What is a MHO Relay : Working & Its Applications

Relay is one kind of electrically operated switch. The main function of this switch is to control the circuit electronically & electromechanically by connecting and disconnecting the circuit. The applications of relays include control panels, automation of building, manufacturing to control the power. There are different factors that required to be considered while selecting a relay within transmission lines are saving power, normal load stability, cost, arc & fault resistances, etc. The major relays that we have used in different applications are reactance, simple impedance, and Mho relay.

What is MHO Relay?

A Mho relay is also called the admittance relay and it is a high-speed relay. In this kind of relay, the operating torque can be attained through the element of Volt-Amperes whereas the controlling part can be developed because of voltage element that means this relay is a directional relay that is controlled through voltage.

Mho Relay Construction
Mho Relay Construction

These relays are used in long transmission, medium, and short transmission lines. The working principle of Mho relay is, it used for long-distance transmission lines because these lines generally experience power swinging transients as well as load shedding problems. Thus, mho relays are used in practice to provide improved accuracy whenever these transients occur.

Operating Characteristic of Mho Relay

In this type of Relay, the working torque can be obtained through the V-I characteristic & limiting torque through the characteristic of voltage that means, this is a directional relay controlled by voltage.

From the equation of universal torque,

T = K1I2+K2V2+K3VICos (Ɵ-Ƭ) +K4


By substituting K1 = 0, K2 = -1 & K4 = 0, then we can get the Torque within this relay like

T = K3*VI*Cos(Ɵ-Ƭ) – K2*V2

To operate the relay, T greater than 0K3VICos (Ɵ-Ƭ) – K2V2 >0

K3*VICos(Ɵ-Ƭ) > K2V2

V2/VI < K3* Cos (Ɵ-Ƭ)/K2

V/I < (K3/K2) * Cos(Ɵ-Ƭ) (Here, Z = V/I)

So, Z < (K3/K2)*Cos(Ɵ-Ƭ)

Operating Characteristics

Once the operating characteristics of this relay are drawn over the diagram of R-X is a loop that passes throughout the source. This can be drawn through the relation like Z< (K3/K2)*Cos (Ɵ-Ƭ).

From the following relay characteristics, we can notice that the relay will work if the impedance observed through it is in the loop. It is very clear from the image of the characteristics that the Mho Relay is the track itself. So we don’t require any directional part for this Relay.

Operating Characteristic of Mho Relay
Operating Characteristic of Mho Relay

The impedance which is observed through the Relay mainly depends on the kind of error. If the error is 3-phase then the relay can observe positive series impedance. If the fault is from line to ground then the Mho Relay can notice the sum of positive, negative & zero series of impedance.

Therefore for the relay actuation, a special impedance setting is necessary for different kinds of error. However, to make the relay include similar sensitivity for all kinds of error it is required for the Mho relay to calculate familiar impedance in all kinds of error. This kind of error is a positive series impedance. So for all kinds of faults, this relay is used to gauge the positive series impedance. Whenever the positive series impedance is detected under the setting within the Relay, then it will generate a Trip command.


The applications of Mho relay include the following.

This relay is used to protect the transmission lines like UHV/EHV. Generally, these are mainly utilized for protecting the long transmission line because of its constancy in the power swing case. Further, no special directional element is necessary in the case of this relay, because these relays are naturally directional.

These are extensively used for protecting the series compensated & uncompensated transmission lines to decide the fault’s location.

Thus, this is all about an overview of the Mho relay, construction, working principle, characteristics, and applications. This relay plays a key role in protecting the transmission lines. An alternate name of this relay is admittance relay and high-speed relay. Here is a question for you, what are the different types of relays used in different applications.