Safety relays sequences

[robert_]

I wonder how that security systems using "normal relay sequences". In a recent project I had the opportunity to be, Mitsubishi-Hitachi developed for it, in security systems a series of sequences of relays (instead of using safety relays) for emergency stops that do not quite know how . In fact, reading in a book entitled: "programmable controllers: an engineer's guide" (comes in google books) http://books.google.com.mx/books?id=h4yIudamf7oC&printsec=frontcover&dq=programmable+controllers+an+engineer#v=onepage&q=programmable%20controllers%20an%20engineer&f=false I found on page 305 to speak precisely about this, supposedly using a method of "sequence relays" for emergency stops (as this the beginning of a traffic light circuit. Someone could explain how this works? Thank you very much in advance.