A SECURITY MONITORING SYSTEM

The invention relates to a security monitoring system for a building or a secured space within a building, the system including:a control unit for controlling, arming and disarming the security monitoring system, and having a radio frequency transceiver;a door giving access to the building or the secured space;an electrically controlled lock which is lockable to lock the door to prevent the door being opened from outside the building or the secured space, and which can be unlocked to enable the door to be opened from outside the building;a disarm node for disarming the system, the node having a user interface to receive a user input to change the status of the system, and a radio frequency transmitter to transmit, in consequence of the user input, a status change message to the control unit, the disarm node being located outside the building or the space;and a portable authentication device to permit hands-free disarming of the security monitoring system, the portable authentication device including a radio frequency transceiver,a memory storing a portable authentication device ID that is registered with the control unit,a motion sensorand a processor operatively connected to the transceiver, the motion sensor and the memory;wherein the motion sensor is configured to sense motion of the portable authentication device and to provide an output based on sensed motion of the portable authentication device, and wherein the processor is configured to determine whether the portable authentication device is stationary or in motion,based on the output of the motion sensor;the disarm node being configured to:(i) transmit from the portable authentication device's transceiver, using a short-range transmission mode, a wake up message including a status change event identifier;the portable authentication device being configured to respond to receipt of the wake up message by transmitting, using the portable authentication device's transceiver, a response message including the status change event identifier and the portable authentication device identity, the portable authentication device's transceiver only transmitting the response message if, when the instruction is received, the portable authentication device is not deemed to be stationary;;the control unit being configured:in response to receiving the response message from the portable authentication device, to :(iv) check whether the status change event identifier is for a current status change event;(v) check whet