Jeffrey D. Lind
In the process of being conceived, prototyped, and readied for final production, a medical device is turned over to regulatory experts or knowledgeable team members in order to undergo regulatory approval. During the approval phase, the device is evaluated based on such technical standards as IEC 60601, which is used to ensure the safety and effectiveness of medical electrical equipment. After production commences, the device is subjected to production line inspection and testing to ensure that it satisfies the controlling agencies’ requirements. Targeting manufacturing engineers on the production line, this article examines dielectric withstand and biphasic surge testing methods using parameters that can be referenced directly from IEC 60601.
The reader may think that the production line testing methods discussed in this article are limited in scope because they can only test the medical device’s primary circuit to discover insulation system issues. To the contrary. While the simple regulatory production line tests presented here are used only to test a device’s primary circuit and, in some cases, the grounding circuit and primary/secondary insulation system, other parts of the device can be tested to satisfy the requirements specified in IEC 60601. In addition, surge tests can be used to uncover insulation system issues in other than the primary circuits and can also be used to test device performance.
Using Dielectric-Withstand Test Methods
The advantages of the dielectric-withstand test are well known: It is easy to conduct, and the result is a simple pass or fail, as determined by the outcome of the leakage current across the insulation system. The numeric pass/fail value is set on the dielectric-withstand tester as a not-to-exceed current flow. If the set current is exceeded, the leakage current circuit in the dielectric withstand is triggered, and a failure is registered.
Most dielectric-withstand testers also have arc-detection circuitry that is triggered if an arc is detected. However, arc-detection circuits have their shortcomings. They cannot differentiate among all the arcs that occur in the insulation system being tested, and they also detect arcs in insulation systems that are not being tested. Moreover, they detect arcs that occur in relays and switches in other equipment. In a production testing environment, these limitations can become assets, however, since a problem in any of the device’s systems can result in an unexpected failure. While IEC 60601-1:2005, paragraph 8.8.3b does not consider arcs a failure, it is advisable to take them into consideration in production line tests.
In addition to their ability to detect arcs and leakage current across the insulation system, dielectric-withstand testers can assess systems other than the primary circuitry. For example, during production line type testing, they can be used to test dielectric-withstand voltage. To determine a possible dielectric-withstand voltage, test personnel can rely on the values shown in Tables 6 and 7 in IEC 60601-1:2005, paragraph 8.8.1, which represent maximum voltages. However, because most insulation systems of interest will already have been tested during the approval process, separate tests conducted on an insulation system that was not part of the type test will presumably not be used to protect a medical device according to this paragraph. Hence, the voltage levels shown in Tables 6 and 7 would not apply to such later tests performed on an insulation system.
Production line personnel wishing to test a wiring harness consisting of wires and connectors can check and compare the factory dielectric-withstand test voltages supplied by the harness manufacturer with the voltage values shown in Tables 6 and 7. If the harness violates the primary/secondary barrier by touching another system, the two means of operator protection (MOOP)/means of patient protection (MOPP) voltage is appropriate. The chief function of this test is to ensure that the part was properly assembled and to determine whether a defective wire or connector was used.
Since testing at too high a voltage can possibly degrade the system, the voltages shown in Tables 6 and 7 should be used. Leakage current levels are not important for this test, since a failure will result in high current flows, and leakage current flow for a good part will essentially be zero. Any convenient value around 1 mA will yield good results. Since arc detection failures are indicative of a problem in the system or the part, they should be investigated. However, because arc detection circuits cannot differentiate between arcs generated in the part and those generated in the fixture, personnel should check before rejecting a part based solely on an arc failure. A good method for finding the arc is to turn out the lights.
Complex insulation systems that isolate two components such as primary and nonprimary circuits are good candidates for dielectric-withstand tests. While most of these insulation systems have withstand voltage prescribed in the type test report, undocumented insulation systems would conceivably require a production line test based on the voltage levels shown in Tables 6 and 7. If a system is important enough to test, it must be a compliant insulation system. Because leakage currents may be present depending on the components used, the leakage current trip level should be set empirically. In such cases, the arc detection feature of the dielectric-withstand tester is important because single arcs may indicate weaknesses in the insulation system, especially in wound inductors.
Biphasic Surge Testing
Surge testing is widely employed in the field for determining the status of motor insulation systems. Used to predict impending motor failure, it is an excellent way to gauge the strength of winding-to-winding insulation in larger motors. In other applications, however, it has not gained much traction as a tool for determining the soundness of the insulation system because surge-testing equipment is complex and expensive. In addition, test results are not easily presented in a pass/fail fashion.
|Surge-test schematic from IEC 60601-1 shows the application of test voltage for testing the delivered defibrillation energy. S = switch for applying the test energy; A, B = switch positions; and RCL = current-limiting resistor.|
However, surge testing can be used to conduct a performance test and an insulation system test simultaneously on medical equipment that is subject to the requirements of defibrillator-proof testing and the energy reduction test in IEC 60601-1. It can also be used to test components that carry, produce, or deliver defibrillator shocks. In such cases, the surge test delivers a minimum shock of 360 J. IEC 60601-1, Figure 11 gives guidance on the construction of such a tester. If a device built to this specification is used to test for defibrillator-proof circuitry by connection to the patient, characterization must be performed before production line testing can commence.
Surge testing can also be used to test for energy reduction. According to IEC 60601-1, paragraph 18.104.22.168, an energy reduction test ensures that a patient-connected device will not absorb more than 10% of the defibrillation pulse when it is delivered to the patient. In order to obtain a baseline, it is necessary to determine how much energy (in joules) a known good device will absorb and ascertain how many joules the tester will deliver.
Equipment featuring circuitry constructed according to IEC 60601-1 Figure 11 delivers between 360 and 390 J depending on the actual component values used to build the tester. If the energy delivered to the internal 100-Ω resistor in the tester is known and the device being tested is characterized so that the energy absorbed is known, the amount of joules absorbed by the 100-Ω device is known. The energy absorbed by the 100-Ω resistor can be determined using an excel spreadsheet, or it can be obtained directly from the tester if this information is presented. Devices that fall outside the characterization window can be evaluated to determine whether there are issues with the patient insulation system or the componentry of the device being tested.
Equipment components that are subjected to a pulse can be tested using the same circuit as that used to test the equipment itself, but the output will be applied directly to the components. In such cases, it is recommend that production line personnel use the circuit shown in IEC 60601-1, Figure 11. While a commercial external defibrillator can be used to perform the testing, the defibrillator might not deliver the entire 360 J of energy because of internal attenuation applied during the pulse, complicating characterization. And as pulse generators, commercial defibrillators have been known to exhibit a short lifespan. Commercially available generators provide monophasic and biphasic shocks that are rated for greater than 1-M cycles continuous duty.
The testing parameters outlined in IEC 60601-1 can be used for more than just type testing. The guidance for dielectric-withstand testing and circuits for surge testing can be used in production line testing as well. Dielectric testers can be used for a quick go/no go evaluation of the insulation systems throughout the device. Surge testers can be used to evaluate a device’s patient insulation and defibrillator-proof systems, as well as the device’s components, ensuring that production units will operate correctly when they are sent into the field.
Jeffrey D. Lind is president of San Diego–based Compliance West, USA. With nearly 35 years of electrical engineering expertise, he approves and implements all business management decisions, runs all financials and budgeting, and oversees the engineering, customer service, and marketing departments. Lind launched his career in the electrical product safety industry, working at Underwriters Laboratories from 1976 to 1982. Subsequently, he worked at Atari as a product safety engineer. He received a BS in electronic engineering from Cal Poly San Luis Obispo in 1976. Reach him at [email protected].