This test is one that has long been required on medical electronic products, as well as on most other electrical devices and appliances before they exit a manufacturer's production floor. The intention of the test is to stress a product's insulation beyond what it would encounter in normal use.

The end goal is to ensure that patients or caregivers never serve as a current path to ground due to faulty insulation or faulty grounding within the product. The test is usually conducted by applying a test voltage that is two times the normal operating voltage plus 1000 V (1250–1500 V ac, depending on whether the product is to be operated from 115 or 240 V).

The test voltage is applied between the shorted-together high (hot) and neutral conductors, and the power line ground as illustrated in Figure 1. This is a test of the basic insulation looking back into the power cord, which is different from the reinforced insulation that is tested at much higher voltages during type testing. Leakage current is monitored by a measuring instrument, commonly known as a hipot tester, with the idea that the insulation withstand a test voltage for a specified time without excessive current flowing across it.

Figure 1. Hipot test.

Today's product safety testing of medical products goes far beyond the traditional withstand test. The requirements for electrical safety testing of medical electronic devices are much more stringent than those for other electronic devices. Increased precaution is needed because a patient may be connected to several devices at the same time, contact with the device may be directly to internal tissue, or a patient may have reduced immune system functionality making the person more susceptible to small leakage currents.

Many standards are used to determine how medical products are to be built and tested. The most widely accepted are IEC 60601-1 (the International Electrotechnical Commission's electrical safety standard for medical electronic equipment) and the U.S. derivation, UL 2601-1. These standards are intended to ensure that safety considerations are taken into account during the design phase of a product. Although most of IEC 60601-1 and UL 2601-1 are applicable to type testing, in the final analysis, safety testing at the production line is the only way manufacturers can be sure that they are shipping safe products. IEC 60601-1 has been used as the basis for many national medical standards worldwide.

In addition to concerns of insulation failure, the integrity of a product's ground connection is equally important. The ground-bond test checks the connection from any user-exposed or user-accessible metal parts to the ground reference on the product's power line cord by measuring the resistance of this connection. The object is to determine whether sufficient current will flow to ground through this connection rather than through the operator in the event that the product should fail, putting the operator into contact with live voltage.

On electrically powered medical products, the test is performed with the application of 25 A of current or 1.5 times the rated current of the product (whichever is greater). This test checks the integrity of the connection—not just the presence of the connection, as is done for many other types of products in which a low-current continuity test is adequate. This bond test is illustrated in Figure 2.

Figure 2. Ground-bond test.

Leakage Tests

Leakage tests are performed under quite different conditions than the hipot or ground-bond tests. For leakage tests, the product is actually powered up under operating conditions. The leakage test is the most-often misunderstood safety test, primarily because it is not a common production test for most electrical products, with the exception of medical products.

There are several ways to perform leakage tests, with the basic difference simply being how or where a person comes into contact with a product. Leakage current is the current that could flow from the point where a person makes contact with a product, through that person's body, and back to ground (or some other point). Acceptable levels of leakage current are generally outlined in the appropriate governing standard for that product. This article examines four tests commonly performed as a final production test on medical products. Leakage current is a general term, which can be further defined by the more specific current types listed below.

Each type of leakage-current test is performed with a measuring device specified by various product standards to simulate the impedance of the human body. Conducting the leakage tests in this fashion provides a good indication of how much leakage current a person could be exposed to in real-world conditions.

Production leakage-current tests must be done with the product at 110% of the highest-rated supply voltage and under a variety of normal and single-fault conditions. Normal conditions are electrical conditions that might occur daily and thus are not considered to be a problem. Single faults are essentially problem conditions that could occur. Because it is unlikely that two faults could occur at the same time, a product does not need to be tested with two or more faults. Examples of normal and single-fault conditions that are often used in a production-test environment include normal power applied (high and neutral), reverse power applied (high and neutral reversed), single fault/normal (neutral open), and single fault/reverse (neutral open, high and neutral reversed). Note that a reversed ac line is considered to be a normal condition, not a fault condition.

Earth Leakage. The earth line leakage test is conducted with a tester circuit similar to that shown in Figure 3, where MD represents the measuring device. This earth leakage is essentially a sum of all leakages in the product under test, or basically the current flowing back to earth ground through the ground conductor of the line cord. This test is only applicable to protective earth products (Class I) that have a three-prong power cord, which applies to most electronic medical products. Class II products, for which other leakage tests are applicable, are those products that employ a two-prong power cord and typically include reinforced insulation.

Figure 3. Earth leakage test (from IEC 60601-1).

For this test, as well as other leakage tests, an isolation transformer (T₁) is recommended to represent worst-case conditions in which the secondary circuit is connected to ground reference of the measuring circuit (r). This test is made under normal conditions (S₁ closed, S₅ to normal), reversed line (S₅ reverse, S₁ closed), single-fault normal (S₁ open, S₅ to normal), and single-fault reverse (S₁ open, S₅ to reverse). The allowable values of leakage current specified in IEC 60601-1 at normal and at single-fault conditions are listed in Table I for this and other leakage tests.¹ Always refer to the standard pertinent to the product being tested.

Enclosure Leakage. Enclosure leakage (or touch/chassis leakage) is essentially the leakage to ground that a person would be subjected to if he or she were to touch non-earth-protected exposed parts of the device. This test is shown in Figure 4, where MD1 represents the measuring device. Exposed parts could be connectors, knobs, screws, or any exterior metal parts that are not at earth ground.

Figure 4. Enclosure leakage test (from IEC 60601-1).

When an enclosure is made completely of insulating material, a piece of conductive foil should be placed in contact with the enclosure surface. This test is also done under the same four conditions discussed in earth leakage measurements: normal, reverse, single-fault normal, and single-fault reverse. In addition, all four tests are often performed with an open ground-fault condition (S₇ open). The allowable values of enclosure leakage current are listed in Table I. Also note in Figure 4 that leakage tests are sometimes made with an external voltage on a signal input or output connection to the product under test. For safety reasons, a series limiting resistor is placed in the connection to prevent current in excess of 5 mA.

Patient Leakage. The patient leakage current test measures the current from parts normally in contact with the patient (applied parts) to earth as shown in Figure 5, where MD represents the measuring device. This test can be one of the most critical tests for medical instrumentation because patient leads are often in direct contact with the skin of the patient and sometimes under the skin, where body resistance is the lowest.

Figure 5. Patient leakage test (from IEC 60601-1).

Testing of applied parts can be a bit more complex. Medical standards usually specify that the leakage current be less than other tests because a device may have several patient connections. This test measures the leakage current patients would be subjected to if they were in contact with a patient lead and happened to make contact with a grounded object. Like the previous tests, this one is also done under the same four conditions: normal, reverse, single-fault normal, single-fault
reverse (with or without an open-ground connection). Allowable values of patient leakage current are listed in Table I, but the table does not reflect the lower values called for on some types of applied parts.

Patient Auxiliary Leakage. The patient auxiliary current is the current that flows between one patient connection and another as shown in Figure 6, where MD represents the measuring device. This test measures the leakage current that a patient would be subjected to between two patient leads that are in contact with the patient. This test is also done under the same four conditions, with or without an open ground connection. Typical limits for this test are also listed in Table I.

Figure 6. Patient auxiliary leakage test (from IEC 60601-1).

The Testing Process

All of this testing can be complex, especially in view of the number of test conditions to consider: normal conditions, fault conditions, switches to open, switches to close, connections to the power cord, and connection to patient leads. How can this be done in a cost-effective and timely fashion in a production environment? Single-function testers can perform the hipot, ground bond, or leakage test, but individual testers do not present an efficient method for electrical safety testing of medical products.

Through the use of microprocessor control and relay switching, it is possible to perform these functions from a single unit. Some commercially available instruments are specifically designed to perform all the required tests automatically. To fulfill this function, a tester must be capable of running sequential tests, and it must automatically configure the connections for each test. This process can be simplified so that it is nearly invisible to the operator. Test conditions and configurations can be easily selected through front-panel menu
programming or remotely by external computer. In either case, incorporating this type of automatic electrical safety testing goes a long way toward ensuring the delivery of safe medical electronic products. It is equally critical to retain test results at the end of the manufacturing process to help protect the manufacturer from liability issues.

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