Medical Device Link.

Medical Electronics Manufacturing Spring 2000

Ensuring the Safety of Medical Electronics

Safety standards in themselves cannot ensure that a medical electronic device is safe to use. The device manufacturer's testing program should do that.

The purpose of safety testing medical electronic equipment is to be sure that a device is safe from electrical hazards to patients and caregivers. Several UL, European, and Canadian standards serve as the ruling authority in determining how medical products are to be tested. One in particular, IEC 601-1 (the International Electrotechnical Commission's electrical safety standard for medical electronic equipment), as a result of global harmonization, is accepted and implemented around the world. IEC 601-1 is mainly intended for the design phase of product development; safety considerations must be taken into account early in the product life cycle. However, much of the standard is applicable to production line testing as well, because it is the only way for manufacturers to be sure they are shipping safe products.

A production safety analyzer performs essential tests to help ensure the safety of medical electronics.

Key areas of safety testing for medical electronic equipment are leakage current, dielectric breakdown, insulation resistance, and ground bond testing. This article examines the purpose of these tests, discusses the technique of each, and differentiates among them.

Ground Bond/Continuity Testing

The ground bond, or continuity, test should be the first electrical safety test performed on a product following visual inspection. This test checks the connection from any exposed or user-accessible metal equipment parts to the ground reference on the product's line cord by measuring the resistance of the connection. The objective is to determine that sufficient current will flow to ground through this connection rather than through the operator in the event that the product should fail and a user-accessible surface come in contact with a live voltage. (See Figure 1 for a schematic of the test setup.)

Figure 1. Setup for ground bond test.

Many product standards require that the presence of this continuity connection be verified during production testing. One way to do this is to perform a low-current continuity test. This test can verify only that the connection is present, not that it is capable of handling high current should the product's insulation fail. Verifying the presence of the connection is one thing, verifying its integrity—via the ground bond test—is another. The two tests should not be confused; ground continuity is generally thought of as a low-current test and ground bond as a high-current test.

The low-current continuity test is adequate for the safety testing of many products but does not satisfy many of the European Norm (EN), IEC, Canadian Standards Association (CSA), and UL standards, particularly those intended for medical equipment. IEC 601-1 specifies that user-accessible conductive parts connected to the safety ground be tested with a current of either 25 A or 1.5 times the product's current consumption, whichever is greater. This current must come from a source with a maximum no-load voltage of 6 V ac. Other medical equipment testing standards allow variations in the maximum voltage ranging up to 12 V, ac or dc. The 12-V maximum is designed to protect the test operator from hazardous voltage levels. The resistance of this ground path is the important parameter that is calculated from the test current and voltage drop. It should be less than 0.1W on equipment with a detachable power cord, or 0.2W when a power cord is permanently attached.

This continuity, or ground bond, test is often a prerequisite for proceeding to the hipot test. It is common sense to verify the ground integrity of the product before applying high voltages that might jeopardize the test operator.

Hipot Testing

The hipot test, often called the voltage breakdown or dielectric withstand test, places an electrical stress on a product's insulation beyond any it might encounter in normal use. The objective is assurance that the product will function as designed and not cause harm to its user. Hipot testing in various forms has been around for decades, governed by many standards that have required an electronic product to be tested before it could be released from the production line.

The requirement in most standards is to apply a test voltage that is two times the normal operating voltage plus 1000 V, this generally being 1250 or 1500 V ac depending on whether the product is to be operated at 115 or 240 V ac. Usually, a sinusoidal ac voltage is applied, but in some cases a dc voltage, typically higher by a factor of 1.414, can be substituted. For hard-wired corded products, the test voltage is applied to a spot between the high (hot) and neutral conductors shorted together and the power ground or exposed metal parts (see Figure 2).

Figure 2. Typical ac hipot test.

The product's power switch should be in the on position for hipot testing, though of course the unit is not powered up and running. The test voltage is raised from zero to the predetermined test voltage and typically held for 1 minute. No breakdown should occur during this test, a breakdown being defined as a rapid increase in current across the tested insulation. Most hipot test equipment allows the operator to program a maximum and a minimum current level. Some leakage current will flow from nearly all products during hipot testing because of inherent capacitance properties and filtering. A product exceeding the maximum limit is considered to have failed. The minimum current limit also serves an important, though less obvious, function. If the device current is below the minimum limit, it may be supposed that the tester is not making proper contact with the device under test.

A product that passes a hipot test is unlikely to cause an electrical shock in normal use. By stressing the product with a high voltage much above what it would normally experience, a large margin of safety for the user can be established.

Line and Earth Leakage Current Testing

Line leakage current is something quite different from the leakage current discussed above in association with the dielectric withstand, or hipot, test. Whereas the hipot test detects excessive leakage current flowing through a product's insulation system as the result of a deliberate overvoltage condition, the line leakage test detects leakage current at an essentially normal operating voltage, not at overvoltage. It measures the current through a simulated human body impedance while the product is powered under normal operating conditions and turned on.

The line leakage test is intended to measure current flow through various parts of the product: through the ground system, from the product enclosure to ground, and into, out of, or between patient-accessible parts of the device under test. If these currents are excessive, electrical shock to the user or patient can result. Safety agencies have consequently set standards for the maximum amount of current that may leak from a nondefective product. Because the more-stringent hipot tests are usually required for every electrical product coming off a production line, line leakage tests are commonly specified as a design test rather than a production test. However, line leakage tests are typically required as a production test for medical products, the purpose being to ensure that normal operation of a device will be safe for the patient and caregiver. Normal medical circumstances such as heart problems can place a patient in a position of higher risk from electrical shocks. Thus, the additional caution is warranted.

Medical device standard IEC 601-1 is the most widely recognized standard containing detailed regulations for the design of safe medical electronic equipment. The extensive leakage testing component of the standard is mainly oriented toward product development, during which comprehensive testing is conducted under many different test conditions. However, some sections are routinely applied in the production line environment.

One of the most important and critical tests specified in IEC 601-1 is conducted with a circuit similar to that shown in Figure 3. It measures earth leakage current, which is essentially a sum of all leakages in the product under test, amounting to the current flowing back to earth ground through the ground conductor of the line cord. The earth leakage test needs to be performed under different normal and single-fault conditions. Normal conditions are electrical conditions that might occur on a regular basis and not be considered a problem. An example is a reversed ac line as is graphically depicted by S1 in Figure 3. A single fault is a problem that could possibly occur. Because it is unlikely that multiple faults would occur simultaneously, faults are tested one at a time. An example of a single fault is an open neutral line as indicated by S2 in Figure 3. This test is made with a normal and reversed line (S1) and an open and closed neutral line (S2).

Figure 3. Line/earth leakage test.

The measurement device chosen for use in making line leakage measurements is important. Standards require the use of meters with very specific loads, where the load simulates the impedance of the human body. An equivalent circuit of the human body consists of a 1000-W resistor in parallel with the series combination of a 0.015-µF capacitor and a 10-kW resistor. Figure 4 shows the test load required.

Figure 4. Human body equivalent impedance.

Power Consumption

In addition to the tests described above, IEC 601-1 specifies other tests that verify the normal operation of a product. One checks current draw, measuring the current that a device under test is consuming through the power cord. This test is undertaken with the power applied and the instrument operating under normal conditions. The actual current must be within 10–25% of the rating marked on the product. Another test, for power consumption, measures the power the product under test is consuming while operating. Power consumption should not be more than 10–15% above the rating marked on the product. This test should be done with equipment controls set for maximum output and should measure true power.

Conclusion

A number of manufacturers produce electrical safety test equipment for the tests discussed above. Many testers perform just one type of test, while in other cases the same box can be used to conduct several tests. Single testers capable of providing ac hipot, dc hipot, ground bond, insulation resistance, and line leakage measurements are not uncommon. These multifunction testers are popular in product development or production contexts where more-comprehensive testing is already being implemented or is anticipated. Technological progress has brought instruments that, for the same or less money, can provide much greater testing capability than before. Product testing is becoming easier, faster, and more complete.

Standards may go far toward ensuring that safe medical products are shipped, but the ultimate responsibility for the safety of these products rests with the manufacturer's testing processes and the equipment employed in carrying them out.

James Richards is marketing engineer for Quadtech Inc. (Maynard, MA).

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