Table of Contents
Advertisement
8-4
Applications
Ring Network Protection
Trip Circuit Supervision
857-UM001A-EN-P – July 2009
Note: Used for protection of ring
main circuit with one feeding point
Figure 8.5 – Feeder Terminals Allen-Bradley 857
Ring networks can be protected with complete selectivity using
directional overcurrent relays as long as there is only one feeding
point in the network. Figure 8.5 shows an example of a ring main
with five nodes using one circuit breaker at each end of each line
section (e.g. a ring main unit). When there is a short-circuit fault in
any line section, only the faulty section will be disconnected. The
grading time in this example is 150 ms.
Trip circuit supervision is used to ensure that the wiring from a
protective device to a circuit-breaker is in order. This circuit is
unused most of the time, but when a feeder device detects a fault in
the network, it is too late to notice that the circuit-breaker cannot be
tripped because of a broken trip circuitry.
The digital inputs of the device can be used for trip circuit
monitoring.
Internal Parallel Digital Inputs
In Allen-Bradley 857-3C7 and Allen-Bradley 857-3C8, the output
relays T5 (DI29), T6(DI30), T7(DI31) and T8(DI32) have internal,
parallel digital inputs available for trip circuit supervision.
Advertisement
Table of Contents
