Safety certification for the T&M world
Preparation is the key to limiting product redesign and obtaining safety certifications on time.
David Lohbeck, National Instruments, Austin, TX -- Test & Measurement World, 8/1/2004 2:00:00 AM
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Read the August 2004 features: Triple play Production test, bench work, and Web-based simulation deliver quality op amps and applications support. Safety certification for the T&M world Preparation is the key to limiting product redesign and obtaining safety certifications on time.EMI@EMC² EMC Corp.'s EMI engineers test computer storage systems in a lab designed to handle a 5000-lb beast. Find reflections in glass CVI Laser, a maker of waveplates used with fiber-optic telecommunication devices, required employees to manually inspect each waveplate after a saw cut it from a substrate. For more information on safety test, visit www.tmworld.com/emc. |
Let's say you've designed a new product, and your marketing department and legal beagles say, "We need safety certification to sell it. And furthermore, we need it yesterday."
Get the picture? You may think the product is ready and should have no trouble passing safety testing for certification. You also may think the testing and certification process is a mere formality and that you needn't worry.
But did you know that if a product fails a single test, uses a noncertified critical component, has a wrong label, or is missing a caution symbol, the product is not certifiable and may not ship? The good news is that safety certification is not as complex as it once was. You just need to familiarize yourself with the process and follow a few simple rules.
The safety certification process I'll outline applies to many products, including Information Technology Equipment (IEC 60950-1) and Test & Measurement Equipment (IEC 61010-1). Figure 1 outlines the general process. The terms "third-party," "agency," and "certification body" are equivalent; all are government-accredited safety testing and certification organizations.
Why Safety Certification?
Safety certification protects the consumer and provides manufacturers with evidence of compliance. Certification is an attestation from an impartial testing body that a product has been evaluated and found to comply with specific standards. Certification allows manufacturers to affix safety marks to products as visual conformity evidence; marks include UL or CSA marks for North America, and VDE, TUV, or Demko marks for Europe. Safety certificates and marks indicate due diligence and are a manufacturer's best defense should the safety of a product come into question by customers, competitors, or market-surveillance authorities.
International (IEC) standards are the focus of safety conformity and form the basis for other standards, such as those promulgated by the two primary types of certification bodies: Nationally Recognized Testing Laboratories (NRTL), which are qualified by OSHA in the US, and Notified Bodies, which are accredited by the European Commission in Europe. UL and CSA are NRTLs, while VDE, TUV, and Demko are Notified Bodies.
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Figure 1. Achieving a certification mark involves many steps, so manufacturers should start the process long before the final product is built. |
Note that a European directive on product safety requires that many electrical products carry the European Conformity (CE) marking. But a CE mark is the supplier's self-declaration (self-test) symbol. CE is not an approval, certification, or mark from an independent certification body. European customers also may expect VDE, TUV, or Demko marks on the products they purchase.
Safety assessment phases
Standards represent the minimum acceptance criteria for safety assessment and certification. You should familiarize yourself with the relevant standards in order to design products for safety compliance. Certification bodies perform safety assessments in three phases:
•Components and construction. The first assessment criteria is verification of safety-critical components—those that typically see hazardous voltages that affect the safety of the product, surroundings, and user. Safety marks on a component are evidence that the component meets safety standards. The certification body also reviews the product's construction and user documentation for compliance to safety standards.
•Testing. Next, the certification body performs safety testing on the product. Which safety tests the agency performs depend on the product's voltage and power rating, design complexity, components, operating environment, and other factors.
•Test report and certification. After achieving successful results, the certification body issues a signed test report and certificate to the manufacturer. The certificate lists the product manufacturer's name, factory(ies), model(s) tested, safety standard(s), and ratings. The manufacturer can then apply the certification body's mark to the product.
Components and construction
A product containing just one wrong safety-critical component—such as a power supply, optoisolator, fuse, relay, or connector—will be found noncompliant. Thus, when selecting safety-critical components, you should look for ones affixed with safety marks and certificates. UL/CSA and IEC/EN standards are not harmonized for many components, so you may need two marks to verify a component meets US and EU requirements. No marks at all suggest a component is not certified for any location!
Always verify a supplier's safety claims. Statements such as "designed to meet . . ." or "in accordance with . . ." are only self-declarations and usually mean a component is not certified. You will need a copy of a component's certificate to verify component number, rating, standard applied, and restrictions for use. The component's rating must meet an end product's rating for use. Check the certificate and look for the mark early. When in doubt, have a substitute ready.
Many certification bodies use the Safety-Critical Components List (Table 1) to identify and check component safety. When submitting a product for testing, you should use the list to identify each part by the component name, manufacturer, part number, and rating. Indicate safety marks in the last column. In addition to completing the list, you should obtain certificates as backup, and then submit the list and certificates to the certification body for product testing.
To conform to the safety standards, a product must be constructed to protect against a number of hazards—shock, fire, heat, mechanical, radiation, explosion—and must be resistant to impact, and in some cases moisture and liquids. Values above 30 Vrms and 42.4 Vp or 60 VDC are considered to be hazardous live voltages. For such voltages, protection against electric shock and fire must be maintained in normal and single-fault conditions—hazardous voltage circuits must not be accessible via test finger or pin. Moving parts must not crush, cut, pierce, or severely pinch the operator's skin. Enclosure construction, especially bottom openings, must limit the spread of fire.
Testing
Testing is a critical part of the certification process and the final checkpoint. Even when a product meets the component and construction requirements, it may fail safety testing. The tests the agency performs depend on the product's function and its operating environment. Tests mandated by a standard are referred to as Type Tests and are carried out on a representative product sample. The agency performs the tests on a product assembled for normal use in the least favorable conditions. Any tests that may damage the product are performed last.
Safety testing, such as dielectric (or hipot) testing, is required in addition to—not in place of—component certification and construction requirements. A product must meet all component, construction, and testing requirements together before it is considered safety compliant.
To minimize the chance of failure, you should arrange for safety pretests before submitting a product to a test body for certification. Examples of safety tests are shown in Table 2.
Preliminary evaluation—the fast track to certification
Compliance with safety standards should be a consideration at every stage of a product's development. Unfortunately, many companies wait to submit their "latest and greatest" model for testing, thinking that safety certification is one of the last steps taken before shipping a product. Safety certification may be one of the last steps in the design process as flow charts go, but it is a step that should be taken much earlier.
Many of those who wait to submit their "final product" and then find that it fails to meet safety standards say that if they had only known of the deviations sooner, they could have fixed them during the product's developmental stages. Waiting only delays the day of product compliance.
To minimize guesswork and obtain answers early, you should submit a product to the testing agency for a preliminary evaluation of components and construction. The cost is typically less than half the cost of a full assessment. In lieu of the finished product, you can submit a prototype for preliminary evaluation. (Click here to download a checklist of what you can expect to submit.)
Changes within safety extra-low voltage (SELV; &42.4 Vp or &60 VDC) circuits usually have no effect on safety, so don't wait for SELV revisions to submit. Remember to provide explanations of any planned corrections or updates so the testing body may review how the potential changes could affect the product's safety. The agency engineer evaluates the product and user documentation and, if requested, can also perform key tests if he or she suspects a failure could occur.
The agency issues a findings letter upon completion. You then correct the deviations and resubmit for a full assessment, testing, and certification. If the deviations are minor, the agency may accept your proposed corrections via drawings or letter without a new sample or further testing.
After initiating a project, you should establish a direct relationship with the agency engineer. Make sure you discuss expectations and how the project will proceed. Don't be afraid to ask questions; this is your chance to learn.
The agency typically forwards its findings to you after the assessment is complete. You can, however, ask the engineer to contact you immediately if he or she find major safety deviations. This way, you may be able to correct problems before the project is completed and end with a passing result. The secret here is good communication and quick response to questions and issues.
You can avoid many pitfalls, time delays, and costly redesign when you understand the standards and become familiar with the safety certification process. Verifying components and construction safety and performing safety tests prior to submittal increase the chance of certification the first time through. The lessons you'll learn by going through the safety certification process typically carry over to new products, making them easier to certify.
FOR FURTHER READING
"Product Certification," National Instruments, www.ni.com/certification.
Table 1. Safety-critical components list
| Component Type (drawing reference or location) | Manufacturer (name/logo on part) | Manufacturer's part number | Technical data and ratings per certificate and markings | Safety Marks (evidence) |
| Power supply (PS1) | XYZ Corp. | X1234-X-Y | Input: 100–240 VAC, 5 A, 50/60 Hz; Outputs: 5 V/20 A, 10 V/5 A; 20 cfmrequired for 35ºC operation | UL-R (QQFU2), CSA, TUV, CB cert #1234 |
| Inlet/filter/fuse holder (J1) | ZYX Inc. | Z-321-10A | 250 VAC, 10 A, 50/60 Hz | CSA, VD |
| • | • | • | • | • |
| • | • | • | • | • |
| • | • | • | • | • |
Click here for a more detailed version of this table.
Table 2. Safety Test Examples
| Test | Purpose | Procedure¹ | Pass/fail criteria¹ |
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Note: 1. Refer to the standards for more information on test procedures and pass/fail criteria. Click here for a more detailed version of this table. |
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| Input | Used to check rating on label. Also used to establish rating. Test monitors the product's power and voltage during testing in normal operation and fault conditions. | Measure power or current during operation with max load; ±10% voltage rating (range). Suggestion: spec the product's rating at about 10% above measured current value for the user documentation and label. | Measured current value shall not exceed 10% of rating on label. |
| Dielectric withstand | Dielectric, aka hipot, stresses the product's insulation system (spacings, materials) to verify high voltage withstand from primary to ground and secondary. Protects user from hazardous voltage breakdown. | Primary to secondary (double) 2300 Vrms/3250 VDC, 1 min; Primary to ground (basic) 1350 Vrms/1900 VDC, 1 min (300 V CAT II). Test performed after humidity preconditioning and when product is well heated. | No breakdown (arc-over) at AC or DC voltage. (Repeat dielectric withstand test after each abnormal test; basic value.) |
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