ABB RELION REX640 Technical Manual page 115

Protection and control

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1MRS759142 F

In example 2, the fail-safe approach aims at securing circuit breaker tripping even

if a relay fails. A relay in a panel measures also the adjacent panel's currents (and

voltages) and receives the necessary primary device's position information. In other

words, the relay in a panel functions as a backup relay for the adjacent panel.

This approach allows service continuation while the failed relay is waiting for spare

parts or a complete replacement. The backup protection features provided by the

adjacent panel's relay do not necessarily fully match the features available in the

Figure 37: Redundant protection fail-safe principle, example 3

In example 3, the fail-safe approach aims at securing circuit breaker tripping even if

one of the redundant relays fails. The scheme is often referred to as the 1-outof- 2

approach. This approach allows service continuation while the failed relay is waiting

for spare parts or a complete replacement. The redundancy in this example covers

relays and circuit breaker tripping coils but it can be expanded to auxiliary power

supplies (two station batteries and isolated distribution), cabling, circuit breaker

failure protection, and so on. Another variant of this approach is to have a main relay

and a backup relay instead of two fully redundant relays. The backup relay does not

Technical Manual

Feeder #1 panel

Feeder #2 panel

Circuit breaker (CB)

Relay trip output #1

Relay trip output #2

Protection relay

CB trip coil 1

CB trip coil 2

Miniature circuit breaker

Circuit breaker (CB)

Protection relay #1

Protection relay #2

Protection relay trip output

CB trip coil 1

CB trip coil 2

Miniature circuit breaker

Basic functions