safety testing of medical electrical equipment with and problems

7/28/2019 Safety Testing of Medical Electrical Equipment With and Problems

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Safety in Biomedical Engineering (KUEU 4130) Prof. Ir. Dr Fatimah Ibrahim

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CHAPTER 1 INTRODUCTION

1.1 What are standards?

The formal definition of a standard that should be adopted in the medical devicedomain is given by the ISO:

Standards are documented agreements containing technical specifications orother precise criteria to be used consistently as rules, guidelines or definitions ofcharacteristics, to ensure that materials, products, process and services are fit fortheir purpose.

Types of specifications in standards

Standards can establish a wide range of specifications for products, processesand services (see www.iso.org for definitions).1. Prescriptive specifications obligate product characteristics, e.g. devicedimensions, biomaterials, test or calibration procedures, as well as definitions ofterms and terminologies.

2. Design specifications set out the specific design or technical characteristics ofa product, e.g. operating room facilities or medical gas systems.

3. Performance specifications ensure that a product meets a prescribed test, e.g.strength requirements, measurement accuracy, battery capacity, or maximumdefibrillator energy.

4. Management specifications set out requirements for the processes andprocedures companies put in place, e.g. quality systems for manufacturing orenvironmental management systems.

A standard may contain a combination of specifications. Prescriptive, design andperformance specifications have been commonplace in standards. Managementspecifications are also rapidly gaining prominence.

Recent years have seen the development and application of what are known asgeneric management system standards, where generic means that thestandards requirements can be applied to any organization, regardless of theproduct it makes or the service it delivers, and management system refers towhat the organization does to manage its processes.Two of the most widely known series of generic management system standardsare the ISO 9000 series for managing quality systems, and the ISO 14000 series


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for environmental management systems. Wide ranging information andassistance related to these standards and their application is available atwww.iso.org. ISO13485 and ISO13488 are specific ISO quality systemsstandards for medical device manufacturing.

Terms such as outcome-oriented standards, objectives standards, function-focused standards and result-oriented standards are also employed. Essentially,these terms indicate that the standards specify the objectives (ends) to beachieved while leaving the methods (means) to the implementers. This canminimize possible constrictive effects of standards.

1. 2 Why do we need standards?

Standards can serve different purposes. They can:

1. Provide reference criteria that a product, process or service must meet.

2. Provide information that enhances safety, reliability and performance ofproducts, processes and services.

3. Assure consumers about reliability or other characteristics of goods or servicesprovided in the marketplace.

4. Give consumers more choice by allowing one firms products to be substitutedfor, or combined with, those of another. Although we take for granted theadvantage of being able to order shoes or clothes simply by referring to a size,this is only possible because manufacturers follow some industrial standards inmaking shoes and clothes. In contrast, incompatibility between electrical plugsand receptacles is a prime example of different countries failing to follow thesame standards. When North Americans want to use a portable computer orother electrical appliance in Europe or Asia, they can be frustrated to find that theplug and voltage are not compatible. With the world becoming a global village,the need and benefits of standardization are becoming more and more importantinternationally for manufacturing, trade and communications. Quality systemsand other management standards can provide common references to the kind ofprocess, service or management practice expected. The Internet functionseffectively because globally agreed-upon interconnection protocols exist. Globalcommunication would be very difficult without international standardization.

Health care workers are well aware of incompatible consumables or replacementpartsin medical devices of similar function that are made by differentmanufacturers (e.g. IV set, X-ray cassettes). The lack of available consumablesand repair parts is an importantcause of medical equipment problems that are constantly encountered indeveloping countries. Most medical devices are used globally. The safety,performance and consistent quality of medical devices is, therefore, an


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international public health interest. Thus, global harmonization of medicaldevice standards and regulations is critical.

1.3 Voluntary and mandatory standards

Most standards are voluntary. However, a standard may be mandated by acompany, professional society, industry, government or trade agreement. Astandard may be called a regulation when it becomes mandatory. This mandatemay, or may not, have a legal basis.

When a standard is mandated by a government or an international tradeagreement, it normally becomes legally obligatory based on regulations or a lawestablished by the government or the contracts between international bodies.Countries that are considering making standards mandatory should take intoaccount the potential consequences under international agreements on technicalbarriers to trade.

1.4 Standards development process

Figure 1 provides an example of the many steps used by standards developmentorganizations (see www.iso.org for ISOs six-step process in the development ofinternational standards). In general, good standards have the following attributes:

1. Their development has been overseen by a recognized body, thus ensuringthat the process is transparent and not dominated by vested interests.

2. The development process has been open to input from all interested partiesand the resulting document based on consensus. Consensus, in a practicalsense, means that significant agreement among the stakeholders is reached inthe preparation of the standard, including steps taken to resolve allobjections.This process implies more than the votes of a majority, but notnecessarily unanimity.

3. Good technical standards are based on consolidated results of science,technology and experience, and are aimed at the promotion of optimumcommunity benefits.

4. Standards do not hinder innovations and must be periodically reviewed toremain in tune with technological advances.


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Figure 1. Typical standard for process development

1.5 Conformity assessment with standards

There are four common industrial methods for assessing conformity to astandard.

1. A products conformity to standards is commonly assessed by direct test ing.2. A process can be assessed by audit. Certification organizations or regulatoryauthorities attest that products or processes conform to a standard by authorizingthe display of theircerti f icationmark.


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3. The conformity to management standard by an organization is known asmanagement systems registrat ion, a relatively new term used primarily in North

America. Formally established audit procedures are followed by certified auditorswho are supported by technical experts of the domain under audit. Management

System Registration bodies (Registrars) issue registration certificates tocompanies that meet a management standard such as ISO9000, or to medicaldevice manufacturers that meet the ISO13485/ISO9001 standards.Note that in North America, the term registration is used for an organizationwhile certification is reserved for products. Many other countries usecertification forboth a product and an organization.

4. Accredi tat ionis used by an authoritative body to give formal recognition thatan organization or a person is competent to carry out a specific task. Forexample, in Europe, Notified Bodies are notified or accredited by the relevantState Competent Authority to carry out conformity assessment of medical

devices. In Canada, a Quality System Registrar needs an accreditation fromHealth Canada before that Registrar begins assessing medical devicemanufacturers for conformity with quality system standards. The InternationalLaboratory Accreditation Cooperation (ILAC) uses accreditation to provide formalrecognition to competent laboratories around the world.

1.6 National and international standards systems

A country may have many voluntary standards bodies. However, normally thereis one official national organization that coordinates and accredits the standardsdevelopment bodies in the country. This official national organization would havethe authority to endorse a document as a national standard in accordance withofficial criteria, and it also represents the country in the various internationalstandards organizations. In the United States, the American National StandardsInstitute (ANSI), a private, non-profit organization, is an official nationalorganization. In Canada, it is the Standards Council of Canada (SCC), a crown(government) corporation. In Europe there is a committee composed of CEN(Comit Europen de Normalisation), CENELEC (the European Committee forElectrotechnical Standardization) and ETSI (the European TelecommunicationStandards Institute) that supercedes the various European national standardsbodies that were in place previously.

For developing countries, reference to a standards system not only helps medicaldevice administration, it is also important for other industrial and economicdevelopments. International development agencies increasingly realize that astandardized infrastructure is a basic requirement for the success of economicpolicies that will improve productivity, market competitiveness and exportcapability.

The three major international standardization organizations are the International


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Organization for Standardization (ISO), the International ElectrotechnicalCommission (IEC), and the International Telecommunication Union (ITU).Generally, ITU covers telecommunications, IEC covers electrical and electronicengineering, and ISO covers the remainder. For information technology, riskmanagement, quality systems and many other areas, joint ISO/IEC technical

committees manage standardization.

Other organizations also produce documents on international standardization.Their documents are usually adopted by ISO/IEC/ITU as international standardsif they have been developed in accordance with international consensus criteria.

Any grouping of five member countries can also propose a standard to beconsidered by ISO for adoption as an international standard. Useful web sitesinclude: www.iso.ch, www.IEC.ch, and www.itu.int/ for the ISO, IEC and ITUrespectively. From here, links to national or regional standard organizations areindicated.

1.7 Identification of standards

Standards are generally designated by an alphabetical prefix and a number. Theletters (e.g. ISO, IEC, ANSI, CAN, EN, DIN) indicate the body that has approvedthem, while the numbers identify the specific standard and the year in which itwas finalized. The standard reference code often gives an indication of adoptionwhere standards are equivalent. For example:1. CAN/CSA-Z386-94 means a standard developed in 1994 by the CanadianStandards Association (CSA, one of four accredited Canadian standardsdevelopment organizations) and designated by the Standards Council of Canada(SCC) as a Canadian national standard.

2. ANSI/AAMI/ISO 15223:2000 means the international standard ISO 15223(established in 2000) adopted by the Association for the Advancement of MedicalInstrumentations in the United States, which in turn is designated by the

American National Standards Institute (ANSI) as an American national standard.

3. UNI EN ISO 9001 indicates an Italian national standard (UNI) which is anadoption of a European standard (EN), which is itself an adoption of theInternational Standard ISO9001.

1.8 Current trends in the use of standards in medical device regulations

Although a standard can be set and mandated by an authority, the current trendis for the adoption of voluntary standards established by consensus from allinterested parties (the stakeholders). The use of voluntary standards originatedfrom the realization that while regulations generally address the essential safetyand performance principles, manufacturers and users still need to know detailedspecifications pertaining to specific products. The provision of such specificationsand detailed requirements for the multitude of devices presents an enormous


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task for regulatory authorities. Fortunately, the wealth of voluntary standardsalready existing or being developed provide such precise specifications. The useof voluntary/consensus standards has many advantages including the following:

1. They are normally developed by experts with access to the vast resources

available in the professional and industrial communities.

2. By taking advantage of such existing resources, the government canovercome its own limited resources for providing product specific technicalrequirements and characteristics.

3. Conformity to standards can also be assessed by an accredited third party(such as a notified body in Europe), which is a well-established industrial practicearound the world.

4. The use of international standards facilitates harmonized regulatory processes

and world trade, and thus improves global access to new technology.

5. As technology advances, it is much easier to update standards than to changeregulations. Timely development and periodic revision by expert groups makemedical device standards effective and efficient tools for supporting health care.

6. Manufacturers have the flexibility to choose appropriate standards or othermeans to demonstrate compliance with regulatory requirements.

Regulatory authorities can recognize a standard, fully or partially, provided theyclearly specify and publicize their intent. Several standards can also berecognized as a group to satisfy the requirements for a particular device. In somecountries, the publication of government-recognized standards mandates productcompliance.

Medical devices intended for global use should follow international standards.For example, the ISO Technical Report (ISO 16142:2000) lists a number ofsignificant international standards that may be suitable for demonstratingcompliance with certain features of the essential principles of safety andperformance of medical devices.

The GHTF has issued the following recommendations regarding the recognitionand use of standards:

International standards are a building block for harmonized regulatory processesto assure the safety, quality and performance of medical devices. To achieve thispurpose, the following principles are recommended:

Regulatory Authorities and industry should encourage and support thedevelopment of international standards for medical devices to demonstrate


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compliance with the Essential Principles of Safety and Performance of MedicalDevices (GHTF document SG1 NO20R5 referred to hereafter as the EssentialPrinciples).

Regulatory Authorities developing new medical device regulations should

encourage the use of international standards.

Regulatory Authorities should provide a mechanism for recognizinginternational standards to provide manufacturers with a method of demonstratingcompliance with the Essential Principles.

When an international standard is not applied or not applied in full, this isacceptable if an appropriate level of compliance with the Essential Principles canbe demonstrated.

While it may be preferable for harmonization purposes to use international

standards, it may be appropriate for Regulatory Authorities to accept the use ofnational/regional standards or industry standards as a means of demonstratingcompliance.

Standards Bodies developing or revising standards for use with medical devicesshould consider the suitability of such standards for demonstrating compliancewith the Essential Principles and to identify which of the Essential Principles theysatisfy.

The use of standards should preferably reflect current, broadly applicabletechnology while not discouraging the use of new technologies.

Standards may represent the current state of the art in a technological field.However, not all devices, or elements of device safety and/or performance maybe addressed by recognized standards, especially for new types of devices andemerging technologies.


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CHAPTER 2 : SAFETY TESTING OF MEDICALEQUIPMENT

2.1 Hazards of Medical Electrical Equipment

Medical electrical equipment can present a range of hazards to the patient, theuser, or to service personnel. Many such hazards are common to many or alltypes of medical electrical equipment, whilst others are peculiar to particularcategories of equipment. Listed below are various types of common hazards.

2.1 Mechanical Hazards

All types of medical electrical equipment can present mechanical hazards. Thesecan range from insecure fittings of controls to loose fixings of wheels onequipment trolleys. The former may prevent a piece of life supporting equipmentfrom being operated properly, whilst the latter could cause serious accidents inthe clinical environment.

Such hazards may seem too obvious to warrant mentioning, but it isunfortunately all too common for such mundane problems to be overlooked whilemore exotic problems are addressed.

2.2 Risk of fire or explosion

All mains powered electrical equipment can present the risk of fire in the event of

certain faults occurring such as internal or external short circuits. In certainenvironments such fires may cause explosions. Although the use of explosiveanaesthetic gases is not common today, it should be recognised that many of themedical gases in use vigorously support combustion.

2.3 Absence of Function

Since many pieces of medical electrical equipment are life supporting or monitorvital functions, the absence of function of such a piece of equipment would not bemerely inconvenient, but could threaten life.

2.4 Excessive or insufficient output

In order to perform its desired function equipment must deliver its specifiedoutput. Too high an output, for example, in the case of surgical diathermy units,would clearly be hazardous. Equally, too low an output would result ininadequate therapy, which in turn may delay patient recovery, cause patientinjury or even death. This highlights the importance of correct calibrationprocedures.


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2.5 Infection

Medical equipment that has been inadequately decontaminated after use maycause infection through the transmission of microorganisms to any person whosubsequently comes into contact with it. Clearly, patients, nursing staff and

service personnel are potentially at risk here.

2.6 Misuse

Misuse of equipment is one of the most common causes of adverse incidentsinvolving medical devices. Such misuse may be a result of inadequate usertraining or of poor user instructions.

2.7 Risk of exposure to spurious electric currents

All electrical equipment has the potential to expose people to the risk of spurious

electric currents. In the case of medical electrical equipment, the risk ispotentially greater since patients are intentionally connected to such equipmentand may not benefit from the same natural protection factors that apply to peoplein other circumstances. Whilst all of the hazards listed are important, theprevention of many of them require methods peculiar to the particular type ofequipment under consideration. For example, in order to avoid the risk ofexcessive output of surgical diathermy units, knowledge of radio frequency powermeasurement techniques is required. However, the electrical hazards arecommon to all types of medical electrical equipment and can minimised by theuse of safety testing regimes which can be applied to all types of medicalelectrical equipment. For these reasons, it is the electrical hazards that are the

main topic of this session.


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CHAPTER 3 : PHYSIOLOGICAL EFFECTS OFELECTRICITY

3.1 Electrolysis

The movement of ions of opposite polarities in opposite directions through amedium is called electrolysis and can be made to occur by passing DC currentthrough body tissues or fluids. If a DC current is passed through body tissues fora period of minutes, ulceration begins to occur. Such ulcers, while not normallyfatal, can be painful and take long periods to heal.

3.2 Burns

When an electric current passes through any substance having electricalresistance, heat is produced. The amount of heat depends on the powerdissipated (I2R). Whether or not the heat produces a burn depends on thecurrent density.

Human tissue is capable of carrying electric current quite successfully. Skinnormally has a fairly high electrical resistance while the moist tissue underneaththe skin has a much lower resistance. Electrical burns often produce their mostmarked effects near to the skin, although it is fairly common for internal electricalburns to be produced, which, if not fatal, can cause long lasting and painfulinjury.

3.3 Muscle cramps

When an electrical stimulus is applied to a motor nerve or a muscle, the muscledoes exactly what it is designed to do in the presence of such a stimulus i.e. itcontracts. The prolonged involuntary contraction of muscles (tetanus) caused byan external electrical stimulus is responsible for the phenomenon where a personwho is holding an electrically live object can be unable to let go.

3.4 Respiratory arrestThe muscles between the ribs (intercostal muscles) need to repeatedly contractand relax in order to facilitate breathing. Prolonged tetanus of these muscles can

therefore prevent breathing.

3.5 Cardiac arrest

The heart is a muscular organ, which needs to be able to contract and relaxrepetitively in order to perform its function as a pump for the blood. Tetanus ofthe heart musculature will prevent the pumping process.


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3.6 Ventricular fibrillation

The ventricles of the heart are the chambers responsible for pumping blood outof the heart. When the heart is in ventricular fibrillation, the musculature of theventricles undergoes irregular, uncoordinated twitching resulting in no net blood

flow. The condition proves fatal if not corrected in a very short space of time.

Ventricular fibrillation can be triggered by very small electrical stimuli. A currentas low as 70 mA flowing from hand to hand across the chest, or 20A directlythrough the heart may be sufficient. It is for this reason that most deaths fromelectric shock are attributable to the occurrence of ventricular fibrillation.

3.7 Effect of frequency on neuro-muscular st imulation

The amount of current required to stimulate muscles is dependent to some extenton frequency. Referring to figure 1, it can be seen that the smallest current

required to prevent the release of an electrically live object occurs at a frequencyof around 50 Hz. Above 10 kHz the neuro-muscular response to currentdecreases almost exponentially.

Figure 1. Current required to prevent release of a live object.


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CHAPTER 4: LEAKAGE CURRENTS

4.1 Causes of leakage currents

If any conductor is raised to a potential above that of earth, some current isbound to flow from that conductor to earth. This is true even of conductors thatare well insulated from earth, since there is no such thing as perfect insulation orinfinite impedance. The amount of current that flows depends on:

a. the voltage on the conductor.

b. the capacitive reactance between the conductor and earth.

c. the resistance between the conductor and earth.

The currents that flow from or between conductors that are insulated from earthand from each other are called leakage currents, and are normally small.However, since the amount of current required to produce adverse physiologicaleffects is also small, such currents must be limited by the design of equipment tosafe values.

For medical electrical equipment, several different leakage currents are definedaccording to the paths that the currents take.

4.2 Earth leakage current

Earth leakage current is the current that normally flows in the earth conductor ofa protectively earthed piece of equipment. In medical electrical equipment, veryoften, the mains is connected to a transformer having an earthed screen. Most ofthe earth leakage current finds its way to earth via the impedance of theinsulation between the transformer primary and the inter-winding screen, sincethis is the point at which the insulation impedance is at its lowest (see figure 2).


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Figure 2. Earth leakage current path.

Under normal conditions, a person who is in contact with the earthed metalenclosure of the equipment and with another earthed object would suffer no

adverse effects even if a fairly large earth leakage current were to flow. This isbecause the impedance to earth from the enclosure is much lower through theprotective earth conductor than it is through the person. However, if theprotective earth conductor becomes open circuited, then the situation changes.Now, if the impedance between the transformer primary and the enclosure is ofthe same order of magnitude as the impedance between the enclosure and earththrough the person, then a shock hazard exists.

It is a fundamental safety requirement that in the event of a single fault occurring,such as the earth becoming open circuit, no hazard should exist. It is clear that inorder for this to be the case in the above example, the impedance between thetransformer primary and the enclosure needs to be high. This would beevidenced when the equipment is in the normal condition by a low earth leakagecurrent. In other words, if the earth leakage current is low then the risk of electricshock in the event of a fault is reduced.

4.3 Enclosure leakage current

Enclosure leakage current is defined as the current that flows from an exposedconductive part of the enclosure to earth through a conductor other than theprotective earth conductor. However, if a protective earth conductor is connectedto the enclosure, there is little point in attempting to measure the enclosureleakage current from another protectively earthed point on the enclosure sinceany measuring device used is effectively shorted out by the low resistance of theprotective earth. Equally, there is little point in measuring the enclosure leakagecurrent from a protectively earthed point on the enclosure with the protectiveearth open circuit, since this would give the same reading as measurement ofearth leakage current as described above. For these reasons, it is usual when


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testing medical electrical equipment to measure enclosure leakage current frompoints on the enclosure that are not intended to be protectively earthed (seefigure 3). On many pieces of equipment, no such points exist. This is not aproblem. The test is included in test regimes to cover the eventuality where suchpoints do exist and to ensure that no hazardous leakage currents will flow from

them.

Figure 3. Enclosure leakage current path.

4.4 Patient leakage current

Patient leakage current is the leakage current that flows through a patient

connected to an applied part or parts. It can either flow from the applied parts viathe patient to earth or from an external source of high potential via the patientand the applied parts to earth. Figures 4a and 4b illustrate the two scenarios.

Figure 4a. Patient leakage current path from equipment.


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Figure 4b. Patient leakage current path to equipment.

4.5 Patient auxiliary current

The patient auxiliary current is defined as the current that normally flows between

parts of the applied part through the patient, which is not intended to produce aphysiological effect (see figure 5).

Figure 5. Patient auxiliary current path.


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CHAPTER 5: CLASSES AND TYPE OF MEDICALELECTRICAL EQUIPMENT

All electrical equipment is categorised into classes according to the method ofprotection against electric shock that is used. For mains powered electricalequipment there are usually two levels of protection used, called "basic" and"supplementary" protection. The supplementary protection is intended to comeinto play in the event of failure of the basic protection.

5.1 Class I equipment

Class I equipment has a protective earth. The basic means of protection is theinsulation between live parts and exposed conductive parts such as the metalenclosure. In the event of a fault that would otherwise cause an exposedconductive part to become live, the supplementary protection (i.e. the protectiveearth) comes into effect. A large fault current flows from the mains part to earth

via the protective earth conductor, which causes a protective device (usually afuse) in the mains circuit to disconnect the equipment from the supply.

It is important to realise that not all equipment having an earth connection isnecessarily class I. The earth conductor may be for functional purposes onlysuch as screening. In this case the size of the conductor may not be largeenough to safely carry a fault current that would flow in the event of a mains shortto earth for the length of time required for the fuse to disconnect the supply.

Class I medical electrical equipment should have fuses at the equipment end ofthe mains supply lead in both the live and neutral conductors, so that the

supplementary protection is operative when the equipment is connected to anincorrectly wired socket outlet.

Further confusion can arise due to the use of plastic laminates for finishingequipment. A case that appears to be plastic does not necessarily indicate thatthe equipment is not class I.

There is no agreed symbol in use to indicate that equipment is class I. Whereany doubt exists, reference should be made to equipment manuals.

The symbols below may be seen on medical electrical equipment adjacent to

terminals.

Figure 6. Symbols seen on earthed equipment.


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5.2 Class II equipment

The method of protection against electric shock in the case of class II equipmentis either double insulation or reinforced insulation. In double insulated equipmentthe basic protection is afforded by the first layer of insulation. If the basic

protection fails then supplementary protection is provided by a second layer ofinsulation preventing contact with live parts.

Reinforced insulation is defined in standards as being a single layer of insulationoffering the same degree of protection as double insulation.

Class II medical electrical equipment should be fused at the equipment end ofthe supply lead in either mains conductor or in both conductors if the equipmenthas a functional earth.

The symbol for class II equipment is two concentric squares indicating double

insulation as shown below.

Figure 7. Symbol for class II equipment.

5.3 Class III equipment

Class III equipment is defined as that in which protection against electric shockrelies on the fact that no voltages higher than safety extra low voltage (SELV) arepresent. SELV is defined in turn in the relevant standard as a voltage notexceeding 25V ac or 60V dc.

In practice such equipment is either battery operated or supplied by a SELVtransformer.

If battery operated equipment is capable of being operated when connected tothe mains (for example, for battery charging) then it must be safety tested as

either class I or class II equipment. Similarly, equipment powered from a SELVtransformer should be tested in conjunction with the transformer as class I orclass II equipment as appropriate.

It is interesting to note that the current IEC standard relating to safety of medicalelectrical equipment does not recognise Class III equipment since limitation ofvoltage is not deemed sufficient to ensure safety of the patient.


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5.4 Equipment types

As described above, the class of equipment defines the method of protectionagainst electric shock. The degree of protection for medical electrical equipmentis defined by the type designation. The reason for the existence of type

designations is that different pieces of medical electrical equipment have differentareas of application and therefore different electrical safety requirements. Forexample, it would not be necessary to make a particular piece medical electricalequipment safe enough for direct cardiac connection if there is no possibility ofthis situation arising.

Table 1 shows the symbols and definitions for each type classification of medicalelectrical equipment.

Type Symbol Definition

BEquipment providing a particular degree of protection againstelectric shock, particularly regarding allowable leakage currentsand reliability of the protective earth connection (if present).

BFAs type B but with isolated or floating (F - type) applied part orparts.

CFEquipment providing a higher degree of protection againstelectric shock than type BF, particularly with regard to allowableleakage currents, and having floating applied parts.

Table 1. Medical electrical equipment types

All medical electrical equipment should be marked by the manufacturer with oneof the type symbols above.


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CHAPTER 6: TESTING REGIMES AND RELEVANT DOCUMENTATION

6.1 Type tests and routine tests

Before discussing the documentation relevant to electrical safety of medicalelectrical equipment, it is important to distinguish between "type tests" androutine tests.

Standards for the manufacture of equipment normally detail tests which areintended to be carried out on a single representative sample of a piece ofequipment for which certification of compliance with a standard is being sought.Such tests are carried out by approved test houses under tightly specifiedenvironmental conditions. These tests are called "type tests" and are not

intended for routine use. Indeed, repetition of many of the tests would certainlycause deterioration in performance and safety of the equipment under test.

Routine tests have an entirely different purpose than that for type tests. Routinetests are intended to provide good indicators to the safety of equipment withoutsubjecting it to undue stress that would be liable to cause deterioration.

6.2 HTM 8

In 1963 the Department of Health and Social Security published HospitalTechnical Memorandum number 8 called "safety code for electro-medical

apparatus". The purpose of the document was to establish adequate standardsfor the design and construction of electro-medical apparatus since no otherrelevant national standard existed at the time. Although the document wasproduced essentially for the guidance of manufacturers, biomedical departmentsin hospitals were quick to adopt tests from the document for the basis of theirown medical electrical equipment safety testing regimes. Although tests detailedin the code were type tests, many of them could be fairly easily be repeatedwithout adverse effects on the equipment as routine tests. Performance of theelectrical safety tests was made easier by the development of specialisedmedical equipment safety testers, specifically, the Liverpool tester.

6.3 BS 5724 or IEC 60601

In 1979, BS 5724 part 1 was published. This document is a comprehensivespecification for safety of medical electrical equipment. Part 1 covers the generalrequirements, i.e requirements common to all medical electrical equipmentregardless of function. A series of part 2's detailing particular requirements forspecific categories of medical electrical equipment followed publication of part 1.


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BS 5724 is a far more detailed document than HTM 8, which it replaced. Like theHTM, the tests contained in the standard are type tests. Some guidance wasgiven in the 1979 edition of the standard on recommended testing duringmanufacture and/or installation. Unfortunately, some routine test regimes basedon BS 5724 tended to be too rigorous for such application and in some cases

caused damage to equipment.

BS 5724 part 1 was revised in 1989, making it identical to the InternationalElectro-technical Committee standard IEC 601 part 1. References to routine testswere made even less specific than in the previous edition. The standard wassubsequently re-numbered as IEC 60601-1.

Any manufacturer obtaining compliance of an item of their equipment to BS5724or IEC 60601 will be in possession of a uniquely numbered certificate issued bythe test house verifying that fact. Compliance to the standard is a commonlyused route used by manufacturers to obtain CE marking.

6.4 Hospital Equipment Information 95

In August 1981, the DHSS issued HEI 95 entitled "Code of practice foracceptance testing of medical electrical equipment". The document wasproduced partly to address the problems that had arisen due to themisapplication of type tests from BS 5724 by some NHS biomedicaldepartments.

As indicated by the title of the document, the code of practice details inspectionand test procedures to be performed on newly acquired medical electrical

equipment before it is put into service. Inspection procedures are clearlyexplained and the standard acceptance test log sheet given in the appendix ofthe document contains references to the explanatory text.

The electrical safety testing recommendations offered in HEI 95 provided atesting regime that was effective whilst being considerably simpler than manytest regimes that were developed from the recommendations of BS 5724. Thereason for this is that the recommended electrical safety tests are generallyapplied under worst-case conditions.

Although designed as a code of practice for acceptance testing the document

has been widely adopted and used as the basis of routine test regimes byhospital biomedical departments.

The document was officially withdrawn in December 1999 on the publication bythe Medical Devices Agency of MDA DB9801 Supplement 1 (see below).


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6.5 DB9801 Supplement 1

In December 1999, the Medical Devices Agency (now the Medicines andHealthcare Products Regulatory Agency or MHRA) published Device Bulletin9801 Supplement 1 entitled "Checks and tests for newly delivered medical

devices". The document is a supplement to Device Bulletin 9801, "Medicaldevice and equipment management for hospital and community basedorganisations", which was published by the Medical Devices Agency in January1998. The supplement supersedes HEI 95 as well as HEI 140 (see 5.9 below) forin vitro diagnostic medical devices.

The document is intended to be applicable to all newly delivered medicaldevices, including equipment which is not electrical, before the equipment isplaced into service. Delivery checks detailed include paperwork checks, visualinspection procedures and functional checks. Electrical safety checks and testsas well as calibration checks are also recommended.

DB9801 Supplement 1 emphasises that new equipment under test should not besubjected to currents or voltages exceeding those experienced under normaloperating conditions. Hence none of the recommended tests involve shortingapplied parts together or applying high voltages to electrodes. It is alsosuggested that medical electrical equipment not having applied parts can besafety tested satisfactorily using a non-specialist portable appliance tester.

Specimen forms for recording the results of checks and tests are given in thedocument. Rationales for the checks and tests prescribed are given in theannexes of the document.

6.6 Medical Devices Directive

Since the Medical Devices Directive (Council Directive 93/42/EEC) became lawin the UK in 1994, it has been mandatory that all medical devices put on to themarket are appropriately CE marked to indicate compliance with the directive. Animportant component of the directive is a list of "essential requirements" to whichall medical devices must comply. Compliance with these requirements can beinterpreted essentially as meaning that the medical device is fit for purpose.

Depending on the risk class under which a particular medical device is classified,

there are various means by which a manufacturer is able to demonstrateconformity with the directive. For devices in the lowest risk category (class I), selfdeclaration is acceptable, whilst for medium risk devices (classes IIa and IIb), theassessment route normally includes auditing of the manufacturers qualityassurance system and independent type testing to a recognised standard (e.g.IEC 60601) of a representative production sample by a "notified body".


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In each member state a "Competent Authority" is authorised by that country'sgovernment to ensure that the requirements of the directive are carried out. Inthe UK, the competent authority is the Secretary of State for Health who hasdelegated day to day running of the competent authority to the Medicines andHealthcare Products Regulatory Agency (MHRA).

As far as the purchaser of equipment is concerned, all medical devicespurchased within any EEC member state should be appropriately CE marked.Conformity to the directive should be confirmed by the equipment supplier bymeans of a "declaration of conformity" prior to purchase.

6.7 ESCHLE

The Electrical Safety Code for Hospital Laboratory Equipment (ESCHLE) wasfirst published in 1977. It was produced by the Department of Health inconjunction with industry as a minimum standard to ensure the safety of

laboratory equipment used in hospital pathology departments and similarenvironments. This was done in response to pressure from industry since prior toESCHLE there was no strictly appropriate standard to form the basis for designand assessment of hospital laboratory equipment. Hospital TechnicalMemorandum 8, which was a safety code for electromedical apparatus was usedfor this purpose prior to 1977. In 1986 the second edition of ESCHLE waspublished by the Department of Health in Hospital Equipment Information 158(HEI158). The title ESCHLE is somewhat misleading since apart from electricalsafety, the document is also designed to ensure safety from overheating, fire,mechanical hazards, and the provision of adequate documentation and clearlymarked controls.

In HEI165 (March 1987) the DHSS recommended that as from January 1989,only laboratory equipment complying to ESCHLE II should be purchased by theNHS. However, ESCHLE itself recognised that some equipment used in hospitallaboratories may comply to other standards applicable to specific types ofequipment (e.g. BS4743/IEC348 for electronic measuring apparatus).

It was recognised that on publication of the IEC 1010 standard in 1990 (seebelow), ESCHLE would "die a natural death".

6.8 IEC 61010

In 1990 the International Electrotechnical Committee published IEC1010-1(subsequently re-numbered as IEC 61010-1), which is a European standardcovering safety requirements for electrical equipment for measurement controland laboratory use. Part 1 deals with requirements relevant to all types ofequipment under its remit and a series of part 2's covers particular requirementsfor specific types of equipment. The standard has been adopted by the British


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Standards Institution (BS EN 61010-1:2001 is the current edition) and haseffectively replaced ESCHLE.

The purpose of the standard is to provide adequate protection to the operatorand the surrounding area against electric shock or burn, mechanical hazards,

excessive temperature, fire, effects of radiation, liberated gases, explosion andimplosion. However, it should be noted that IEC61010-1 is not specificallyintended for hospital applications of laboratory equipment.

6.9 Hospital Equipment Information 140

Part II of HEI140 is virtually an equivalent to HEI95 except that it is designed forhospital laboratory equipment. Part I of the document summarised theimplementation of the first edition of ESCHLE and added some amendments tothat standard.

Again, although HEI140 Part II is a code of practice for acceptance testing, it ishighly relevant to routine testing and has been widely used for this purpose. Thedocument was written with ESCHLE in mind but was still considered to be arelevant code of practice for equipment complying to IEC61010, since noequivalent document relating specifically to that standard was produced.

MDA DB9801 Supplement 1 is stated to replace HEI 140 part II for in-vitromedical diagnostic medical devices from December 1999, "pending furtherguidance in this area". No guidance has been given by the Medical devices

Agency as to the status of HEI 140 with regard to hospital laboratory equipmentthat is not classified as an in-vitro diagnostic device.

In March 2002, the MDA published Device Bulletin DB2002(02) "Management ofin-vitro diagnostic medical devices". Although this document gives some generalguidance on the need for acceptance testing, it does provide any specific detailon appropriate electrical safety tests.

6.10 When to test

Medical electrical equipment should be inspected and tested on the followingoccasions.

a. On newly acquired equipment prior to being accepted for use

b. During routine planned preventative maintenance.c. After repairs have been carried out on equipment.

A patient should never be connected to a piece of equipment that has not beenchecked.

The testing regime used in the case of acceptance testing will be slightly differentto that used on other occasions particularly as regards condition of packaging,


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presence of relevant documentation and accessories. Having said this, theimportance of visual inspection prior to all electrical testing cannot be overstressed. Most equipment that has become hazardous or defective due todamage exhibits visible signs of such.

6.11 Recording test results

The acceptance test log sheet given in appendix A of HEI 95 is a good exampleof a comprehensive format for its purpose and many biomedical departmentshave used it successfully or modified it slightly to suit their own requirements.

Log sheets for routine safety testing are generally less extensive since the bulk ofthe information they need to record relates purely to the electrical safety tests.Examples of forms in use are given in appendix 2. One important feature of suchforms is that they should allow changes in results over a period to be seen at aglance. This facility enhances the usefulness of routine safety testing since it can

often allow developing problems to be spotted before they present a hazard.

6.12 Equipment Management Software Packages

There are on the market a number of equipment management softwarepackages suitable for use with medical electrical equipment. SEMS and QA-MAPare some such packages.

Some software packages allow the downloading of test results from automatictesters to a database and the production of hard copy test result forms. Thesefacilities can be very useful if used correctly although there are also inherent

dangers. Computers are able to handle large amounts of information very quicklyand the temptation exists with automatic testers to perform large numbers oftests and store all results on computer. Such information is of no use whatever ifthere is no human contact with the information. This may seem to be stating theobvious, but where large amounts of information are presented, it becomes veryeasy to miss important details.


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CHAPTER 7: ELECTRICAL SAFETY TESTS

7.1 Normal condition and single fault conditions

A basic principle behind the philosophy of electrical safety is that in the event of asingle abnormal external condition arising or of the failure of a single means ofprotection against a hazard, no safety hazard should arise. Such conditions arecalled "single fault conditions" (SFC's) and include such situations as theinterruption of the protective earth conductor or of one supply conductor, theappearance of an external voltage on an applied part, the failure of basicinsulation or of temperature limiting devices.

Where a single fault condition is not applied, the equipment is said to be in"normal condition" (NC). However, it is important to understand that in this

condition, the performance of certain tests may compromise the means ofprotection against electric shock. For example, if earth leakage current ismeasured in normal condition, the impedance of the measuring device in serieswith the protective earth conductor means that there is no effectivesupplementary protection against electric shock.

Many electrical safety tests are carried out under single fault conditions sincethese represent the worst case and will give the most adverse results. Clearly thesafety of the equipment under test may be compromised when such tests areperformed. Personnel carrying out electrical safety tests should be aware that thenormal means for protection against electric shock are not necessarily operative

during testing and should therefore exercise due precautions for their own safetyand that of others.

7.2 Protective Earth Continuity

The resistance of the protective earth conductor is measured between the earthpin on the mains plug and a protectively earthed point on the equipmentenclosure (see figure 6). The reading should not normally exceed 0.2 O at anysuch point. The test is obviously only applicable to class I equipment.

In IEC60601, the test is conducted using a 50Hz current between 10A and 25Afor a period of at least 5 seconds. Although this is a type test, some medicalequipment safety testers mimic this method. Damage to equipment can occur ifhigh currents are passed to points that are not protectively earthed, for example,functional earths. Great care should be taken when high current testers are usedto ensure that the probe is connected to a point that is intended to be protectivelyearthed.


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HEI 95 and DB9801 Supplement 1 recommend that the test be carried out at acurrent of 1A or less for the reason described above. Where the instrument useddoes not do so automatically, the resistance of the test leads used should bededucted from the reading.

If protective earth continuity is satisfactory then insulation tests can beperformed.

Applicable to Class I, all types

Limit: 0.2

DB9801 recommended?: Yes, at 1A or less.

HEI 95 recommended?: Yes, at 1A or less.

Notes: Ensure probe is on a protectively earthed point

Figure 8. Measurement of protective earth continuity.

7.3 Insulation Tests

IEC 60601-1, clause 17, lays down specifications for electrical separation of partsof medical electrical equipment compliance to which is essentially verified by

inspection and measurement of leakage currents. Further tests on insulation aredetailed under clause 20, "dielectric strength". These tests use AC sources totest equipment that has been pre-conditioned to specified levels of humidity. Thetests described in the standard are type tests and are not suitable for use asroutine tests.

HEI 95 and DB9801 recommend that for class I equipment the insulationresistance is measured at the mains plug between the live and neutral pins


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connected together and the earth pin. Whereas HEI 95 recommends using a500V DC insulation tester, DB 9801 recommends the use of 350V DC as the testvoltage. In practice this last requirement could prove difficult and it isacknowledged in a footnote that a 500 V DC test voltage is unlikely to cause anyharm. The value obtained should normally be in excess of 50M but may be less

in exceptional circumstances. For example, equipment containing mineralinsulated heaters may have an insulation resistance as low as 1M with no faultpresent. The test should be conducted with all fuses intact and equipmentswitched on (see figure 9).

Applicable to Class I, all types

Limits: Not less than 50M

DB9801recommended?:

Yes

HEI 95recommended?:

Yes

Notes: Equipment containing mineral insulated heaters may givevalues down to 1M. Check equipment is switched on.

Figure 9. Measurement of insulation resistance for class I equipment

HEI 95 further recommends for class II equipment that the insulation resistancebe measured between all applied parts connected together and any accessibleconductive parts of the equipment. The value should not normally be less than50M (see figure 10). DB9801 Supplement 1 does not recommend any form of

insulation test be applied to class II equipment.


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Applicable to Class II, all types having applied parts

Limits: not less than 50M.

DB9801 recommended?: No

HEI 95 recommended?: YesNotes: Move probe to find worst case.

Figure 10. Measurement of insulation resistance for class II equipment.

Satisfactory earth continuity and insulation test results indicate that it is safe toproceed to leakage current tests.

7.4 Leakage current measuring device

The leakage current measuring device recommended by IEC 60601-1 loads theleakage current source with a resistive impedance of about 1 kO and has a halfpower point at about 1kHz. The recommended measuring device was changedslightly in detail between the 1979 and 1989 version but remained functionallyvery similar. Figure 11 shows suitable arrangements for the measuring device.The millivolt meter used should be true RMS reading and should have an inputimpedance greater than 1 M. In practice this is easily achievable with mostgood quality modern multimeters. The meter in the arrangements shownmeasures 1mV for each A of leakage current.


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Figure 11. Suitable arrangements for measurement of leakage currents.

7.5 Earth Leakage Current

For class I equipment, earth leakage current is measured as shown in figure 12.The current should be measured with the mains polarity normal and reversed.

HEI 95 and DB9801 Supplement 1 recommend that the earth leakage current bemeasured in normal condition (NC) only. Many safety testers offer theopportunity to perform the test under a single fault condition such as live orneutral conductor open circuit.

Applicable to Class I equipment, all types

Limits:0.5mA in NC, 1mA in SFC or 5mA and 10mA respectivelyfor permanently installed equipment.

DB9801recommended?:

Yes, in normal condition only.

HEI 95recommended?:

Yes, in normal condition only.

Notes:Measure with mains normal and reversed. Ensureequipment is switched on.

Figure 12. Measurement of Earth Leakage Current.


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7.6 Enclosure leakage current

Enclosure leakage current is measured between an exposed part of theequipment which is not intended to be protectively earthed and true earth asshown in figure 13. The test is applicable to both class I and class II equipment

and should be performed with mains polarity both normal and reversed. HEI 95recommends that the test be performed under the SFC protective earth opencircuit for class I equipment and under normal condition for class II equipment.DB9801 Supplement 1 recommends that the test be carried out under normalcondition only for both class I and class II equipment. Many safety testers alsoallow the SFC's of interruption of live or neutral conductors to be selected. Pointson class I equipment which are likely not to be protectively earthed may includefront panel fascias, handle assemblies etc.

Applicable to Class I and class II equipment, all types.

Limits: 0.1mA in NC, 0.5mA in SFC

DB9801recommended?:

Yes, NC only

HEI 95recommended?:

Yes, class I SFC earth open circuit, class II NC.

Notes:Ensure equipment switched on. Normal and reversemains. Move probe to find worst case.

Figure 13. Measurement of Enclosure Leakage Current.

7.7 Patient leakage current

Under IEC 60601-1 and HEI 95, for class I and class II type B and BF equipment,the patient leakage current is measured from all applied parts having the samefunction connected together and true earth (figure 14). For type CF equipment


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the current is measured from each applied part in turn and the leakage currentleakage must not be exceeded at any one applied part (figure 15).

DB9801 Supplement 1 recommends that patient leakage current be measuredfrom each applied part in turn for all types of equipment, although the

recommended leakage current limits have not been revised to take into accountthe changed test method for B and BF equipment.

Great care must be taken when performing patient leakage currentmeasurements that equipment outputs are inactive. In particular, outputs ofdiathermy equipment and stimulators can be fatal and can damage testequipment.

Applicable toAll classes, type B & BF equipment having appliedparts.

Limits: 0.1mA in NC, 0.5mA in SFC.

DB9801recommended?: No

HEI 95 recommended?:Yes, class I SFC earth open circuit, class II normalcondition.

Notes:Equipment on but outputs inactive. Normal and reversemains.

Figure 14. Measurement of Patient Leakage Current with applied partsconnected together.


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Applicable toClass I and class II, type CF (B & BF for DB9801 only)equipment having applied parts.

Limits: 0.01mA in NC, 0.05mA in SFC.

DB9801recommended?:

Yes, all types, normal condition only.

HEI 95recommended?:

Yes, type CF only, class I SFC earth open circuit, class IInormal condition.

Notes:Equipment on but outputs inactive. Normal and reversemains. Limits are per electrode.

Figure 15. Measurement of patient leakage current for each applied part in turn

7.8 Patient auxiliary current

Patient auxiliary current as defined in section 3.5 is measured between anysingle patient connection and all other patient connections of the same moduleconnected together. It is not usual to test all possible combinations since togetherwith all possible single fault conditions this would give an exceedingly largeamount of data of questionable value.


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Applicable toAll classes and types of equipment having appliedparts.

Limits:Type B & BF - 0.1mA in NC, 0.5mA in SFC.Type CF - 0.01mA in NC, 0.05mA in SFC.

DB9801

recommended?: No.

HEI 95 recommended?: No.

Notes:Ensure outputs are inactive. Normal and reversemains.

Figure 16. Measurement of patient auxiliary current.

7.9 Mains on applied parts

By applying mains voltage to the applied parts, the leakage current that wouldflow from an external source into the patient circuits can be measured. Themeasuring arrangement is illustrated in figure 18.

Although the safety tester normally places a current limiting resistor in series withthe measuring device for the performance of this test, a shock hazard still exists.Therefore, great care should be taken if the test is carried out in order to avoidthe hazard presented by applying mains voltage to the applied parts.

Careful consideration should be given as to the necessity or usefulness ofperforming this test on a routine basis when weighed against the associatedhazard and the possibility of causing problems with equipment. The purpose ofthe test under IEC 60601-1 is to ensure that there is no danger of electric shockto a patient who for some unspecified reason is raised to a potential above earthdue to the connection of the applied parts of the equipment under test. Thestandard requires that the leakage current limits specified are not exceeded.There is no guarantee that equipment performance will not be adversely affectedby the performance of the test. In particular, caution should be exercised in thecase of sensitive physiological measurement equipment. In short, the test is a

"type test".


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Applicable to Class I & class II, types BF & CF having applied parts.

Limit: Type BF - 5mA; type CF - 0.05mA per electrode.

DB9801recommended?:

No.

HEI 95 recommended?: No

Notes:Ensure outputs are inactive. Normal and reverse mains.Caution required, especially on physiologicalmeasurement equipment.

Figure 17. Mains on applied parts measurement arrangement.

7.10 Leakage current summary

The following table summarises the leakage current limits (in mA) specified byIEC60601-1 for the tests most commonly performed as routine tests. Limits forDB9801 recommended tests are underlined. Limits for HEI 95 recommendedtests are given in bold type.

Leakage currentType B

NC SFC

Type BF

NC SFC

Type CF

NC SFC

Earth 0.5 1 0.5 1 0.5 1

Earth for fixedequipment

5 10 5 10 5 10

Enclosure 0.1 0.5 0.1 0.5 0.1 0.5

Patient 0.1 0.5 0.1 0.5 0.01 0.05

Mains on appliedpart

- - - 5 - 0.05

Patient auxiliary 0.1 0.5 0.1 0.5 0.01 0.05

* For class II type CF equipment HEI95 recommends a limit for enclosureleakage current of 0.01mA as per the 1979 edition of BS 5724.

Table 2. Leakage current limits summary.


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7.11 Comparison of HEI 95 and DB 9801 Supplement 1recommendations

Test HEI 95 DB9801 Supplement 1

Earth continuityUse test current of 1A orless

Limit 0.2ohm

Use test current of 1A orless

Limit 0.2ohm

Insulation for Class 1equipment

Measure between L and Nconnected together and Eusing 500v DC tester.Limit > 50M. Investigatelower values

Measure between L and Nconnected together and Eusing 350v DC tester.Limit > 20M. Investigatelower values

Insulation for Class IIequipment

Measure between appliedparts and accessibleconductive parts of theequipment.

Limit > 50M. Investigatelower values

No recommendation.

Earth leakage currentMeasure in normalconditionLimit < 0.5mA

Measure in normalconditionLimit < 0.5mA

Enclosure leakage current

Measure in SFC, earthopen circuit for Class-1,NC for Class-IILimit


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CHAPTER 8: GENERAL POINT ON SAFETY

Many Electrical safety tests are performed under single fault conditions such that

a means for protection against electric shock has been removed. In the case ofpatient leakage current with mains on applied parts, a hazard is actuallyintroduced. For these reasons no equipment under test should be touched whilsttests are being undertaken, as parts of the equipment may be hazardous live.For similar reasons, tests should be conducted on suitable non-conductivesurfaces and conductive objects should be kept well clear of the equipment. Thepotential hazard is exacerbated by the use of automatic testers when running inautomatic or semi-automatic modes since hazardous voltages may appear onthe equipment under test at any time without any warning.

Many categories of medical electrical equipment can produce outputs for

treatment purposes that, if applied incorrectly to a person can prove fatal, or atleast cause serious injuries. Examples of these categories include surgicaldiathermy machines, nerve and muscle stimulators, short-wave therapy units anddefibrillators. Persons who have not had specific training on such equipmentsufficient to enable them to avoid the hazards should not be allowed to performelectrical safety testing on it.

The tests applied in the course of routine safety testing can cause damage toequipment if carried out incorrectly or inappropriately. Such damage may leaddirectly or indirectly to patient injuries or death if the equipment is put back intoservice in this condition. It is clear that only maintenance personnel who are

sufficiently trained to avoid such occurrences arising should carry out electricalsafety testing of medical equipment.

References1. Note adopted fromhttp://www.ebme.co.uk/arts/safety/part7.htm 2. World Health Organization (WHO), Medical Devices Regulations:

Global overview and guiding principle
http://www.ebme.co.uk/arts/safety/part7.htmhttp://www.ebme.co.uk/arts/safety/part7.htmhttp://www.ebme.co.uk/arts/safety/part7.htmhttp://www.ebme.co.uk/arts/safety/part7.htm

safety testing of medical electrical equipment with and problems

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