9171000990 Rev E BE1-50 Functional Description 3-1
SECTION 3 • FUNCTIONAL DESCRIPTION
INTRODUCTION
BE1-50 Instantaneous Overcurrent relays are static devices that respond to the current magnitude of the
monitored circuit. BE1-50 relay functions are illustrated in Figure 3-1 and described in the following
paragraphs.
Figure 3-1. Function Block Diagram
STEP-DOWN TRANSFORMER
Monitored system currents are applied to the primaries of internal current transformers and stepped down
to internal circuit levels. The internal current transformers provide 1,500 V of isolation for the twelve-volt
logic circuits of the relay.
FULL-WAVE RECTIFIERS
Outputs from each step-down transformer are full-wave rectified and applied to resistor networks to
develop voltages that represent the magnitude of the monitored system currents.
RESPONSE CHARACTERISTICS
Input current signals are rectified and passed through a low pass filter. Filtering smoothes out current
spikes and provides a degree of security against operation on short-term transients.
Filtering also smoothes out the harmonic component effects on current signals. Harmonic component
effects are reduced approximately by 1/n where n is the order of the harmonic. For example, the third
harmonic effect is reduced by filtering to 1/3 of what is would be without filtering. With filtering, the
response characteristics are similar to those of electromechanical relays.
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