Trapped key interlock systems help eliminate human error

Frequently, human error is listed as the contributing factor in safety and environmental incidents. Training and procedures can improve the situation, but the best way to eliminate risk is to make it impossible for people to perform actions that lead to dangerous situations. One way to eliminate human error is to use trapped key interlock systems.

06/01/1999


Frequently, human error is listed as the contributing factor in safety and environmental incidents. Training and procedures can improve the situation, but the best way to eliminate risk is to make it impossible for people to perform actions that lead to dangerous situations. One way to eliminate human error is to use trapped key interlock systems.

Trapped key interlock systems are mechanical systems, which can be used in nearly any location, or hazardous environment. Based on the premise that one key can't be in two places at once, systems are configured to ensure a preset sequence of events takes place to safely remove hazards.



A dual line system must ensure an uniterrupted flow of materials
requires a specified order of opening and closing valves.

For example, the simplified diagram illustrates a header valve system with Valves 1 & 2 locked closed (LC) and Valves 3 & 4 locked open (LO) (see Valve sequence diagram). As shown, maintenance can safely be conducted on Line-2 while Line-1 operates. To maintain Line-1, an enforced sequence of opening and closing valves ensures a safe sequence is followed to place Line-1 in a safe state. To divert flow from Line-1 to Line-2, the following steps are taken:

  1. Collect Key-A from the control room, and insert into Valve-1's valve position lock. When Valve-1 reaches full open, Key-B can be removed. Note: Key-A cannot be removed while the valve is open, and the valve cannot be closed until Key-B is reinserted.

  2. Key-B is inserted into Valve-2's valve position lock. When Valve-2 reaches full open, Key-C can be removed.

  3. Key-C is inserted into Valve-3's valve position lock. When Valve-3 reaches full- closed, Key-D can be removed.

  4. Key-D is inserted into Valve-4's valve position lock. When Valve-4 reaches full- closed, Key-E can be removed and returned to the control room.

Line-2 has been safely placed in service and Line-1 safely removed from service.

In this example, no more than one key is free at a time and each fits in only one predetermined lock. Machine, process uses Responsibility for safe design and implementation of safety interlocking systems rests with the machine designer, system installer, and end-user. This responsibility includes conducting and documenting a risk assessment to identify all hazards and estimate risk. For hazards that can't be eliminated, a safety system should be implemented per relevant industry or national standards.

Safety devices available for machine and process guarding include light curtains, active mats, switches, two-hand controls, and process safeguards, including trapped key interlock systems. Devices available for trapped key interlock systems include:

  • Master/slave key control cabinets that require keys be removed and reinserted in sequence;

  • Trapped key electrical and pneumatic power isolators, with or without electronic time delays;

  • Switchgear adapters are interchangeable with standard switchgear levers and locks;

  • Single and dual key deadbolt switches with or without electrical position indicators;

  • Multiturn and quarter-turn valve interlocks; and

  • Single- and dual-key chain interlocks.

They do, trapped key interlock devices, fitted with an emergency override, permit operation of the interlocked device without a key.

Companies like Castell Interlocks (Erlanger, Ky.) and Scientific Technologies (Fremont, Calif.) explain how to provide machine and process safeguarding to protect against human error, in various documents available on the Internet.



Author Information

Dave Harrold, senior editor dharrold@cahners.com




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