BE1-50/51M - Functional Description 3-1
SECTION 3 • FUNCTIONAL DESCRIPTION
GENERAL
BE1-50/51M 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 BE1-50/51M 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 precision +5 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
microprocessor is active and executing code, ACTIVE/PICKUP LED is green. When the input current falls below
an acceptable level, the supervisor circuit interrupts the microprocessor and halts further operation, and turns
OFF the ACTIVE/PICKUP LED.
Output pulses from the microprocessor are applied regularly to the watchdog timer during normal program
operation. If something occurs to disrupt the microprocessor, these pulses stop, the watchdog timer times out
and sends a reset pulse to the microprocessor. The microprocessor then resets and resumes normal operation.
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 ACTIVE/PICKUP LED turns from Green to
RED. TIME contacts (51) are closed in accordance with the time characteristic equation. If the sensed current
exceeds the INST PICKUP setting, the INST contacts (50) are closed.
Courtesy of NationalSwitchgear.com
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