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