Chapter 8: Equipment Construction

Medical Device Directive
IEC60601 International Standard
Isolation and Leakage Currents
Dielectric, Creepage and Clearance
CE Marking
Electromagnetic Compatibility
Emissions and Immunity Standards
Wired versus Wireless
System-level Integration
Grounding and Isolation Considerations
Low Power Design
Circuit Design Methods, PCB Ground Traces, Star Grounding
Wire and Cable Inductance, Capacitance and Resistance Calculators

Medical Device Directive

The European Medical Device Directive, MDD 93/42/EEC, has been in place since June 14, 1998. This Directive establishes the essential requirements and harmonized standards for the manufacture, design, and packaging of medical devices. Directive 2007/47/EC of the European Parliament and of the Council, amended MDD 93/42/EEC on September 5th, 2007. As per this amendment, a “medical device” means any instrument, apparatus, appliance, software, material or other article, whether used alone or in combination, including the software intended by its manufacturer to be used specifically for diagnostic and/or therapeutic purposes and necessary for its proper application, intended by the manufacturer to be used for human beings for the purpose of:

• diagnosis, prevention, monitoring, treatment or alleviation of disease
• diagnosis, monitoring, treatment, alleviation of or compensation for an injury or handicap
• investigation, replacement or modification of the anatomy or of a physiological process
• control of conception

and which does not achieve its principal intended action in or on the human body by pharmacological, immunological or metabolic means, but which may be assisted in its function by such means.

IEC60601-1 International Standard

The IEC60601-1 international standard describes the requirements to satisfy the accepted standards for medical equipment capability and construction. The European Medical Device Directive (MDD) and the United States Food and Drug Administration (FDA) both consider IEC60601-1 to be a consensus or harmonized standard. However, compliance with IEC 60601-1, and/or the relevant national version, does not equal medical device approval. Nevertheless, compliance with IEC60601-1 is a recognized step towards medical device approval in nearly all markets across the world and is an essential requirement in most markets for: product registration; safety marking – such as CE, CSA or UL; contracts; and defense again claims. The guiding philosophy of the IEC60601-1 harmonized standard is that equipment must be safe in both normal operating conditions (NOC) and single fault conditions (SFC).

Definitions:

1. Applied Part – the part of the equipment that connects to the subject

2. Basic Insulation – a physical insulation barrier

3. Supplemental Insulation – a physical insulation barrier

4. Double Insulation – Basic Insulation combined with Supplemental Insulation

5. Reinforced Insulation – a redundant physical insulation barrier

6. Creepage – the spacing between conductors along a surface (typically a PCB)

7. Clearance – the spacing between conductors separated by air

8. Protective Impedance – is a circuit component that provides protection

9. Protective Earth – is a mains-grounded part that provides protection

10. Class I Equipment – uses a Protective Earth

11. Class II Equipment – does not use a Protective Earth, but relies on Double Isolation, for protection

Isolation and Leakage Currents

The IEC60601-1 international standard incorporates a host of requirements related the the idea of electrical isolation. Power distribution systems, worldwide, terminate in two types of generally and commonly accessible electrical power. Countries have adopted either 120 VAC @ 60 Hz or 240 VAC @ 50 Hz mains power, as that which is commonly accessible in homes, offices and laboratories.

Primarily, as related to isolation, IEC60601-1 is concerned with establishing safety barriers between mains power and the subject. Because mains power is high voltage and supports substantial electrical current flow, mains power can be lethal if coupled to a subject. Accordingly, IEC60601-1 indicates the required level of isolation if a subject is to be connected to some equipment which is mains powered.

The IEC60601-1 isolation requirements specify the maximum current flows between mains power and subject for a variety of test cases, subject to both direct and alternating currents (DC and AC). The test cases cover the equipment in normal operating conditions (NOC) and also under single fault conditions (SFC). Under a SFC, the allowable current flows are the same or higher then they would be for normal conditions.

Dielectric, Creepage and Clearance

The dielectric, creepage and clearance specifications associated with the IEC60601-1 international standard are an important part of the isolation requirement. These specifications are concerned with equipment construction methods which directly support the required isolation. Accordingly, not only is it important that currents are limited to some level, but that the equipment be designed to satisfy other physical requirements to further support the required level of isolation. The dielectric, creepage and clearance specifications are concerned with choices of physical insulators, such as air, fiberboard, plastics or ceramics and the physical location of isolated electrical conductors associated with those insulators.

CE Marking

The French phrase “Conformite Europe”, translated as “European Conformity”, are abbreviated by the letters “CE”. CE marking on a product is a manufacturer’s declaration that the product complies with the essential requirements of relevant European health, safety and environmental protection laws, as defined by the product directives. The product directives contain the essential requirements, performance levels and harmonized standards to which the products must conform. Harmonized standards are technical specifications, established by a recognized European Standards Organization, that provide descriptions for attaining compliance.

A Declaration of Conformity (DoC) is an essential aspect of CE marking. A DoC is a document in which the manufacturer verifies that the product satisfies the essential requirements of the applicable product directives. A DoC is issued after completion of all the relevant conformity assessment procedures and must be realized before the product is placed on the market.

The CE marking procedure is generally one of self-certification. The only situations where it’s not possible for the manufacturer to self-certify are when the relevant product directives state that a Notified Body must be involved in the assessment of the products. Notified Bodies are organizations, designated by governments of member states, that are determined competent to make independent judgments about a product’s compliance with the essential safety requirements defined by the product directives.

Electromagnetic Compatibility

Changing voltages, in a conductor, create an electric wave. Changing currents, in a conductor, create a magnetic wave. After traveling for a distance, of approximately one-sixth of their wavelength, both electric and magnetic waves turn into electromagnetic waves. A electromagnetic wave is a combined electric and magnetic wave that travels with a specific ratio in their intensities. This ratio is named the “wave impedance”. As the electromagnetic wave travels, the wave impedance varies in accordance to the travel medium.

As the electromagnetic waves passes through the medium, the electric wave creates a magnetic wave and the magnetic wave creates an electric wave. If the electromagnetic wave is traveling through vacuum, or air, the electric wave intensity (E – electric wave field strength) divided by the magnetic wave intensity (H -magnetic wave field strength) is 377 ohms.

Standardization

Under the General Agreement on Tariffs and Trade (GATT) and its successor, the World Trade Organization (WTO), member countries are obliged to adopt international standards for national use wherever possible. International standards concerning EMC are primarily developed by the International Electrotechnical Commission (IEC) and the International Special Committee on Radio Interference (CISPR). The new extensive series developed by IEC includes

IEC 61000-1 Introduction, terms, and conditions
IEC 61000-2 Classification of electromagnetic environments
IEC 61000-3 Limits and disturbance levels
IEC 61000-4 Testing and measurement techniques
IEC 61000-5 Installation and mitigation guidelines
IEC 61000-6 Generic standards

Electromagnetic Compatibility Standards – three types

Basic Standards – General conditions and fundamental rules for meeting the EMC requirements. Basic EMC standards serve as building blocks for the IEC technical committees that develop EMC Product standards. Examples are contained in the 61000-4 series of EMC Standards.

Generic Standards – Environmentally specific standards which describe the EMC requirements for industrial and residential environments. Generic EMC standards specify a limited number of essential emission and immunity tests, as well as minimum test levels. Generic EMC standards refer to Basic EMC standards for detailed measurement and test methods. Examples are the 61000-6 series of EMC Standards.

Product Standards – Standards for the EMC requirements relating to specific products or types of products. Product standards are coordinated with generic standards. When no product standard exist, then the generic standard is used.

EMC Emissions and Immunity Standards

Medical equipment EMC Standards are specified by IEC60601-1-2. This standard references CISPR 11 for emissions and a variety of IEC 61000-4-X standards for immunity.

Generic EMC Standards

IEC 61000-6-1 (Immunity) and IEC 61000-6-3 (Emissions)
Immunity and Emissions standards for residential, commercial and light-industrial environments.

CISPR 22/ EN 55022 – Class B
The scope of CISPR 22 applies to information technology equipment (ITE). CISPR 22 defines procedures for the measurement of spurious signals generated by the ITE.

IEC 61000-6-2 (Immunity) and 61000-6-4 (Emissions)
Immunity and Emission standards for industrial environments.

CISPR 11/ EN 55011 – Class A
The scope of CISPR 11 applies to electrical equipment intended for industrial, scientific or medical purposes. Equipment usage environments, which would fall under this category, include: standard laboratories, test and measurement facilities or areas that are specifically used for analysis, calibrating, repairing or testing. CISPR 11 defines procedures for the measurement of spurious signals generated by relevant equipment.

Conducted emissions: 0.15 – 30 MHz
Measures unintentional emissions conducted from the product, to the AC power mains, in normal operating mode.

Radiated E-field emissions: 30-1000 MHz
Measures unintentional E-field emissions from the product, to the environment, in normal operating mode.

Generic EMC Immunity Standards

Electrostatic Discharge (ESD): IEC 61000-4-2
Performed to determine immunity of product to ESD. The IEC 61000-4-2 international standard specifies a desired nature of immunity of electronic equipment to electrostatic discharge (ESD). IEC61000-4-2 is the most commonly used ESD testing standard for most electronic product categories.

Radiated RF Immunity: IEC 61000-4-3
Performed to determine immunity of product to fields generated by intentional transmitters (AM radio, TV, cell, etc.)

Electrical Fast Transient/Burst: IEC 61000-4-4
Performed to determine immunity of product to switching and transient noise.

Surge Immunity: IEC 61000-4-5
Performed to determine immunity of product to switching and lightning-induced transients.

Conducted RF Immunity: IEC 61000-4-6
Performed to determine immunity of product to low frequency fields generated by intentional transmitters (AM radio, TV, cell, etc.)

Voltage Dips and Interruptions: IEC 61000-4-11
Performed to determine immunity of product to fluctuations on AC power input.

Wired versus Wireless

Wireless medical devices have many advantages over wired devices with equivalent subject-measuring capability. The primary advantage is the range of motion provided by a wireless link. A secondary advantage is the intrinsic subject electrical safety provided by a wireless, battery-powered, monitoring or controlling system.

Wireless, short and medium-range, medical device types:

1. Radio-Frequency Implants: body-implanted, typically inductive, low-frequency (less than 1MHz), passive devices that communicate, to an external terminal, over short distances (less than 25 cm).

2. Medical Body Area Network Devices: body implanted or externally-worn, active controllers or sensors that operate over a personal area, high frequency (2360-2400 MHz), network consisting of multiple body sensors, over a short distance (less than a meter).

3. Medical Micro-power Network Devices: body implanted, active, electrical stimulators for the purposes of mobility restoration for previously paralyzed subjects. These medium frequency (413-457 MHz) devices communicate over short distances (less than a meter).

4. Medical Radio-communication Device Implants: body implanted, medium frequency (401-406 MHz), active devices that communicate to an external terminal over short distances (less than 3 meters).

5. Bluetooth, WiFi and Zigbee Devices: body implanted or externally-worn, active devices, high frequency (902-928 MHz, 2400-2483 MHz and 5725-5850 MHz) that communicate over unlicensed bands with phones, tablets and personal computers over medium distances (less than 100 meters).

6. Ultra-Wideband Medical Devices: body implanted or externally-worn, active devices that communicate over wide bandwidths, to an external terminal, using low power over short distances (less than a meter).

System-level Integration

Any medical device requires consideration of system-level integration requirements. Example areas to consider include:

1. The extent and nature of other equipment that may be simultaneously attached to the subject

2. The extent and nature of other equipment that communicates with the primary equipment

3. Mains power supply voltage range and frequency

4. Environmental operating conditions

Grounding and Isolation Considerations

Medical equipment for subject monitoring is categorized on the basis of how it is designed to connect to the subject via the Applied Part. There are several types of subject Applied Parts:

Type B Applied Part:

A part that is mains-ground referenced. Type B Applied Parts are not suitable for direct cardiac application.

Type BF Applied Part

A part that has a higher degree of protection, against electric shock, than a Type B Applied Part. Type BF Applied Parts are electrically isolated (floating) from mains-ground and other parts of the medical equipment. Type BF Applied Parts are not suitable for direct cardiac application.

Type CF Applied Part

A part that has a higher degree of protection, against electric shock, than a Type BF Applied Part. Type CF Applied Parts are electrically isolated (floating) from mains-ground and other parts of the medical equipment. Type CF Applied Parts are suitable for direct cardiac application.

Type BF and CF equipment, designed to IEC60601-1, incorporate two or more separate grounds and several isolation barriers. Primary isolation occurs between mains power and the equipment’s accessible conductive parts, including computer communications ports. The equipment’s accessible conductive parts are typically connected to mains ground. The floating applied part, in BF and CF equipment, has a ground reference that is galvanically isolated from mains ground. Communication between circuits referenced to different grounds occurs over optical, capacitive or transformer coupled links. Power distribution typically occurs over transformer coupled links, such as DC-DC converters.

Low Power Design

Low power design solutions are nearly always more optimal than higher power circuit design. Lower power means lower supply current, lower heat dissipation and reduced conducted and radiated electromagnetic interference (EMI). Trade-offs in choosing lower versus higher current designs tend to involve the following issues:

Analog considerations – higher current consumption typically increases

bandwidth
slew rate
common-mode rejection ratio
power supply rejection ratio
noise performance
with supply voltage

Digital considerations – higher current consumption typically increases

clocking speed
drive capability
with supply voltage

Circuit Design Methods, PCB Ground Traces, Star Grounding

PCB traces have three electrical circuit qualities, resistance, capacitance and inductance.
PCB Trace Characteristics –

Thicker
Lower resistance
Unchanged capacitance and inductance

Wider
Lower resistance
Higher capacitance
Lower inductance

Narrower
Higher resistance
Lower capacitance
Higher inductance

When designing mixed signal printed circuit boards (PCBs), it’s of primary importance to be aware of the flow of current. From any given power source, the source current will equal the return current. Current flowing through PCB traces will generate associated voltage drops, because of the PCB trace’s resistance. Also, switching currents through traces will generate voltage spikes along the traces because of the trace inductance, subject to V = L di/dt. If the PCB trace is running over a ground plane, then voltage changes on the trace will generate current spikes from trace to plane, subject to I = C dv/dt. To stabilize power supply voltages at the point of the functional block, relatively high-valued capacitance is placed between source and return on the functional block. The capacitance acts in concert with the series resistance and inductance of the source and return traces to establish a lowpass Pi filter between power supply and functional block.

High performance, mixed signal, PCBs incorporate separate current source and return lines that tie back to the power supply output and ground, respectively, for each powered functional block on the board. The resistance and inductance qualities of PCB traces can be used to advantage when isolating for power supply and functional block noise. Sometimes, the current return path of all functional blocks is replaced by a ground plane, but this action is not always helpful, as the flow of current in a ground plane is not particularly well-defined when a superposition of return currents are flowing. Accordingly, a mixed signal PCB with a ground plane can exhibit spurious effects, depending on the current requirements of any particular functional block. Optimal isolation between blocks (especially between analog and digital blocks) is best managed by providing separate current source and return PCB traces for each block. In this circumstance, the current demands from any functional block do not interfere with the supply voltages applied to other blocks.

If it’s not practical to send separate source and return traces to each functional block, then the next best strategy is to consider general source and return PCB trace “trees”. The trunk of the tree is at the power supply and the traces increasingly branch out to feed the current requirements of each functional block. Near the power supply, the source and return traces are more capacitive and have reduced resistance and inductance. As the traces fan out, near the peripheral blocks that perform primary, low-level, amplification, the source and return current traces become less capacitive and have increased inductance and resistance. This power distribution approach minimizes coupling capacitance between sensitive, high impedance, inputs and low impedance, noisy, power supply lines.

Wire and Cable Inductance, Capacitance and Resistance Calculators

Inductance Calculators

At lower frequencies, wire inductance consists of the combination of inductances both internal and external to the structure of the wire. The low frequency inductance of a straight wire is:

L = 2 * l * [ln(2 * l/ r) – 0.75] nH

At higher frequencies, skin effects in the wire cause the internal inductance to go to zero and so the high frequency inductance becomes:

L = 2 * l * [ln(2 * l/r) – 1.00] nH

Where L is the inductance in nanohenries (nH), l is the length of the wire in cm, and r is the radius of the wire in cm. This calculation is based on the assumption that the length “l” is greater than radius “r” by a factor of 10 or more.

Capacitance Calculators

C = 2 * Pi * L * E / (log (Ra / Rb)

Where E is permittivity of insulator (pF / meter), L is cable length (meters), Ra is radius of cable (mm), Rb is diameter of center conductor (mm), C is capacitance per unit meter.

Resistance Calculators

R = (rho * L) / A

Where rho is the resistivity of the conductor (ohms * m), L is the cable length (meters), A is the conductor cross-sectional area (meters^2).