An EN ISO 13849-1 category 4 immediate stop system design requires supply shut-off device redundancy and activation monitoring.
Wiring of open contacts (K3, K4) allows every activation request to be checked.
It is compulsory for the contacts of relays (K3, K4) to be mechanically linked.
EN ISO 13849-1 category 3 wiring means:
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no wiring of auxiliary contacts in the feedback loop (a strap links terminals Y1 and Y2/S33),
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standard switches, with non-guided contacts, are sufficient.
2-switch set-up (EN ISO 13849-1 category 4):
