than a relay with a slope of 40 %. Although we have used lower-case symbols for the currents,
signifying the instantaneous values, it should be clear that the corresponding relations exist between
the r.m.s. values of the currents as well. Thus, equations (8.1)–(8.5) also hold for the r.m.s. currents.
A practical percentage differential characteristic is shown in Figure 8.4. The relay slope determines
the trip zone. The three sources of differential current during normal transformer operation
are shown, as is the margin of safety used in arriving at the slope. The relay has a small pickup
current setting, i.e. the relay does not operate unless the differential current is above this pickup
value. The pickup setting is usually set very low: typical values are 0.25 A secondary. This accounts
for any residual CT errors at low values of transformer load current.