RESTRAINT CHARACTERISTICS

At high current levels, the inevitable difference in the

saturation characteristics between current transform

ers requires a compensating decrease in relay

sensitivity. The design of the BE1-87G provides a

restraint factor that is proportional to input current.

Specifically, the BE1-87G compares the two sensed

currents (in and out) to determine which is less. This

lower current level becomes the restraining current.

The difference between the two sensed currents (I0

) is compared to a reference established by the sensitivity

setting and adjusted by an amount proportional to the

restraining current. (The typical ratio is given in the

formula on page 3.) This makes the BE1-87G more

sensitive to low current internal faults, for which the

relay should operate, and less sensitive to external

faults with high levels of through current for which the

relay should not operate.