Access Control Wiring Best Practice: Dry-Contact Relays & Dedicated Lock Power
Best practice: Run all access control relay outputs as dry contacts from the main controller, and let a listed 12/24 VDC power supply provide the actual lock power. This protects your hardware, reduces nuisance failures, and supports life-safety integration.
What’s a “Dry Contact” (and why it matters)?
A dry contact is a relay that does not source power—it simply opens/closes a circuit. Use the controller’s relay to switch the lock power coming from a dedicated supply instead of feeding the lock directly from the panel.
Why not power the lock from the panel relay?
- Inrush & arcing: Strikes and mag locks draw high inrush; arcing burns relay contacts and shortens panel life.
- Isolation: Separate lock power prevents brownouts and keeps control electronics stable during surges/faults.
- Load ratings: Most controller relays aren’t rated to drive inductive loads continuously.
- Code & listing alignment: Using a UL 294/UL 603 listed supply with battery backup aligns with common AHJ expectations.
Recommended Door Circuit (Typical)
- Controller relay (dry contact) → switches +V from the listed lock power supply.
- Per-door protection: Fuse or PTC on each output; home-run grounds.
- Suppression: Flyback diode (DC) or MOV (AC) across the lock to protect contacts and boards.
- Life safety: Fire alarm interface (FAI) / emergency release to drop power on alarm while preserving free egress.
- Sensing: Door position switch (DPS) + request-to-exit (REX) as required.
- Power: 24 VDC recommended for mag locks; size supply for continuous and inrush current with battery backup.
Fail-Safe vs. Fail-Secure (Quick Guide)
| Lock Type | Power Loss Behavior | Common Use |
|---|---|---|
| Fail-safe (e.g., mag locks) | Unlocks on power loss | Where egress priority & AHJ require unlock |
| Fail-secure (e.g., electric strike) | Stays locked on power loss (egress still free) | Perimeter/security doors (verify with AHJ) |
Video: Dry-Contact Switching in Practice
Field Checklist
- Controller relay = signal only (dry contact)
- Listed power supply with battery backup for locks
- Per-door fusing and suppression (diode/MOV)
- DPS/REX wired and supervised as required
- Fire interface for emergency release; confirm with AHJ
- Labeled terminations, updated as-builts, and documented tests
Need help designing or retrofitting a compliant door? Explore Access Control or request a site visit.
Informational only. Requirements vary by jurisdiction and hardware listings. Always follow manufacturer instructions and consult your AHJ.