Leakage current is one of the most stringent, yet telling, parameters of possible danger to patients or caregivers. It does not take much electric current flowing through the human body to cause harm. This is especially true for patients with weakened immune systems. The potential risk is why measurement of leakage current in electrical medical products is so critical.
|Robert M. Brown|
The IEC 60601-1 standard, “Medical Electrical Equipment—Part 1: General Requirements for Safety and Essential Performance,” describes tests for leakage current, as do a number of related national standards.1 This article aims to simplify these tests and the requirements of related standards and explain the rationale behind them. For an overview on other tests in the IEC 60601-1 standard, please refer to “A Primer for IEC 60601-1.”2
As noted in NFPA 99: “Standard for Health Care Facilities,” 2002 Edition, just three conditions occurring simultaneously can result in a shock to patient or caregiver:
• One part of the body is in contact with a conductive surface.
• A different part of the same body is in contact with a second conductive surface.
• A voltage source drives current through the body between those two points of contact.3
Figure 1 illustrates these three conditions along with eight separate conditions that should be analyzed when evaluating the electrical safety of medical devices.
Leakage current is measured to ensure that direct contact with the medical equipment is highly unlikely to result in electrical shock. The tests are designed to simulate a human body coming in contact with different parts of the equipment. The measured leakage current values are compared with acceptable limits. These limits are based on the type of product being tested, the point of contact with the product (earth, enclosure, patient), and the operation of the product under normal and single-fault conditions.
|Figure 1. Electrical shock and analysis points for medical devices (click to enlarge).|
Leakage current measurements are performed with the product energized and in all conditions, such as standby and full operation. The mains supply voltage is normally delivered via an isolation transformer to the product. Per IEC 60601-1, the mains supply voltage should be at 110% of the highest-rated supply voltage and at the highest-rated supply frequency. This means that a product rated for operation at 115 V ac, 60 Hz and 230 V ac, 50 Hz would be tested at 253 V ac and a line frequency of 60 Hz.
|Figure 2. Human body model for IEC 60601-1 (click to enlarge).|
The measuring device as defined in IEC 60601-1 is made up of two parts. One is a voltmeter with a Å½1-MÅ¾ input impedance and a frequency characteristic that is flat from dc to 1 MHz. The instrument must show the true rms value of the voltage across the measuring impedance. Indicating error must not exceed ±5%. The second part of the measuring device is a circuit, as shown in Figure 2. The circuit provides a resistance of approximately 1000 Å¾ and frequency characteristics that take into account the human body and the risk of ventricular fibrillation.
The frequency characteristic of the circuit is based on information from a number of different studies of how electric current relates to ventricular fibrillation. Most of these studies were performed in the late 1960s through the 1970s.
|Figure 3. Frequency characteristic for human body model of IEC 60601-1 (click to enlarge).|
The study data showed that the risk of ventricular fibrillation is highest for frequencies from 10 to 200 Hz. The risk is slightly reduced at 1000 Hz. It rapidly decreases for frequencies above 1000 Hz. The frequency characteristic of the circuit, which is shown in Figure 3, is designed to mimic the risk of ventricular fibrillation. It has a relatively flat frequency response to 1000 Hz, then a rapid roll-off.
A number of commercially available instruments are designed to perform leakage current measurement. These instruments should have the capability to measure with the correct accuracy, input impedance, and frequency characteristic.
To illustrate the various types of leakage currents and the points from which they are measured, the measuring device in this article will be represented in the figures by a cartoon character called MD. This cartoon character will touch the various points to show where connections would be made for leakage tests.
Leakage Current Measurement Conditions
Leakage currents are measured during both normal conditions and single-fault conditions.
Normal conditions are those in which all protection against safety hazards is intact. The leakage current test is performed with the medical equipment under normal use conditions. The equipment is energized in both standby and full operation. Reversing line and neutral on the supply mains is considered a normal condition, as this occurs frequently.
There are a number of single-fault conditions. These include the opening of protective ground and the opening of each conductor on the mains supply, one at a time.
For medical devices, additional single-fault conditions may be required, depending on the classification of the medical equipment. These can include 110% of mains voltage applied on signal input/output parts (SIP/SOP) during patient leakage and enclosure leakage tests. Mains voltage on applied parts is another fault condition.
Connection for most tests is straightforward, with the measuring device connected to the conductive point under test. For example, if measuring equipment with a metal enclosure, the measuring device is connected to an unpainted portion of the enclosure. To achieve measurements on a product that has an enclosure or other measurement point made of an insulating material, a piece of conductive foil no larger than 20 ¥ 10 cm (simulating the size of the palm) is placed in direct contact with the measurement point. If the surface contacted by the patient or operator is larger than 20 ¥ 10 cm, the size of the foil is increased accordingly. The foil is normally shifted to determine the highest value of leakage current.
Acceptable Levels of Leakage Current
|Figure 4. Earth leakage current (click to enlarge).|
IEC 60601-1 specifies allowable limits for leakage current measurements. These limits depend on the test being performed, the classification of applied parts, and whether under normal or single-fault conditions. Leakage limits for IEC 60601-1 are shown in Table I.
In this section of this article, leakage current is simplified to illustrate typical measurements for each type of leakage test. This section is not a substitute for IEC 60601-1, related national standards, or any particular standards covering the specific medical equipment under test.
|Figure 5. Enclosure leakage current (click to enlarge).|
Earth Leakage Current. The earth leakage current test measures the leakage current flowing from the protective earth of the medical device through the patient (in this case, the measuring device) back to the protective earth conductor of the power cord. This is the total leakage current from all protectively earthed parts of the product. This test applies to Class I devices.
As shown in Table I, there are three different sets of limits for earth leakage current. The first set is for general equipment. The second is for equipment that has no accessible protectively earthed parts and no means for the protective earthing of other devices. These limits also apply to mobile X-ray equipment and mobile equipment with mineral insulation. The third set of limits is for devices with a permanently installed protective earth conductor.
|Figure 6. (a) Patient leakage current for a Type B applied part, (b) patient leakage current for a Type BF applied part, and
(c) patient leakage current for a Type CF applied part (click to enlarge).
Figure 4 illustrates the basic measurement of earth leakage current in a piece of medical equipment using a standard detachable power cord. Such measurements are taken during normal conditions as well as with single-fault conditions, which is the interruption of one power supply con-
ductor (line or neutral) at a time.
Enclosure Leakage Current. The enclosure leakage current is measured from any part of the enclosure through the measuring device to earth, and between any two parts of the enclosure. This applies only to parts of the enclosure not connected to protective earth. See Figure 5.
Enclosure leakage current is measured during normal conditions as well as during single-fault conditions, in which one supply conductor at a time is interrupted, and, if applicable, the protective earth conductor is opened.
Patient Leakage Current. This is the leakage current measured from any applied part to ground. Depending upon the type of applied part (B, BF, or CF), there are different requirements for how the leakage tests are performed and the type of fault conditions. Type CF applied parts have the most stringent test requirements.
|Figure 7. Mains voltage on applied parts (click to enlarge).|
The leakage current for Type B applied parts is measured between all applied parts tied together and ground, as illustrated in Figure 6a.
Type BF applied parts must be separated into applied parts having different functions. The leakage current is measured between all applied parts with similar function and ground. See Figure 6b.
Leakage current for Type CF applied parts must be measured from each applied part to ground individually. See Figure 6c.
Patient leakage is measured during normal conditions as well as during single-fault conditions consisting of the interruption of one supply conductor at a time and the opening of the protective earth conductor, if applicable.
|Figure 8. Mains voltage on SIP/SOP (click to enlarge).|
Mains Voltage on Applied Parts. Type F applied parts have an additional IEC 60601-1 requirement. The leakage current of each applied part is measured while applying 110% of mains voltage through a current-limiting resistor. During this test, signal input and output parts are tied to ground. The polarity of the mains voltage to the applied part is reversed, and leakage current is measured for both conditions. See Figure 7.
Mains Voltage on Signal Input and Signal Output. Type B applied parts must have the additional single-fault condition of 110% of mains applied to all signal input and signal output parts during patient leakage measurement. This is only applicable to Type B applied parts if inspection of the circuit shows that a safety hazard exists. See Figure 8.
Patient Auxiliary Leakage Current. This test measures the leakage current between any single applied part and all other applied parts tied together. Patient auxiliary leakage current is measured under normal as well as single-fault conditions. See Figure 9.
National Differences on Leakage Current
|Figure 9. Patient auxiliary leakage current (click to enlarge).|
United States. There are three major differences between IEC 60601-1 and UL 60601-1 for the measurement of leakage current.4 The UL standard incorporates the requirements of NFPA 99 and ANSI/AAMI ES1, “Safe Current Limits for Electromedical Apparatus.”5 NFPA 99 incorporates the requirements of the U.S. national electrical code (NFPA 70) that relate to healthcare facilities. ANSI/AAMI ES1 defines safe leakage current limits within the three parameters of frequency, equipment function, and intentional contact with patient. It is likely that ANSI/AAMI ES1 will be withdrawn when the third edition of IEC 60601-1 is adopted in the United States by ANSI/AAMI.
UL 60601-1 differentiates between patient-care equipment (6 ft around and 7.5 ft above the patient) and non-patient-care equipment for leakage current tests. The typical leakage current values for a Class I device are 300 µA in a patient-care area and 500 µA outside that area. For a Class II device, the values are 150 µA in a patient-care area and 250 µA outside that area.
UL 60601-1 allows opening of the earth conductor and one of the supply connections simultaneously for non-patient-care equipment. This would be considered a double fault under IEC 60601-1.
European Union and Australia. There are currently no differences between IEC 60601-1 and EN 60601-1 and AS/NZS 3200.1 with respect to leakage current.6
|Figure 10. Japanese leakage current measurement circuit (click to enlarge).|
Canada. There is one difference between IEC 60601-1 and CAN/CSA C22.2 No. 601.1 on leakage current.7 If the medical device is to carry the CSA mark, production-line leakage tests are required.
Japan. There are only a few minor differences between IEC 60601-1 and JIS T 0601-1 on leakage current.8
In order to differentiate between the various patient leakage measurements (normal and single-fault), JIS T 0601-1 adds the clarifying nomenclature Patient Leakage I for patient leakage in a normal condition, Patient Leakage II for patient leakage in a single-fault condition of mains voltage on SIP/SOP, and Patient Leakage III for patient leakage in a single-fault condition of mains voltage on a floating patient applied part.
JIS T 0601-1 also specifies that the risk of external voltage on SIP/SOP is very low for a device that has been evaluated to IEC 60601-1-1 with its accessories. Hence the leakage current measurements in a single-fault condition with mains applied on SIP/SOP need not be performed for such a product.
There is only one significant national deviation for leakage current measurement in Japan. For leakage currents with a frequency component greater than 1 kHz, the leakage currents must not exceed 10 µA. The IEC 60601-1 measuring device is used, but with the 10-kÅ¾ resistor bypassed by a switch. See Figure 10.
A key step before performing leakage testing is to determine the class of the medical equipment under test and to identify the type of applied parts. Once these are determined, the appropriate tests and corresponding limits can be established. The applicable leakage tests can then be conducted under the appropriate single-fault conditions.
The leakage testing outlined here is based on the compliance testing requirements of IEC 60601-1. There are no specific requirements in that standard for leakage current measurements during production testing. Nonetheless, the manufacturer should do such testing. This can take the form of good manufacturing practice, routine production testing, or sampling.
1. IEC 60601-1, “Medical Electrical Equipment—Part 1: General Requirements for Safety and Essential Performance” (Geneva: International Electrotechnical Commission, 1995).
2. Leonard Eisner, Robert M Brown, and Dan Modi, “A Primer for IEC 60601-1,” MD&DI 25, no. 9 (2003): 48–58.
3. National Fire Protection Association, NFPA 99, “Standard for Health Care Facilities” (Quincy, MA: NFPA, 2002).
4. UL 60601-1, “Medical Electrical Equipment, Part 1: General Requirements for Safety” (Northbrook, IL: Underwriters Laboratories, 2003).
5. ANSI/AAMI ES1:1993, “Safe Current Limits for Electromedical Apparatus” (Arlington, VA: AAMI, 1993).
6. AS/NZS 3200.1, “Medical Electrical Systems” (Sydney: Standards Australia, 1998).
7. CAN/CSA-C22.2 NO. 601.1, “Medical Electrical Equipment—Part 1: General Requirements for Safety” (Mississauga, ON, Canada: Canadian Standards Association, 1995).
8. JIS T 0601-1, “Medical Electrical Equipment—Part 1: General Requirements for Safety” (Tokyo: Japanese Standards Association, 2000).
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