Why Test?

The first steps in finding the best match between your product and an environmental test laboratory are determining what must be tested and why. In general, you will need to ensure that safety regulations have been met, the product actually performs the function intended, and it will continue to do so in the environment where it will be used. And, don’t forget to test the packaging you’ve designed to protect the product during transportation from your factory to a sales or installation location.

Safety

In addition to a manufacturer’s concern about legal liability in case of user injury, there are several reasons to test for compliance with safety regulations. For example, your product may be regulated by the occupational safety and health administration (OSHA) and covered by the General Industry Standards (Part 1910 of Title 29, Code of Federal Regulations—29 CFR Part 1910).

Two major causes of injury are fire and accidentally dropped heavy items. So, fire alarm and sprinkler systems, storage and dispensing equipment for flammable liquids, and hoisting equipment must meet appropriate OSHA standards before they can be used in workplaces.

To carry out product testing, OSHA has instituted the nationally recognized testing laboratory (NRTL) program. Private-sector organizations and companies that meet the necessary qualifications specified in the regulations for the program are designated as NRTLs. An NRTL determines that specific equipment and materials (products) meet relevant safety standards.

These organizations and companies currently are recognized by OSHA as NRTLs:

• Applied Research Laboratories (ARL)
• Canadian Standards Association (CSA) (also known as CSA International)
• Communication Certification Laboratory (CCL)
• Curtis-Straus LLC (CSL)
• Detroit Testing Laboratory (DTL)
• Electro-Test (ETI)
• Entela (ENT)
• Factory Mutual Research Corp. (FMRC)
• Intertek Testing Services NA (ITSNA) (formerly ETL)
• MET Laboratories (MET)
• NSF International (NSF)
• National Technical Systems (NTS)
• SGS U.S. Testing Company (SGSUS) (formerly UST-CA)
• Southwest Research Institute (SWRI)
• TUV Product Services GmbH (TUVPSG)
• TUV Rheinland of North America (TUV)
• Underwriters Laboratories (UL)
• Wyle Laboratories (WL)

Each NRTL designate has a unique certification mark that it authorizes manufacturers of complying products to affix. An example is the familiar UL mark molded into plastic electrical plugs.

The individual consumer also is represented. In 1972, Congress established the U.S. Consumer Product Safety Commission (CPSC). It is an independent federal regulatory agency created to “protect the public against unreasonable risks of injuries and deaths associated with consumer products.” The commission’s activities include regulation through standards development, education of consumers about potential safety risks, research on possible product hazards, and recall of faulty and dangerous products.

For example, in September 1998, a recall of power-strip surge protectors was announced. “In cooperation with the CPSC, First Choice Products Inc., of City of Industry, CA, is voluntarily recalling about 194,200 power-strip surge protectors. The power strips have undersized, cracked, or corroded wiring and misaligned plugs, which present fire, shock, and electrocution hazards. CPSC and First Choice Products are not aware of any injuries involving these power strips. This recall is being conducted to prevent the possibility of injury.”¹ Even without any claims for damage or injury, such a recall could be very expensive to carry out.

As an example of a CPSC standard, consider federal regulation Title 16, Part 1204, Safety Standard for Omnidirectional Citizens Band Base Station Antennas. The stated purpose of the document is “to reduce the risk of electrocution or serious injuries occurring if the antenna contacts an electric power line while the antenna is being put up or taken down.” To that end, the antenna or mast must be insulated “so that a harmful electric current will not pass from the antenna to a person in contact with the mast.”²

What may be a completely reasonable safety requirement has become a testing issue. As the standard points out, many antennas will have been in use for several years before they are taken down. During this time, insulating materials may deteriorate due to environmental conditions. For that reason, the standard suggests that the insulating material should include an ultraviolet stabilizer or screen and have heat resistance up to 212°F without loss of elasticity and moisture absorption of not more than 0.2%.

If heat-shrinkable sleeving is used as insulation, it must maintain flexibility to -40°C without cracking. The American National Standards Institute (ANSI) and the American Society for Testing and Materials (ASTM) tests are listed: ANSI/ASTM D 746-79 for 212°F heat resistance and ANSI/ASTM D 570-77 for moisture resistance. MIL-I-23053C, 20 May 1976 is required for the heat-shrinkable tubing test.

If you are in the business of manufacturing or importing citizens bandbase station antennas, the antennas need to use materials that pass these tests. But, that’s not all. The completed antenna assembly must be shown to be safe if it were to come in contact with a power line. For the purposes of this standard, a 14.5-kV rms, 60-Hz, single-phase voltage is specified as representing a typical high-voltage power line.

Section 4 of the standard details the tests to be performed and starts with a list of safety precautions:

• Two people should be present at each test, at least one of them with cardiovascular resuscitation training.
• The test area should be secure against accidental intrusion by other people.
• Indoor test areas should be ventilated to avoid potentially dangerous buildup of gaseous byproducts that may result from the test.
• Fire extinguishers should be readily accessible.
• High-voltage test warning devices should be activated before a test starts.
• Emergency phone numbers should be posted.

Having read the list of precautions, if a manufacturer is less than enthusiastic about performing the required tests, a qualified test lab may carry out the tests. However, the manufacturer ultimately is responsible for ensuring that all certification testing has been properly performed with passing or acceptable results. In addition, the manufacturer must maintain all records of the tests in accordance with Section 1204.17 of this standard.

Performance

Almost all products are subject to vibration and changes in temperature. Figure 1 shows a typical vibration test setup including accelerometers and cabling. The environment can be severe, such as in a truck’s engine compartment or much less so as in a typical office. In either case, a well-designed product should continue to operate correctly when subjected to temperature and vibration that exceed normally experienced values.

During development, a project team may use highly accelerated life testing (HALT) to improve the design’s robustness. The process involves increasing the stress applied—typically temperature, vibration, and higher or lower supply voltage—until the product fails. The failure is analyzed and corrected. The stress is applied at a higher level; the product fails for a different reason, is repaired, and so on. Ultimately, the design should be better because many weaknesses have been removed.

For example, some faults may be easily remedied by installing a higher quality capacitor or an integrated circuit from a different manufacturer. Others will require alterations to the design. The HALT process is complete when any further improvement simply requires too large a design change. At this point, the margin by which the performance exceeds the specification is as great as it’s going to be without extensive redesign.

Many test labs offer HALT and its production-related highly accelerated stress screening (HASS) cousin. These labs use multiaxis pneumatic impact-type vibration platforms and very high rate-of-change temperature chambers. Rather than mimic the power spectral density (PSD) of the actual application, as is common in conventional vibration test, HALT and HASS attempt to excite resonances in all six axes simultaneously.

Beyond temperature and vibration, many more specialized test capabilities are provided by test labs. For example, aerospace applications require testing at reduced atmospheric pressure. Some labs specify an altitude chamber directly in terms of its range of pressures.

Trace Laboratories has an altitude chamber with a 3- to 10-Torr rating. One Torr is equivalent to 1 mm of mercury (mm Hg), and the standard atmospheric pressure is 760 mm Hg. So, 3 Torr corresponds to 0.395% of one atmosphere. Because pressure varies with altitude (H in feet) approximately as

Pressure = e^(-H/24278)

A pressure of 3 Torr would be found at an altitude of about 134,000 ft. Other labs simply refer to the height their chambers simulate, and 100,000 ft is a common specification.

Humidity is another often specified test, especially in combination with high temperature to simulate a tropical climate. Depending on how well protected the product is when installed, it may be important to include wind, rain, salt spray, and fungus growth tests. At the other extreme of very low humidity, some labs have wind, sand, and dust test facilities to simulate desert conditions.

Acceleration, such as an electronic control module might be subjected to in a car or spacecraft, is simulated by a centrifuge. Shock, like the sudden impact a product receives when dropped, can be developed mechanically, hydraulically, electrodynamically, or pyrotechnically. Each method has its own characteristics, pyrotechnics offering the highest g force but not as narrow a pulse width as other techniques.

The test capabilities that have been discussed are the more popular ones and offered by many labs. If a lab has combined these capabilities in programs that address specific test applications, you may be able to obtain required tests without becoming an expert yourself.

For example, the International Safe Transit Association (ISTA) publishes a series of tests ranging from basic procedures to help you compare the relative performance of packaging and product designs to actual road transport-based simulation. If a lab states that it is ISTA-certified, you can be sure your packaging tests will be run to well-established and industry-recognized standards.

Labs also offer calibration services and test-fixture design. After all, these two items are key to achieving accurate test results, and many labs have become expert at designing their own fixtures and maintaining and calibrating their own test equipment. Some labs have extended the usual testing capabilities to include these services.

Of course, if you need to deviate from temperature, vibration, and humidity, the number of labs in your area that can handle a product weighing 10,000 lb or is 50 ft long may be limited. In some industries, because of the nature of the products and the required tests, only a few labs have the necessary capabilities, regardless of location.

So, if you do have an unusual test requirement, it is best to identify a suitable test lab and visit it well before you expect to start testing. Your development schedule may change once you consider the time required for transportation to the lab; installation, which might involve a separate power feed from the local utility for really high power products; and the tests themselves and any equipment scheduling conflicts within the lab.

The chart accompanying this article outlines the test capabilities of several labs. In each case, if a lab can perform a test, it generally has a range of equipment even though only the largest specification value may be shown in the chart.

If you find a lab that appears to satisfy most of your requirements, contact them to ask about any further tests you may need. There are so many specialized test capabilities available, including nuclear radiation, seismic earthquake simulation, and hydrostatic pressure equivalent to deep-water submersion, that it’s not possible to list all of them.

References

1. NEWS from CPSC, cpsc.gov/cpscpub/prerel/prhtml98/98168.html

2. Safety Standard For Omnidirectional Citizens Band Base Station Antennas, www.access.gpo.gov/nara/cfr/waisidx_00/16cfr1204_00.html

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Published by EE-Evaluation Engineering
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without the express written consent of the publisher.

October 2001