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Product Design: How to Get the Design Right the First Time

1403 product-design coverMany manufacturers design a product first, then attempt to “fix” the design later to meet the applicable safety standard. Prior knowledge of the applicable safety standard and its requirements for the product will help meet deadlines, keep design costs down, and result in a properly designed product.

You spend months, or even years, designing a product. After it’s all ready to be shipped to your customer, you find out that you need a safety certification mark. So in a panic, you send the product off to a test lab for evaluation. The shipment is sitting in your loading bay waiting for the final certification to arrive and then the bad news arrives. Your test lab tells you that it fails! This is not only heartbreaking, but time, effort and money are wasted in redesign. Not to mention the delay in shipping your product to the customer! Everyone is looking at you and wondering why it wasn’t initially designed correctly. If only you had a manual entitled “Things I need to know to design my product to ensure that it will pass safety testing”! Oh wait…you do! It’s called a safety standard. It may have been published by the IEC, UL or CSA, but it contains everything you need to know, right there, in black and white.

If designers have access to safety standards, why is it that most products submitted for certification have a flaw of some sort that causes the product to fail the safety evaluation? Sometimes these flaws are minor in nature (e.g. missing label, wrong wire color used) which don’t take much time to fix. But sometimes the flaws require a complete redesign (e.g. replace the power supply, redesign circuit boards, redesign the enclosure). Why don’t designers pay more attention to the requirements? A variety of reasons come to mind: lack of time to research the requirements, lack of knowledge that the safety standard exists, miscommunication within the design team, etc. Even if the designer doeslook at the standards, it is often difficult to understand the requirements (if you can find them). Anyone who has read a safety standard will agree that they are not easy to understand, and tend to bring on lengthy discussions when it comes to interpretation of the requirements.

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What is a designer to do? There are a variety of steps that the designer can take to help insure against costly redesigns.

Determine the Market Where the Product Will Be Sold

The first thing to find out is exactly where your company will want to sell this product. Your marketing department may have already determined this (but may not have shared this with the design team). North American manufacturers will often focus on sales in North America, only later to be surprised when they find out the extent of the redesign required to comply with European requirements. Knowing the target market may affect many aspects of the design: voltage ratings, component selection, wiring methods, etc. For example, selecting an auto-ranging power supply (100-240V) will allow your product to be used in Europe (220-240V), Japan (100V) and North America (120V). If you’ve designed for the North American market only, you may have neglected the other voltage options, resulting in a costly redesign.

Now that you know what countries you will be targeting, determine what safety related marks are required. The United States and Canada have a variety of options available; many certifiers (e.g. CSA, UL, TUV Rheinland, etc.) are accredited by both the Standards Council of Canada (SCC) and the Occupational Safety and Health Administration (OSHA) as a Nationally Recognized Test Lab (NRTL). Knowing that you can talk to a single certifier to gain simultaneous marks for both countries will make things much easier.

Europe uses a self-evaluation method called the CE Mark. The CE Mark declares compliance to all the directives applicable to the product (e.g. Low Voltage Directive, EMC Directive, and Machinery Directive). Because it is a self-evaluation mark, manufacturers can evaluate the product themselves (with a high level of risk to the manufacturer), or use an agency to evaluate the product on their behalf (low level of risk). Europe has some extra requirements to consider, namely the RoHS and WEEE directives, which have specific restrictions on toxins (mercury, lead commonly used in solder, etc.) and requirements for disposal methods. Many component manufacturers have lead-free alternates to be selected for European markets.

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Some countries, such as Japan, have a list of products that need to be certified. Any product not on this list does not need to be evaluated for safety. It’s important to look into this beforehand so you can learn the requirements (if any) before designing your product.

Determine the Correct Safety Standard for the Product

Now that you know where your product will be sold and the certification marks required for each market, you can determine the safety standard(s) that apply to your product. If you are designing Information Technology Equipment (ITE), you are fortunate because many countries have adopted the same safety standard (IEC 60950-1) [1] and only tweaked it slightly to meet with their National Electrical Codes. Often meeting the requirements for one country will meet the requirement of other countries sharing the same standard.

Some products are not so lucky and have different standards in each country. For example, Industrial Control Equipment has a standard in the United States (UL 508) [2], a very different standard in Canada (CSA C22.2 No. 14) [3], and a completely different standard in Europe (EN 61010-1) [4]. In circumstances such as this, you may need to design with three different standards in mind!

Knowing what the applicable standards are will allow you to purchase them, review their requirements, and use your new knowledge in the design of your product.

Select Components That Are Suitable for the Standard

Most safety standards contain a list of component standards that are acceptable for compliance. Use these when selecting components! The safety standards generally allow for two choices: (1) evaluate the component to the applicable component standard (as listed in the standard), or (2) evaluate the component to the product’s safety standard. If the component is already certified to the applicable component standard, you can be assured that the component is suitable and will not require additional testing. An uncertified component (including CE marking because it is self-declared) will require additional testing. In general, this testing is at an additional cost and will extend the amount of time allotted for certification.

Keep in mind that each standard may have different requirements for components. For example, the ITE standard for the United States will list many UL standards that need to be met. Since UL standards are used in the United States only, these certifications alone will not be suitable for the European market.

Obtain Component Licenses

Many component datasheets and catalogues state the safety certification marks and safety standards that the components have been evaluated to. Don’t believe them. The marketing teams that produce these datasheets and catalogues make mistakes, incorrect assumptions, or use outdated information. You need to collect proof that each component is certified according to their claims.

Some agencies, such as UL, CSA and TUV Rheinland, have powerful databases on their webpage that allow you to search for licenses. Use these online tools for all your components!

Now that you are sure that the certifications are valid, you need to ensure that you are using the component according to its ratings. Often these ratings are listed on the agency websites and are easy to check. However, for some components, finding the listing on the agency website is not enough. Sometimes the certification record is vague, doesn’t list the exact standard, doesn’t include things like current and voltage ratings, etc. The only practical way to be sure that the component will be acceptable is to get the licenses from the manufacturer. Component licenses will sometimes include an important section entitled “Conditions of Acceptability”, because the component evaluation is not a complete product evaluation. The Conditions of Acceptability include conditions that will need to be met in the end-use product (e.g. enclosure requirements, wiring details) and assumptions that were made during certification (e.g. required airflow, fusing). UL provides this for every component certification in their UL Recognition Program. Other certifiers may provide the conditions, but not always. It is crucial to obtain the Conditions of Acceptability for key components such as power supplies, dc-dc converters and transformers.

Read the Conditions of Acceptability and license and ask yourself “Am I using this component according to its rating?” If you will be using a power supply in a 60°C environment, but the license states a rating of 40°C, then you are using that supply outside its ratings. Never mind that the manufacturer may have provided a derating curve in their datasheet. If it hasn’t been evaluated by a certifying agency, consider it to be unproven and therefore unreliable. Using a component outside of its ratings will void the certification of the component and result in retesting of the component in your specific equipment. This is an extra cost and hassle that should be avoided if possible. One simple way of correct this is to source a more suitable component with the correct ratings or adjust your equipment ratings.

Also ask yourself if you are meeting all the conditions stated in the Conditions of Acceptability. If the Conditions of Acceptability state that there must be airflow over the power supply, make sure you are providing that same airflow. If the Conditions of Acceptability state that a terminal block is not for field wiring, you cannot use that terminal for field wiring! You must evaluate these Conditions of Acceptability as they apply to your product with a critical eye!

One more thing to consider is to make sure the licenses you receive from the manufacturer are current! Manufacturers are eager to send you agency licenses that show compliance to old standards or cancelled certificates. Always double check that the component is still certified (confirm on the agency website), and ensure that the standard (and the edition of the standard) used for compliance is listed in your product’s safety standard.

Remember, even if the component is certified, if it’s not certified to the correct component standard, used outside of its ratings, or certified to an older version of the standard, consider it to be uncertified. If you include that component into your design, you will have increased certification costs to cover the extra evaluation and testing.

Design a Suitable Enclosure

There are many things to look for when designing an enclosure for your product. Not only does it have to match the “look” that your marketing department desires, but it has to be functional and pass the tests of the appropriate safety standard. There are a variety of things to look at, including material selection, material thickness, openings (including ventilation) and sturdiness necessary to pass the tests of the standard.

Material Selection
Are you considering a plastic enclosure or metal enclosure? Plastic enclosures have some additional requirements to consider, such as flammability ratings of the plastic. These details are described in the safety standard. Consider the plastic to be a component and look it up on the agency website (UL has an excellent online database for plastics). Make sure the specific plastic you are using is listed there, with the appropriate flammability rating and in the correct color. If your plastic is not listed on this website, not only will you be required to have flammability testing conducted, but annual confirmation tests will also be required (at additional cost to you).

Material Thickness
Plastics that are certified will have been tested at a specific thickness. Often the flammability rating will differ depending on the thickness of the plastic. Making sure that the minimum thickness in your enclosure is greater than that listed on the agency certification is critical.

Openings
Openings in the enclosure, generally for ventilation purposes, create a few challenges: (1) if they are too big the user may be able to touch the circuit inside, creating a shock hazard, (2) if the enclosure is providing a fire enclosure the openings may allow flaming particles to exit or enter the enclosure, thereby defeating the purpose of the fire enclosure, and (3) large openings that house a fan or moving part could introduce pinch hazards without suitable shielding. Ensure that all your openings comply with the requirements of the standard.

Tests
Enclosure tests are commonly conducted in safety evaluations. The enclosure must be sturdy enough that it won’t allow a hazard to occur after falling, being leaned on, stood on, impacted, heated, cooled, exposed to UV radiation, or any other foreseeable situation that may affect the safety of the product. You need to consider all the possible tests that will be conducted, as described in the safety standard, and design accordingly.

Determine the Required Spacings

Knowing what spacings are required between different types of circuits, or between a circuit and an accessible part (i.e. the enclosure) is critical. Planning and designing your wiring boards when you know what is required will save you much time and effort, and will avoid that costly redesign.

Identification of Circuits
The first step is to identify different circuits and accessible parts (i.e. mains circuit, unearthed secondary circuit, earthed enclosure, floating enclosure, etc.).

Create a Block Diagram
Each of these circuits and parts can be considered (and drawn as) a block. Include components that bridge these different blocks (i.e. a transformer, capacitor, relay, etc.). Litter your block diagram with arrows between blocks to indicate where insulation is required. See Figure 1 for a sample block diagram.

1111_F2_fig1

Figure 1:  Sample block diagram

 

Determine the Level of Insulation Required
Referencing the safety standard, determine the type of insulation required between each of the blocks identified with an arrow. Examples of insulation include: basic insulation, reinforced insulation, and supplementary insulation.

Using Tables in the Standard
Determine the creepage distances and clearances required for each of the locations indicated with an arrow. These requirements are found in the safety standard, generally in tables. The required distances will differ depending on the working voltage and the type of circuit.

After determining the required spacings, ensure you are applying these when laying out printed wiring boards. Also consider clearances between boards and enclosures or between adjacent boards.

Single Fault Examination

Knowing what single fault tests will be conducted on your product will help immensely during your design. You need to design your product so it can withstand the fault applied and remain safe. A fire or a shock hazard is unacceptable. Single fault tests include shorting and overloading transformer windings, short circuiting or open circuiting components (i.e. capacitors, legs of optocouplers, transistors, resistors, etc.), blocking air ventilation openings and stalling fans. Anticipating these faults and designing protection devices (such as fuses) into your design will be extremely beneficial.

Other Standard Requirements

Every safety standard is different. You, as the designer, need to thoroughly go through the standard to make sure all requirements are met. There will be clauses about earthing methods and bonding tests, requirements for the sizes of wire used, disconnect devices, fusing requirements, touch current requirements, electric strength testing requirements, etc. Knowledge of these requirements will improve your design.

Using Consultants Who Understand the Requirements of Your Safety Standards

Consultants familiar with your safety standard can be a genuine asset for your design team. They have experience with the safety standard and agencies. They know what requirements you need to consider and can identify common pitfalls. They can advise you on the suitability of the components selected and assist with the design of your product (i.e. enclosure design, circuit board layout, etc.). Relying on a consultant will allow you to focus on other aspects of the design, feeling confident that the design will not result in failures during safety certification and evaluation.

Summary

It’s critical to know the market your product will be shipped to before the product design is started. Once you know this, you can use the appropriate safety standards when designing your product. Using consultants to assist with understanding the safety standard is another option to be considered.

If you are unfamiliar with the appropriate safety standards that will be used to evaluate your product during safety certification testing, your design will most likely fail. Your components may not be suitable, your enclosure may be inadequate, your circuits may need to be redesigned, etc. When your product fails during safety certification, you will be charged more for extra evaluation. Furthermore, certification failure significantly delays your time to market while you spend time and effort to fix the problems.

Designing to meet the safety standard is the smartest thing you can do! favicon

References

  1. IEC 60950-1: 2005, Information Technology Equipment – Safety – Part 1: General Requirements.
  2. UL 508, Seventeenth Edition, UL Standard for Safety for Industrial Control Equipment.
  3. CAN/CSA-C22.2 No. 14-05, Tenth Edition, Industrial Control Equipment.
  4. EN 61010-1: 2001, Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use – Part 1: General Requirements.


© 2009 IEEE Reprinted with permission from the author and the IEEE from the 2009 Symposium on Product Compliance Engineering Proceedings

author_forbes-cherie Cherie Forbes
is an electrical engineer focused on helping manufacturers gain safety certifications such as UL and CSA. She has worked in the product safety industry for fifteen years and is currently an independent consultant at CertAssist Consulting Inc. She was formerly Manager of Engineering for Lamothe Approvals and has published numerous articles and conducted seminars to aid manufacturers with product design. She is currently a member of IEEE Product Safety Engineering Society (PSES), Society of Manufacturing Engineers (SME), Professional Engineers of Ontario (PEO) and Silicon Halton. She can be reached at cherieforbes@certassist.ca or www.linkedin.com/in/forbescherie.

 

 

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