Trapped key interlocking
Encyclopedia
Trapped key interlocking utilizes lock
s and key
s 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(1880-1953) 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 station
s, 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.
, 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.
s. 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:
Lock (device)
A lock is a mechanical or electronic fastening device that is released by a physical object or secret information , or combination of more than one of these....
s and key
Key (lock)
A key is an instrument that is used to operate a lock. A typical key consists of two parts: the blade, which slides into the keyway of the lock and distinguishes between different keys, and the bow, which is left protruding so that torque can be applied by the user. The blade is usually intended to...
s 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
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
, 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(1880-1953) or American R. L. Kirk invented the concept of trapped key interlocking. Both worked in the power generation
Electricity generation
Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
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 station
Power station
A power station is an industrial facility for the generation of electric energy....
s, oil & gas processing plants and platforms, railway signalling
Railway signalling
Railway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop...
and points control, electrical substations, ships, electrical switchgear and factories across all sectors as a response to Occupation Health and Safety
Occupational safety and health
Occupational safety and health is a cross-disciplinary area concerned with protecting the safety, health and welfare of people engaged in work or employment. The goal of all occupational safety and health programs is to foster a safe work environment...
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 kilnKiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
, 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 systemControl system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
s. Malfunctioning of control systems can lead to unacceptable risk, jeopardising both capital investments
Investment
Investment has different meanings in finance and economics. Finance investment is putting money into something with the expectation of gain, that upon thorough analysis, has a high degree of security for the principal amount, as well as security of return, within an expected period of time...
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).