Lighting device

A method for controlling a lighting apparatus comprising at least one light source and primary and secondary power connections which provide power to a light source; wherein the method comprises controlling power delivery to at least one light source such that in event of power failure from the primary power connection, power from the secondary power connection is used to power a light source; the external impedance of the primary power connector is sensed using measurement pulses and the power delivery is controlled in response to these impedance measurements. Sensing of the impedance may be used to distinguish between a user opening a switch connected to the primary power connection or a failure of the power supply to the primary power connection. The method may be used to provide emergency lighting. Additional lighting devices are also disclosed: devices which vary brightness in response to user configuration of primary and secondary operating modes; devices wherein a primary connection powers a primary light source and secondary connection powers a secondary light source and devices including an isolator to isolate sensing circuitry from other components. sources using power from the secondary power supply.