Test & measurement
SAFETY CRITICAL COMPONENTS
By Dennis Butcher, senior engineer at TÜV SÜD
O 60
ne of the main causes of project delays is safety critical component approval. So, it is very important to understand what these are, and what can or
cannot be accepted. In general, a safety critical component is a component which, if changed, could compromise the safety of the end product. A common misunderstanding is that these are usually mains powered components, but in many cases they are secondary components.
During abnormal and single fault condition (SFC) testing, some components become safety critical because they limit hazardous energy to a safe level or prevent access to it. Safety critical components must be used within their specifications, to ensure reliable and repeatable operation during normal, abnormal or single fault conditions in the end product. This can be determined from datasheets and component certifications, but in all cases they are assessed in the end products to ensure that they are suitable in that application.
DEFINITION IECEE Operational Document OD-2020 defines a safety critical component as: “When the design of the product is such that a determined component, in case of failure, can compromise the safety aspect, this component is defined as “safety critical”. Other definitions of safety critical components include:
Components which have specific requirements in the applied end product standards. Some common examples of these are: mains input fuses, safety capacitors, fire enclosures.
Components which have requirements in the National Standards included in the evaluation. Some countries or regions have different or additional requirements for safety critical components.
Materials which are critical to maintain compliance with the standard, such as corrosion requirements and flammability. For example, plastics need a minimum flammability rating depending on their function e.g. a fire enclosure or barrier. All plastic components and parts need a minimum level of flammability, unless they can be exempted e.g. by size or mass.
Terminals and screws for some purposes must be protected from corrosion, so may need plating or to be made of specific materials.
Electrical insulation must be adequate for the working voltages within the end product.
Professional judgment of the CB Scheme Test Laboratory or National Certification Body. This would usually be something that was relied on as a safeguard but isn’t specifically mentioned in the standard.
June 2025 Instrumentation Monthly
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