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Castell - Machine Guarding & Switchgear interlocking explained…

Trapped Key Interlocking for Machine Guarding

Castell Safety International designs and manufactures the world’s widest range of industrial safety interlocking systems ensuring that industry can operate safely around the world.

Castell interlocking systems are designed to be robust, durable and are proven in all types of operating environments that meet the demands of the harsh locations our customers operate in. Above all, they are designed to protect personnel and assets where the risk of injury and damage are high.

Trapped key interlocking ensures that a process is followed and cannot be circumvented or short cut. The transfer of a key ensures that wherever personnel find themselves, in either starting or shutting down operations, they can be assured that they are safe. There are three simple steps in designing a trapped key system, what is being isolated, how many access points are there and what type of access is required. A key is used to start the process and remains trapped whilst the machine is running. The only way to remove the key is to isolate the hazard. This key is then used to gain access to the dangerous area and remains trapped in position while the gate or door is opened. The key can only be removed when the gate or door has been shut. In this way the key is either trapped when the machine is running and access cannot be gained, or the key is trapped while access is gained and the machine cannot be started.

Designing interlock systems

To design an interlock system there are a number of key questions that need to be addressed, these are:-

  • What is the operational flow to start and stop equipment?
  • What is being Isolated?
  • Is there more than one system that needs to be isolated to make access safe?
  • Is there a time delay required for safe access?
  • How many access points are there?
  • What is the type of access, full body or part body?
  • Severity of the possible injuries?
  • What is the possibility of avoiding the hazard?
  • What is the nature if the hazards?
  • What are the energy sources present?
  • What is the operating environment?

Some areas where Castell products can be found are: Food & Beverage, Aggregates & Mining, Pharmaceutical & Chemical, Paper, Pulp & Wood, Steel, Aluminum & Precious Metal, Automotive & Electronics, Water & Recycling.

Typical application areas:

Robot Cells I Conveyors I Palletizers I Gas Turbines I Freezer Rooms I Automated Warehouse I Power Press I Spray Booths I Industrial Mixers I Pressure Vessels I Recycling I Rotating machinery I Mixers & Blenders, Crushers I Bottling I Packaging.


The original Castell interlock concept was developed for the electrical switchgear industry. This remains today a very important part of the Castell product portfolio. Castell delivers solutions across the electrical network from power stations to transmission equipment and from sub stations to incomer rooms. The ability to work across HV, MV and LV means that a Castell system can be used as a single solution to provide personnel safety and ensure equipment is used in the correct mode.

Castell products has been developed to provide the industry with interlocks of high quality and integrity. Working closely with key switchgear manufacturers has enabled Castell to produce interlocks designed specifically for use on the leading manufacturers own breakers, isolators, disconnectors and earth mechanisms. Castell products are available in a range of materials and finishes, to ensure the correct specification interlocks. High temperature locks for use on electrostatic precipitators, stainless steel and weatherproof locks for use in sub-stations, chrome plated locks for areas where aesthetics are important and brass as standard for locks in dry, clean, non-corrosive environments. Castell products are used in the following areas:-

  • Wind Turbine Isolation & Earthing
  • Rail Electrification Systems
  • Electrostatic Precipitators
  • UPS Systems
  • LV Distribution & Busbar Systems
  • HV Transmissions & Distribution Sub Stations
  • Transformer Isolation & Earthing Systems

Castell have developed products to suit the following companies’ equipment:-

ABB I Alstom I Terasaki I Schneider Electric I Siemens I Hawker Siddeley I George Ellison

For further information on the Castell safety system or to discuss your application with one of our product specialists please contact us.

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