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SAFETY TESTING

In the medical industry—where engineers, technicians, and laymen alike need to understand and operate complex electrical safety testing equipment—it is no wonder that there is a general sense of confusion surrounding line leakage testing. Most often specified to be performed as a type test in the laboratory or on the production line before shipment, the line leakage test is more involved and can be more complicated than related tests, such as hipot and ground-bond tests.

Indeed, the setup and training requirements to perform the test correctly are more complex than those of other electrical safety tests, but the line leakage test is not necessarily so different. In fact, it is an extremely important and valuable test that benefits manufacturers, customers, and patients alike. This article breaks down the test and explains how and why it is performed. It also describes the test's crucial elements, such as measuring devices and fault-simulating relays, and explains the various subtests involved at each step.

Background

The line leakage test, also known as the touch current test, is most commonly specified by safety agencies to be performed on all electrical products in two different scenarios: as a type test and as a routine production line test. Although type tests are usually specified for many different electrical products, the line leakage test is usually performed on medical products in a production environment.

Most electrical products, including anything from appliances to handheld tools, must be tested during the design and development phase to receive a safety agency listing. In these laboratory environments, the line leakage test is used to help ensure correct manufacturing processes and assembly practices. Along with other common tests, such as the hipot and ground-bond tests, the line leakage test is used in these situations primarily as an indicator of design quality.

Design and Type Test

Consider the following excerpt from IEC/UL 60601-1:

The electrical insulation providing the protection against electric shock shall be of such quality that currents flowing through it are limited to the specified values . . . in normal conditions and in the specified single fault conditions . . . with a supply voltage equal to 110% of the highest rated mains voltage.

Some products that are designed for sensitive applications, such as medical equipment, should be tested as a 100% routine production line test. These types of electronic instruments often have parts or components that come into direct contact with a patient. Because of the sensitive nature of these applications, rigorous testing must be performed as a routine test.

Regardless of the test environment, the goal is the same: to determine whether a product's insulation has the integrity to prevent any current from reaching the operator. In the case of a medical device, this would also extend to the patient. When current finds its way through or across any part of a product's insulation system, it is known as leakage.

Manufacturers often choose to use reinforced- or double-insulation systems and provide sufficient spacing between current-carrying conductors; however, leakage current will still be present to some degree. The electrical relationships between the materials used in a product's construction are what account for a substantial portion of any resulting leakage. Although the resistance of the insulation accounts for some leakage, using an insulating material in between conductors creates a certain amount of distributed capacitance, which helps to facilitate leakage to ground. This current looks to travel through a product's insulation system and return to ground by any means available, whether that is through a safety earth ground connection or through an operator or patient who is at ground potential.

For example, certain medical devices that run off line power have components that are in direct contact with a patient. In this case, a wall outlet—an almost unlimited power source—has a direct connection to a patient who may already be sick or frail. In such situations, it is vitally important that the leakage current produced by the product be small enough so as not to be perceived by the individual to whom the device is connected. More importantly, the device insulation must prevent any current from reaching the patient.

The Test

Some tests, such as the dielectric withstand (hipot) test or insulation resistance (IR) test, determine whether a product is manufactured correctly with good insulation. However, these tests don't indicate how much leakage current may be flowing through a product while it is running. Moreover, they fail to indicate how that leakage current would change if the product were connected to a power source incorrectly. What would happen if the operator plugged the product into an outlet that is wired incorrectly? What would happen if the neutral side of the line opened up? The line leakage test was developed to answer these types of important questions.

The line leakage test is actually a general term that is used to describe a series of tests. There are four different types of line leakage tests: earth leakage test, enclosure leakage test, patient leakage current test, and patient auxiliary current test. Each type of test is discussed in detail later in this article. These tests are performed under nominal operating conditions as well as in a variety of fault conditions. The fault conditions provide valuable information about how a product will behave if it is operated incorrectly.

Figure 1. The internal switching network for a line leakage tester. (click to enlarge)

Although the setup may vary from test to test, the actual test methodology doesn't change. One-box solution testers come equipped with the components and relay switching networks to perform all leakage tests in an automated sequence with little or no input from the test operator. Some testers are also capable of recording leakage using peak or rms measurements, a feature beneficial for manufacturers that must comply with standards such as IEC 60990. Figure 1 shows the internal line leakage test network built into a line leakage tester.

The measuring device is one of eight as specified by safety agency standards. Relays S1, S2, and S3 are used to simulate the various fault conditions during testing. S1 corresponds to the open neutral condition, S2 to the reversed polarity condition, and S3 to the open ground condition.

Measuring Devices. Because the line leakage test is designed to measure the leakage current of a product while it is running, the way in which the current is measured is critical. In fact, placement of the measuring device is the factor that distinguishes one type of line leakage test from another. Measuring devices are specified by safety agencies depending on product classification and the standard to which the product is being tested.

Figure 2. A schematic for the network that should be used when testing to EN 60601-1. (click to enlarge)

For the most part, measuring devices are resistive and capacitive networks designed to simulate the impedance of the human body in certain conditions. During touch current tests, for example, measuring devices can be used to approximate full hand-to-hand or hand-to-foot contact. Figure 2 shows the network that should be used when testing to EN 60601-1.

Many safety agencies differ in their interpretation of how the test's measuring device should be configured. Regardless of where the measuring device is placed or how it is configured, the test should measure the amount of leakage current that would flow through a person if the person were to come into contact with the device under test (DUT).

Fault Conditions. The line leakage test is performed in both normal and single-fault operating conditions. Measuring leakage current during fault conditions is important to determine whether a product fails safely, if it fails at all. A product that fails safely will not produce excessive leakage current even if a combination of fault conditions occurs. Table I shows various combinations of relay closures that simulate up to eight different testing scenarios.

Test Setup and Procedure

Earth Leakage Test. The earth leakage test places the measuring device from the earth ground pin of the DUT to the neutral side of the line (which is referenced to ground). In this configuration, power is applied to the DUT at 110% of the nominal voltage level using an isolation transformer. The measuring device tracks the amount of leakage current flowing from the mains-input line and returning to ground through the product's insulation under normal and single-fault conditions.

Enclosure Leakage Test. The enclosure leakage test places the measuring device from one or more points on the DUT's chassis to the neutral side of the line. In this configuration, power is applied to the DUT at 110% of the nominal voltage level using an isolation transformer. The measuring device tells the amount of leakage current flowing from the enclosure, excluding applied parts accessible to the operator or patient to ground or to another part of the enclosure under normal and single-fault conditions.

Applied Part Leakage Test. The applied part leakage test, also known as the patient leakage test, places the measuring device from a patient-applied part (usually some sort of probe or meter that comes into direct contact with a patient's body) to the neutral side of the line. In this configuration, power is applied to the DUT at 110% of the nominal voltage level using an isolation transformer. The measuring device monitors the amount of leakage current flowing from applied part to ground under normal and single-fault conditions.

Patient Auxiliary Leakage Test. The patient auxiliary leakage test places the measuring device in between two different applied parts that come into contact with a patient's body. In this configuration, power is applied to the DUT at 110% of the nominal voltage level using an isolation transformer. The measuring device records the amount of leakage current flowing from one applied part to another under normal and single-fault conditions.

Conclusion

Determining what agency listing needs to be obtained and which standard should be used is the first of many steps on the road to electrical safety compliance. Examine the standards thoroughly and determine whether line leakage testing is right for each particular application.

Although the line leakage test can seem somewhat confusing at times, it is a vitally important test that should be given due consideration in any electrical safety testing routine. With a little research and the right equipment, line leakage testing can be performed as quickly and easily as the other more commonly performed tests. Adding the line leakage test to a safety testing routine will help make medical products safer and more reliable for users and patients.