Model 17®, 27, 37 Fail-locked 17®Universal electric strike, standard, with FaFix® (FF) The door can only be opened while a contact is made. In alternating current operation, a humming noise is audible. In direct current operation,... Hold-open function 27The latch bolt controlled locking pin in the centre of the electric strike latch keeps the electric strike unlocked until the door is opened, even after contact has been made. Fail-unlocked 37The electric strike is locked for as long as the power is on. If the power is switched off or if there is a power cut, the electric strike latch can be moved and the door can be opened.