Electric strike vs maglock: how do I wire the lock relay on a Mercury LP1502 (fail-safe vs fail-secure)

Updated Source: PLT-05244 Rev A.7Source: PLT-05244 Rev A.7
Quick answer

On a Mercury LP1502 an electric strike that unlocks when powered goes on the relay's Normally Open (NO) and Common (C) poles, so the relay delivers power to unlock. A maglock, which unlocks when power is removed, goes on the Normally Closed (NC) and Common poles, so the relay momentarily removes power. Put a protection diode (1N4002 typical for 12 or 24 V DC) across the lock to stop feedback from damaging the relay, and check local building codes for egress.

Steps

From: Mercury LP1502 Controller Installation a install guide, pp. 4, 15

  1. Know what the relays are
    The LP1502 has four Form-C dry relays for door locks or alarm signaling. Each has a Common pole (C), a Normally Open pole (NO) and a Normally Closed pole (NC). They switch your lock power supply; they don't supply power.
  2. Strike that unlocks with power (fail-secure):
    Wire the lock supply through C and NO. The relay closes on a valid read and delivers power to the strike.
  3. Maglock (fail-safe):
    Wire the lock supply through C and NC. The magnet holds while powered, and the relay momentarily removes power to unlock the door.
  4. Add a diode across the lock
    Lock coils feed back into the relay and cause damage and premature failure. Choose a diode rated 1x the strike current and 4x the strike voltage; for a 12 V DC or 24 V DC strike a 1N4002 (100V/1A) is typical.
  5. Mount the protection close to the lock
    On the MR52-S3B Mercury calls for the protection circuit within 12 inches (30 cm) of the load; it loses effectiveness farther away.
    Source: HID Mercury MR52-S3B Reader Interface In install guide · p. 10
  6. Size the wire
    Use wire of sufficient gauge for the lock current to avoid voltage loss at the door.
  7. Check egress
    Mercury tells you to check local building codes for proper egress door installation before you decide between a strike and a maglock.

Specs & limits

From: Mercury LP1502 Controller Installation a install guide, pp. 4, 15

RelaysFour Form-C (dry) on LP1502
StrikeNO and C poles
MaglockNC and C poles
Diode current1x strike current
Diode voltage4x strike voltage
Typical diode1N4002 (100V/1A) for 12 or 24 V DC

Common mistakes

From: Mercury LP1502 Controller Installation a install guide, p. 15

  • Wiring a maglock to NO. The door then stays unlocked until a valid read and locks when the relay fires.
  • Leaving out the diode. Lock feedback can cause premature relay failure and affect the controller's operation.
  • Putting the protection circuit back at the panel on a long run. Keep it within 12 inches (30 cm) of the lock.
    Source: HID Mercury MR52-S3B Reader Interface In install guide · p. 10

Applies to

LP1502 · PLT-05244 rev A.7MR52-S3B · PLT-06451

Sources

  1. Mercury LP1502 Controller Installation and Specifications, p. 15 (PLT-05244 Rev A.7)
  2. Mercury LP1502 Controller Installation and Specifications, p. 4 (PLT-05244 Rev A.7)
  3. HID Mercury MR52-S3B Reader Interface Installation and Specifications, p. 10 (PLT-06451 Rev A.1)
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