6-10 B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALMETERING CHAPTER 6: ACTUAL VALUES6Figure 6-2: UR phase angle measurement convention6.4.2 Bus zoneACTUAL VALUES METERING BUS BUS ZONE 1(6)These values display the differential and restraint currents phasors for each bus zone. The magnitudes are displayed inprimary amperes.6.4.3 CurrentsACTUAL VALUES METERING CURRENTSThe metered current values for each terminal are displayed in this menu. BUS ZONE 1 BUS ZONE 1 DIFF:0.000 A 0.0° BUS ZONE 1 REST:0.000 A 0.0°There is no cutoff level applied to the differential and restraint currents computed by the B90. Therefore, a smalldifferential current reflecting CT inaccuracies and bus leakage current can be present during balanced conditions.This is done to achieve a more accurate differential balance and to provide a direct indication of the real differentialcurrent, as excluding small currents or not accounting for CT errors can impose significant unaccounteddifferential current. It is advisable to set the minimum pickup for 87B protection greater than the maximumdifferential current during normal load conditions. CURRENTS F1 CURRENT:0.000 A 0.0° F2 CURRENT:0.000 A 0.0° S8 CURRENT:0.000 A 0.0°
