Types of Locking Devices

Risk reduction is the phase that concludes the efforts made during all stages of risk analysis and assessment: it’s the phase where dangerous phenomena are acted upon directly and concretely.

The risk reduction strategy as described in ISO 12100 and CSA Z432 requires three steps in a specific order:

1- Application of inherent prevention measures

2- Safeguarding and complementary prevention measures

3- Information for use

Movable guards are generally part of the risk reduction measures in the second stage (safeguarding and complementary prevention measures)—namely to reduce a residual risk that couldn’t be adequately reduced through inherent prevention measures and, at the same time, one that is too significant to be managed by information for use alone, and where risk analysis and evaluation have shown that risk reduction through safeguarding is applicable.

This article specifically addresses one type of movable guards—that is, guards equipped with locking and interlocking devices.

By definition, a locking guard is a movable guard that simultaneously meets three criteria:

1- Stopping the machine when opening the guard.

2- The machine remains stopped as long as the guard is open or has been moved.

3- Putting back in place/closing the guard doesn’t directly cause the machine to restart (for this condition, there are specific cases when it might be possible to use locking guards with closure initiating machine restart, but very particular conditions must be met to justify their use that will not be addressed in this article).

The text and wording of the definitions of this type of guard may vary slightly between standards (ISO 12100, ISO 14120, ISO 14119, CSA Z432) and regulations, but the logic is exactly the same.

Interlocking guards are a more advanced version than locking guards. They meet the same conditions, but interlocking guards remain locked in the closed position until the dangerous phenomenon has completely disappeared.

Types Locking Devices

Designers generally have four choices (4 types) of locking devices to add to a guard (panel, door, barrier) to create a locked guard in accordance with ISO 141119.

Type 1 This type includes mechanical position switch devices with a non-coded actuator.

The actuator in this context refers to a component of the locking device that transmits the status of the guard (closed or open) to the actuation system.

The actuator is considered to be coded when it is specially designed (based on its physical characteristics, shape, magnetic field, etc.) to operate a specific position switch.

Coding is a critical aspect of preventing locking devices from being bypassed and neutralized because it entails the difficulty of finding another (free) actuator that can operate the same switch.

There are three levels of coding.

Low coding level: coded actuator for which 1 to 9 code variants are available

Medium coding level: coded actuator for which 10 to 1,000 code variants are available.

High coding level: coded actuator for which more than 1,000 variants are available.

Rotating cam devices and liner cam devices are excellent examples of a type 1 locking device.

Type 2: This type includes locking devices with mechanically operated position switches with coded actuators, where the coding in this case is generally done by shape.

Tongue devices and key transfer systems are the easiest examples to find for this type of locking device.

The two types we’ve covered so far rely on mechanical control. The key distinction with type 3 and 4 locking devices lies in their use of electrical/electronic control, allowing for contactless actuation.

Type 3: This type includes contactless position switch locking devices with non-coded actuators; well-known examples of this type are proximity sensors (inductive/capacitive).

Since the concept of coding isn’t applicable to this type of device, they are associated with a very high risk of neutralization. For this reason, they must be used cautiously while checking all possible scenarios where a reasonably foreseeable neutralization could occur.

Type 4: This type of device is a locking device with non-contact position switches and coded actuators.

This type is the most commonly used in new machines due to its reliability, suitability, compatibility with various types of environments and ease of integration into electronic and electrical control systems.

Devices with coded magnets and position switches operated by RFID tags are examples of this type of device.

The purpose of this article is to define the various types of locking devices as solutions to be implemented in various situations rather than to compare them to determine the best or worst ones. Each type of device could be the best or worst depending on its application, the environment where it will be used and its suitability for reducing risks based on a properly conducted risk analysis.