mains?
Normally, IO1 and IO2 would see ~22v at roughly 2mA.
Thank you kindly