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Designing Medical Electrical Equipment to Meet
Safety Certification and Regulatory Requirements
Note - On April 25, 2003, UL issued "UL 60601-1" to
replace "UL 2601-1". There are no requirement
differences between UL 60601-1 and UL 2601-1 (format and name changes only).
Medical equipment is highly regulated and held to a
higher level of safety than nearly all other types of equipment on
the market. The main reasons for this are that medical
equipment may be used on patients who are not able to respond to
hazardous conditions or pain, an actual electrical connection between
the equipment and patient may exist, and certain types of medical
equipment function as life support, the failure of which could result
in the death of the patient.
While Engineers spend years in school and the workplace
learning about how to design equipment, they usually do not learn
about the certification and regulatory requirements that the
equipment must meet to comply with the US and international codes and
laws. Understanding these requirements before the design phase
of the equipment will result in a reduction of product development
cost, faster certification turnaround, and increased product safety.
This information is intended to increase awareness of
product safety certification requirements by exploring the
requirements for medical equipment both in the US and
internationally. We will look at the applicable safety
standards and review their philosophy of safety, then show the
process of evaluation and documentation. We will then discuss
the most common noncompliances seen when evaluating medical equipment
to safety standards.
MEDICAL EQUIPMENT FOR THE US AND CANADA (FDA, UL)
In the United States, the Food and Drug Administration
(FDA) sorts devices into three categories (Class I, II, or III),
depending upon the degree of regulation necessary to provide a
reasonable assurance of their safety and effectiveness. Class I
devices are subject to premarket notification, registration and
listing, prohibitions against adulteration and misbranding, and rules
for good manufacturing practices. Class II devices also need
performance standards, and Class III devices need premarket approval
from the FDA. A 510(k) is a collection of documents and forms
used to show substantial equivalence to a device that was either in
commercial distribution before May 28, 1976 or has been reclassified
into Class I or II.1 The FDA or accredited Third Party Reviewer
examines the documentation and determines whether the device is
substantially equivalent to the specified predicate device or
not. If the device is found to be substantially
equivalent, it can be marketed and sold in the US. If the
device is not found to be substantially equivalent (due to new
technology or differences in intended use), then the submitter must
present information, such as clinical trial data, statistical data,
and safety testing results to the FDA to show that the device is safe
and effective. If the FDA finds the information and data
adequate, they will grant premarket approval for the device.
The FDA Federal Food, Drug, and Cosmetic Act requires
that all medical devices be "safe and effective," and
recognizes safety standards as a means to support a declaration of
conformity. Many "Authorities Having Jurisdiction"
(AHJ) and purchasers of medical electrical equipment in the US and
Canada require a safety certification mark on the equipment.
Therefore, a product that carries a safety certification mark will
usually reach its full market potential.
Underwriters Laboratories Inc. (UL) is the major product
safety certification organization in North America.
Manufacturers of medical equipment submit product samples and
information to UL for evaluation to applicable safety standard(s) and
products that meet these requirements are authorized to apply the
appropriate UL Mark for the US and/or Canada.
MEDICAL EQUIPMENT FOR THE EUROPEAN UNION (CE Marking)
All but low-risk, non-measuring, non-sterile medical
devices used in Europe must bear the CE mark, reviewed by a Notified
Body (CE mark with the Notified Body's identification number).
A Notified Body is a third party designated by European authorities
to assess compliance with the Medical Device Directive (93/42/EEC
). The Medical Device Directive is essentially the European
"law" for medical devices. This assessment by a
Notified Body evaluates compliance with the Medical Device Directive
requirements for safety, performance, suitability for intended use,
and risk analysis. Manufacturers can choose from several
conformity assessment routes, most involving a quality assurance
assessment of the manufacturer's facilities.
MEDICAL ELECTRICAL SAFETY STANDARDS
Product Safety certification agencies use safety
standards to evaluate many different types of products. These
safety standards are consensus documents, which define the minimum
construction and performance requirements. Table 1 provides an
example of UL Standards that cover medical and related product
categories. A complete list of UL standards, covering more than
5,000 product categories, can be found at http://ulstandardsinfonet.ul.com.
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UL
187
X-Ray Equipment (withdrawn end of 2010, product freeze at end of 2004)
UL
198
Fuses
UL
498
Appliance Inlets
UL
544
Medical and Dental Equipment (withdrawn end of 2010, product freeze
at end of 2004)
UL 1577
Optical Isolators
UL 60950-1 Information Technology
Equipment - Computers, etc.
UL 2111
Motors
UL 2601-1 (Changed to
"UL 60601-1")
UL 60601-1 Medical Electrical Equipment
UL 61010-1 Electrical Equipment for
Laboratory Use |
Table 1. Example of UL Standards
Since 1972, electrically operated medical equipment used
in the US has been evaluated to the UL544 Standard for Medical and
Dental Equipment. This standard will be withdrawn at the end of
2004, requiring both new and current UL544 equipment to be evaluated
to the UL 2601-1 Standard to continue to apply the UL Mark.
Underwriters Laboratories published UL2601-1 in 1994. UL2601-1
was written as an IEC601-1 (renamed IEC60601-1) harmonized
standard. Prior to this harmonization initiative, manufacturers
were required to comply with different standards for different
countries. This often required that multiple product models had to be
designed and manufactured to meet different national standards if
they were to be used in more than one country. Using an
internationally harmonized safety standard meant that a product could
be designed and evaluated for compliance with a single standard, such
as UL2601-1, and also be eligible for use in many different
countries. Other countries that use an IEC60601-1 harmonized
standard include the European Union, Canada, Brazil, Japan, Korea,
and Australia. In addition to being the base of so many harmonized
standards, IEC60601-1 is an FDA recognized consensus standard, used
to support a manufacturer's declaration of conformity. Visit the FDA
website to see all the FDA-CDRH recognized standards at http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfStandards/search.cfm.
The UL2601-1 safety standard contains the full text of
IEC60601-1 and adds US deviations, as shown in Figure 1. The US
deviations contain national requirements, such as those for the mains
circuit (National Electric Code, UL), component requirements (ANSI,
UL), lower leakage current limits (AAMI, UL), enclosure flame ratings
(UL), and production line testing (OSHA, UL). Since these
deviations do not conflict with the base standard, the equipment is
still in compliance with IEC60601-1.
Figure 1. Structure of UL2601-1
The second edition of IEC60601-1 currently has two
amendments that were published in 1991 and 1995. These
amendments are additions and corrections to the base standard.
The standard also has collateral (horizontal) standards, numbered
IEC60601-1-x, and particular (vertical) standards, numbered
IEC60601-2-xx. The collateral standards include requirements
for specific technologies and/or hazards and apply to all applicable
equipment, such as medical systems (-1-1), EMC (-1-2), radiation
protection in diagnostic X-ray equipment (-1-3), and software
(-1-4). The particular standards apply to specific equipment
types, such as Medical Electron Accelerators (-2-1), High Frequency
Surgical Equipment (-2-2), and hospital beds (-2-38). Figure 2
illustrates the organization of the collateral and particular
IEC60601 standards. The US deviations, amendments, collateral, and
particular standards are used together to evaluate the medical
electrical equipment. They all use the same clause numbering
system, which allows cross-referencing of the requirements, and each
may have amendments.
Figure 2. Organization of the IEC60601-1 Standard
IEC60601-1 REQUIREMENTS
Philosophy
The underlying philosophy of the IEC60601-1 harmonized
standards is that equipment must be safe in normal condition (NC) and
single fault condition (SFC). To understand the electrical
safety requirements, we need to first define a few terms:
- An Applied Part is any pieces of the equipment that can
intentionally or unintentionally be brought in contact with the patient.
- Creepage is spacing along a surface (as an ant crawls).
- Clearance is spacing through the air (as a bug flies).
- LOP is a level of protection (2 required by standard).
- Basic Insulation (BI) is a spacing or a physical insulation barrier
providing 1 LOP.
- Supplemental Insulation (SI) is also a spacing or a physical
insulation barrier providing 1 LOP.
- Double Insulation (DI) is BI + SI and provides 2 LOP.
- Reinforced Insulation (RI) is a single spacing or physical
insulation barrier that provides 2 LOP.
- Protective Impedance is a component (such as a resistor) that
provides 1 LOP.
- Protective Earth (PE) is a well-grounded part that provides 1 LOP.
- Class I Equipment is defined as using PE as 1 LOP.
- Class II Equipment (also known as Double Insulated) is defined as
not using PE as 1 LOP.
For electrical safety, the standard requires 2 LOP
against excessive unintentional current, defined as leakage current,
passing through the patient or operator. Figure 3 graphically
depicts the 2 LOP between the live part (mains) and the patient (1A
and 2A), and between the live part and the enclosure (1B and 2B). In
the case of 1A and 2A, the levels of protection are BI and SI.
For 1B and 2B, they are BI and PE.
Figure 3. Two Levels of Protection (2 LOP)
Table 2 provides an example of the minimum spacing
requirements and dielectric (hipot) requirements for these
barriers. If the insulation does not meet both the dielectric
and the spacing requirements, it cannot be considered as a level of
protection and can be shorted as a normal condition. Note that
BI and SI spacing requirements are the same, however the SI
dielectric values are greater than the BI values. To be
considered protectively earthed, the grounding path of the equipment
must pass 15 Amps or 1.5* rated current for 5 seconds from the
protectively earthed part to the earth connection, with
0.1 Ohms resistance for equipment with a detachable power supply cord
or 0.2 Ohms for equipment with a non-detachable power
supply cord. The Canadian requirement changes the current to 30
Amps or 2* rated current and changes the time to 2 minutes.
Since this is the only major difference between the US and Canadian
standards, the test is typically done at 30 A for 2 minutes as the
"worst case" for testing protective earthed parts.
Table 2. Insulation Spacing and Dielectric Requirements
(excerpt from full tables)
To demonstrate that medical equipment is safe in normal
and single fault condition, the following conditions must be
addressed when evaluating the equipment. These conditions are
specified in the Standard and need to be addressed when designing
medical equipment and/or selecting components.
Likely to Occur (Normal Condition)
· Reverse polarity of supply mains
· Failure of insulation less than basic
Could Occur (Single Fault Condition)
· Interruption of protective earth
· Interruption of one supply conductor
· Mains voltage on floating (F-type) applied part(s)
· Mains voltage on communication ports
· Failure of electrical components, one at a time
· Failure of mechanical parts, one at a time
· Failure of temperature limiting devices, one at a time
· Shorting of basic or supplemental insulation
· Overload of mains supply transformers
· Interruption and short circuit of motor capacitors
· Locking of moving parts
· Impairment of cooling (fans, vents)
Unlikely to Occur (Not evaluated)
· Total breakdown of double or reinforced insulation
· Loss of protective earth on permanently installed equipment
· More than one Single Fault Condition at a time
· Failure of a UL Recognized optocoupler barrier
· Failure of a UL Recognized Y1 capacitor, acting as a barrier
EVALUATION OF MEDICAL EQUIPMENT
The process of evaluating medical equipment for
compliance with the requirements in UL2601-1 includes not only the
equipment itself, but the user's manual, markings, software (if it
mitigates a hazard), biocompatibility of applied parts and
electromagnetic compatibility (EMC). Before submitting
equipment for evaluation, the following information should be developed:
· Does equipment fit the scope of UL2601-1?
· Does equipment fit the scope of IEC60601 Collateral or
Particular standards?
· List all equipment functions and accessories that can be used
with the basic product.
· Is the medical equipment connected to other equipment, such as
a computer or printer?
- Any other equipment must have IEC certification
(evaluated to IEC standard) or be part of the medical equipment evaluation.
- Does equipment have electrical Signal
Input/Output Parts (SIP/SOPs)?
- What could be connected to the SIP/SOPs?
- Computers and other IT equipment are considered
to have 50 V in Normal Condition, Mains in Single Fault Condition on
their data ports.
· Create insulation diagram (graphic illustration of the LOPs)
· Determine classifications from the standard
· Document components that cross all barriers per insulation diagram
· Verify creepage and clearance spacing requirements, per the
insulation diagram (printed wiring boards, transformers, relays, etc.)
· Examine enclosure openings
- IEC test finger (access to live parts)
- IEC test pin (top openings to live parts)
- Need for a tool to access any live parts
· Determine potential mechanical hazards, pinch points, sharp edges
· Determine potential hazards under abnormal use
· Document components that must meet a nationally recognized
standard, such as ANSI in the US:
- Primary circuit components (including wiring),
up to mains transformer(s)
- Lithium batteries (also requires reverse charge
protection circuitry)
- CRTs > 5 inches
- Printed wiring boards with > 15 W available
- Wiring/tubing with > 15 W available
- Optical isolators with > 15 W available
and/or acting as barrier per insulation diagram
- Conductive coating process
· Verify that component ratings meet the equipment's ratings
· List enclosure materials
- UL 94 flame rating requirements for polymeric
enclosures if there is > 15 W available power in the enclosure
- V-2 min. for mobile, portable equipment
- V-0 min. for fixed or stationary equipment
· Verify mains fuse requirements (equipment or wall plug-in
power supply):
- Class I: Line and neutral
- Class II with functional earth: Line and Neutral
- Class II: Line only
- Permanently installed equipment: Line only
· Verify protective earth conductors are green with yellow stripe
· Verify wires secured from hazardous movements
· Verify equipment marking requirements (labels)
· Verify accompanying document requirements
· Provide illustrations of equipment, complete with all
accessories, showing critical components (digital * .JPG files)
Once this information is developed, the safety evaluation of the
equipment can be initiated. One or more samples are required,
depending on the equipment type and time requirements. Multiple
samples of components may be needed to perform destructive tests
(transformers, relays, plastic enclosures, motors etc.). For
medical equipment, it may be advantageous to conduct a preliminary
evaluation at the manufacturer's facilities. This allows for more
expedient changes to the equipment if there are construction or
test-related noncompliances.
The following are the steps for a typical evaluation of medical
electrical equipment:
· Review the information previously identified
· Evaluate construction of equipment
· Perform required testing
- Electrical, Mechanical, Temperature, Abnormal
Condition Testing, etc.
· Software evaluation (IEC60601-1-4) + (ISO/IEC12207) +
(ANSI/UL1998, 2nd Edition)
- Required if mitigating fire, shock, mechanical
hazards, or requirements of particular standard(s)
· Electromagnetic Compatibility (EMC) testing (optional for UL Classification)
- Per collateral standard (IEC60601-1-2)
· Review of biocompatibility documentation on patient contact
parts (optional for UL Classification)
- Per ISO 10993-1 standard
· Develop critical component list
- If it affects test results or requirements in
the standard, it's a critical component
- May be design specifications (spacings, colors, etc.)
· Prepare the technical report and optionally, the Informative
Test Report
- Preferred document for CE Marking technical files
THE PROCESS OF DOCUMENTATION
The documentation developed as a result of a safety
evaluation depends on the manufacturer's requirements. The three
common types of documentation are a UL report, an Informative Test
Report, and a Certified Body (CB) report. A UL report (consisting of
a product description and test report) authorizes the manufacturer to
apply the UL/C-UL (US/Canada) Mark to products covered in the report.
It describes the equipment evaluated and its critical components. UL
conducts quarterly audits using this report to verify that the
equipment bearing the UL/C-UL Mark is the same as the equipment that
was tested. An Informative Test Report is a complete documentation of
all the requirements in a standard (N/A, Pass, or Fail), a test
record, insulation diagram, illustrations, equipment markings, and
other applicable information. It is the preferred document for MDD
technical files (required for CE marking) and required by some
international hospitals and clinics for equipment purchases. A CB
report, or National Certification Body report, is similar to an
Informative Test Report, but also contains a certificate from the
issuer, who is required to be a member of the IECEE CB Scheme. A CB
report is used to obtain certification marks in different countries
without repeating the full evaluation of the equipment. The
Informative Test Report or CB Report are very important to have for
the equipment's technical files, and act as an international
"passports" for a device.
COMMON NONCOMPLIANCES
There are many common noncompliances that could have
been easily avoided had the designers been aware of the safety
standard requirements early in the design phase. The most
common noncompliance item relates to the accompanying documents.
All the '601 standards have very specific requirements for
inclusions in the accompanying documents. Since most companies
have separate departments that create these documents, they are often
not aware of the requirements. The next most common (and likely
the most costly) noncompliance is the power supply selection.
The best advice is to use a UL 2601-1 Recognized power supply
(evaluated to UL2601-1 by UL). By doing this, compliance with
spacing, leakage current, and mains component requirements is
assured. Also, the cost to evaluate the power supply and the
required UL quarterly inspections at the power supply manufacturer is
avoided. Many designers begin with a non-UL recognized power
supply, only to change to a UL recognized one when they discover the
associated costs of using a non-recognized power supply, or when they
realize that it does not comply with the requirements. When
designing medical equipment, it is also important to be aware that
there are minimum spacing requirements for electrical barriers.
Inadequate spacings on circuit boards are another typical
mistake. An example of this is DC-DC converters. Nearly
all DC-DC converters, including UL recognized models, do not provide
the spacing or insulation barrier required by these standards.
Make sure you get the specifications on the spacings or barriers from
input to output. For equipment with plastic enclosures, there
will also be flammability requirements for the plastic material.
Make sure the plastic chosen for the enclosure has at least a UL
Recognized V-0 flame rating for fixed equipment, and a UL Recognized
V-2 flame rating for all other types of equipment. The last
typical mistake relates to indicator lights. Red indicator
lights can only be used for a warning, yellow for caution. Keep
this in mind when selecting LEDs for any indicator lights.
These common noncompliances can be easily avoided with knowledge of
the applicable standards and they are the major reasons that
preliminary investigations of medical equipment are routinely done in
the early design phase.
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