The requirement for the electrical safety testing of medical equipment is essential to ensure that the apparatus is safe for operators and patients alike. Here we will take a look at the latest tests used to assess the integrity of insulation in medical electrical devices.
It is accepted that electrical currents are a necessary part of medical electrical devices and that faulty or excessive currents can cause a serious hazard to the patient, operator, or medical device. The electrical industry and international community have therefore implemented stringent procedures and requirements to ensure the safe and effective operation of medical devices, embodied in the IEC 60601 standard for medical electronic devices.
The risk of unacceptably high electrical fault currents can be minimized through design criteria, such as through effective levels of electrical insulation/isolation between the operator or patient and live parts or potentially live parts in a fault condition. Such insulation can be achieved through physical spacing (air gaps), dielectric materials, or component choice in order 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), while the level of isolation is often tested using a dielectric or insulation test. During a dielectric, or hipot test, a potentially high voltage (up to 4,000V AC) is applied across different parts of the electronic design in order to stress the dielectrics.
The results are displayed in mA or µA—similar to that in leakage current measurements. An insulation resistance test applies a lower DC voltage, typically between 250 to 500V DC, across different parts of the electronic design. The results are displayed in Mega ohms (MO).
Unlike the dielectric testing at high voltage, conventional insulation resistance measurement (insulation test) has been the traditional method of completing preventive inspections of the insulation levels in medical devices. While a 500V DC insulation test is not specified within the IEC 60601 standard (type testing), the insulation test is now an optional part of the recently published standard for routine testing of medical devices, the IEC 62353. Fotodrobės ir paveikslai https://interdeko.lt/produktai/paveikslai
Despite the traditional merits of a 500V DC insulation test to verify the level of insulation, it has also been recognized that this method can be problematic in some circumstances causing damage to the equipment under test and also not indicate the true state of the insulation when presented with an alternating voltage.
Therefore, there is a new alternative leakage test within IEC 62353 that applies a typical line voltage (~230V) and frequency (50Hz) as the insulation test source rather than DC. Both have their relative merits and place in periodic testing, provided the different limitations of each test method are understood.
|Figure 1: Insulation test on input.|
Insulation resistance is normally checked by applying 500V DC between:
1) Input (live conductors, phase, and neutral, connected together) and enclosure (protective earth in class I) (figure 1).
2) Output (applied parts) and enclosure (protective earth in class I) (figure 2).
3) Input (phase and neutral) and output (applied parts) for floating type applied parts (BF and CF) (figure 3).
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 2: Insulation test on applied parts (output).|
With all measurements of insulation resistance, the appliance under test must have the power switch (on/off switch) “on” before performing the test. Otherwise, the test voltage does not pass beyond the mains switch, in which case only the mains in the cord will be tested.
However, since the insulation resistance test does not power up the appliance, which could be seen as an advantage (reducing the time taken to test and eliminating 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.
In addition, appliances fitted with electronic mains switches or RCD plugs cannot be tested in this manner because it is not possible to close the mains switch (as they require mains to be present).
In some cases, sensitive electronic devices and particularly older IT equipment, which does not comply with BS EN60950, may be damaged by 500V. However, in practice, this may not be a significant issue as BSEN 60950 has been around longer than most IT equipment currently in use.
|Figure 3: Insulation test, input against output.|
While the outcome of a 500V DC insulation test is quick and safe to do, 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.
This is due to the increased use of switch mode power supplies that may indicate very high DC insulation resistances (>100MO), but, when measured with AC supply, could indicate a potentially high leakage. A greater influence of capacitive and inductive leakage experienced in these devices is more likely than resistive leakage as in a heating element.
Infinity readings are common when performing DC insulation tests and so provide no information as to 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 MO (higher) result is “safer” than a 10 MO (lower) result, considering the equipment has been exposed to a voltage at which it was not designed to operate. Furthermore, the 50 MO (higher) device might have been designed to measure 100 MO and has thus lost 50% of its insulation level. This could lead to higher leakage currents and unsafe conditions.
|Figure 4: Equipment leakage class I, alternative method.|
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, indicating an erroneous failure of the test. On the plus side, the insulation resistance test is relatively quick and easy to perform, which is why it is probably the most widely used.
To verify the effectiveness of insulation while maintaining the speed and safety of a traditional insulation test, an alternative leakage method is contained in the recently published IEC 62353 standard for routine testing of medical devices. The alternative leakage test is similar in setup as 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.
|Figure 5: Equipment leakage class II, alternative method|
As with the IEC 60601 leakage test, the alternative leakage test is done at mains potential and frequency, thus representing operational conditions unlike the 500V DC insulation test and the dielectric tests. This effective and safe method involves the application of a test voltage between the input and output of a medical device.
Equipment Leakage— Alternative Method
The equipment leakage is performed by placing an AC voltage (~230V, 50Hz) between the mains input (live conductors, phase, and neutral, connected together and protective earth in class I) against output (applied parts) including the enclosure as shown in the diagrams (figures 4 and 5).
Applied Part Leakage— Alternative Method
|Figure 6: Applied part leakage class I, alternative method.|
|Figure 7: Applied part leakage class II, alternative method.|
Applicable to floating applied parts (BF and CF) only, the applied part leakage is performed by placing the test voltage (~230V, 50Hz) between the output (applied parts only) and enclosure (protective earth in class I) and input (phase and neutral) together. This is shown in the diagrams (figure 6 and figure 7).
The test voltage is at mains potential and frequency of 50 Hz (Europe) or 60 Hz (US mains frequency), 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 by taking into account capacitive and inductive elements.
However, 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 affect measurements may not be activated.
The results measured with the alternative test can be compared to the IEC 60601 leakage measurements performed with the “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 is roughly twice the expected leakage during normal conditions, similar to the IEC 60601 “open neutral” measurements. Any variation in leakage is easily noted and you rarely get infinity readings—as with the DC insulation tests—so this is far more reliable in ascertaining the safety of the equipment under test.
In fact an infinity reading probably tells you the unit is not switched on or has “active” circuits to power up the device. These indications—the unit is not switched on or has “active” circuits—would not be indicated in a DC insulation test because a DC insulation test often gives infinity readings. When infinity readings occur, it is either because the equipment is not connected to the test device (ie, testing nothing), the mains switch is not in the “on” position, or a switch mode supply (which is active) is present in the device under test. In these cases a DC voltage used in the test would result in an infinity reading.
As 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. As 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, make for a safer and reliable test method.
Testing Comparisons Summarized
Below is a summary of the arguments for and against DC insulation testing verses IEC 62353 alternative testing.
DC Insulation Test
Pro: Quick and simple.
Con: May cause damage to device under test.
Con: Frequent infinity readings may mask unit is not switched on.
Con: Not suitable for use with “active” power circuits.
Con: Results do not relate to real-world leakage measurements.
IEC 62353 Alternative Test Method
Pro: Quick and simple.
Pro: Unlikely to damage device under test.
Pro: Results relate to real-world leakage measurements.
Pro: Zero reading indicates unit is not on or has “active” power circuit.
Con: Not suitable for use with “active” power circuits.
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Importantly, by stipulating various test methods and pass/fail limits, IEC 62353 provides the basis for consistent data collection and the development of formal preventive maintenance procedures. Because IEC 62353 also stipulates that a comparison is made between previous and current test results, it will be more obvious when a device is likely to fail. 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 biomedical/clinical engineering departments, medical physics, and other technical departments.
In particular, with the introduction of the new standard, care should be taken in the specification and selection of medical safety analyzers 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.
John Backes is the divisional manager for Rigel Medical, Peterlee, England. He is one of the UK representatives on the IEC 60601 committee (WG14) responsible for producing and maintaining international standards for the electrical safety of medical electronic devices. Rigel Medical’s A Practical Guide to IEC 62353 is available at www.rigelmedical.com and summarizes the standard’s methods and requirements. For more information, contact .