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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

Use this article if you experience an issue if 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 can be seen with a power controller whose inputs are normally closed. In this case, the lock remains continually locked and you can see the opposite results in the troubleshooting steps.

Notes:

  1. The solution is the same in both cases, connecting an isolator relay between the AC41 and the power controller.

  2. The AC42 is not susceptible to this ground loop scenario.

Issue

The lock state does not change as expected.

Troubleshooting steps

To verify that this problem is occurring:

  1. Use a multimeter to test the continuity of the AC41’s lock relay (NO/COM at 0V):
    * Not continuous in a normal state.
    * Trigger lock button causes the relay to be continuous.

  2. Test the access power controller inputs.

  3. Short the inputs of the access power controller. The lock should unlock.

  4. Remove the short. The lock should re-lock.

  5. 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 breaks the ground loop, causing the lock to unlock/lock, as expected. See example below:

Case Study

Issue

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. To remedy the issue, a relay has been added between the AC41 and the access power controller inputs.

Troubleshooting steps

  1. Check the door's access history in Verkada Command and verify to see if the door was on a locked schedule.

  2. Go to the physical wiring.

Assumptions

For this case study, there are 2 assumptions:

  • Assumption 1: The configuration is what was assumed to keep the door locked.

  • Assumption 2: The AC41's lock relay may not be working.

Assumption 1

  • 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. See example below:

Assumption 2

The AC41's lock relay may not be working. To troubleshoot, these are the steps tried:

  1. Disconnected the wires from the AC41 to do some multimeter testing.

  2. 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 non-triggered state.

  3. Pressed TRIGGER LOCK and the high-pitched hum confirmed the circuit was now continuous. This validated that the AC41 was functional.

  4. 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).

  5. Pressed TRIGGER LOCK 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.

  6. Disconnected the AC41 and shifted the focus on to the access power controller. There were still 2 wires hanging off of the access power controller’s inputs.

  7. Separated the wires and checked the lock state: locked, which was. different from when the AC41 was connected.

  8. Touched the wires together caused the circuit to short and unlock the lock. This test mirrored what the AC41 should be doing, and a different result occurred than with the AC41 attached.

The access power controller monitors its 2 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.

Resolution

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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