An apparatus includes a mounting base and a compatible electrical unit removably carried on the base. The base carries a transducer and a sensor external to the unit. The transducer emits radiant energy toward the unit. The sensor receives radiant energy modulated by the unit. A local control unit can store the received, modulated radiant energy as a base-line profile. Subsequently, additional profiles can be generated and compared to the stored base-line to determine is a spatial characteristic of the electrical unit has changed.