A variable percentage relay requires more operating current at the higher through-currents, as shown
in Figure 7.4. Regardless of the specific design, however, conceptually, the contact-closing torque
(for an electromechanical relay) or tripping action (in a solid-state or digital relay) caused by the
current in the operating coil is proportional to the difference between the secondary currents. In
contrast, the contact-opening torque, or nontripping action, caused by the through-current in the
restraint coils is proportional to the sum of the two currents, with the additional requirement that
there must be current in both restraint windings.