Siemens Argus 7SR21 Technical Manual page 601

The Effects of Capacitance Current
The binary inputs have a low minimum operate current and may be set for instantaneous operation. Consideration
should be given to the likelihood of mal-operation due to capacitance current. Capacitance current can flow
through the BI for example if an earth fault occurs on the dc circuits associated with the relay. The binary inputs
will be less likely to mal-operate if they:
1 Have both the positive and negative switched (double-pole switched).
2 Do not have extensive external wiring associated with them e.g. if the wiring is confined to the
relay room.
Where a binary input is both used to influence a control function (e.g. provide a tripping function) and it is
considered to be susceptible to mal-operation the external circuitry can be modified to provide immunity to such
disturbances, see fig 1.2-2.
AC Rejection
The default pick-up time delay of 20 ms provides immunity to ac current e.g. induced from cross site wiring.
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7SR210 & 7SR220 Applications Guide
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