November/December 2019

34 n MACHINE SAFETY November/December 2019 www.drivesncontrols.com Trapped-key interlocks get their own standard L ast year, the International Standards Organisation (ISO) published the first international document devoted specifically to trapped-key interlocking. The standard, ISO/TS 19837:2018 - Safety of Machinery – Trapped Key Interlocking Devices – Principles for design, selection and configuration, answers many of the questions that have arisen relating to performance levels, key coding, and power interlocking. It is a significant step forward which will provide more choice for users of interlock devices and systems. Trapped-key interlocking has its roots in the rail and power distribution industries where sequential control was, and still is, critical to safe, continuous operation. In the 1960s, trapped-key interlocking was gradually adopted by the manufacturing sector where there was a need to isolate machinery before gaining access to it. Trapped-key interlocks can be used to safeguard machinery with multiple energy sources and access points. Products are available that can isolate air and hydraulic supplies as well as electrical power in sequence before allowing access points to be opened. Confusing When ISO14119:2013 Safety of Machinery – Interlocking device associated with guards, Principles for design and selection was published, trapped-key interlocking had become no more than an annex. It was clear that the standard did not provide all of the specific requirements for trapped-key systems. In many respects, ISO14119:2013 was confusing to users and potential users familiar with decades of trapped-key interlock installations. ISO/TS 19837:2018, which took three years to develop, is therefore a welcome addition to the existing suite of machinery safety standards. Perhaps the most critical question it addresses is where mechanical or hybrid mechanical/electrical systems sit in relation to performance levels introduced in ISO 13849. Power interlocking is now more properly referred to as “isolation control through direct mechanical action” and falls within the safety-related part of a control system. This, coupled with a section on “validating performance level”, makes evaluating performance levels for trapped-key systems much clearer. Following the confusion created by ISO14119:2013, the new standard clears up the difference between actuators and keys, and both are addressed in the “terms and definitions” and “symbols and abbreviated terms” sections. The need for a schematic detailing the trapped-key system operation and establishing key coding appears under a section called “key transfer plan”, and provides users with an excellent tool to use during the system design phase, as well as a documented system layout after installation. The new standard also addresses key coding and management. A much-needed standardisation of symbols to represent trapped-key interlocks will make it easier for users to understand schematics, and the excellent pictorial examples throughout the document are useful for those building trapped-key interlock systems. The publication of ISO/TS 19837 will help to keep trapped-key interlocking at the forefront of machine safety. It is good to finally have a document that helps users to understand better how trapped-key interlocks fit within international machinery safety standards. n ISO/TS 19837:2018 is intended to be used in conjunction with ISO 14119. Copies of the new standard can be obtained from the ISO at https://www.iso.org/standard/66335 A recent international standard is the first devoted specifically to trapped-key interlocking. Jo Smith, managing director of Fortress Interlocks, explains the significance of the document. The new international standard answers many of the questions that have arisen about trapped-key interlocks, relating to performance levels, key coding, and power interlocking