Abstract: A monitoring means for apparatus having a number of functional stations, said means comprising a plurality of sensors or groups of sensors located to monitor the operation of at least some of said functional stations, each sensor having a critical condition indicator adjacent the respective station, a central monitor having a power supply connected via a common connection to each of the sensors and a common signal line connected to the output of each of said sensors wherein each sensor provides an output on activation of the critical condition indicator, said central monitor having an alarm means activated through said signal line wherein each sensor and/or group of sensors provides an output signal to said signal line characteristic of said sensor or group of sensors to cause the alarm means to be activated in a manner characteristic of said sensor or group of sensors.