The Effects Of Capacitance Current; Ac Rejection - Siemens 7SR18 Solkor Manual

1.2.2

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 Figure 1.2.4-1.
1.2.3

AC Rejection

The default pick-up time delay of 20 ms provides immunity to ac current for dc applications e.g. induced from
cross site wiring.
Binary inputs can be configured for intentional operation from an ac power supply by setting pickup (PU) and
drop-off (DO) timers for each binary input e.g. 0 ms PU and 25 ms DO. If wiring to AC operate binary inputs is
required to have a total length of more than 10 metres, screened twisted pair cable should be used.
If additional pickup or drop-off time delays are required by the scheme logic, this functionality can be achieved by
programmable logic within the device.
Example: -
An AC operated Binary input is required to inhibit the 50-1 protection element with 100 ms minimum pickup delay
and 200 ms minimum drop-off delay.
1) Set Binary Input 1 for AC operation:
2) Set quicklogic equation E1 to operate from Binary Input 1 and apply required delays:
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7SR18 Applications Guide
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