Siemens SIRIUS 3SK1 Manual page 41

Series connection of position switches
In general, position switches may be connected in series if measures ensure that several
protective doors are not regularly opened simultaneously (otherwise a fault cannot be
detected).
For safety level SILCL3 in accordance with IEC 62061, SIL3 in accordance with IEC 61508,
and PL e (Cat. 4) in accordance with ISO 13849-1, however, they must never be connected
in series, because every hazardous error must be detected (independently of the operating
personnel).
3SK1 safety relays
Manual, 04/2013, A5E02526190021A/RS-AA/01
Safety systems - General information
4.3 Basic terminology
41