Trapped-key interlocking

Operations which are safe or harmless if carried out in the correct sequence can potentially be dangerous if that sequence is not followed. Trapped key interlocking utilizes locks and keys for sequential control of equipment and machinery to ensure safe operation. A simple system might consist of two locks and a single key. Normally inserting the key into a lock would enable a power supply, thus in this simple system only one power supply could be enabled at any one time. Trapped key interlocks are widely used to ensure safe access to potentially live or dangerous plant or equipment in an industrial setting.

Safe access is enabled through transfer of keys that are either trapped or released in a safe sequence. For example a key is used to the isolate power, this key is then released and can then be used to gain access through a gate or door to a high risk area by inserting it in to an access lock. The key will then remain trapped until the gate or door is closed. A personnel or safety key can be released from the access lock, this ensures that the gate or door can not be closed and the initial key released until this personnel or safety key is returned. This provides increased operator safety.

Claims have been made that Englishman James Harry Castell or American R. L. Kirk invented the concept of trapped key interlocking, both worked in the power generation and distribution industries in the early part of the 20th Century, and both pioneered the use of trapped key interlock for switchgear control. Trapped key interlocks were used prior to this though, in the 1890s, in the French railway system to control track switching operations.

Today trapped key interlocks can be found in many industrial settings including power stations, oil & gas processing plants and platforms, railway signalling and points control, electrical substations, ships, electrical switchgear and factories across all sectors as a response to Occupation Health and Safety legislation.

Application

The principles of trapped key technology apply to all industries where it is essential that all energy sources are isolated before gaining access to machinery. The following sequence can be used in creating a correct interlocking scheme.

• Identify the energy sources to be isolated and/or any hazard that cannot immediately be isolated such as; heat, pressure, radiation or machine rundown time

• Identify the type and number of access points

• Identify the type of access point; part body or full body access doors with or without the use of personal safety keys (to prevent accidental lock in)

Example

For example, to prevent access to the inside of an electric kiln, a trapped key system may be used to interlock a disconnecting switch and the kiln door. While the switch is turned on, the key is held by the interlock attached to the disconnecting switch. To open the kiln door, the switch is first opened, which releases the key. The key can then be used to unlock the kiln door. While the key is removed from the switch interlock, a plunger from the interlock mechanically prevents the switch from closing. Power cannot be re-applied to the kiln until the kiln door is locked, releasing the key, and the key is then returned to the disconencting switch interlock. A similar two-part interlock system can be used anywhere it is necessary to ensure the energy supply to a machine is interrupted before the machine is entered for adjustment or maintenance.

Machine safety standards

The safety of machines, plants and systems employed in industry depends more than ever before on the correct functioning of electronic control systems. Malfunctioning of control systems can lead to unacceptable risk, jeopardising both capital investments and people. To eliminate such risks as far as possible, the relevant control systems must meet exacting requirements in terms of functional safety.

These requirements can include the use of safety gate switches and or trapped key interlock systems. The standard EN/IEC 9541 (Safety of machinery, Safety related parts of control systems, Part 1: General principles for design) is due to expire on the 31st October 2009. It is to be replaced by the following standards:

• EN/ISO138491 (Safety of machinery, Safetyrelated parts of control systems, Part 1: General principles for design)
• EN/IEC 62061 (Safety of machinery, Functional safety of safety related electrical, electronic and programmable electronic control systems).