Dry contacts and supervised circuits

A dry contact reports an electrical state. Its meaning depends on the wiring, configuration and system design, so record those details before interpreting it as a door event.

Overview#

A relay contact is one of the most interoperable physical security interfaces, but it carries almost no semantics by itself. “Closed” can mean alarm, normal, energised, bypassed, unlock, fault, or nothing, depending on the approved design.

Terms#

• Dry/volt free contact: switch contact that doesn't intentionally supply voltage. Verify ratings and isolation; never assume a terminal is dry from its label alone.
• NO / NC / COM: normally open, normally closed, and common relative to the relay's defined normal/de energised state, not necessarily the system's normal condition.
• Digital input: senses an electrical state; it may supply a wetting voltage and have polarity or threshold requirements.
• End of line (EOL) supervision: components at the circuit end allow the receiver to distinguish more than open/closed states.

Separate electrical and semantic state#

Define a mapping table in the approved design:

Single EOL, double EOL, and product specific schemes differ. Resistor values, tolerances, topology, cable resistance, input thresholds, debounce, and tamper semantics must come from exact documentation and approved drawings.

Relay isn't confirmation#

An output energizing doesn't prove the physical operation. Use a separate, appropriately designed monitor where the outcome matters:

unlock request -> controller accepts -> lock output changes
               -> lock monitor changes (if present)
               -> door contact changes (if opened)

Each arrow has separate timing and failure modes. A door contact indicates leaf position, not lock security; a gate limit indicates position, not a clear path.

Safety and isolation#

Don't connect, bridge, force, meter, or actuate live fire, egress, lift, gate, lockdown, alarm, or emergency circuits based on generic guidance. Use qualified people, approved drawings, isolation boundaries, contact ratings, surge/fault protection, test windows, manual override, and independent observation.

Avoid driving an inductive load directly from an interface relay unless the product and electrical design explicitly support it. Suppression components and polarity can affect release timing and monitoring; this is an electrical engineering decision.

Integration design#

• Name signals by meaning and polarity, not input1 or relay2 alone.
• Record normal/degraded/unknown behaviour during power loss, controller restart, cable fault, and communication loss.
• Debounce without hiding rapid fault transitions; preserve raw transition count where useful.
• Rate limit automated outputs and require operation level authorisation.
• Keep observation inputs physically/logically separate from actuation outputs.
• Audit request, controller result, output state, independent sensed state, and time quality separately.

Dry contacts are intentionally simple. Don't invent identity, encryption, message integrity, sequence, or supervision properties they don't provide.

Source boundary#

• IEC 60839-11-1:2013 supplies the official catalogue scope for electronic access control system and component requirements.
• IEC 62642-1:2010 supplies the official catalogue scope for intrusion and hold up alarm system requirements.
• NIST SP 800-82 Rev. 3 supplies the OT safety, reliability, segmentation, and operational framing used here.

Catalogue scope doesn't supply wiring values or product acceptance criteria. Resistor values, circuit classes, terminals, ratings, release behaviour, and acceptance criteria must come from the applicable licensed normative text, approved design, and exact product documentation.

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