Production-Line Testing:
A 100% Solution to Product Safety

John Jansen

Electrical safety testing in the manufacturing environment is critical. It can be achieved quickly, easily, and cost-effectively.

With verification of the safe operation and functionality of electrical products being vital to ensure compliance with established industry standards and to maintain customer confidence, the focus has switched from whether safety testing is needed to the actual extent of testing required.

The requirement to ensure conformance with standards through the manufacturing process is made clear by regulatory and legislative authorities, but manufacturers still commonly react with, "Does this mean I have to do 100% testing?" This reaction is followed by a quick retreat into discussions that aim to reduce an erroneously perceived time and cost burden, often quoting ISO 9000 procedures and focusing on sample testing as a suitable solution. This approach tends to exclude a closer evaluation of the concept of 100% testing—which is designed to ensure and maintain product safety.

Batch sampling is essentially designed to determine whether type-test and build instructions are being maintained via a set of working standards. Such sampling relies on a traceable scientific relationship between the sample and the rest of the batch. The assumption is that if the sample shows conformance, then the rest of the batch also complies. In many manufacturing processes, sampling can provide a satisfactory confidence level, but when the issue is customer safety, is such a risk level acceptable?

To maintain a proper scientific relationship to the type-approved product, batch-sample testing should involve a repeat of the type test. Maintenance could involve:

Batch Testing

A typical batch-sampling routine can present many challenges—and hidden costs. For example, risk analysis could determine a procedure for testing one sample product for every 100 that come off the assembly line. Suppose a sample is sent to the laboratory where it undergoes rigorous testing and fails. Strictly speaking, production should then be halted until the cause and extent of the fault is identified. This should include recalling and testing not only the remaining 99 items of that particular batch, but any items produced since the sample was taken. The cost of this exercise can be worked out in terms of:

This scenario presents a very bleak picture, and it might be argued that this worst-case example applies only if the sample fails. But would anyone feel comfortable knowing that an electric drill or an appliance used in a workshop has only a 1 in 100 chance of not causing electrocution?

Similarly, it is clearly in the interests of manufacturers of finished products that the safety-critical components used to assemble a product are satisfactory—preferably before being incorporated into the product. Batch testing has many identifiable shortcomings, making 100% testing a more attractive alternative.

100% Testing

Against this grim background, increasing numbers of manufacturers of electrical products are checking supplied components before or during their own product assembly. Such companies have essentially rejected batch sampling as a viable test method.

It is important to define what is meant by 100% testing. Testing 100% of components allows for identification of problems and defects before assembly, increases confidence in finished products, and reduces likelihood of product rework. It also allows the cost of failures to be recovered from suppliers more easily.

By completing the manufacturing cycle with 100% product testing, the information gathered can be used to improve and refine manufacturing processes and techniques. Identifiable causes of product failures can be highlighted and acted upon quickly. Even simple fault counters can indicate particular areas of the build phase that may require further investigation. It should be noted that the three main tests required to ensure product safety are high-current earth bond (ground) measurement, high-voltage flash (hipot or dielectric strength) test, and insulation-resistance measurement.

The implementation of 100% testing enables companies to develop a competitive advantage. Such testing enables component suppliers to offer fully tested components or products, which reduces the testing burden on manufacturers.

A number of concerns have been raised questioning the viability of a 100% testing approach. These concerns usually surround time and cost issues and are easily addressed.

Misconceptions arise between type-testing requirements and the established practices for 100% production-line testing. For example, comprehensive test stations are available that can apply all three basic safety tests in cycles of as little as 2–3 seconds per product. Referring back to the example discussed earlier, all 100 products could have been tested in just 5 minutes.

A number of production-line test systems available are reasonably priced. With simple-to-use setup and control features, these test stations can be readily incorporated into a production environment without the need for highly skilled labor. Equipment for type testing, on the other hand, can be quite expensive.

The Transtar Example

Given the tremendous variety in size, shape, and performance characteristics of electrical products, customized electrical safety-testing solutions must often be developed to meet specific production-line requirements. For example, Clare Instruments (Goring-by-Sea, UK) recently customized a system for lighting equipment manufacturer Transtar (Newcastle Upon Tyne, UK).

For more than 50 years, Transtar has manufactured control gear for the lighting industry. The company has extended its product range to include a wide variety of control gear for fluorescent and high-intensity-discharge lamps, including high-frequency ballasts. Lighting control ballasts are the drive units for gas discharge lamps and use either an electromagnetic (50-Hz) circuit or high-frequency electronics. One of the company's most recent additions is a low-power, low-pressure sodium ballast that gives users the benefits of installing a high-power-factor unit without the need for connecting a separate capacitor across the mains input. New testing facilities were required to test this range of 50-Hz control ballasts.

Electromagnetic ballasts have become more popular among luminaire manufacturers because of their energy-saving features, and because they comprise an iron core with insulated-copper winding. The lamination stack consists of electrical-grade mild steel. It is attached to a mild-steel base plate to which terminal blocks are fixed. The whole unit is vacuum impregnated with an unsaturated polyester resin system to improve reliability, reduce noise, and ensure high thermal and insulation performance with good knock resistance.

A range of lighting control units from Transtar.

Testing Demands

When used in luminaires, ballasts are grounded through the base plate. Therefore, at the end of the production process, effective earth bond and flash tests were necessary to ensure the electrical safety of each unit. It was particularly important to ensure that reliable contact had been established between the lamination stack and the base plate.

The presence of the external resin coating produced some problems, particularly with the earth bond test. Because the earth bond test is conducted to ensure the proper and secure connection of the metal case to the mains earth reference using test probes, each external metal surface must be tested—a process rendered nearly impossible by the polyester resin coating.

The Solution

Considerable challenges were involved in the development of a customized solution to meet the quality assurance requirement for fully integrated end-of-line testing. For example, testing was to include checking both the electrical safety and functional parameters of the product. To address this particular challenge, the design team configured various test modules into a dedicated system.

The major hurdle, however, was to achieve successful testing in a quick, simple, and operator-friendly manner. The use of a safety enclosure is regarded as essential for such applications for several reasons. It provides for operator safety and ease of product location. It can also be readily interfaced to the testing instruments for initiating automatic sequencing of required tests.

Incorporating fixturing that would automatically apply the test probes to the various test points—lamination stack, mounting plate, and ballast winding terminals—also required careful consideration given the nature of the product. For this particular application, pneumatically driven test heads were selected to provide the additional power required for effective and reliable contact.

Because the base material of the laminations and mounting plate was encased in a resilient, insulating, resin coating, extensive research and experimentation were also required to develop an optimal probe-tip design. The design had to satisfy the criteria for making effective contact through the resin to the base metal; moreover, it had to stand up to the high-volume throughput of busy production cells.

Other features incorporated into the overall design were developed to provide clear identification of faulty product through audible and visual warnings. In addition, isolation of all test outputs was achieved automatically whenever the enclosure was opened.

The end result was a fully integrated test and measurement system that provides quick and simple testing of finished ballast control products. Coupled with operator-safety measures, this solution provided maximum production efficiency at minimal unit cost.

John Jansen is the vice president of test equipment manufacturer Clare Instruments US Inc. (Tampa, FL). He can be reached at usa@clareinstruments.com.