Application of the Ohm and Mho Principles to Protective Relays

THE mho relay unit was first used 15 years ago as the directional unit for an early-type reactance relay. Thus it can be said that “mho” applied to relays is a new term applied to an old principle. The mathematical justification for the term has been given in a paper1 describing the use of the mho unit in a carrier-current relaying scheme. The mho unit in its present form is a directional impedance unit whose accuracy is comparable with that of the reactance ohm unit used in a distance relay.