Most electric strikers are fail secure as you would want your gate or front door to remain locked in the case of power failure for security reasons. However, there is also a place and time for the Fail Safe configuration, such as Fire Exits. In case of fire or any emergency, power is likely cut off and the fire exit door will be unlocked making sure people can evacuate.
To get more technical with this, Fail Safe electric striker requires a Normally Closed (NC) relay and Fail Secure requires a Normally Open (NO) relay. So, you will also need a Power Supply Control Unit (FK785) to "connect" or "break" the circuit relay depending on which configuration your electric striker is. Simple diagrams below explain how all the components work together.
Normally Open - Fail Secure
In a 'resting' state, there is no electricity running to the electric striker (FK660) in Fail Secure configuration. The striker is normally in a 'locked state'. When the button is pressed, a pulse is sent from the button to the Power Control Unit (FK785), the Control Unit then closes the relay of the electric striker enabling electricity to flow through causing it to unlock for an amount of time, which can be set. Once the time runs out, the electricity is cut off meaning the relay comes back to 'Open State', causing the striker to go back to its 'locked state'.
Normally Close - Fail Safe
In Fail Safe Configuration, there is always electricity running to the striker or Electromagnetic Lock to keep them locked in their 'resting' state. Power is needed for the two sides of Magnetic Lock to become magnets so without power, they are just pieces of metal. Once the button is pressed, Power Control Unit breaks the closed relay stopping electricity flowing through, causing the striker to unlock and electromagnetic lock to become 'non-magnetic' allowing the door or gate to be opened. Once the time runs out, relay is closed back enabling electricity to flow through causing striker to go back to locked state and magnetic lock to become magnetic again.