9252000991 Rev N
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 overcurrent relay at
terminals 5 and 6. 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 relay 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
microprocessor is active and executing code, the ACTIVE/PICKUP LED is green. When the input current
falls below an acceptable level, the supervisor circuit interrupts the microprocessor, halts further
operation, and turns OFF the ACTIVE/PICKUP LED. 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, INST DELAY
switches, and RESET CHAR switch 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 and 51 AUX) are closed in accordance with the TIME characteristic
equation. If the sensed current exceeds the INST PICKUP setting, the INST contact (50) is closed.
Power-Off Sensing
In 200 series relays, 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.
www
. ElectricalPartManuals
. com
