Rayleigh instruments ri f368 series single and three phase meters rayleigh instruments ri f368 21 the rayleigh instruments ri f368 21 is a din 96 panel mounting class 1 kilowatt hour meter that can measure single phase or three phase 3 or 4 wire network unbalanced loads the input is via a 5a or 1a current transformer driven 230 415v 50 60hz system it has a pulse output rayleigh instruments ri f368 21 single and three phase meter pulse output