BE1-50/51B-Functional Description
3-1
SECTION 3 • FUNCTIONAL DESCRIPTION
GENERAL
BE1-50/51B Overcurrent Relays are microprocessor based non-directional relays that measure ac current
to provide secure and reliable instantaneous and time overcurrent protection for power systems.
FUNCTIONAL DESCRIPTION
Sensing Input
Single phase ac current from system current transformers (CT) is brought into the BE1-50/51B Overcurrent
Relay at terminals eight and nine. Refer to Figure 3-1 to follow the functional description. The input current
is applied to internal power and signal CTs.
Power Supply
Current from the power CT is rectified, filtered, and supplied to all of the internal circuitry for operating power.
A pre5 Vdc supply also serves as a reference for automatic calibration.
Instantaneous Signal
Current from the signal CT is rectified and applied to the instantaneous scaling resistors controlled by the
INST PICKUP selector switches. The analog voltage of the instantaneous input signal developed across
the scaling resistors is filtered and applied to the multiplexor (MUX).
Time Signal
Current from the signal CT is also rectified and applied to the time scaling resistors controlled by the TIME
PICKUP selector switches. The analog voltage of the time input signal is also filtered and applied to the
multiplexor.
Microprocessor
Operating power from the power supply is applied to the microprocessor supervisor circuit. When the input
current falls below an acceptable level, the supervisor circuit interrupts the microprocessor and halts further
operation. A microprocessor watchdog feature resets the microprocessor program when the program flow
is interrupted.
Information from the TIME DIAL selector switches, the TIME CURVE selector switch, and the 50/60 Hz, INST
DELAY, and RESET CHAR switches is also applied to the microprocessor. The microprocessor uses these
inputs to set the operating parameters.
When the microprocessor is ready for analog information from the multiplexor, microprocessor control
signals cause the multiplexor to route the desired input through to the output. The output is converted from
an analog value to a digital value and applied to the microprocessor.
The microprocessor performs the program operations based on the inputs and the internal software
program. When the sensed current exceeds the TIME PICKUP setting, the TIME PICKUP LED is turned
ON. TIME contacts (51 and 51 AUX) are closed in accordance with the time characteristic equation. If the
sensed current exceeds the INST PICKUP setting, the INST contacts (50) are closed.
Power-Off Sensing
In series 200 relays, the power-off sensing circuits measure the decaying voltage to determine the length
of time that power is removed (zero current). This provides information for the integrating reset function
even when power has been entirely removed.
