BE1-51/27R - Functional Description
3-3
M
I
I
t
×
V
N
V
Current Sensing - Continued
Three-phase-and-neutral sensing units, however, have one of four possible combinations of input sensing
ranges with one range for neutral and one range for the phases.
Five ampere CTs have: 0.5 A to 4.0 A and 1.5 A to 12.0 A.
One ampere CTs have: 0.1 A to 0.8 A and 0.3 A to 2.4 A.
A front panel TAP selector and TAP CAL control are provided for selection and precise adjustment of the
time overcurrent pickup point. The front panel TAP selector, a ten-position BCD weighted rotary switch,
selects the desired current sensing pickup point, while the front panel TAP CAL control provides precise
adjustment between the selected setting and the next lower setting.
Voltage Sensing
Input voltages for each phase to be monitored are applied to the voltage sensing circuits. Each voltage
sensing circuit consists of an input transformer, rectifier, and filter. Analog voltages from the voltage sensing
circuits are applied to the multiplexor. Note that neutral is not monitored for voltage.
Multiplexor
The multiplexor sequentially switches a voltage representing each of the sensed inputs (sensed currents
or sensed voltages) to the analog-to-digital converter and level detector.
Analog-To-Digital Converter and Level Detector
Analog dc voltages representing the sensed currents or sensed voltages from the multiplexor are converted
to binary numbers (successive approximation) and applied to the voltage restraint circuits and the time trip
comparator and scaler circuits.
Time Trip Comparator and Scaler
This circuit accepts both the binary number representing the detected current level and the desired time
delay characteristic selected by the front panel TIME DIAL, then computes the required time delay before
the timed output relay will be energized. Time delay characteristics are shown in the curves located in
Section 1. If extended timing range options 2-D or 2-E is present, the time delay characteristic curves are
modified so that the time delay is approximately 5.7 times the derived value.
Time delay computations are updated continuously so that changes in the overcurrent condition are moni-
tored and result in a corresponding change in the time delay. A software counter begins counting when the
initial binary number is received from the analog-to-digital converter and level detector. The counter mea-
sures the elapsed time of the overcurrent condition, and resets if the current decreases below the pickup
point. This continuously increasing binary number is then passed to the comparator.
Voltage Restraint
Voltage restraint compares the binary number representing the monitored voltage with the fixed nominal
voltage limit (100 Vac for 50 hertz systems and 120 Vac for 60 hertz systems). When the voltage is between
100 percent and 25 percent of the nominal voltage, the circuit automatically lowers the selected time
overcurrent pickup point proportionally. Instantaneous overcurrent operation, if present, is not affected.
Neutral is not monitored for voltage, nor is the neutral time overcurrent pickup point restrained. When the
voltage is above the 100 percent limit, the pickup point is restrained to 100 percent. When the voltage is
below the 25 percent limit, the pickup point remains at the 25 percent point.
For a given level of current above pickup, a lowering of the pickup point, via voltage restraint effectively
increases the multiples of current. This shifts the time delay characteristic to the right on the multiples-of-
pickup-current axis (characteristic curves) as voltage drops from 100 percent to 25 percent. To find the
effective multiple-of-pickup-current point on the axis, use Formula 1.
Formula 1