Do E-Stop switches/buttons normally operate relay switches?
To start with, the NC contact of an E-stop button served to de-energise a motor contactor / clutch / brake or a pneumatic / hydraulic solenoid valve, in order to shut down a machine during an emergency.
Accidents, as a result of failure to shut down caused by a faulty E-stop button, were prevented by multiple E-stop button NC contacts wired in series.
Other safety systems were then progressively introduced e.g. safety guard interlocks, two-hand push button operation, light curtains etc.
The key component used in the functioning and integration of these systems is the relay. Fail-safe dual redundant relays are in turn monitored by a third relay to prevent a machine start, should one of them have failed.
The ultimate solution would be a safety PLC which can handle not only the functions of a standard PLC but also the safety functions that have been integrated into it.