3-2
BE1-51 Functional Description
9137200997 Rev D
Power Supply
Operating power for the relay circuitry is supplied by a wide range, electrically isolated, low-burden power
supply. Power supply operating power is not polarity sensitive. The front panel power LED and power
supply status output indicate when the power supply is operating. Power supply specifications are listed
in Table 1-1.
Current Sensing
All relay models (except three-phase-and-neutral units) have two sensing ranges for each phase. Each
high/low sensing range has its own set of input terminal connections.
•
Five ampere CTs have: LOW (0.5 A to 4.0 A) and HIGH (1.5 A to 12.0 A).
•
One ampere CTs have: LOW (0.1 A to 0.8 A) and HIGH (0.3 A to 2.4 A).
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.
Tap Select and Tap Cal
Front panel
TAP
selectors and
TAP
CAL
controls are provided for selection and precise adjustment of
the phase and neutral (if provided) time overcurrent pickup points. The front panel
TAP
selectors, ten-
position BCD weighted rotary switches, select the desired current sensing pickup point, while the front
panel
TAP CAL
controls provide precise adjustment between the selected setting and the next lower
setting.
Neutral Defeat
Front panel mounted switch allows neutral current sensing to be disabled. When the switch is set to ON,
a current imbalance in the neutral circuit will not trip the output contacts.
Multiplexor
The multiplexor sequentially switches a voltage representing each of the sensed input currents to the
analog-to-digital converter and level detector.
Analog-To-Digital Converter and Level Detector
Analog dc voltages representing the sensed currents from the multiplexor are converted to binary
numbers (successive approximation) and applied to 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
Appendix A. 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
monitored 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 measures 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.
Microprocessor (Not Shown)
Some of the circuitry already discussed is part of the microprocessor and no definite lines are drawn to
separate the functions. The microprocessor compares the desired time delay (from the time trip
comparator and scaler) with the actual elapsed time from the counter. When the elapsed time reaches the
intended delay, the timed output relay is energized. During the time delay period, the front panel TIMING
indicator (i.e. pickup) associated with the detected phase is illuminated.
If targets are present, the front panel
TIME FUNCTION
target will be tripped, and the
A, B, C,
or
N
ELEMENT
target associated with the detected phase will be tripped. See Table 2-3 for the types of
targets that are present (depending upon relay configuration).
