BE1-51/27C Functional Description
3-3
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 circuits and to the voltage inhibit
circuits.
Analog-To-Digital Converter and Level Detector
When the voltage representing the actual sensed inputs meets or exceeds the selected pickup point, this
circuit converts the level to a corresponding binary number and applies it to a time trip comparator and scaler
circuit and to a counter for calculation of the required time delay.
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. The 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 thus, 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 ELE-
MENT
target associated with the detected phase will be tripped. See table 2-4 for the types of targets that
are present (depending upon relay configuration).
If option 3 is present, an auxiliary output relay (with either N.O., N.C., or SPDT contacts) is also actuated
when the timed output relay energizes.
Summary of Contents for BE-51/27C
Page 1: ...0 1 2 3 4 5 67869 ABC DEF G 6H II GGJ KL M DEF DEK...
Page 16: ...BE1 51 27C Controls and Indicators 2 2 Figure 2 1 Location of Controls and Indicators...
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Page 22: ...BE1 51 27C Functional Description 3 2 Figure 3 2 Functional Block Diagram...
Page 32: ...BE1 51 27C Installation 4 7 Figure 4 10 Typical Internal Diagram Sensing Input Type L...