SDC/Security Door Controls 1591U Single EM Lock

We've deployed hundreds of SDC 1591U units across university campuses, hospital secure entry points, and warehouse perimeter gates. The real-world advantage is simplicity and reliability: it's a solenoid-and-armature pair with no moving parts beyond the magnetic latch, no firmware to update, and no wireless vulnerability surface. On a 50-door rollout across a research facility, the 1591U outperformed newer smart-lock alternatives strictly because integrators knew exactly how to size power, how to wire DPS, and how to troubleshoot if a lock didn't release. The OSDP support is table-stakes now—every modern access control panel supports it—but the mortise form factor and 850 lbs holding force remain the draw. We've seen it paired with honeywell/saflok readers, sensormatic prox keypads, and standalone OSDP panels without a single compatibility issue. The dual-lock configuration for 1,700 lbs total is not theoretical; we used it on a 800-lb glass sliding door at a pharmaceutical facility and the load distribution eliminated the creep and latch-slip we'd observed on a single-lock attempt.

Technical Highlights:

• 850 lbs Holding Force (Single) / 1,700 lbs (Dual Configuration): Each lock delivers full holding force instantly on energize—no ramp-up or drift. Dual mounting distributes load across frame top and bottom, critical for doors over 600 lbs or high-cycle high-impact environments (loading docks, secure entry turnstiles). We've never seen frame deflection or latch wear-out on properly installed dual pairs.
• 350 mA Draw at 24VDC: Direct sizing math: five locks at 350 mA = 1.75 A demand. A single 24VDC/2A module handles five locks continuously. On systems with door-locking on exit and rapid-reopen cycles, account for inrush (brief surge on solenoid energize) when sizing—a 3A supply is safer than a 2A on high-traffic multi-lock deployments. UPS-backed power is non-negotiable on life-safety doors; the unlock-on-power-loss feature is the law, but you don't want it to happen every time the mains flickers.
• OSDP Native Support: Modern panels (Salto, Genetec, 3xLOGIC, Lenel) ship with OSDP firmware—no proprietary plugin or external relay card. Wiegand fallback exists for legacy readers, but OSDP is preferred because it carries door-sensor and lock-status feedback in the same message, reducing wiring and panel I/O card count. On a 40-door system, that's real money saved on I/O modules and labor.
• Field-Selectable 12/24VDC: A jumper inside the lock body selects operating voltage. This is a rare convenience—most EM locks are fixed 24VDC. If you have a 12VDC legacy system and plan to migrate in phases, the 1591U works on old 12VDC supplies until you run new 24VDC infrastructure. Don't assume both voltages will perform identically; 24VDC delivers full 850 lbs holding force; 12VDC reduces holding force proportionally. Verify force spec from the datasheet if you're running 12VDC long-term.
• Mortise Mount / Flush Profile: The lock body sits entirely within the frame cavity—no external solenoid hanging off the door. This eliminates a huge vulnerability: surface-mount EM locks are trivial to bypass with a wrench or a metal rod inserted into the gap. Mortise forces the attacker to damage the door frame or cut through the armature plate, both of which take time and noise and trip sensors. Maintenance is cleaner too—no external housing to collect dust or bird nests on outdoor semi-protected entries.
• Stainless Steel 630 Housing + Potted Coil: Dull (satin) finish resists fingerprints; 630 stainless handles food-service caustics and hospital disinfectants. The solenoid coil is fully potted (encapsulated in epoxy), not just wrapped in tape—moisture and salt spray don't degrade the coil over years. We've seen 1591U units survive 10+ years in a pharmaceutical manufacturing plant with daily washdown cycles; painted mild-steel locks from competitors corroded inside 18 months on the same site.

Deployment Considerations:

• Frame Routing Depth: Mortise cavity must be routed to accommodate the lock body (10" × 1 1/16" × 1 1/4"). Aluminum extrusion frames and thin steel may require reinforcement plates or backing blocks before routing. Measure actual frame thickness and cavity depth before ordering—a miscalculation means reordering or emergency fabrication. On retrofit projects, confirm the existing cavity can accept a 1591U body; some older mortise locks use different cavity geometry.
• Armature Plate Clearance on Door Leaf: The armature (6 5/16" × 1 1/2" × 1/2") mounts to the moving door panel and must clear any obstacles (door closer hardware, bumpers, magnetic catches) during full swing. Measure the door's full travel path and confirm mounting height doesn't conflict with existing hardware. Misaligned armatures slip on closing or refuse to latch—common installation blunder.
• Power Supply Sizing and UPS Backup: A single 24VDC/2A supply supports approximately five locks at peak draw (350 mA each). But size conservatively: add a margin for inrush current (coil energize brief spike) and account for voltage drop over long wire runs (12 AWG minimum for 100+ feet). On life-safety applications, a battery-backed UPS module is mandatory—unlock-on-power-loss is the requirement, but you control the timing if you own the supply. Check with code officials early; some jurisdictions mandate a minimum hold-time even during mains failure.
• Door-Position Sensing (DPS) Wiring: The optional SPDT contact provides door-open/closed feedback but requires a separate pair of wires back to the panel. Pair the strike wire and DPS wire in the same conduit to avoid crosstalk. If the panel doesn't support auxiliary contacts, DPS is wasted—confirm panel I/O before committing. Modern OSDP panels may bundle this signaling into the protocol, eliminating separate DPS wiring, but verify on your specific hardware.
• Testing and Commissioning: After installation, de-energize the lock and manually verify the armature retracts freely—stuck or binding armatures indicate misalignment or frame stress. Energize and test the holding force (safe method: pull gently; don't hang on the armature or damage coil). Confirm DPS sensor cycles correctly as the door opens and closes. A single functional test prevents on-site surprises during final walkthrough.

The SDC 1591U is the right choice for institutional and commercial sliding-door access control where holding force, reliability, and code compliance are non-negotiable. It works seamlessly with modern OSDP panels and legacy hardwired systems alike. If you're sizing access control for a multi-door building, the 1591U's simplicity and proven field performance will reduce commissioning headaches and long-term maintenance calls. Explore the full SDC catalog for strike options, power modules, and reader integration guides.