External monitoring of plc sequence

Hello guys, I need to monitor a critical shutdown sequencer in order to guard against for example programming errors. If the sequencer does not "do its job" I have the option of enabling the backup relay logic shutdown system. I need some way to monitor along the lines of:
after x seconds a has not happened -> enable backup system
after y seconds b has not happened -> enable backup system.. and so on

Is this a known problem, how does one go about solving it? hardwired timing relays?

any help appreciated, thanks

Use a safety PLC and rest much easier. If you do need a secondary system use redundancy modules or another plc of a different brand for cross checking and takeover.

This is how safety used to be done in many cases with 2 plcs of different brands in conjunction with hard wired safrty's also.

I would do a risk assessment first as a single safety plc will likely fit the bill in most cases.