This article describes the tests used to assess the integrity of insulation in electrical appliances. Each test method has its relative merits and place in periodic testing, providing the different limitations of each are understood.
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In search of effective testing
It is accepted that electrical currents are a necessary part of medical electrical devices and faulty or excessive currents can cause a serious hazard to the patient, operator or medical device. Therefore, stringent procedures and requirements, embodied in the IEC 60601 series of standards1 for medical electronic devices, have been implemented to ensure the safe and effective operation of medical devices.The risk of unacceptably high electrical fault currents can be minimised through design criteria, that is, through effective levels of electrical insulation or isolation between the operator or patient and live parts, or potentially live parts in a fault condition. This insulation can be achieved through physical spacing (air gaps), dielectric materials or component choice to achieve the highest possible level of insulation, while ensuring the device operates properly.
The effectiveness of electrical insulation is tested through electric leakage measurements (results in mA or µA) and the level of isolation is often tested using a dielectric or insulation test. During a dielectric test, also known as the hipot test, a potentially high voltage (up to 4000 V AC) is applied across different parts of the electronic design to stress the dielectrics. The results are displayed in mA or µA, similar to those for leakage current measurements. An insulation resistance test applies a lower DC voltage, typically between 250 and 500 V DC across different parts of the electronic design. The results are displayed in Mega ohms (MΩ).
Unlike the dielectric testing at high voltage, conventional insulation resistance measurement (an insulation test) has been the traditional method of completing preventative inspections of the insulation levels in medical devices. A 500 V DC insulation test is not specified in IEC 60601 (type testing); however, an insulation test is an optional part of IEC 62353, the standard for routine testing of medical devices.2
Despite the traditional merit of a 500 V DC insulation test to verify the level of insulation, it has been recognised that this method can be problematic in some circumstances. It can cause damage to the equipment under test and not indicate the true state of the insulation when presented with an alternating voltage. The alternative leakage test in IEC 62353 applies a typical line voltage (approximately 230 V) and frequency (50 Hz) as the insulation test source rather than DC. Both test methods have their relative merits and place in periodic testing, provided the limitations of each are understood.
Measuring insulation resistance
Insulation resistance is normally checked by applying 500 V DC between:
- Input (live conductors, phase and neutral, connected together) and enclosure (protective earth in Class 1)
- Output (applied parts) and enclosure (protective earth in Class 1)
- Input (phase and neutral) and output (applied parts) for floating type applied parts: body floating (BF) and cardiac floating (CF); BF is typical for applied parts being connected to patients in a noninvasive way, CF can be used in invasive applications.
The resistance is measured and compared with the minimum acceptable value to assess pass or fail conditions, which can vary greatly depending on design and test voltage variations.
| Figure 1: Equipment Leakage Alternative Method, Class I. | Figure 2: Equipment Leakage Alternative Method, Class II. |
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| Figure 3: Applied Part Leakage Alternative Method, Class I. | Figure 4: Applied Part Leakage Alternative Method, Class II. |
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With all measurements of insulation resistance, the appliance under test must have the power switch (On/Off switch) “On” before performing the test. If not, the test voltage does not pass beyond the mains switch, in which case only the insulation in the mains cord will be tested. However, because the insulation resistance test does not power up the appliance, which could be seen as an advantage (it reduces the time taken to test and eliminate the danger of moving hazardous parts), extra care should be taken to ensure the equipment switch is in the “On” position to complete a meaningful test. Appliances fitted with electronic mains switches or residual current device plugs cannot be tested in this manner, because it is not possible to close the mains switch (they require mains to be present).
In some cases, sensitive electronic devices and particularly older IT equipment, which does not comply with EN 60950, Safety of Information Technology Equipment, may be damaged by 500 V. However, in practice, this may not be a significant issue because EN 60950 has existed longer than most of the IT equipment currently in use.
Limitations of DC insulation test
Although the outcome of a 500 V DC insulation test is quick and safe to perform, in most cases it does not provide a real indication of the effectiveness of the insulation in modern medical devices, or the expected leakage values that may be experienced during normal or typical operation. The increased use of switch mode power supplies may indicate high DC insulation resistances (more than 100 MΩ), but when these are measured with an AC voltage could indicate high leakage. The high resistance of more than 100 MΩ results from the greater influence of capacitive and inductive leakage experienced in these devices rather than resistive leakage, which is found in a heating element.
Infinity readings are common when performing DC insulation tests and provide no information on whether the unit was actually switched “On” or “Off.”
This makes the test results meaningless from a safety point of view. It is a matter of debate as to whether a 50 MΩ (higher) result is safer than a 10 MΩ (lower) result, bearing in mind that the equipment has been exposed to a voltage at which it is not designed to operate. Furthermore, the 50 MΩ (higher) device may have been designed to measure 100 MΩ and thus has lost 50% of its insulation level. This could lead to higher leakage currents and unsafe conditions. Finally, in some electrical equipment, components connected to the live/neutral conductors for electromagnetic compatibility (EMC) filtering or surge protection can significantly influence the measurement and thus indicate an erroneous failure of the test. Conversely, the insulation resistance test is relatively quick and easy to perform, which is why it is probably the most widely used.
Alternative insulation test
To verify the effectiveness of insulation while maintaining the speed and safety of a traditional insulation test, an alternative leakage test method is included in IEC 62353. The alternative leakage test is similar in setup to the dielectric strength test (high voltage), a DC insulation test and the IEC 60601 earth/enclosure leakage test in the “open phase/neutral” single fault condition. As with the IEC 60601 leakage test, the alternative leakage test is performed at mains potential and frequency, thus it represents operational conditions unlike the 500 V DC insulation test and the dielectric tests. This effective and safe method involves the application of a test voltage to the input and output of a medical device.
The alternative leakage test is applicable to both the Equipment Leakage Test and the Applied Part Leakage Test in the IEC 62353. The Equipment Leakage Test is performed by placing an AC voltage (approximately 230 V, 50 Hz) between the mains input (live conductors, phase and neutral, connected together and protective earth in Class 1) against output (applied parts) including the enclosure as shown in Figures 1 (Class 1 equipment) and 2 (Class 2 equipment).
Applicable to floating applied parts (BF and CF) only, the Applied Part Leakage Test is performed by placing the test voltage (approximately 230 V, 50 Hz) between the output (applied parts only) and enclosure (protective earth in Class 1) and input (phase and neutral) together. This is shown in Figures 3 (Class 1 equipment) and 4 (Class 2 equipment).
The test voltage is at mains potential and at a frequency of 50 Hz, which means the leakage paths will be similar to those present when the equipment is in operation. This avoids the problems associated with EMC filtering or surge protection affecting DC insulation tests, and provides a more accurate reading of the true insulation, taking into account capacitive and inductive elements.
Pros and cons of alternative test
Yet, the alternative leakage test still has some limitations, because any electronic switches present will not be “on” (as with insulation testing) and relays or other active circuitry that may effect measurements may not be activated.
The results measured with the alternative test can be compared with the IEC 60601 leakage measurements performed with open neutral, single fault condition. During a comparison between IEC 60601 earth leakage and the IEC 62353 alternative method, results showed a consistent relationship between the two measurements.
The alternative method measures approximately twice the expected earth leakage during normal conditions, similar to the IEC 60601 “open neutral” measurements. Any variation in leakage is easily noted and there are never infinity readings as with the DC insulation tests. Thus, this is more reliable in ascertaining the safety of the equipment under test. In fact, a zero reading probably informs the engineer that the unit is not switched “On” or has active circuits to power up the device. This would not be indicated in a DC insulation test, because this often gives infinity readings.
Because the equipment under test is not powered up, the alternative method is therefore a safe and quick method of verifying the effectiveness of the insulation and thus the expected safety. Because both mains phases are shorted together during the test, no mains reversal needs to be performed, which saves time. This, coupled with the more accurate and realistic data, makes for a safe and reliable test method.
Towards consistency and prevention
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. IEC 62353 also stipulates that a comparison is made between previous and current test results, thus it will be more obvious when a device is likely to fail.
Although the onus remains with medical device manufacturers to advise on appropriate tests for their equipment, IEC 62353 will clearly have a significant impact on medical service companies as well as biomedical departments, medical physics, clinical engineering and other technical departments. In particular, when considering implementation of IEC 62353, care should be taken in the specification and selection of medical safety analysers to ensure that they can be used to test in accordance with the IEC 62353 requirements and that they are capable of performing accurate and repeatable test routines.
References
1. IEC 60601-1, Medical Electrical Equipment, Part 1: General Requirements for Safety, 3rd 2005 (International Electrotechnical Committee).
2. IEC 62353, Medical Electrical Equipment, Recurrent Test and Test after Repair of Medical Electrical Equipment, 2007 (International Electrotechnical Committee).
John Backes is Divisional Manager at Rigel Medical, 18 Bracken Hill, South West Industrial Estate, Peterlee SR8 2SW, UK
tel. +44 191 587 8745
e-mail: johnb@rigelmedical.com [5]
www.rigelmedical.com
tel. +44 191 587 8745
e-mail: johnb@rigelmedical.com [5]
www.rigelmedical.com