During the stages of product development it is a good idea to consider safety design constraints, material selection and best practices as early as possible. There is possibly nothing more costly and embarrassing as having finished a project on time and on budget only to learn that some key part of your design is not acceptable to the safety agency. Worse, are the costs and lost time in recovering from a revelation such as this. Below is a collection of the more common activities you should be prepared to do when certifying your product. ~ Doug
Obtain and familiarize yourself with the safety standard(s):
- Identify and obtain an up-to-date copy of all applicable product safety standards.
- With internationally harmonized standards you will discover parts of the world that implement these standards on different timelines
- Depending on those parts of the world where you want to market your product, you may also find national differences which require additional measures for acceptance
- Read the standard in enough detail to be familiar with those parts which are applicable to your product and the markets where you are going
- Apply the applicable clauses of the standard during the early stages of product design
- Itemize areas of special concern or high risk of failure when presenting your product to the safety agency for evaluation
- Test critical high-risk areas and do not rely on computer modelling alone
Component selection:
- Use components in safety-critical areas that are certified by an acceptable third party for the intended application
- NRTL certified parts usually meet this requirement
- Listed, CE Marked and CB-Certified components may require additional investigation
- Always use components within their certified ratings
- Always obtain “conditions of acceptability” for critical components
- In general, military or mil-spec ratings are not acceptable
- Custom or specially fabricated components must be assessed as early as possible
For example, a custom molded connector housing may need to have the plastics evaluated by several tests which can add months to your schedule (Hot Wire ignition, Ball Pressure, Flammability, etc.)
Circuit isolation and spacings:
- Be very clear about each circuit type, how it will be classified for potential hazards and its proximity to other types of circuits:Mains/Secondaries/SELV/PELV/TNV/other
- Determine and design to required spacings (physical distances) for insulation:
- Clearance – distance through air
- Creepage – distance over surfaces
- Solid insulation – thickness through insulation, voltage strength, impulse ratings, etc.
- Understand the environmental conditions:
- Overvoltage Category – transient spikes routinely occur on the power grid. These can propagate into a building and cause damage to equipment.
- Pollution degree – depending on the conditions where your product will be used dirt, dust, humidity, moisture and weather can increase spacings requirements.
- Altitude – most standards use 2,000 meters as the default. Many populated areas of the world are higher than this and clearances will have to be increased accordingly.
- What is the voltage involved when assessing spacing requirements:
- Is the voltage sinusoidal, direct current or a combination?
- Are there recurrent peaks such as those produced by a switch-mode power supply?
Polymers and printed wiring boards:
- Most plastics will require a minimum flame rating
- If a material is in direct contact with live circuits or used as a critical insulator, the thermal rating is very important to know
- Consider the plastic’s use: enclosure? insulator?
- Comparative tracking index (CTI) of a material may change spacings requirements
- Special requirements apply to the use of conformal coatings
- Many of the same ratings apply to the laminates used in printed wiring boards
Mechanical hazards and moving parts:
- Pinch points, crushing, sharp edges, fast moving parts, high pressure and breaking glass are just a few of the potential hazards
Sources of radiation:
Chemical hazards:
- Caustic or corrosive substances can be damaging or hazardous which means leaks and spills must be controlled
- Toxins and various gases can also present a hazard
- Some chemicals can cause unexpected reactions, for example leaking battery electrolytes can react with some metals and explosive gases result
Outdoor use and immersion of products:
- For products of this type used in North America it may be necessary to apply the requirements of UL 50 (USA) and CSA C22.2 No. 94 (Canada).
- Other parts of the world have additional requirements found in IEC 60529 or EN 60529 for ingress protection from dirt, dust and liquids.
- These standards have requirements for assessment of the plastics for UV exposure and of gaskets relied upon for ingress protection
Product Markings and User Documentation:
Place labels and markings on the product as intended or provide mechanical drawings drawings showing where markings will be placed. For example:
- Manufacturer or trademark, model, country of origin
- Electrical ratings
- Safety warnings
- Installation instructions & user/operating instructions will require a thorough review.
Risk Assessment:
- Many of the latest edition UL standards, IEC standards and their national derivatives now require a formal risk assessment for all potential hazards which are not explicitly covered by the safety standard
- This can be leveraged by an existing FMEA / FMECA process or by other means
Engineering Change Control:
- As with the original product design, any changes to safety critical material, components or function must be approved by the safety agency
Factory audits:
- NRTLs require at least four unannounced factory surveillance audits per year
- You will receive four audits at each registered factory location, including locations for factories that are outsource manufacturing
- Be prepared to show that the certified product remains compliant over time
- A demonstration of production line safety testing will be required; this usually includes electric strength testing and earthing continuity testing
- When you get a copy of the test report, review it; this is what the factory inspectors use to conduct their inspections
As always, if you need assistance with any aspect of safety review and testing, feel free to contact us.
