NYC Building Code: 2.19.2

2.19.2.1 Purpose.
Protection shall be provided with a means to detect unintendend car movement (1.3) and stop the car movement, as a result of failre in any of the following:
  (a) electric driving-machine motor, brake, coupling, shaft or gearing
  (b) control system
  (c) any other component upon which intended car movement depends, except suspension means and drive sheave of the traction machine

2.19.2.2 Where Required and Function.
All electric traction elevatros shall be provided with a means (see 2.26.2.30) that shall
  (a) Detect unintended car movement in either direction away from the landing with the hoistway door not in the locked position and the car door or gate not in the closed position.

    Note [2.19.2.2(a)]: Freight elevators provided with combination mechanical locks and contacts on the hoistway door shall detect the closed position of the hoistway and the closed position of the car door or gate.

    (1) If the detection means requires electrical power for its functioning then
      (a) a loss of electrical power to the unintended movement detection and control means shall cause the immidiate actication of the emergency brake as required in 2.19.2.2(b)
      (b) the occurence of a single ground, or the failre of any mechanically operated switch that does not meet the requirements of 2.26.4.3, any single magnetically operated switch, contactor, or relay, or any single solid-state device, or software system failure, shall not render the detection detection means inoperative.
    (2) The failure of any single mechanicallt operated switch taht does not meet the requirements of 2.26.4.3, shall not render the detection means inoperative.
    (3) When a faule specified in 2.19.2.2(a)(1)(b) or 2.19.2.2(a)(2) is detected, the car shall stop at or before the next landing for which a demand was registered, and shall not be permitted to restart.
    (4) Once actuated by unintended car movement, the detection means shall remain actuated until manually reset, and the car shall not start or run unless the the detection means is reset.
  (b) Upon detection of unintended car movement, stop and hold the car, with any load up to the rated load [see also 2.16.8(h)], by appying an emergency brake conforming to 2.19.3. The stopped position of the car shall be limited in both directions, to a maximum of 1220 mm (48 in) as measured from landing sill to the car sill. The car shall not start to run unless the emergency brake provided for the unintended movement detection is reset.