Use Of Binary Inputs In Control And Tripping Circuits; Capacitance Discharge Current In Centre Tapped Battery Installations - Siemens 7SR12 Argus Manual

7SR11 & 7SR12 Applications Guide
1.2.3

Use of Binary Inputs in control and tripping circuits

Where a binary input is used to as part of a dc control function, for example tripping or closing a circuit breaker
and the wiring connection consists of direct connection of long cross-site cable runs, it may be desirable to
provide an enhanced level of immunity to prevent maloperation due to induced voltages.
Where the battery voltage allows its use, the 88V binary input will give an added measure of immunity, compared
to the 19V binary input, due to its higher minimum pickup voltage.
One method of enhancing the immunity of the binary input is to switch both positive and negative connections;
however this is often not possible or desirable.
As a guide to suitable degrees of enhanced immunity, we have adopted the parameters laid down in U.K.
standard ENA TS 48-4 which is applied to separate dedicated Trip relays. This standard identifies two levels of
immunity:
Low Burden: may be adopted for connections in high noise environments with double pole switching.
High Burden: should be used for connections with significant wiring runs outside the relay enclosure in very high
noise environments. This category also gives immunity to capacitive discharge currents, see 1.2.4.
The Figure 1.2-3 show the external resistors which should be fitted to allow the binary input to comply with either
of the above categories.
Fitting these components will raise the current required to operate the binary input, and hence makes it less
susceptible to maloperation.
Where required, the minimum pickup delay for the binary input is stated on the diagram.
1.2.4

Capacitance Discharge Current in Centre Tapped Battery Installations

The binary inputs have a low minimum operate current and can be set for instantaneous operation. Consideration
should be given to the likelihood of mal-operation due to capacitance current if very fast operation is required. If
an earth fault occurs between the wiring on the switched pole of the binary input and earth, charge developed
between the other pole and earth will be discharged through the binary input causing operation.
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
protection panel or 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 1.2-2.
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