How you detect a fault depends on the nature of the fault. A stuck contact on an e/stop is detected by the safety relay seeing one channel open but not the other. A welded-in safety contactor is detected by wiring the NC auxiliary of your safety contactor in series with your reset loop (and of course, making sure you're using the appropriate mechanically-linked contactors so you can't have the NC closed while the main poles are welded shut). A short circuit to 24V on a guard switch is detected by the use of a test pulse. And so on.
I 100% disagree that Cat 4 is "a goal and not a standard". I've put in hundreds of systems that reach Cat 4, many of them externally validated as Cat 4. Many of my clients do risk assessments on their machines and require a Cat 4 safety system implemented. I'll admit to having my tongue firmly in my cheek here, but a decent amount of my company's work at one client involves getting in new machines from the USA and completely re-doing the safety systems
On a simple system, you can reach Cat 4 with nothing more than an off-the-shelf safety relay, an e/stop, and some safety contactors. In larger, more complex systems, you might need a full safety PLC to do the job. I've had systems validated with a single guardmaster safety relay, and others with a Guard Logix and multiple remote safety I/O racks. Horses for courses.
Whether or not you can put things in series (and what safety category you end up with) is a very broad question without a simple answer. It depends an enormous amount on the specifics of the system. A general rule of thumb would be that you can put e/stops in series and meet any safety category, but if you put guard switches in series, you can generally only meet Cat 3, and even then it's not a sure bet. Depends on a lot of other factors. Here's another thread that discusses a lot of these sort of issues - in my post #23 there's an explanation of fault masking and how that applies to putting things in series.
I 100% disagree that Cat 4 is "a goal and not a standard". I've put in hundreds of systems that reach Cat 4, many of them externally validated as Cat 4. Many of my clients do risk assessments on their machines and require a Cat 4 safety system implemented. I'll admit to having my tongue firmly in my cheek here, but a decent amount of my company's work at one client involves getting in new machines from the USA and completely re-doing the safety systems
On a simple system, you can reach Cat 4 with nothing more than an off-the-shelf safety relay, an e/stop, and some safety contactors. In larger, more complex systems, you might need a full safety PLC to do the job. I've had systems validated with a single guardmaster safety relay, and others with a Guard Logix and multiple remote safety I/O racks. Horses for courses.
Whether or not you can put things in series (and what safety category you end up with) is a very broad question without a simple answer. It depends an enormous amount on the specifics of the system. A general rule of thumb would be that you can put e/stops in series and meet any safety category, but if you put guard switches in series, you can generally only meet Cat 3, and even then it's not a sure bet. Depends on a lot of other factors. Here's another thread that discusses a lot of these sort of issues - in my post #23 there's an explanation of fault masking and how that applies to putting things in series.