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Originally Published MEM Fall 2002

Safety Testing

Automating Complex Electrical Safety Testing of Medical Devices

Improvements in test instrumentation help simplify the complex safety testing requirements for medical electronics.

Jim Richards

The medical electronics marketplace continues to undergo technology changes. Electrical safety test capabilities are evolving as well to stay abreast of these changes. Most notable are changes in safety test instrumentation that enable productivity improvements in the overall testing process.

The best-known electrical safety test, the dielectric withstand or hipot test, has long been required on medical electronic products, as well as on most other electrical devices and appliances before they exit a manufacturer's production floor. The intention of this test is to stress a product's insulation beyond what it would encounter in normal use, with the end goal being assurance that a patient or caregiver never serves as a current path to ground because of faulty insulation or faulty grounding within the product.

Today's product safety testing of medical products goes far

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beyond the traditional withstand test. Besides the concerns of insulation failure, of equal importance are the concerns over the integrity of a product's ground connection and how dangerous a product might be to the user under an actual fault condition. This article examines these more-stringent testing requirements and how manufacturers can perform these tests in a timely and cost-effective manner.

Tougher Testing Requirements
Figure 1. Hipot test. Voltage is applied between the high and neutral conductors and ground.
(Click to enlarge)
The requirements for electrical safety testing of medical electronic devices are much more stringent than are those for other electronic devices. Some of the reasons for this increased precaution include the following conditions: a patient may be connected to several devices at the same time; a patient's contact with the device may be directly to internal tissue; or a patient may have a reduced immune system, increasing the susceptibility to small leakage currents.

Many standards serve as the ruling authority in determining how medical products are to be built and tested. The one most widely accepted and implemented worldwide is IEC 60601-1 (the International Electrotechnical Commission's electrical safety standard for medical electronic equipment). This standard is intended to ensure that safety considerations are taken into account during the design phase of a product. However, much of the standard is applicable to production-line testing, and, in the final analysis, this is the only way manufacturers can be certain of safe products.
Figure 2. Ground bond test. An application of high current checks the integrity of the connection.
(Click to enlarge)
This standard has been used as a base for many national standards. Many countries have national deviations that modify some of the requirements of the 60601 standard; therefore, it is important to be aware of related standards. Products must be designed to meet the deviations that apply to the country where the product will be sold. For example, electronic medical equipment sold in the United States must meet the safety requirements of UL 2601-1 (IEC 60601-1 with U.S. national deviations). The older U.S. standard, UL 544, is in the process of being phased out over the next few years.

The bottom line is that there is no substitute for being familiar with the standard that governs the product in any given marketplace. The Internet is a good place to find listings and ordering information (e.g., http://www.ulstandardsinfonet.ul.com or http://www.iec.ch are good resources for UL or IEC standards, respectively). But remember, don't expect to find a particular standard in its entirety on-line—agencies make money by selling them.

Every manufacturer strives for improved productivity. Conducting electrical safety testing efficiently is no exception. Because medical products require more-stringent testing and typically undergo more types of tests, there are plenty of opportunities for increasing productivity. Six tests are commonly performed at a production level:
  • Figure 3. Earth leakage current test. This test is conducted under the following conditions: normal (S1 to the right, S2 closed), reversed line (S1 to the left, S2 closed), single-fault normal (S2 open neutral, S1 to the right), and single-fault reverse (S2 open neutral, S1 to the left)
    (Click to enlarge)    .
    Hipot or dielectric withstand test. For this test, a high voltage is applied (ac or dc) and leakage current is monitored between insulated parts to ensure that the insulation will withstand the voltage for a specified time.
  • Ground bond test. Commonly known as a high-current continuity test, it applies a 25-A ac current through a product's ground system to verify the connection.
  • Earth leakage current test. With the unit under power, this test measures the leakage current flowing back through the ground conductor on the power cord through an impedance that simulates the impedance of the human body.
  • Enclosure leakage test. This test measures the current from an enclosure back to ground through an impedance that simulates the impedance of the human body.
  • Patient leakage test. Also known as applied-part leakage, this test measures the current that flows from patient connections to ground.
  • Patient auxiliary leakage test. This test measures the current that flows between patient connections.

Hipot Test

For the hipot test, the requirement in most medical standards is to apply a test voltage that is two times the normal operating voltage plus 1000 V (1250–1500 V ac, depending on whether the product is to be operated from 115 or 240 V). For hard-wired corded products, this test voltage is applied between the high (hot) and neutral conductors shorted together and the power line ground (see Figure 1). Because the high and neutral must be shorted together, the product is not powered up during this test. From a medical product point of view, this test is not unique. It must be performed on any electrical product per applicable standard.

Ground Bond Test

This test checks the connection from any user-exposed or user-accessible metal parts to the ground reference on the product's power-line cord by measuring the resistance of this connection. The object of this test is to determine that sufficient current will flow to ground through this connection rather than to the operator in the event that the product fails and the operator comes into contact with a live voltage.
Figure 4. Human body equivalent impedance. This shows the equivalent circuit of the human body.
(Click to enlarge)

For medical products, this test is performed with the application of high current, which checks the integrity of the connection and not just the presence of the connection. For many other types of products, a low-current test is adequate. This bond test is illustrated in Figure 2. The product is not powered up for this test.

Leakage Tests

The leakage tests are performed under quite different conditions than the hipot or ground bond tests. For leakage tests, the product is actually powered up under operating conditions. These electrical safety tests are the most often misunderstood, primarily because they are not a production test commonly performed on most electrical products—with the exception of medical products.

Leakage current is the current that flows from the point where a person makes contact with a product, through that person's body, and back to ground (or some other point). Different types of currents are discussed below. The basic difference between them is simply how or where a person comes into contact with a product. Depending on the type of equipment, acceptable levels of leakage current are generally outlined in the governing standard for that product.

Leakage testing can be quite extensive during the design and development phase of a product. This is called compliance testing, which is carried out by a National Recognized Test Lab (NRTL). This article examines four tests that are feasible and recommended for every product during the final test phase before it is shipped to the end-user.
Figure 5. Enclosure leakage current test. S3 is the open or closed ground connection. S1 represents the reversed line, and S2 is the open neutral condition.
(Click to enlarge)

In a production environment, all tests are usually performed at 110% of the highest rated supply voltage and under a variety of normal and single-fault conditions. Normal conditions are electrical conditions that might normally occur on a daily basis and thus are not considered to be a problem. Single faults are essentially problem conditions that could occur, but because it is unlikely that two faults could occur at the same time, a product does not need to be tested with two or more faults. An example of normal and single-fault conditions often used in a production-test environment are described and illustrated. Note that a reversed ac line is considered to be a normal condition, not a fault. Conditions include:

  • Normal power applied (high and neutral).
  • Reverse power applied (high and neutral reversed).
  • Single fault/normal (neutral open).
  • Single fault/reverse (high and neutral reversed with neutral open).

Earth Line Leakage. The earth line leakage test is conducted with a tester circuit similar to that shown in Figure 3. This test essentially measures a sum of all leakages in the product under test, or basically the current flowing back to earth ground through the ground conductor of the line cord. This test is, of course, only applicable to protective earth products with a three-prong power cord. Most medical devices fit in this category.

Figure 6. Patient leakage current test. This test measures the current from parts normally in contact with the patient to earth.
(Click to enlarge)
This leakage test is done under the different normal and single-fault conditions discussed above. This test is made under normal conditions (S1 to the right, S2 closed), reverse line (S1 to the left, S2 closed), single-fault normal (S2 open neutral, S1 to the right), and single-fault reverse (S2 open neutral, S1 to the left).

When performing leakage measurements, the measurement device is subject to certain requirements. Standards require the use of meters with very specific loads because the load simulates the impedance of the human body. An equivalent circuit of the human body is shown in Figure 4.

Enclosure Leakage. Another leakage test is enclosure leakage (or touch/chassis leakage), which is essentially the leakage to ground that a person would be subjected to if they were to touch non-earth-protected exposed parts of the device. These exposed parts can be any exterior metal parts such as connectors, knobs, and screws. When an enclosure is made completely of insulating material, a piece of conductive foil should be placed in contact with the enclosure surface for this test. This test is usually done with the product's ground closed and open under normal and fault conditions. Referring to Figure 5, S3 is the open or closed ground connection and S1 and S2 the reverse line and open neutral.
Figure 7. Patient auxiliary leakage current test. This test is also conducted under the following conditions: open or closed ground connection, reversed line, and open neutral.
(Click to enlarge)
Patient Leakage. The patient leakage current test measures the current from parts of the device normally in contact with the patient (applied parts) to earth. For medical instrumentation, this test can be critical because patient leads are often in direct contact with a patient (and sometimes under the skin) where body resistance is the lowest. Testing of applied parts can be a bit more complex because medical standards usually specify that the leakage current must be less than other tests. It is further complicated when a device has several patient connections. This test measures the leakage current that patients would be subjected to if they were in contact with a patient lead and happened to make contact with a grounded object. Like the previous test, this test is also done under conditions of open or closed ground connection, reversed line, and open neutral (see Figure 6).

Patient Auxiliary Leakage. Patient auxiliary current is the current that flows between a single patient connection and all other patient connections that are connected together. Like the other leakage tests, this test measures the leakage current that a patient would be subjected to between two patient leads that are in contact with the patient. This test is also done under conditions of open or closed ground connection, reversed line, and open neutral (see Figure 7).

How Can All This Testing Be Done?

Because so much testing must be completed, the process sounds a bit complex: normal conditions, fault conditions, switches to open, switches to close, connections to the power cord, connections to patient leads, and so on. It seems impossible to do it all in a cost-effective and timely fashion. Although there are single-function testers that perform the hipot, ground-bond, or leakage tests, using single-function testers is no longer the most efficient method for medical product safety testing.

Through the use of microprocessor control and relay switching, it is now possible to perform all these functions from one unit, rather than from individual testers. Commercially available instruments are specifically designed to perform all the required tests automatically. Multifunction testers are finding their way into many production environments, making the process easier, faster, and more complete.

Test Mode
Leakage (selects the test type)
Human body mold
UL 2601-1 or other (see Figure 4)
Input voltage to device
Normal or reverse
Fault condition
Open or closed neutral
Leakage measurement type
Patient leakage: tells the measuring unit where to connect for current measurement, patient connection to ground (see Figure 6)
Current Limit
Program limit in milliamps (current in excess of this indicates product failure)
Table I. Test conditions and configurations required by the operator.

A multifunction tester must be able to run sequential tests and must be able to automatically configure the connections for each. This process can be simplified so that it is nearly invisible to an operator if the test conditions and configuration can be selected easily. The simplest, most error-free way to do this is through menu-selection programming within the measuring instrument. The example in Table I shows some actual selections required by the operator. After being set up the first time, these conditions can be saved in memory for later recall.

Conclusion

As the medical electronics marketplace undergoes technology changes, electrical safety test capabilities are evolving as well. Productivity improvements in the overall testing process are key to simplifying the complex battery of tests required.

Today's product safety testing of medical products goes far beyond the traditional withstand test. However, manufacturers can perform the more-stringent tests in a timely and cost-effective way.

Jim Richards is marketing engineer for QuadTech (Maynard, MA). He can be reached at jrichards@quadtech.com.

Copyright © 2002 Medical Electronics Manufacturing