The Universal Torque Equation is a equation which governs the application of all types of relays. The equation has variables and constants which can be ignored for specific functions.
This equation can be used to describe the operation of any Electrical Relay by changing the signs of some of the terms or ignoring them entirely.
For example, to describe the overcurrent relay, K2 and K3 can be considered zero while K will be negative as it is used to describe the restraining torque.
The Equation will then become
T=K1I2-K
In the case of a directional power relay, K1 and K2 can be considered to be zero while K can be considered to be negative.