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Dry lock configuration does not lock/unlock as expected
Dry lock configuration does not lock/unlock as expected
Dry lock setup on an AC41 not locking/unlocking due to a ground loop
Updated over a week ago

This article covers the issue where a lock stays continuously unlocked where an access power controller powers the lock. The power controller's inputs require a normally open connection. This same issue could be seen with a power controller whose inputs are normally closed, but in that case, the lock would remain continually locked. In that instance, you would see the opposite results in the troubleshooting steps. The solution is the same in both cases, connecting an isolator relay between the AC41 and the power controller.


  • Issue: Lock state does not change as expected

  • Steps to verify this problem is occurring

    • Use a multimeter to test the continuity of the AC41’s lock relay (NO/COM, at 0V)

      • Not continuous in normal state

      • Trigger lock button causes the relay to be continuous

    • Test the access power controller inputs

      • Short the inputs of the access power controller - the lock will unlock

      • Removed the short - the lock will re-lock

    • Connect the AC41 back to the access power controller

      • The lock state will not change when triggering the lock on the AC41

  • Analysis: A ground loop is causing the access power controller to not detect the trigger events from the AC41

  • Resolution: Use an isolator relay between the AC41 and the access power controller. This will break the ground loop causing the lock to unlock/lock as expected. Diagram showing the fix below:

Case Study:


In a new access controller (AC41) deployment, one door would not lock. A grounding difference between the AC41 and the access power controller was to blame. This is typically referred to as a ground loop. We added a relay between the AC41 and the access power controller inputs to remedy the problem.


We first checked the door’s access history in Command and could see the door was on a locked schedule. Next, we had to go to the physical wiring as the config would keep the door locked.

I asked the installer to go over their wiring. It was a standard dry setup for this lock. The AC41’s normally open (NO) and common (COM) ports connect to the inputs on an access power controller. The power controller supplies power to the maglock.

We thought the AC41's lock relay may not be working. We disconnected the wires from the AC41 to do some multimeter testing. We measured continuity across the NO and COM ports of the AC41’s cassette. The multimeter read open-loop (OL), as it should while the relay is in an un-triggered state. We pressed TRIGGER LOCK and the high-pitched hum told us the circuit was now continuous. This validated that the AC41 was functional.

We reconnected the AC41 to the access power controller (NO COM). Initially, the lock continued to stay locked as expected (as if the wires were not touching). Then we pressed the trigger lock button and heard the AC41 click the lock relay over. The power controller immediately unlocked the maglock as expected. However, the lock did not relock after the AC41 relay clicked and switched back.

We disconnected the AC41 and shifted the focus onto the access power controller. We still had the two wires hanging off of the access power controller’s inputs. We separated them and checked the lock state, locked - different from when we had the AC41 connected. When we touched them together, shorting the circuit, the lock unlocked. This test mirrored what the AC41 should be doing, and we saw a different result than with the AC41 attached.

The access power controller monitors its two inputs to see if it should unlock the maglock. It puts a voltage on an input and measures the voltage across an internal resistor to check. If the voltage is high, the power controller knows the connection between the two inputs is open. When we connect the inputs together the voltage is low triggering the unlock.

From our tests, the AC41 has the lock relay open, but the power controller detects a closed circuit.

The AC41 is able to supply power to the lock relay as many installs use the AC41 to power the lock. Due to this, the lock relay doesn’t act like a pure switch in a dry configuration. This relay connects to the AC41's ground. This normally does not cause issues as the grounding of the AC41 and power controller match.

In this instance, the AC41 and the power controller had different earth grounds. This meant the reference voltage of AC41 did not match the power controllers. It’s the difference in potential that causes electricity to move. With a non-common ground, electricity moved from one ground to the other. This resulted in the power controller incorrectly detecting the input circuit as complete.


We connected an isolator relay between the AC41 and the access power controller. This separated the devices to keep electricity from flowing from one ground to the other. Immediately after installing this relay, the maglock began to function as expected.

Here is the typical wiring for adding an isolator relay between the AC41 and an access power controller. You’d power the relay from the AC41 cassette, and then connect the common and NO or NC to the inputs on the power controller. Use NC if the power controller expects a normally closed circuit, and NO otherwise.

Wiring of the AC41 cassette to the relay:

Diagram of the fix:

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