Chapter 4 - Electromagnetic Compatibility And Medical Devices

Have you heard about the wheelchair that moved on its own every time a police car passed
by? No, it’s not part of a joke. This actually happened, and several people were seriously
injured when radio signals from the two-way communications equipment on emergency
vehicles and boats, CB, and amateur radios interfered with proper operation of the control
circuitry of powered wheelchairs, sending some off curbs and piers. Similar reports of
improper operation of apnea monitors, anesthetic gas monitors, and ECG and EEG monitors
due to electromagnetic interference prompted government agencies to look carefully
at these occurrences and establish regulations by which equipment must possess sufficient
immunity to operate as intended in the presence of interference.

Complying with these regulations is not easy. The technologies involved in modern circuit
design have considerably blurred the boundaries between the digital and analog
worlds. Suddenly, multihundred megahertz and even gigahertz clocks became commonplace
in high-performance digital circuits, making it necessary to consider every connection
between components as an RF transmission line. At the same time that the need for
higher performance pushes designers toward high-speed technology, the marketplace is
demanding more compact, lighter, and less power-hungry devices. With smaller size, analog
effects again enter into consideration, because as components and conductors come
into close proximity, coupling between circuit sections becomes a real problem.

Obviously, self-interference within a circuit must be eliminated to make the product
workable, but this still does not make the product marketworthy. This is because strict regulations
concerning electromagnetic compatibility are now being enforced around the
world in an effort to ensure that devices do not interfere with each other. In the United
States the FCC regulates the testing and certification of all electronic devices that generate
or use clock rates above 9 kHz [Dash and Strauss, 1995]. In principle, the FCC’s charter is
to protect communications from unwanted electromagnetic interference (EMI). In the
European Common Market, on the other hand, an electromagnetic compatibility (EMC)
directive is now in effect, which not only establishes requirements against causing undue
interference to radio and telecommunications equipment, but also institutes requirements
by which equipment must possess sufficient immunity to operate as intended in the presence
of interference [Gubisch, 1995].

Regulatory bodies around the world have developed standards and regulations covering
both emissions and immunity that designers must take very seriously. Failure to comply
with EMI and EMC regulations can have a serious impact on everyone associated with a
product, starting with the designer, through the manufacturer, the marketing and distribution
network, and extending even to customers. The consequences of noncompliance
include halting manufacturing and distribution, levying fines, and the publication of public
notices of noncompliance to warn potential customers and other agencies. These considerations
become especially important in the case of medical equipment, since it often
involves sensitive electronics that can be affected adversely by electromagnetic interference,
leading to potentially serious hazards to patients and health-care providers.

The European Community regulates emissions and immunity of medical devices
through the EN-60601-1-2 standard (Medical Electrical Equipment—Part 1: General
Requirements for Safety; Section 2: Collateral Standard: Electromagnetic Compatibility—
Requirements and Tests) as well as the EN-55011 standard (Limits and Methods of Measurement
of Radio Disturbance Characteristics of Industrial, Scientific and Medical Radio
Frequency Equipment). In EN-60601-1-2, pass/fail criteria are defined by the manufacturer.
As a result, the manufacturer may chose to classify a failure mode that does not pose
a hazard to the patient as a “pass.” In the United States, the FDA is adopting many of the
IEC-60601-1-2 requirements but is imposing restrictions on a manufacturer’s ability to
adopt pass/fail criteria. The FDA prescribes that a passing result corresponds to maintaining
clinical utility. In addition, there are discrepancies between the immunity levels recommended
by European authorities and the FDA. Because of these differences in opinion,
as well as because the standards are relatively new, changes occur often, and we advise
engineers to keep updated on the latest versions.

Assuring compliance with the rules involves an extensive series of tests. The EMI and
EMC standards enforced by the various regulatory agencies clearly define the construction
of test sites as well as the test procedures to be followed. Even a fairly spartan facility
capable of conducting these tests ends up costing over $100,000 just to set up, and for this
reason, most companies hire an outside test lab at the rate of $1500 to $3000 per day to
conduct testing. Considering how fast charges can accumulate during testing, it is obviously
not a smart move simply to hire a test lab and wait for the results. Rather, designers
should familiarize themselves with the relevant EMI and EMC standards and make sure
that compliance requirements are considered at every stage in the design process.

In this chapter we present the major EMI/EMC requirements for medical devices, look
at the theory of how circuits produce EMI, and describe some low-cost tools and methods
that will allow you to identify and isolate the sources of EMI that inevitably make it into
a circuit.

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