As described in section 2.2, the ability to differentiate between a fault in one direction or another
is obtained by comparing the phase angle of the operating current phasor, which varies directly
with the direction of the fault, and some other system parameter that is not dependent on the fault
location. This constant parameter is referred to as the polarizing quantity. For phase relays, the
polarizing quantity is, almost invariably, the system voltage at the relay location. Depending on
the voltage and current connections, a directional relay will operate either for normal load current
or fault current. In an electromechanical relay, the maximum torque on an induction disc
occurs when the two torque-producing fluxes are 90◦ apart in time and space. The space criterion
is easily obtained by the location of the flux-producing coils around the relay. The time
criterion is achieved by creating an appropriate phasor difference between the two operating quantities.