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Description
of operation
The combined overcurrent and earth-fault re-
lay is a secondary relay device to be connected
to the current transformers of the feeder to be
protected. The three-phase overcurrent unit and
the non-directional earth-fault unit continu-
ously measure the phase currents and the neu-
tral current of the protected feeder. In fault situ-
ations these units initiate external auto-reclose
functions or trip the circuit-breaker, depending
on the selected protective scheme.
When a phase current exceeds the starting value
of the low-set overcurrent unit, the unit starts,
simultaneously starting the corresponding tim-
ing circuit. When the set operating time has
elapsed, a circuit-breaker tripping command is
delivered. Correspondingly, the high-set stage
of the overcurrent unit starts when its starting
value is exceeded, starting its timing circuit and
performing a tripping when the set time has
elapsed.
The low-set stage of the non-directional earth-
fault unit operates in the same way. Depending
on the protective scheme it either signals, per-
forms a tripping or initiates a function of an
external auto-reclose relay.
IL1
IL2
IL3
Io
Blocking or
reset
Fig. 1. Protective functions of the overcurrent and earth-fault relay SPAJ 142 C.
The low-set stage of the overcurrent unit and
the low-set stage of the earth-fault unit may be
given definite time or inverse definite minimum
time (IDMT) characteristics. When the IDMT
characteristic is to be chosen six curve types are
available in the relay. Four of the curves types
comply with BS 142 and IEC 60255 and are
named normal inverse, very inverse, extremely
inverse and long-time inverse. The two addi-
tional curves are named the RI curve and the
RXIDG curve.
By appropriate programming of the tripping
relay matrix, the starting signals of the overcur-
rent and non-directional earth-fault modules are
received as contact functions. This contact in-
formation is used e.g. for the blocking of co-
operating protective relays located upstreams.
The relay comprises one external logic control
input, which is actuated by a control signal of
the auxiliary voltage level. The influence on the
relay by the control input is determined by pro-
gramming switches in the measuring module.
The control input can be used either for block-
ing one or more of the protective stages, for re-
setting a latched output relay in the manual re-
set mode or for selecting a new group of relay
settings by remote control.
Three phase definite time or
dependent time low-set
overcurrent protection
Three phase instantaneous or
dependent time high-set
overcurrent protection
Definite time or dependent time
low-set earth-fault protection
Instantaneous or definite time
high-set earth-fault protection
Remote reset, remote setting control or
blocking input for the different current stages
Circuit breaker failure protection
Serial communication port
Trip
51
Signal 1
50
51 N
Signal 2
50 N
Start 1
Start 2
IRF
51BF
Serial I/O
3
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