COMPONENT DESIGN Safety Critical Components
One 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.
D
uring 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.
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 for example, a fire enclosure or barrier. All plastic components and parts need a minimum level of flammability, unless they can be exempted 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. Safety critical components should be approved to the relevant component standards, if any. They must also comply with any specific requirements in the end product standard, and must be used within its ratings such as voltage, current or power, rated max. temperature. In addition to an accredited test report, components approved to an IEC standard must also have a CB Certificate if that standard is included in the CB scheme. Other international or national approvals may sometimes be accepted in lieu of CB Certification and reports, provided that the standard applied is technically equivalent to the IEC component standard.
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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”.
One example is to use standards with different prefixes where the number that follows is identical, as in this example: “If a required standard for a safeguard component is IEC 61058-1 switches, then the Cenelec standard EN 61058-1 may be used instead, as the main body of the Cenelec version is technically equivalent to the IEC standard, including test and compliance criteria. Note that the edition or date of publication may not be the same as the IEC standard, so this must be checked to ensure that it is the equivalent version.”
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Some components or modules might be previously approved to an end-product standard instead of a component standard. This is usually the case where there is no appropriate component standard, or where the ‘component’ is an end product in its own right, for example, motherboards, power supplies, pumps, battery packs. In this case, the approval must also be acceptable for the end-product standard being used and the same ‘rules’ apply for acceptance as for any other component. Some components in this section may also need additional verification to ensure that tests and compliance criteria are sufficient to meet the requirements for the end product. It is essential that, in addition to certification, the full test reports are also available. Alternate national standards may be required for some countries or regions. Some common examples are North America, the European Union, United Kingdom, Japan, Australia/New Zealand. It is also worth noting that a manufacturer’s Declaration of Conformity or a self-issued Certificate of Conformance is not acceptable in lieu of third-party accredited testing and certification. In some cases, an end product standard may allow for some components to be assessed in the end-use application. This will usually require additional in- application testing.
FEBRUARY 2024 | ELECTRONICS FOR ENGINEERS 25
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