Lock Status Monitoring Tutorial

Author: Brian Rhodes, Published on May 12, 2012

Just because your doors look secure does not mean they are. Unless access systems are using lock status monitoring, the doors and areas they protect may not have any certainty of being secured. Can you tell whether the door below is locked? How?

Lock status monitoring addresses a key vulnerability of many access control systems, but its value is commonly ignored or misunderstood. Inside this note, we explain:

  • How Lock Status Monitoring Prevents Security Risks
  • Why Shut Doors Do Not Mean Secure Doors
  • Tradeoffs Of Door Position vs.or Lock Monitoring
  • Typical Solution Cost
  • Examining Latchbolt Monitoring vs Latchbolt Strike Monitoring
  • Explaining Maglock Bond Sensors

[Note: This tutorial was originally published in 2012 and substantially revised in 2017]

The Importance Of Lock Monitoring

Simply put, lock monitoring checks to see whether an opening's lock is reporting itself as 'locked'. Because most electrified locks can only lock properly when the door is shut, Lock Monitoring tells access sytems the door is both closed and secured.

Even with sophisticated electronic access locks, door hardware is vulnerable to tampering. Unless the lock hardware itself is monitored as being locked, tampering can defeat their strength and be undetected by the system.

For example, electric strikes can be neutralized by placing foreign objects in the strike so the latchbolt never fully engages, or held in with tape so they never extend at all.

And while maglocks generate large amounts of holding force, it works only when the magnet and the armature have full contact. The rated holding force drops drastically if their piece are not allowed to contact each other, even just through covering the surface with tape or paper, dropping the bond from thousands or pounds to just a few hundred, allowing for doors to be easily kicked open.

Shut Doors Do Not Mean Secure

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The scary part is that these doors may otherwise appear fully closed to an access system, but be significantly insecure and unlocked or weakly locked in reality. Even a door equipped with Door Position Switches can only tell access operators if a door is open or shut, leaving the assumption that 'closed' also means 'locked'.

Unfortunately, many attempts to defeat access controls come from insiders, often for convenience and not malice, as noted in our Propped Doors Access Control Tutorial. However, even if done without criminal motivation, tampering with the access lock can leave free, uncontrolled entry through the door to employees or outsiders alike.

Door Position or Lock Monitoring

In many cases, using lock monitoring costs moderately more than door position switches. For many access Door Controllers, either Door Position Switches or Lock Monitors are connected at the 'Door Monitor' input, usually a simple Normal-Closed circuit on the controller board. Here is an example for an HID VertX controller:

The cost of adding a surface-mount door contact can cost as little as $5 per set, and with a few minutes of install work be connected to an access system. However, specifying integrated lock monitoring in the lock itself can add 5% - 10%, often $20 or $30 to the item's cost. The cost difference may explain why lock monitoring is not as commonly used as Door Position Switches.

In the strike below, the integrated latch monitor is connected to controllers by way of the yellow and green wires:

The wiring harness for these components in all types of locks (ie: strikes, maglocks, or electric latches) is often totally separate from power wires, and leaving latch monitoring unconnected generally still leaves the lock normally operational.

Three Common Forms

Lock monitoring is usually deployed in three ways:

  • Latchbolt Monitoring (LBM)
  • Latchbolt Strike Monitoring (LBsM)
  • Maglock Bond Sensors

While the first two seem to be labeled almost the same, the monitoring methods are drastically different. And even a lock with no moving parts, like a maglock, can be monitored by checking its bond strength. We examine the three methods below:

Latchbolt Monitoring

One way to check if doors are closed and their latches engaged is by 'latchbolt monitoring'. In this method, a mechanical or inductive switch built inside the strike checks whether or not the door's latchbolt or deadbolt is extended. If the latch is thrown, the LBM pressed in. If the latch is not thrown, the door is unlocked, and the switch remains unpressed or uncontacted by the latch.

Here's an image showing the position of the monitoring switch inside a lock:

Users should expect approximately this feature adds 5% - 10% per lock, often $20 or $30, and adding it usually requires replacing existing locks.

Latchbolt Strike Monitoring

Another method of monitoring strikes involves checking the internal solenoid position, as that in turn indicates whether or no the strike itself is rigid or flexible in retaining lock latches. The advantage of LBsM is that they are less exposed, and less prone to malfunction and breakage, but they are not as common as LBM options on strikes.

Some strikes offer both as an option, like this HES 1006 Series strike:

Bond Sensors

The most common method of monitoring maglocks involves a sensor that checks the field strength of the electromagnetic coil inside the unit. When the field is strongest or within spec, an integrated LED or wire output loop signals the lock is firmly bonded. The image below shows the 'green is good' LED indicator on one example:

In many cases, bond sensors are a default option, and using them adds no cost since they are already included. However, these sensors can go 'bad' over time, and the sensitive circuits fail with regular use. Periodic replacement involving a technician onsite and a replacement sensor costing a few dollars may be needed once or twice in the span of five years for these maglocks.

Using Both DPS and Lock Monitoring Together

For highest security, some installers use both door position and lock monitoring at the same time by wiring the contacts in series with the door position switch. Instead of simply knowing a door is open or closed, the system would see it as open and not secure or closed and secure.

Optionally, lock monitoring outputs may be run to separate general controller inputs. If users want to display door status separately, this method may also be needed to get accurate status. For example, if doors unlocked during the day should be kept closed, separate monitoring from both DPS and lock monitoring is required.

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