Manufacturers of medical electrical equipment are required to test to the international electrical safety standard IEC 60601 Medical Electrical Equipment, General Requirements for Safety, to ensure that the design of the equipment is intrinsically safe. The standard specifies various type testing requirements for protection against potential electric hazards.
In the absence of a recognised international standard for testing the safety of electronic medical devices once they have entered service in a hospital or other medical facility, many electro biomedical engineering (EBME) departments have traditionally used IEC 60601-1 tests and limits as the basis for the routine testing of electrical devices.
In addition, some countries have taken matters a step further with the development of their own guidelines and protocols, for example, those for testing newly delivered medical devices, which is also known as acceptance testing. Other countries have specified the safety tests to be performed at regular intervals, referred to as routine testing or preventative product maintenance; and some have specified test requirements directly following service or repair. Some examples of these localised testing standards include MDA DB9801 in the United Kingdom, VDE 750/751 in Germany, AS/NZ 3551 in Australia and New Zealand, and NFPA-99 in the United States.
The new demands of IEC 62353
The introduction of IEC 62353, Medical Electrical Equipment, Recurrent Test and Test After Repair of Medical Electrical Equipment, seeks to streamline this situation and harmonise all standards that specify measures to ensure the safety of electromedical devices used in the diagnosis and treatment of patients. In particular, IEC 62353 defines a series of test requirements and associated pass/fail limits that are designed to ensure the in-service safety of electromedical devices and systems. The standard contains tables with allowable values relating to different editions of IEC 60601-1 and includes general requirements and further clauses for special types of medical electrical equipment and systems.
Although complementary with IEC 60601, the new standard is less complex and incorporates tests beyond those of type testing. Significantly, IEC 62353 recognises that the laboratory conditions described in IEC 60601 may not always be relevant to working locations and that some type test specifications could potentially damage equipment when it is used in service. The important differences introduced by IEC 62353 are described below.
Earth bond testing
One of the main differences is the use of a minimum test current of 200 mA for earth bond testing, compared with 25A required in IEC 60601-1. This test is designed to test the integrity of the low resistance connection between the earth conductor and any conductive metal parts, which in Class I devices may become live when there is a fault.
The use of a minimum 200 mA test current recognises the potential for high test currents to potentially be destructive or damaging to parts of the device under test that are connected to the protective earth, but which have a functional purpose such as screening.
Historically, problems linked to contact resistance have sometimes caused erroneous pass/fail readings when using test currents. However, new high intensity low current test technology is now available that overcomes this problem and enables valid earth bond tests to be successfully performed using highly portable battery powered testers.
Insulation resistance test
Unlike IEC 60601-1 tests, IEC 62353 provides details of methods for testing the insulation of the medical device. Three different test methods are detailed for assessing the insulation between mains parts and earth, between applied parts and earth and, finally, between applied parts and mains parts.
Research has shown that it is current rather than voltage that is the source of electricity related injuries or deaths. As a result there are stringent rules on the design of medical equipment to ensure that the patient or operator is not exposed to those currents that do not form part of the functional operation of the device. Those currents are called leakage currents. In the interests of helping to guarantee safer practice and the repeatability of test measurements, IEC 62353 defines two different types of leakage current tests:
Equipment Leakage Current. This is regarded as the total leakage deriving from applied parts, enclosure and mains parts combined to real earth.
Applied Part Leakage Current. This is the total leakage deriving from the combined patient connections within an applied part to earth and any conductive or nonconductive parts of the enclosure.
IEC 62353 specifies three methods that can be used to determine these types of leakage current. Each of these methods has certain benefits and disadvantages for use with certain types of equipment and careful consideration needs to be given to the selection of test method made in relation to individual circumstances.
- Direct Method. This is identical to the method used in IEC 60601-1 measuring the true leakage through a body model measuring device to earth.
- Differential Method. This measures the leakage current as an imbalance in current between the live and neutral conductors
- Alternative Method. This is similar to a dielectric strength test at mains potential, using a current limited voltage source at mains frequency. The live and neutral conductors are shorted together and the current limited voltage is applied between the mains parts and other parts of the equipment.
Formalising preventative care
Importantly, by stipulating various test methods and pass/fail limits, IEC 62353 provides the basis for consistent data collection and the development of formal preventative maintenance procedures.
Although the onus will remain on the manufacturers of medical devices to advise on appropriate tests for their equipment, the new standard will clearly have a significant impact on medical service companies as well as EBME, medical physics, clinical engineering and other technical departments.
With the introduction of the new standard, care should be taken in the specification and selection of the medical safety analyser that is employed. If IEC 62353 is adopted, the tester must be properly configured to meet all the requirements of the new standard in terms of functionality, accuracy and repeatability. In particular, when using low current earth bond test methods, it is important to ensure that contact resistance is taken into account.
To help those likely to be affected by the introduction of IEC 62353, a free booklet, “A Practical Guide to IEC 62353,” is available that summarises the standard’s methods and requirements: www.rigelmedical.com.
John Backes is Product Manager at Rigel Medical.