How Do I Know It’s Safe?

We are a product-oriented society. Day after day, we buy products for our homes and offices. Our lives are built around the comfort that they provide, so the safety of these products is terribly important to us.

As consumers, we are at the mercy of our suppliers. Should I buy this product? How do I know it’s safe? As suppliers, we recognize the obligation to our customers and the need to convey assurance as we put our products on the market.

Product safety consciousness evolved over the entire 20th century. Underwriters Laboratories (UL) was formed in 1894 to establish de facto safety standards and test products to those standards. The American Society for Testing and Materials (ASTM) was established in 1898 to recommend standards for safety and other matters.

Product manufacturers began implementing those standards. Retailers started to demand more proof of the safety of the products they sold. Consumers learned to look for symbols certifying that the product designs had been tested and validated for safety. Last, but most powerful, governments began to legislate safety standards.

Product Safety Items of Concern

Some unusual events happen in the life of a typical product in a typical home. Uncle Joe spills a drink on it. Dad bumps it when he is emptying the trash. Mom pushes it as she rearranges the furniture. The baby puts a little finger in every opening he can find to see what makes it hum. Susan drops the hair dryer on it.

Products for use outside the home have an even tougher challenge. Dad leaves the electric hedge clippers outside when the sprinkler is running. Pete gets a vine tangled in the lawn mower. Andy puts the gas grill together incorrectly. Someone puts the tent too near the campfire. Do we just have an interesting story to share later, or do we call 911?

The U.S. Consumer Product Safety Commission reported that in 1998, some 141,000 children were treated in emergency rooms because they were injured by unsafe products. Yes, product safety testing has a high priority.

Sources of Specifications for Safety Testing

Through many years of study and design, several agencies have become prominent in the product safety area:

• American National Standards Institute (ANSI).
• ASTM with safety testing recommendations including the specification on toy safety.
• Canadian Standards Association (CSA).
• The European Community with the Conformit‚ Europ‚enne (CE) Marking, an indicator of compliance required on certain types of products for sale in the 18-country European market.
• U.S. Consumer Product Safety Commission (CPSC) which monitors the safety of various products, notably toys.
• International Electrotechnical Commission (IEC), with standards relating to household audio/video devices, medical instruments, automatic electrical controls, hand-held electrical tools, computers and television sets, and laboratory and measurement equipment.
• UL with a myriad of product safety and other specifications negotiated with manufacturers over the last 105 years.

While manufacturers would like to see specifications for similar items harmonized, they are slow in coming. However, one arrangement that helps to gain wider acceptance for a product in the international market is the Certified Body (CB) scheme. Under this program, a CB member body can issue a certificate on an item that may help in obtaining approval in another country without more testing. There is no guarantee of acceptance, but about 50% of all requests are granted fully or result in a shortening of the retesting process.

The number of products covered by the CB scheme is limited but includes standards for information technology, audio/video, medical/dental, and test/measurement equipment. All requirements must be met for a particular piece of equipment in its intended environment. For example, a standard may be met for an equipment laboratory yet be insufficient for the same product installed in a hazardous location.

Manufacturers recognize that when their products pass safety tests, this does not protect them from the possibility of lawsuits. However, they feel less vulnerable than if they could not produce safety testing results.

Safety Tests and Equipment for Electrical Products

Fortunately, sophisticated tests are available to evaluate designs and assess the safety of electrical products. Here is information on some of the more common tests in this category:

Hipot or Dielectric-Withstand Test

The hipot test is a part of virtually every standard, which indicates its importance. This high-voltage test stresses insulation in the product to a point far beyond what it will encounter in normal use to confirm that it is acceptable. The test also detects other potential defects such as conductors that are too close together.

Even if a product works when it is new, these conditions eventually may lead to current leakage and create a shock hazard after dust or other contaminants collect in the circuitry. This is most critical in high-humidity environments.

The rule of thumb says to test at twice the typical operating potential plus 1,000 V. Most hipot tests use AC; but for a product with an EMI filter, it is necessary to test with DC voltage.

The versatile hipot tester generates up to 3,000 VAC or 5,000 VDC. Voltage is applied for a selected interval of 1 s to 60 s. Arcs are detected and indicated, and excessive current flow will cause an indicator to trip.

Ground Continuity or Ground Bond

The ground continuity test determines whether, if the insulation becomes defective, the safety ground circuit of the UUT can handle fault current safely and protect the user from dangerous shock. The tester puts out a relatively low voltage with a test current of perhaps 25 A for 1 s to 60 s. A display indicates the resistance of the high-current path to ground and generally is set to alarm if the value is more than 0.1 W .

Insulation Resistance

Insulation resistance is a quantitative assessment of UUT insulation when a high voltage is applied. The tester, commonly called a megger, generally has an analog readout. Full-scale measurement can be as great as 100,000 MW when 500 V to 5,000 V is applied.

Leakage Current

Leakage current flows from user-accessible parts of the product to ground when the unit is operating. If the current is excessive, it can cause an electrical shock.

A leakage current tester measures leakage while simulating the impedance of a human body, generally about 1,500 W in parallel with 0.15 µF. The measurement reference point is switchable to the high, neutral, or ground wire of the power cord. Current is measured through the ground connector from the case to one of the lines or from one point to another on the case. Most standards set a limit of 0.50 to 0.75 mA for this current.

Medical equipment carries a different set of measurement conditions. The simulation resistance is 1,000 W, and the current limit generally is 1.0 mA.

Creepage/Clearance

Creepage/clearance are measurements of the surface distance (creepage) and air distance (clearance) between two conductive parts. Inadequate separation can cause operating problems after the product has been in service for a few months or years. A measurement kit includes a family of feeler gauges in the 1-mm to 8-mm range, an optical comparator measuring in 0.005″ increments, and a digital caliper with 0.0005″ resolution.

X-Radiation Tests

Because products incorporating cathode-ray tubes may have excessive and potentially harmful radiation, many safety standards require measurement of radiation levels. A hand-held, battery-operated instrument for this evaluation is calibrated for a measurement distance of 5 cm.

Operating Temperature Measurements

Because excessive temperatures in a normally operating product may be a precursor of developing problems, the UUT is examined with a temperature probe. There are no safety limits for this test, but the results are used by the product design engineer to analyze potential trouble spots.

Response to Improper Airflow

If a product requires cooling air from vents or a blower, and if that flow is restricted by a foreign object at the vent or a malfunctioning blower, overheating is likely to occur. The safety test involves simulation of the possible problem and evaluation of the result.

Response to Excessive Input Voltage

On the assumption that excessive input voltage may be applied to the product, response to excessive input voltage is simulated and the results are analyzed.

Plug/Socket/Cord Test

Since electrical plugs, cords, and sockets are sometimes subjected to abuse, it is helpful to simulate such treatment in the laboratory to evaluate durability. Flexibility can be checked by putting the cables and cords through a repetitive life test. Cord anchorage on nondetachable flexible cords is checked by a torque tester. The typical life test simulation is 60,000 load cycles.

Glow-Wire Contact

If one of the internal components of a product, such as a resistor, could overheat, the nearby components are tested with a glow wire to see if they are likely to ignite from contact with the hot component. The 4-mm glow wire can be set at a temperature ranging from 50°C to 960°C.

Multiple Electrical Tests With One System

Since equipment for product safety testing must serve a broad range of requirements and a large volume of business, there is a noticeable trend toward automation under computer control. “Automation simplifies setup of instruments, enhances operator safety, and streamlines the storage of test results,” according to Dwayne Davis, technical services manager at Associated Research. Software also plays a major role in automation.

On a product where multiple certifications are needed to sell it in different markets, test equipment often is available to perform the unique tests for each specification. Generally, the product manufacturer does a few routine tests on each production unit.

In many cases, a single tester can perform several electrical tests. Figure 1 (right) shows a system that performs AC hipot, DC hipot, insulation resistance, ground bond, and leakage current tests on a UUT with one connection.

Safety Tests and Equipment for Other Characteristics

Other test equipment relates to nonelectrical characteristics of products. Generally, the specifications for this equipment are not so thoroughly defined, and the equipment has less commonality from one manufacturer to another.

Accessibility of Hazard

To ensure that a user is protected from hazards accessible through ventilation holes or other openings in the product enclosure, an object the size of a small child’s finger is used as a testing device. This inspection is mandated by most safety standards to evaluate the possibility of electrical shock, contact with a moving part such as a fan blade, or burning on a hot component. Probe kits are compiled to cover the most common standards.

Stability

To assess the possibility that the product will fall if tilted during abnormal use, an angular measurement indicator is attached and the UUT is rocked from side to side. Top-heavy products may require safety designs to improve stability.

Ingress Protection

In many cases, the level of protection provided by a product’s enclosure is indicated by a two-digit code. The first digit relates to solid foreign objects and the second to water.

Equipment is available to evaluate the effectiveness of this protection. Water is injected with a hand-held calibrated sprayer or an oscillating sprayer in an enclosed chamber, a splasher apparatus, a calibrated rainfall of one hour duration, or a dripping box. Dust chambers of various sizes are used to test for conformance to specific standards.

Ignition Potential

Some products, notably camping equipment, are likely to be used in proximity to open flames. Standards specify that these items be checked for potential to ignite from direct exposure to flame of a specified intensity set at a specific angle.

Resistance to Mechanical Shock

Because of their typical usage, some products are likely to be subjected to mechanical shock. Test equipment is available to simulate such shock so the designer can evaluate the effect on the product. A calibrated hammer with 1,250g mass can be set to provide an impact energy of 0.2 J to 1.0 J. For evaluating the effect of a severe mechanical disturbance, a tumbling barrel is used.

Sharp-Edge Test

To evaluate the sharpness of edges on a product as defined by certain industry standards, a test kit contains a clamshell housing with layered tape caps. The clamshell housing is moved across all areas where a discontinuity is suspected, and a sharp edge snags the tape cap to indicate any problem area.

Temperature/Humidity Environment

Since some products will be used in relatively unfriendly environments and those environments may cause unsafe conditions, temperature/humidity chambers are used in a safety testing program. Typically, the temperature can be raised to 400°F and possibly decreased to lower-than-ambient temperatures. Humidity control also is part of this test capability.

Label Validation

Since many products have practical limitations on their use, part of the testing sequence involves verification that the proper warning labels are in place. For example, for a toy, this means specifying the minimum age at which it is safe for a child.

Conclusion

Equipment and techniques are available to prove the safety of the products that we use everyday. All of us are more secure because of the responsible manufacturers, equipment designers, and test engineers that make the safety of our products their top priority.

Acknowledgments

Published by EE-Evaluation Engineering
All contents © 1999 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

November 1999