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Safety systems - General information
4.3 Basic terminology
3SK1 safety relays
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Manual, 04/2013, A5E02526190021A/RS-AA/01
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Figure 4-2 Two-channel sensor connection
* = positively-driven auxiliary contacts/mirror contacts
4.3.2 Cross-circuit detection
Cross-circuit detection is a diagnostic function of a safety relay that detects short-circuits and
cross-circuits between the input channels (sensor circuits) during two-channel sensing or
reading. A cross-circuit can be caused, for example, by a cable casing being squashed. In
devices without cross-circuit detection, this can mean that a two-channel EMERGENCY
STOP circuit does not trip even though only one NC contact is faulty (secondary error).
With 3SK1 devices, a cross-circuit is detected in the sensor circuits by means of signals with
different clock pulses. If the clocked signals overlap, the device detects a cross-circuit. With
3SK1 basic units, cross-circuit detection can be deactivated to allow electronic sensors to be
evaluated (that monitor themselves as well as the cable to the evaluation unit.
4.3.3 Enabling circuit
An enabling circuit provides a safety-related output signal. From an external viewpoint,
enabling circuits usually act as NO contacts (however, in terms of functionality, safety-
oriented opening is always the most important aspect).
An individual enabling circuit that is redundantly configured internally in the safety relay can
be used for SIL 3 / PL e.
Note: Enabling current paths can also be used for signaling purposes.
3SK1 safety relays are only equipped with enabling circuits with NO functionality. This
means that when the safety function is triggered, or a fault is detected, the enabling circuits
will always transfer to the safe state (NO contact open).