TESTING

(click to enlarge)A technician tests medical equipment using a Rigel 277 Plus.

However, such use is likely to change with the introduction of the newly published IEC 62353. This standard specifically describes the test requirements for in-service and postrepair testing of medical devices.

What are the implications of IEC 62353, and how does it differ from the well-established and widely understood requirements of IEC 60601?

IEC 60601

IEC 601 was introduced by the International Electrotechnical Commission to govern the design and development of medical equipment. The international safety standard, "Medical Electrical Equipment—General Requirements for Safety," was first published in 1977 and became widely known as IEC 60601.¹

Manufacturers of medical equipment are required to test to IEC 60601 to ensure that the design of that equipment is intrinsically safe. The standard specifies the type-testing requirements for protection against potential electric hazards including protective grounding (ground continuity), ground leakage currents, patient leakage currents, and patient auxiliary currents.

As a type-testing standard, the document describes a range of measurements that are intended to prove the safety of an item of electromedical equipment during its expected useful life. These measurements include a combination of stress and destructive tests that must be performed under certain environmental conditions.

In many cases, IEC 60601 has been translated into local national standards for use in certain countries. Examples are EN 60601 in the European Union, UL 2601-1 in the United States, CSA C22.2 in Canada, and AS/NZ 3200-1 in Australia and New Zealand.

Clearly, safety testing at the design stage and at the end of the production line are vitally important, but what about when the equipment enters service? In the absence of a recognized international standard for in-service testing, a number of countries have already introduced their own national test recommendations.

For example, some countries have produced their own technical standards or guidelines for safety testing of newly delivered medical devices (sometimes referred to as acceptance testing). Others have specified the tests at regular intervals (also referred to as preventive maintenance), and some have testing requirements directly following service or repair. Examples of such guidelines are MDA DB9801 (UK), VDE 750/751 (Germany), AS/NZ 3551 (Australia and New Zealand), and NFPA-99 and AAMI standards (United States).

In essence, all of these standards are linked by the aim to control the safety of medical devices used in treatment, care, and diagnosis for both patients and users. However, in countries without any national guidance or code of practice for in-service testing, the convention has been to follow the manufacturer’s instructions, which invariably require IEC 60601-1 test requirements and limits to be repeated.

IEC 62353

(click to enlarge)Medical testing to IEC 62353 can be done using equipment such as this Rigel 2288 electromedical tester.

IEC 62353 incorporates tests beyond those of type testing. The standard recognizes that the laboratory conditions described in IEC 60601-1 cannot always be guaranteed when in-service testing of medical devices is undertaken. As a result, test measurements that require certain environmental conditions may not always be applicable or consistent for the testing of equipment that is already in use. Another factor raised is that applying type-test specifications when the device is in service could damage equipment and pose a danger to users.

Technical Considerations. The main goal of IEC 62353 is to provide a uniform standard that ensures safe practice and reduces the complexity of the current IEC 60601-1 standard.

For example, one of the main differences is in ground bond testing. IEC 62353 proposes a minimum test current of 200 mA instead of the 25 A required in IEC 60601-1. This means that, provided sufficient consideration is given to potential contact resistance, test equipment can be smaller and more lightweight compared with current practices. Also, under insulation resistance testing, unlike IEC 60601, IEC 62353 provides methods for testing the insulation of medical devices. Three different test methods are detailed for assessing the insulation between mains parts and ground, between applied parts and ground, and between applied parts and mains parts.

Leakage Measurements

In terms of assessing leakage currents, IEC 62353 incorporates a number of different measurement methods to help guarantee safer practices and the repeatability of measurements.

IEC 62353 defines two different kinds of leakage current tests. Equipment leakage current is described as the total leakage derived from the applied parts, enclosure, and mains parts, combined with real ground. It is different from the applied part leakage current, which is the total leakage derived from the combined patient connections within an applied part to ground, and any conductive or nonconductive parts on the enclosure.

Three different methods are described in IEC 62353 to measure these leakage currents. These are direct leakage, the current flowing directly to ground via a measuring device (body model); differential leakage, which is the result of an imbalance in current between the live and neutral conductors; and the alternative method, a test similar to a hipot test at mains potential. There are various advantages and disadvantages associated with each of these current leakage test methods.

Direct Leakage Method. The direct leakage method can measure both ac and dc components. It enables direct comparison with the manufacturer’s IEC 60601-1 tests and typically can measure leakage values of less than 100 µA. The direct leakage method has the highest accuracy compared with other methods.

However, this method does have some areas that need consideration. For example, when a 1-K? resistor forming the measuring device or body model is connected in series with the protective ground conductor, a potential hazard can be caused when testing faulty equipment.

To avoid secondary ground paths and the potential for faulty equipment to register a pass, the device being tested must be fully isolated from ground during measurement. The direct method must be performed on terre neutral (TN) supply, and leakage measurements must be taken in each polarity of the mains supply. Differential Leakage Method. With the differential leakage method, potential secondary ground connections are included in the total measurement and the equipment does not need to be isolated from ground.

The main advantage of using the differential leakage method is that the ground conductor remains intact during the measurement, thus providing safer working conditions. Differential measurement of leakage also does not require an isolated device under test. It compares the difference in current between the live and neutral conductors to measure the complete leakage of the device, including leakage caused by secondary connections.

Leakage currents of less than 75 µA are difficult to measure using the differential leakage method. The method is also unsuitable for measuring conductive ungrounded parts and for those instances in which leakages are expected to be below 75 µA.

Measurements can also be influenced by external magnetic fields or the analyzer’s own internal magnetic field. Both the direct and alternative methods provide high accuracy and broad frequency response, which are required for measuring trends in low-leakage conditions.

Alternative Leakage Method. If the device under test is not connected to the mains supply, the alternative leakage method provides the safest possible test conditions for the operator. Also, this measurement is only taken in a single polarity and is similar to a dielectric test at mains potential using a current-limited mains-frequency supply.

Because of current-limiting resistors, the actual measuring voltage is dependent on the test load. The measured leakage current is scaled in proportion to the actual output voltage to predict the actual leakage current flow at full mains potential.

Because the live and neutral conductors are combined, the mains polarity has no influence and only one measurement is required. Measurements are not influenced by secondary ground connections and are highly repeatable, providing a good indication of deterioration in the dielectrics of the medical device under test. However, because the equipment is not activated while being tested, measurements cannot be made of actual leakage currents on equipment with switched circuits. In addition, measurements cannot be compared with those of previous IEC 60601-1 tests.

Preparation is Vital

All those involved in the planning, management, and implementation of electrical safety testing procedures for medical equipment need to fully understand the requirements set forth by the IEC 62353 standard.

Although the onus inevitably falls on the manufacturers of medical devices to advise on appropriate in-service test procedures for their own equipment, the new standard will clearly have an effect on medical service companies and on biomedical, medical physics, clinical engineering, and other technical departments.

It is essential to be familiar with the standards to make the best decisions for medical equipment testing.

John Backes is business development and product manager for Rigel Medical (Peterlee, County Durham, UK), a division of the Seaward Group. He can be contacted at johnb@seaward.co.uk.