You designed a safe product, but did manufacturing build it correctly? A safety tester provides the proof.
Electrical product safety testing ensures that your new power drill or toaster won’t electrocute you. A safety test doesn’t deal with the correct functioning of a product or operator competency, so it won’t prevent burned bagels or holes drilled in the wrong places. Safety tests only can determine how well an appliance meets certain electrical specifications. If the product hasn’t been designed with safety in mind, testing won’t improve its quality.
“The requirement to ensure safety conformance through manufacture is clear from many different directives and standards,” said John Jansen, vice president of Clare Instruments US. “However, concern and confusion still seem to exist regarding the full extent of the required safety testing. Factors affecting the sampling vs. 100% testing decision include engineering and production considerations, quality, cost increases caused by additional testing, and ISO 9000 implications.”
“Sampling of random products from the production line is intended to determine that original-type test performance and build instructions are being maintained,” he explained. “This approach relies on a traceable scientific relationship between the sample and the rest of the batch. Although this technique is satisfactory in many instances, when the issue is customer safety, can a manufacturer afford not to do 100% testing?”
The throughput and cost aspects of safety testing are similar to those of all types of product testing. However, it is the possible consequences of user injury or death caused by a faulty appliance that make safety testing different. For that reason, manufacturers have adopted test methods that provide 100% safety checking quickly and economically. Automation plays a large part and has the added benefit of helping improve product quality.
“In electrical safety testing, automation is extremely practical,” said Dwayne Davis, technical services manager at Associated Research. “In fact, many customers using our automated products and software have seen a significant increase in production-line throughput. Several tests can be set up and run with only one DUT connection, such as hipot, ground bond, and a functional test.
“In the case of medical electronics manufacturers, many electrical safety tests are required including eight separate line-leakage tests,” he continued. “Through automation, the test sequence can be set up to comply with common agency specifications such as IEC 601-1, UL 2601-1, IEC 1010, UL 3101, UL 3111-1, UL 544, and IEC 950. Automating the test process speeds up the test time and provides more consistent testing.”
Safety test automation comes in many flavors. Kevin Clark, Vitrek’s CEO, suggested that it starts with semi-automatic testing in which an operator selects and runs a previously stored test program. Although an operator still is closely involved, errors are minimized and test consistency is greatly improved.
(See the Product Safety Test Comparison Chart.)
The capability to run several tests in succession until a failure occurs represents a higher degree of automation. If only the pass/fail indication is available, a programmable logic controller or a PC can continue testing until a product has passed all the tests or failed at least one of them.
Full and complete test automation requires that safety testers have bidirectional communications capability. “The user needs to download specific test parameters, execute a test, monitor test status, retrieve results, and record actual readings,” explained Mr. Clark. “This highest level of test automation inherently provides automated data acquisition and supports the implementation of statistical process control.”
There’s no doubt that hipot, ground-bond, insulation-resistance, and leakage testing help ensure product safety. But, testing doesn’t only occur after the manufacturing process is complete. The component parts that form a subassembly each may need to be tested before they can be used.
This is the case with the multilayer ceramic substrates manufactured by General Ceramics in Anaheim, CA. George Mendoza, the quality assurance manager at the company, said that the parts are subjected to test voltages of 100 V to 500 V to determine if any shorts or opens are present. Only known-good parts make it to the subassembly stage.
Sometimes, the nickel layer deposited prior to the gold-plating process may contaminate the area around the actual conductors, causing shorts. This type of fault can be repaired by removing the unwanted metal. Then the part is retested. In other cases, for example when a short or open occurs on an inner layer, the part simply is scrapped.
Hipot is a relative term for testing these small, ceramic substrates. Voltage of 100 V to 500 V is high in comparison to the 5-V signals normally carried. This application really doesn’t have a safety aspect to it but rather is about component reliability. The hipot test finds faults that might not be apparent at low voltage.
Because there are several different types of parts being tested and the batch size of each is small, testing has not been fully automated. The test voltages present little operator hazard, and the number of points to be tested is relatively small. General Ceramics uses a semi-automated tester developed by Compliance West.
Making a printed circuit board (PCB) involves a complex mix of mechanical and chemical processes. Getting things only slightly wrong can cause a bare PCB to have shorts or opens.
To help PCB manufacturers guarantee product quality, Global Technology, a subsidiary of GCA Technology in Pembroke, MA, has developed a fixtured, high-voltage, bare-board tester. It incorporates a QuadTech Guardian hipot tester, custom fixturing, and software.
Complex digital PCBs, for example, often have very tight trace spacing, which limits the test voltage to 200 V maximum. This voltage level can be supplied by existing shorts and opens testers already used by PCB manufacturers.
But, many PCBs, such as those used in power supplies, backplanes, and communications equipment, must reliably handle much higher voltages. Hipot testing at 1,500 V and above quickly sorts out the boards with problems from those that are good, explained Jerry Antoine, the president of Global Technology.
Testing starts by placing a bare board onto a test fixture. Mr. Antoine said that most fixtures are single-sided and very low cost, contain a relatively small number of probes, and require only a small weight placed on the board to ensure good contact. This is in contrast to the expensive vacuum fixtures often used in populated board testing and the hundreds of pins needed for in-circuit component test.
Test software then runs a number of tests in sequence to check for shorts, near shorts, and dielectric breakdown. In this application, reliability is the primary concern. On the other hand, faults in a power-supply subassembly have obvious safety implications. If a board passes the Global Technology tests, it shouldn’t cause problems later when assembled into a final product.
If you’re in the hospital recovering from a heart attack, the last thing you need is an electrical shock from the monitoring equipment. To ensure patient safety, leakage specifications for this type of medical equipment must be as low as 50 µA.
At GE Medical Systems Information Technologies in Milwaukee, WI, line leakage, ground continuity, and hipot measurements are handled by an automated test system. Dennis Sauer, a test engineer at the company, said, “We use the automated test system for almost all the products that we make because it simplifies and speeds up the testing process.
“On a fairly simple product, automatic safety testing might take about two minutes compared to four or five if done manually,” he continued. “But on more complicated products, there are many more tests to run. Even the automated system may take about six minutes just because it has to switch between so many tests.”
The test-system application software runs on a PC under Windows 2000 or NT. Both the software and some of the test hardware it controls are proprietary. The system includes a QuadTech Guardian 6100 with a built-in scanner for leakage measurements. A companion Model 5000-2 Scanner provides up to eight channels of hipot and four channels of ground-continuity test capability.
Has the development of a specialized test system paid off? Mr. Sauer certainly thinks so: “Usually, we can test the next generation of patient monitoring products with little or no change to the test system. An addition may be required to accommodate a new product feature. Otherwise, changes are limited to different test cables.”
The adoption of 100% safety testing by so many companies has prompted new trends in product design. To ensure that testing really does safeguard against product liability problems and reduce or eliminate costs associated with product recall, some testers include automated result recording. Any required report can be generated from the tester’s database that correlates test results and product serial numbers. For detailed information on many of the product safety testers available today, see the comparison chart that accompanies this article.
Testing accuracy has been improved through the use of bar codes that identify the type of product and the tests relevant to it. And, once a test is set up, graphic LCDs help simplify the operator’s role.
The new displays require fewer abbreviations so misinterpretation has been reduced. The screens generally are larger than a simple two-row alphanumeric LCD so more complete instructions and prompts can be presented. Finally, because very large characters can be displayed, warnings or a simple pass/fail message are easily read from a distance.
Some improvements already are common in other types of instruments. These include autocalibration and full programmability. Other changes are specific to safety testing, such as better ground-fault interrupt circuits to help protect the operator.
For applications that can’t be efficiently served by general-purpose testers, there are special models. For example, in medical equipment testing, many different leakage checks are required, so an instrument with multiple connections speeds and simplifies testing.
In addition, software such as Associated Research’s AutoWARE™ sets up test sequences in compliance with common medical specifications. For telecommunications testing, a product such as Compliance West’s PT-600 Telephone Over Voltage Test System performs Telcordia and UL safety tests.
These trends all follow from the acceptance of the need for 100% testing. As Clare’s Mr. Jansen put it, “Only 100% testing and traceability can show 100% conformance.”
Electrical Safety Testing Reference Guide, QuadTech, 2002, www.quadtech.com/est/
Safety Tests Defined
The ground-bond test measures a product’s ground path resistance and verifies that a 25-A or 30-A fault current can be carried. If a fault occurred that allowed the AC line voltage to become connected to a user-accessible metal part, built-in circuitry should protect the operator. Because the ground path can withstand high current, the protection device, typically a fuse or a circuit breaker, will have time to operate.
The ground-continuity test only determines that exposed conductive parts are connected to ground. The resistance of the ground path can be measured, but because the test current is less than 1 A, fault conditions cannot be simulated.
Dielectric Strength or Hipot
A high AC or DC voltage is applied between the AC power-input primary circuits and ground to ensure that the insulation barrier does not break down under fault conditions. A measure of dielectric strength is the leakage current that flows to ground during a hipot test. Hipot testing generally is performed on 100% of production units.
If insulation breakdown does not occur during a DC hipot test, the insulation-resistance value can be used as an indication of product safety: Insulation resistance = test voltage/leakage current. During a hipot test, high-frequency current transients indicate arcing and imminent insulation breakdown. Any arcing causes test failure, but stopping the test before damage occurs may aid determination of the fault and its correction.
Leakage refers to current that flows to ground similar to the current produced in a hipot test. However, in this test, leakage current is measured under normal operating conditions, typically 120 VAC, not the 1,500 VAC or higher often used in a hipot test. Very stringent limits on leakage current apply to medical equipment that is connected to patients.
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