An anesthetized patient is wheeled toward an OR slider that closes on the gurney — while, two doors down, a pharmacy airlock lets both doors stand open at once, collapsing the pressure cascade. Which subsystem failed? Different ones in each case: presence detection at the first door, interlock logic at the second. Sensors and electronic locking are small line items that carry most of the functional risk of a medical door system.

9.1 Activation and presence sensors

Automatic pedestrian doors rely on two distinct sensor roles, both mandated by EN 16005 (Europe) and ANSI/BHMA A156.10 (US) with monitored self-testing:

  • Activation sensors — microwave radar (Doppler) detects approaching motion; it sees through nothing but reacts fast and ignores stationary objects. It must NOT be the only protection: radar cannot see a person standing still in the doorway.
  • Presence sensors — active infrared curtains (light grids over the threshold) or TOF/laser scanners detect stationary persons and objects in the danger zone during closing. On swing doors, presence sensors must guard the swing path on both sides.

Selection guidance for healthcare: radar-only activation is acceptable for corridors; doors crossed by slow-moving beds, wheelchairs, or walking-aid users need extended presence-hold time and full threshold curtains. Reflective stainless door skins can confuse cheap IR sensors — specify sensors with adjustable background suppression and test against the actual door finish. Common mistake: mounting height above the sensor's rated envelope to "get a cleaner look," creating a dead zone at foot level.

9.2 Access readers and locking

  • Readers: RFID (13.56 MHz smart card preferred over 125 kHz proximity for security), keypad, or hybrid; hands-free long-range readers suit sterile areas where staff cannot touch anything.
  • Electromagnetic locks (maglocks): no moving parts, silent, high holding force (300–600 kg typical), inherently fail-safe (power off = unlocked) — which makes them correct for egress but WRONG as the only containment for a pharmacy or controlled-substance store, where fail-secure electric strikes or electrified mortise locks (per A156.13, Chapter 7) are appropriate. Code note: maglock release on egress must comply with IBC 1010.2.12/2.13 — sensor release, push-button release ("PUSH TO EXIT", 30 s), fire-alarm release, and power-loss release.
  • Electric strikes: keep the mechanical latch, retrofit-friendly, but tolerate door/frame misalignment poorly — a sagging steel leaf (Chapter 7 hinge wear) shows up first as strike binding.

9.3 Door interlocks (airlocks)

An interlock controller prevents two or more doors of an airlock from being open simultaneously — the functional backbone of cleanroom pressure cascades, gowning rooms, and isolation suites. Typical implementations use maglocks or electric strikes wired through a dedicated interlock relay panel with:

  • Mutually exclusive open states (door B unlocks only when door A is closed AND latched — monitor the latch contact, not just the lock status; a door resting against gaskets reads "closed" but leaks).
  • Timed release / purge delay between doors for ISO-classified spaces.
  • Emergency override: fire-alarm input releases all locks (fail-safe), and local break-glass or pull-station release per jurisdiction. Override events should be logged — auditors ask.

Trade-off: hard interlocks guarantee the cascade but create queuing and can trap staff if a door faults; a "soft" mode (interlock with timed override alarm) is sometimes negotiated with infection control for high-traffic transfer hatches. Never disable the interlock silently — log and alarm every override.

9.4 Emergency release and system coordination

Safe operation depends on coordination between five layers: activation sensor → controller → operator (Chapter 8) → locking device → fire/BMS system. Failure modes cluster at the interfaces:

Failure Cause Prevention
Door opens but won't re-lock Strike misalignment, relay weld Monitored lock contacts, door position sensor
Interlock deadlock Both doors report fault; logic latch-up Watchdog reset, manual release per leaf
Fire alarm drops locks, door won't close Operator not receiving alarm input Single alarm panel output to BOTH lock PSU and operator
False presence triggers Curtain sway, reflective floor Sensor masking, re-commissioning after floor refinishing

Commissioning checklist: (1) verify monitored sensors fail to a safe state on power cut; (2) test egress release via sensor, button, fire alarm, and power loss — all four; (3) walk the interlock through every door-state combination; (4) record holding-force and sensor-envelope tests for the AHJ; (5) re-test after any flooring, gasket, or closer change.

Recommendation: for B2B procurement, demand a single-point responsibility matrix in the quotation naming who integrates operator, locks, readers, and fire-alarm relays, plus the interlock logic diagram as a deliverable drawing. Doors where "the electrician sorts it out on site" are the ones that fail acceptance testing.

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