A crash detector arrangement comprises a crash sensor(1) in the form of an accelerometer which provides a signal to a micro-processor(2) which uses a first algorithm to determine if the signal is representative of a crash of a predetermined severity and which consequently provides an output(8) rendering a first switch(9) conductive. The output of the accelerometer(1) is provided also to the micro-processor(2) through a high pass filter. This input to the micro-processor is processed by another, different, algorithm determined to differentiate a crash from a signal error in the accelerometer. A second output(7) is generated by the micro-processor which is used to assist in the control of a further switch(13). The two switches(9,13) are in a series connection incorporating a source of power(10) and a resistor(12). When current passes through the resistor, the safety device is activated. The second output(7) of the micro-processor(2) passes through an AND-gate(6) which also receives an input from a threshold detector(4) and from a check circuit(14).