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Physiological Effects of Electricity Hazards Physiological Effects Leakage Currents Classes & Types Standards & Guidance Electrical Safety Test & Inspection General Points on Safety Bibliography 2.1 Electrolysis The movement of ions of opposite polarities in opposite directions through a medium is called electrolysis and can be made to occur by passing DC current through body tissues or fluids. If a DC current is passed through body tissues for a period of minutes, ulceration begins to occur. Such ulcers, while not normally fatal, can be painful and take long periods to heal. 2.2 Burns When an electric current passes through any substance having electrical resistance, heat is produced. The amount of heat depends on the power dissipated (I2R). Whether or not the heat produces a burn depends on the current density. Human tissue is capable of carrying electric current quite successfully. Skin normally has a fairly high electrical resistance while the moist tissue underneath the skin has a much lower resistance. Electrical burns often produce their most marked effects near to the skin, although it is fairly common for internal electrical burns to be produced, which, if not fatal, can cause long lasting and painful injury. 2.3 Muscle cramps

Classes and Types of Medical Electrical Equipment

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Page 1: Classes and Types of Medical Electrical Equipment

Physiological Effects of Electricity Hazards Physiological Effects Leakage Currents Classes & Types Standards & Guidance Electrical Safety Test & Inspection General Points on Safety Bibliography

2.1 Electrolysis

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

2.2 Burns

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

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

2.3 Muscle cramps

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

2.4 Respiratory arrest

The muscles between the ribs (intercostal muscles) need to repeatedly contract and relax in order to facilitate breathing. Prolonged tetanus of these muscles can therefore prevent breathing.

2.5 Cardiac arrest

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

2.6 Ventricular fibrillation

The ventricles of the heart are the chambers responsible for pumping blood out of the heart. When the heart is in ventricular fibrillation, the musculature of the ventricles 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 current as low as 70 mA flowing from hand to hand across the chest, or 20µA directly through the heart may be sufficient. It is for this reason that most deaths from electric shock are attributable to the occurrence of ventricular fibrillation.

2.7 Effect of frequency on neuro-muscular stimulation

The amount of current required to stimulate muscles is dependent to some extent on 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 frequency of around 50 Hz. Above 10 kHz the neuro-muscular response to current decreases almost exponentially.

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

2.8 Natural protection factors

Many people have received electric shocks from mains potentials and above and lived to tell the tale. Part of the reason for this is the existence of certain natural protection factors.

Ordinarily, a person subject to an unexpected electrical stimulus is protected to some extent by automatic and intentional reflex actions. The automatic contraction of muscles on receiving an electrical stimulus often acts to disconnect the person from the source of the stimulus. Intentional reactions of the person receiving the shock normally serve the same purpose. It is important to

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realise that a patient in the clinical environment who may have electrical equipment intentionally connected to them and may also be anaesthetised is relatively unprotected by these mechanisms.

Normally, a person who is subject to an electric shock receives the shock through the skin, which has a high electrical resistance compared to the moist body tissues below, and hence serves to reduce the amount of current that would otherwise flow. Again, a patient does not necessarily enjoy the same degree of protection. The resistance of the skin may intentionally have been lowered in order to allow good connections of monitoring electrodes to be made or, in the case of a patient undergoing surgery, there may be no skin present in the current path.

The absence of natural protection factors as described above highlights the need for stringent electrical safety specifications for medical electrical equipment and for routine test and inspection regimes aimed at verifying electrical safety.

Leakage currentsMost safety testing regimes for medical electrical equipment involve the measurement of certain "leakage currents", because the level of them can help to verify whether or not a piece of equipment is electrically safe. In this section the various leakage currents that are commonly measurable with medical equipment safety testers are described and their significance discussed. The precise methods of measurement along with applicable safe limits are discussed later under paragraphs at 6.

3.1 Causes of leakage currents

If any conductor is raised to a potential above that of earth, some current is bound to flow from that conductor to earth. This is true even of conductors that are well insulated from earth, since there is no such thing as perfect insulation or infinite 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 earth and from each other are called leakage currents, and are normally small. However, since the amount of current required to produce adverse physiological effects is also small, such currents must be limited by the design of equipment to safe values.

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

3.2 Earth leakage current

Earth leakage current is the current that normally flows in the earth conductor of a protectively earthed piece of equipment. In medical electrical equipment, very often, the mains is connected to a transformer having an earthed screen. Most of the earth leakage current finds its way to earth via the impedance of the insulation between the transformer primary and the inter-winding screen, since this 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 metal enclosure 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 is because the impedance to earth from the enclosure is much lower through the protective earth conductor than it is through the person. However, if the protective earth conductor becomes open circuited, then the situation changes. Now, if the impedance between the transformer primary and the enclosure is of the same order of magnitude as the impedance between the enclosure and earth through the person, 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 in order for this to be the case in the above example, the impedance between the mains part (the transformer primary and so on) and the enclosure needs to be high. This would be evidenced when the equipment is in the normal condition by a low earth leakage current. In other words, if the earth leakage current is low then the risk of electric shock in the event of a fault is minimised.

3.3 Enclosure leakage current or touch current

The terms "enclosure leakage current" and "touch current" should be taken to be synonymous. The former term is used in the bulk of this text. The terms are further discussed in connection with the electrical test methods under paragraphs 6.6 (Part 6). Enclosure leakage current is defined as the current that flows from an exposed conductive part of the enclosure to earth through a conductor other than the protective earth conductor.

If a protective earth conductor is connected to the enclosure, there is little point in attempting to measure the enclosure leakage current from another protectively earthed point on the enclosure, since any measuring device used is effectively shorted out by the low resistance of the protective earth. Equally, there is little point in measuring the enclosure leakage current from a protectively earthed point on the enclosure with the protective earth open circuit, since this would give the same reading as measurement of earth leakage current as described above. For these reasons, it is usual when testing medical electrical equipment to measure enclosure leakage current from points on the enclosure that are not intended to be protectively earthed (see figure 3). On many pieces of equipment, no such points exist. This is not a problem. The test is included in test regimes to cover the eventuality where such points do exist and to ensure that no hazardous leakage currents will flow from them.

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Figure 3. Enclosure leakage current path

3.4 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 via the patient to earth or from an external source of high potential via the patient and the applied parts to earth. Figures 4a and 4b illustrate the two scenarios.

Figure 4a. Patient leakage current path from equipment

Figure 4b. Patient leakage current path to equipment

3.5 Patient auxiliary current

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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 a physiological effect (see figure 5).

Figure 5. Patient auxiliary current path

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

4.1 Class I equipment

Class I equipment has a protective earth. The basic means of protection is the insulation between live parts and exposed conductive parts such as the metal enclosure. In the event of a fault that would otherwise cause an exposed conductive part to become live, the supplementary protection (i.e. the protective earth) 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 a fuse) in the mains circuit to disconnect the equipment from the supply.

It is important to realise that not all equipment having an earth connection is necessarily class I. The earth conductor may be for functional purposes only such as screening. In this case the size of the conductor may not be large enough to safely carry a fault current that would flow in the event of a mains short to 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 of the mains supply lead in both the live and neutral conductors, so that the supplementary protection is operative when the equipment is connected to an incorrectly wired socket outlet.

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

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There is no agreed symbol in use to indicate that equipment is class I and it is not mandatory to state on the equipment itself that it is class I. Where any 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.

4.2 Class II equipment

The method of protection against electric shock in the case of class II equipment is either double insulation or reinforced insulation. In double insulated equipment the basic protection is afforded by the first layer of insulation. If the basic protection fails then supplementary protection is provided by a second layer of insulation preventing contact with live parts.

In practice, the basic insulation may be afforded by physical separation of live conductors from the equipment enclosure, so that the basic insulation material is air. The enclosure material then forms the supplementary insulation.

Reinforced insulation is defined in standards as being a single layer of insulation offering the same degree of protection against electric shock as double insulation.

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

The symbol for class II equipment is two concentric squares illustrating double insulation as shown below.

Figure 7. Symbol for class II equipment

4.3 Class III equipment

Class III equipment is defined in some equipment standards as that in which protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present. SELV is defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc.

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

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If battery operated equipment is capable of being operated when connected to the 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 SELV transformer should be tested in conjunction with the transformer as class I or class II equipment as appropriate.

It is interesting to note that the current IEC standards relating to safety of medical electrical equipment do not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of the patient. All medical electrical equipment that is capable of mains connection must be classified as class I or class II. Medical electrical equipment having no mains connection is simply referred to as "internally powered".

4.4 Equipment types

As described above, the class of equipment defines the method of protection against electric shock. The degree of protection for medical electrical equipment is defined by the type designation. The reason for the existence of type designations is that different pieces of medical electrical equipment have different areas of application and therefore different electrical safety requirements. For example, it would not be necessary to make a particular piece medical electrical equipment safe enough for direct cardiac connection if there is no possibility of this situation arising.

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

Type Symbol Definition

B

Equipment providing a particular degree of protection against electric shock, particularly regarding allowable leakage currents and reliability of the protective earth connection (if present).

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

CFEquipment providing a higher degree of protection against electric shock than type BF, particularly with regard to allowable leakage currents, and having floating applied parts.

Table 1. Medical electrical equipment types

Equipment standards, guidance and legislation5.1 Type tests and routine tests

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

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Standards for the manufacture of equipment normally detail tests which are intended to be carried out on a single representative sample of a piece of equipment for which certification of compliance with a standard is being sought. Such tests are carried out by approved test houses under tightly specified environmental conditions. These tests are called "type tests" and are not intended for routine use. Indeed, repetition of many of the tests would certainly cause deterioration in performance and safety of the equipment under test.

Routine tests have an entirely different purpose than that for type tests. Routine tests are intended to provide good indicators to the safety of equipment without subjecting it to undue stress that would be liable to cause deterioration.

In summary then, it should be understood that International Electrotechnical Commission (IEC) or British Standards (BS) manufacturers' standards for medical electrical equipment referred to below are intended only for type testing and should not be used for acceptance, in-service or routine testing of equipment. However, any tests that are used for the latter purposes should ideally be consistent with the standards to which the equipment has been manufactured. Routine tests and test limits may therefore be derived (in modified form) from the standards, with the strict proviso that any such tests should not damage or even stress the equipment under test.

5.2 HTM 8

In 1963, the Department of Health and Social Security published Hospital Technical Memorandum number 8 called "safety code for electro-medical apparatus". The purpose of the document was to establish adequate standards for the design and construction of electro-medical apparatus since no other relevant national standard existed at the time. Although the document was produced essentially for the guidance of manufacturers, biomedical departments in hospitals were quick to adopt tests from the document for the basis of their own medical electrical equipment safety testing regimes. Although tests detailed in the code were type tests, many of them could be fairly easily be repeated without adverse effects on the equipment as routine tests. Performance of the electrical safety tests was made easier by the development of specialised medical equipment safety testers, specifically, the Liverpool tester. The HTM was withdrawn on publication of BS5724 part 1 (see below).

5.3 BS 5724 or IEC 60601

In 1979, HTM 8 was superseded by the British Standard BS 5724 part 1. This document is a comprehensive specification for safety of medical electrical equipment. Part 1 covers the general requirements, i.e. requirements common to all medical electrical equipment regardless of function. A series of part 2's detailing particular requirements for specific categories of medical electrical equipment followed publication of part 1 (see Annex 1).

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

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BS 5724 part 1 was revised in 1989, making it identical to the International Electro-technical Commission standard IEC 601-1: 1988. References to routine tests were made even less specific than in the previous edition. The standard was subsequently re-numbered as IEC 60601-1.

Any manufacturer obtaining compliance of an item of their equipment to BS5724 or IEC 60601 will be in possession of a uniquely numbered certificate issued by the test house verifying that fact. Compliance to the standard is a commonly used route used by manufacturers to obtain CE marking (see paragraphs at 5.6.1 below).

5.3.1 The Third Edition of IEC60601-1

The third edition of IEC60601-1 was introduced in December 2005. The standard has been renamed "General requirements for basic safety and essential performance" to reflect the fact that inadequate equipment performance may give rise to hazards. The new standard is stated to replace the second edition, although it is recognised that, in practice, due to the references made to the general standard by particular standards (part twos), there is likely to be a fairly long transitionary period for compliance by equipment manufacturers.

There are some significant changes in the new standard, some of which are worth noting here.

The new standard states that the manufacturer must have in place a risk management process that complies to the requirements of ISO 14971 in order to ensure that the equipment design process results in equipment that is suitable for its intended purpose and that any risks associated with its use are acceptable.

Certain changes in terminology and numbering systems have been introduced in order to make the standard more compatible with other IEC standards, in particular IEC 60950-1 (Information technology equipment).

Collateral standards for medical electrical systems (IEC 60601-1-1) and programmable electrical medical systems (IEC 60601-1-4) have been incorporated into the body of the new standard as new clauses.

5.4 Guidance from the UK Department of Health

The Department of Health has, in the past, issued two stand alone documents giving detailed guidance on acceptance testing or pre-use checks on medical devices. Although both of these documents have been superseded, they are discussed briefly below because they have been used by many equipment user organisations as the basis for acceptance testing regimes, and even for routine testing regimes. Additionally, a number of manufacturers of medical equipment safety testers have incorporated protocols derived from these guidance documents into their testers' firmware.

A comparison between the test recommendations of both documents is provided in annex 2 for information.

5.4.1 Hospital Equipment Information 95

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In August 1981, the DHSS issued HEI 95 entitled "Code of practice for acceptance testing of medical electrical equipment". The document was produced partly to address the problems that had arisen due to the misapplication of type tests from BS 5724 by some NHS biomedical departments.

As indicated by the title of the document, the code of practice detailed inspection and test procedures to be performed on newly acquired medical electrical equipment before it was put into service. Inspection procedures were clearly explained and the standard acceptance test log sheet given in the appendix of the document contained references to the explanatory text.

The electrical safety testing recommendations offered in HEI 95 provided a testing regime that was effective whilst being considerably simpler than many test regimes that were developed from the recommendations of BS 5724. The reason for this is that the recommended electrical safety tests are generally applied 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 by hospital biomedical departments.

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

5.4.2 DB9801 Supplement 1

In December 1999, the Medical Devices Agency (now the Medicines and Healthcare Products Regulatory Agency or MHRA) published Device Bulletin 9801 Supplement 1 entitled "Checks and tests for newly delivered medical devices". The document was a supplement to Device Bulletin 9801, "Medical device and equipment management for hospital and community based organisations", which was published by the Medical Devices Agency in January 1998. The supplement superseded HEI 95.

The document was intended to be applicable to all newly delivered medical devices, including non-electrical equipment, before being placed into service. Delivery checks detailed included paperwork checks, visual inspection procedures and functional checks. Electrical safety checks and tests as well as calibration checks were also recommended.

DB9801 Supplement 1 emphasised that new equipment under test should not be subjected to currents or voltages exceeding those experienced under normal operating conditions. Hence none of the recommended tests involved shorting applied parts together or applying high voltages to electrodes. It was also suggested that medical electrical equipment not having applied parts could be safety tested satisfactorily using non-specialist portable appliance testers.

Specimen forms for recording the results of checks and tests were given in the document. Rationales for the checks and tests prescribed were also given in the annexes of the document.

DB0801 and its supplements were replaced by DB2006(05) in November 2006 (see below).

5.4.3 Device Bulletin DB2006(05)

In November 2006, the MHRA published (on their website only) Device Bulletin DB2006(05) - "Managing Medical Devices - Guidance for healthcare and social services organisations". The

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document updates and replaces guidance previously given in DB9801 ("Medical device and Equipment Management for hospital and community based organisations") and its supplements. Section 4 of the new guidance addresses "Delivery of a new piece of equipment" and hence replaces guidance previously given in DB9801 Supplement 1. Having said that, much of the basic philosophy behind, and recommendations from, the latter document have been retained.

The guidance stresses the importance of acceptance checks as a means of improving efficiency and reducing risk. It also emphasises the necessity of recording checks and test results in order to meet health and safety requirements, possible litigation demands and to enable safe and effective future device management.

Delivery checks relevant to medical electrical equipment on delivery are divided into administrative tasks ("paperwork/database") and visual inspection. Recommended administrative checks and tasks include:-

device compatibility with purchase specification inclusion and appropriateness of user and service instructions inclusion of compliance and calibration certificates and test results adding device details to equipment management records check for special requirements such as need for decontamination before use

The guidance further recommends that functional checks, electrical safety tests and calibration checks (where appropriate) should be carried out prior to the equipment being placed into service.

No specific detail is given on safety tests, other than to emphasise that "pre-use tests should not exceed the bounds of normal use". In connection with this, it states that the tests described in IEC 60601-1 are "type tests" and are therefore not suitable for pre-use or maintenance tests (see paragraphs at 5.1 above).

The guidance does, however, point out the legal requirements for electrical safety testing under the Health and Safety at Work etc Act 1974 and the Electricity at Work Regulations 1989. The guidance states that "Responsible organisations should ensure that they have implemented electrical safety testing procedures to comply with this legislation". The legal requirements are further discussed in these notes under paragraphs at 5.6 below.

The full text of DB2006(05) is available free of charge on the MHRA website at www.mhra.gov.uk

5.5 Future Guidance

The International Electrotechnical Commission has been preparing IEC 62353 Edition 1: "Medical Electrical Equipment - Recurrent test and test after repair of medical equipment" for some years. Publication of this document is expected in the near future.

Also in preparation by the Institute of Physics and Engineering in Medicine (IPEM) is a publication called "Electrical Safety Testing: A Workbench Guide".

5.6 UK Legislation

There are a number of items of legislation applicable in the UK that impact in a fairly direct way on maintenance procedures for medical electrical devices. These are discussed briefly below.

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5.6.1 Medical Devices Directive

Since the Medical Devices Directive (Council Directive 93/42/EEC) became law in the UK in 1994, it has been mandatory that all medical devices put on to the market are appropriately CE marked to indicate compliance with the directive. An important component of the directive is a list of "essential requirements" to which all medical devices must comply. Compliance with these requirements can be interpreted 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 demonstrate conformity with the directive. For devices in the lowest risk category (class I), self declaration is acceptable, whilst for medium and higher risk devices (classes IIa, IIb and III), the assessment route is more rigorous and may include auditing of the manufacturers' quality assurance system and independent type testing to a recognised standard (e.g. IEC 60601) of a representative production sample by a "notified body". Each notified body may be identified by a unique number that appears to the top right of the CE mark on medical devices.

In each member state a "Competent Authority" is authorised by that country's government to ensure that the requirements of the directive are carried out. In the UK, the competent authority is the Secretary of State for Health who has delegated day to day running of the competent authority to the Medicines and Healthcare Products Regulatory Agency (MHRA). The Medical Devices Directive is enshrined into UK law by the medical Devices Regulations 2002.

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

5.6.2 Health and Safety at Work etc. Act 1974

The Health and Safety at Work etc. Act 1974 (HASAWA) act may be regarded as the "catch all" act that covers all aspects of health and safety in the workplace. It places responsibility on employers and employees for the health, safety and welfare of all persons that may be affected by activities of an employer (including NHS Trusts). The overarching nature of the act is illustrated by part 1, section 3, paragraph 1 of the act that states:

"It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health and safety".

There are many sets of Regulations that are made under the act that spell out in detail what must be done to meet the requirements of the act. The Regulations are said to be "made under the Act", and non-conformity to any such regulations is therefore an offence under the HASAWA.

5.6.3 Electricity at Work Regulations 1989

Particular requirements with regard to electrical equipment are imposed by the Electricity at Work Regulations 1989. Some significant extracts from the regulations are quoted below. It should be noted when reading them that the word "systems" refers to electrical installations and any equipment capable of being made live by them.

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Regulation 4(1) "All systems shall at all times be of such construction as to prevent, so far as is reasonably practicable, danger".

Regulation 4(2) "As necessary to prevent danger, all systems shall be maintained so as to prevent, so far as is reasonably practicable, such danger".

Regulation 4(3) "Every work activity, including operation, use and maintenance of a system and work near a system, shall be carried out in such a manner as not to give rise, so far as is reasonably practicable, to danger".

Regulation 16 "No person shall be engaged in any work activity where knowledge or experience is necessary to prevent danger or, where appropriate, injury, unless he possesses such knowledge or experience, or is under such degree of supervision as may be appropriate having regard to the nature of the work."

Although the Electricity at Work Regulations clearly put requirements on employers and employees with regard to the necessity for maintaining electrical safety, the means by which this should be done are not spelt out in the Regulations.

5.6.4 Management of Health and Safety at Work Regulations 1999

The Management of Health and Safety at Work Regulations 1999 set out the need for organisations to develop formalised management systems for health and safety. These systems will form a part of the organisations health and safety policy.

The policy should detail arrangements for effective planning, organisation, control, monitoring and review of protective and preventative measures. Hence protocols for electrical safety inspection and testing of medical equipment should be a part of this policy.

A major plank of the regulations is prescription of the use of risk assessments as a tool in managing health and safety effectively. The prime obligation under health and safety legislation is to eliminate or minimise risks to health and safety of anyone who may be affected by work activities. Safe systems of work and effective preventative measures to achieve this can only be developed following effective risk assessments.

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

Electrical Safety TestsThe following paragraphs and diagrams describe the electrical safety tests commonly available on medical equipment safety testers. Please note that although HEI 95 and DB9801 are no longer current, they are referred to in the text since many medical electronics departments have used them as a basis for local acceptance testing and even routine testing protocols. Protocols based on both sets of guidance are also available on many medical equipment safety testers.

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6.1 Normal condition and single fault conditions

A basic principle behind the philosophy of electrical safety is that in the event of a single abnormal external condition arising or of the failure of a single means of protection against a hazard, no safety hazard should arise. Such conditions are called "single fault conditions" (SFCs) and include such situations as the interruption of the protective earth conductor or of one supply conductor, the appearance of an external voltage on an applied part, the failure of basic insulation 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 even in this condition, the performance of certain tests may compromise the means of protection against electric shock. For example, if earth leakage current is measured in normal condition, the impedance of the measuring device in series with the protective earth conductor means that there is no effective supplementary protection against electric shock.

Many electrical safety tests are carried out under various single fault conditions in order to verify that there is no hazard even should these conditions occur in practice. It is often the case that single fault conditions represent the worst case and will give the most adverse results. Clearly the safety of the equipment under test may be compromised when such tests are performed. Personnel carrying out electrical safety tests should be aware that the normal means for protection against electric shock are not necessarily operative during testing and should therefore exercise due precautions for their own safety and that of others. In particular the equipment under test should not be touched during the safety testing procedure by any persons.

6.2 Protective Earth Continuity

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

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

HEI 95 and DB9801 Supplement 1 recommended that the test be carried out at a current of 1A or less for the reason described above.

Where the instrument used does not do so automatically, the resistance of the test leads used should be deducted from the reading.

If protective earth continuity is satisfactory then insulation tests can be performed.

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Applicable to Class I, all types

Limit: 0.2Ω

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.

6.3 Insulation Tests

IEC 60601-1 (second edition), clause 17, lays down specifications for electrical separation of parts of medical electrical equipment compliance to which is essentially verified by inspection and measurement of leakage currents. Further tests on insulation are detailed under clause 20, "dielectric strength". These tests use AC sources to test equipment that has been pre-conditioned to specified levels of humidity. The tests described in the standard are type tests and are not suitable for use as routine tests.

HEI 95 and DB9801 recommended that for class I equipment the insulation resistance be measured at the mains plug between the live and neutral pins connected together and the earth pin. Whereas HEI 95 recommended using a 500V DC insulation tester, DB 9801 recommended the use of 350V DC as the test voltage. In practice this last requirement could prove difficult and it was acknowledged in a footnote that a 500 V DC test voltage is unlikely to cause any harm. The value obtained should normally be in excess of 50MΩ but may be less in exceptional circumstances. For example, equipment containing mineral insulated heaters may have an insulation resistance as low as 1MΩ with no fault present. The test should be conducted with all fuses intact and equipment switched on where mechanical on/off switches are present (see figure 9).

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Applicable to Class I, all types

Limits: Not less than 50MΩ

DB9801 recommended?:

Yes

HEI 95 recommended?:

Yes

Notes:Equipment containing mineral insulated heaters may give values down to 1MΩ. Check equipment is switched on.

Figure 9. Measurement of insulation resistance for class I equipment

HEI 95 further recommended for class II equipment that the insulation resistance be measured between all applied parts connected together and any accessible conductive parts of the equipment. The value should not normally be less than 50MΩ (see figure 10). DB9801 Supplement 1 did not recommend any form of insulation test be applied to class II equipment.

Applicable to Class II, all types having applied parts

Limits: not less than 50MΩ.

DB9801 recommended?: No

HEI 95 recommended?: Yes

Notes: Move probe to find worst case.

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

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

6.4 Leakage current measuring device

The leakage current measuring device recommended by IEC 60601-1 loads the leakage current source with a resistive impedance of about 1 kΩ and has a half power point at about 1kHz. The recommended measuring device was changed slightly in detail between the 1979 and 1989 editions of the standard but remained functionally very similar. Figure 11 shows the arrangements for the measuring device. The millivolt meter used should be true RMS reading and should have an input impedance greater than 1 MΩ. In practice this is easily achievable with most good quality modern multimeters. The meter in the arrangements shown measures 1mV for each µA of leakage current.

Figure 11. Arrangements for measurement of leakage currents.

6.5 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 recommended that the earth leakage current be measured in normal condition (NC) only. Many safety testers offer the opportunity to perform the test under single fault condition, neutral conductor open circuit. This arrangement normally gives a higher leakage current reading.

One of the most significant changes with regard to electrical safety in the 2005 edition of IEC 60601-1 is an increase by a factor of 10 in the allowable earth leakage current to 5mA in normal condition and 10mA under single fault condition. The rationale for this is that the earth leakage current is not, of itself, hazardous.

Higher values of earth leakage currents, in line with local regulation and IEC 60364-7-710 (electrical supplies for medical locations), are allowed for permanently installed equipment connected to a dedicated supply circuit.

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Applicable to Class I equipment, all types

Limits:0.5mA in NC, 1mA in SFC or 5mA and 10mA respectively for equipment designed to IEC60601-1:2005.

DB9801 recommended?:

Yes, in normal condition only.

HEI 95 recommended?:

Yes, in normal condition only.

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

Figure 12. Measurement of earth leakage current.

6.6 Enclosure leakage current or touch current

Enclosure leakage current is measured between an exposed part of the equipment which is not intended to be protectively earthed and true earth as shown 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 95 recommended that the test be performed under the SFC protective earth open circuit for class I equipment and under normal condition for class II equipment. DB9801 Supplement 1 recommended that the test be carried out under normal condition only for both class I and class II equipment. Many safety testers also allow the SFC's of interruption of live or neutral conductors to be selected. Points on class I equipment which are likely not to be protectively earthed may include front panel fascias, handle assemblies etc.

The term "enclosure leakage current" has been replaced in the new edition of the IEC 60601-1standard by the term "touch current", bringing it into line with IEC 60950-1 for information technology equipment. However, the limits for touch current are the same as the limits for enclosure leakage current under the second edition of the standard, at 0.1 mA in normal condition and 0.5 mA under single fault condition.

In practice, if a piece of equipment has accessible conductive parts that are protectively earthed, then in order to meet the new requirements for touch current, the earth leakage current would need to meet the old limits. This is due to the fact that when the touch current is tested from a protectively earthed point with the equipment protective earth conductor disconnected, the value will be the same as that achieved for earth leakage current under normal condition.

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Hence, where higher earth leakage currents are recorded for equipment designed to the new standard, it is important to check the touch current under single fault condition, earth open circuit, from all accessible conductive parts.

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

Limits: 0.1mA in NC, 0.5mA in SFC

DB9801 recommended?:

Yes, NC only

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

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

Figure 13. Measurement of enclosure leakage current

6.7 Patient leakage current

Under IEC 60601-1, for class I and class II type B and BF equipment, the patient leakage current is measured from all applied parts having the same function connected together and true earth (figure 14). For type CF equipment the current is measured from each applied part in turn and the leakage current leakage must not be exceeded at any one applied part (figure 15).

HEI 95 adhered to the same method, however, DB9801 Supplement 1 recommended that patient leakage current be measured from each applied part in turn for all types of equipment, although the recommended leakage current limits were not revised to take into account the changed test method for B and BF equipment.

Great care must be taken when performing patient leakage current measurements that equipment outputs are inactive. In particular, outputs of diathermy equipment and stimulators can be fatal and can damage test equipment.

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Applicable to All classes, type B & BF equipment having applied parts.

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

DB9801 recommended?: No

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

Notes: Equipment on, but outputs inactive. Normal and reverse mains.

Figure 14. Measurement of patient leakage current with applied parts connected together

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.

DB9801 recommended?:

Yes, all types, normal condition only.

HEI 95 recommended?: Yes, type CF only, class I SFC earth open circuit, class II normal condition.

Notes:quipment on, but outputs inactive. Normal and reverse mains. Limits are per electrode.

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

6.8 Patient auxiliary current

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Patient auxiliary current is measured between any single patient connection and all other patient connections of the same module or function connected together. Where all possible combinations are tested together with all possible single fault conditions this yields an exceedingly large amount of data of questionable value.

Applicable to All classes and types of equipment having applied parts.

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 reverse mains.

Figure 16. Measurement of patient auxiliary current.

6.9 Mains on applied parts (patient leakage)

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

Although the safety tester normally places a current limiting resistor in series with the 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 avoid the hazard presented by applying mains voltage to the applied parts.

Careful consideration should be given as to the necessity or usefulness of performing this test on a routine basis when weighed against the associated hazard and the possibility of causing problems with equipment. The purpose of the test under IEC 60601-1 is to ensure that there is no danger of electric shock to a patient who for some unspecified reason is raised to a potential above earth due to the connection of the applied parts of the equipment under test. The standard requires that the leakage current limits specified are not exceeded. There is no guarantee that equipment performance will not be adversely affected by the performance of the test. In particular, caution should be exercised in the case of sensitive physiological measurement equipment. In short, the test is a "type test".

Most medical equipment safety testers refer to this test as "mains on applied parts", although this is not universal. One manufacturer refers to the test simply as "Patient leakage - F-type". In all cases there should be a hazard indication visible where the test is selected.

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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.

DB9801 recommended?: No.

HEI 95 recommended?: No

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

Figure 17. Mains on applied parts measurement arrangement

6.10 Leakage current summary

The following table summarises the leakage current limits (in mA) specified by IEC60601-1 (second edition) for the most commonly performed tests. Most equipment currently in use in hospitals today is likely to have been designed to conform to this standard, but note that the allowable values of earth leakage current have been increased in the third edition of the standard as discussed above.

The values stated are for d.c. or a.c. (r.m.s), although later amendments of the standard included separate limits for the d.c. element of patient leakage and patient auxiliary currents at one tenth of the values listed below. These have not been included in the table since, in practice, it is rare that there is a problem solely with d.c. leakage where that is not evidenced by a problem with combined a.c and d.c. leakage.

Leakage currentType B

NC SFCType BF

NC SFCType CF

NC SFC

Earth 0.5 1 0.5 1 0.5 1

Earth for fixed equipment

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 applied part - - - 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 enclosure leakage current of 0.01mA as per the 1979 edition of BS 5724.

Table 2. Leakage current limits summary.

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6.11 Comparison of HEI 95 and DB 9801 Supplement 1 recommendations

Test HEI 95 DB9801 Supplement 1

Earth continuityUse test current of 1A or lessLimit 0.2ohm

Use test current of 1A or lessLimit 0.2ohm

Insulation for Class 1equipment

Measure between L and N connected together and E using 500v DC tester.Limit > 50MΩ. Investigate lower values

Measure between L and N connected together and E using 350v DC tester.Limit > 20MΩ. Investigate lower values

Insulation for Class IIequipment

Measure between applied parts and accessible conductive parts of the equipment.Limit > 50MΩ. Investigate lower values

No recommendation.

Earth leakage currentMeasure in normal conditionLimit < 0.5mA

Measure in normal conditionLimit < 0.5mA

Enclosure leakage current

Measure in SFC, earth open circuit for Class-1, NC for Class-IILimit <0.5 mA for Class1<0.1 mA for class II

Measure in NC onlyLimit < 0.1 mA

Patient leakage current

Measure from all applied parts connected together for B & BF equipment and from each applied part in turn for type CF.Measure under SFC, eart open circuit for Class 1, NC for classII.Limits :

Class I, B& BF < 0.5 mA Class II, B& BF < 0.1

mA Class I, CF < 0.05 mA

per electrode

Class II, CF < 0.01 mA per electrode

Measure from each applied part in turn, for all types of equipmentMeasure under NC onlyLimits

Type B & BF <0.1 mA per electrode

Type CF < 0.01 per electrode

Test and Inspection Protocols7.1 When to test

As discussed at paragraphs under 5.6 above, user organisations should design and implement electrical safety inspection and test regimes on the basis of risk assessments.

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In practice, most user organisations have found it necessary to carry out electrical inspection and safety testing on medical electrical equipment on the following occasions.

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 been checked.

The testing regime used in the case of acceptance testing will be slightly different to that used on other occasions particularly as regards checks on the condition of packaging, presence of relevant documentation and accessories. However, it is useful to use the acceptance testing procedure to lay down baseline data for comparison when the equipment is tested on future scheduled services and after repairs.

7.2 Example inspection and test protocol

Annex 3 contains a test record sheet that is used to record inspection and test results produced by a simple electrical safety protocol. It is not intended to be in any way prescriptive, but is included here simply to illustrate many of the important features of an effective protocol.

Details of the equipment under test are recorded at the top of the form including the device serial number and a plant number ascribed by the user organisation. This ensures that the record can be linked to the particular item of equipment. The class and type/s of the equipment under test are also recorded here to ensure that appropriate test limits are applied.

The details of the test equipment used are also recorded at the top of the form together with the calibration date. This information is important for traceability since test results can only be proved to be accurate if it can be demonstrated that the test equipment was in calibration.

The visual inspection checklist provides a record that the relevant parts of the equipment have been inspected. This is very important since, in practice, the visual inspection is likely to flag up problems far more often than the electrical safety tests themselves. It is also important that a record of visual inspection is kept. Where user organisations use electronic means to record data downloaded from electrical safety testers, it is important to add information on visual inspection to the record.

The electrical safety tests that are used in this particular protocol are few in number and are the same tests, derived from IEC60601-1, that were selected for HEI 95. The earth continuity test is obviously important for all class I equipment. The insulation test is intended to look at the insulation between the mains part and the earth of the equipment under test, and may be regarded as a pre-test to verify that it is safe to apply mains power in order to measure leakage currents.

Earth leakage current here is only measured under normal condition (NC). Note that "normal" and "reverse" here mean that the leakage current is measured with L1 and L2 the right way round and the wrong way round. Both of these conditions are defined as "normal condition". This test will not usually produce as high a reading as if the test is conducted with under single fault condition, neutral open circuit. However, in most cases, if there is no problem with earth leakage current under normal condition, there is unlikely to be one under the single fault condition.

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Enclosure leakage and patient leakage currents are both recommended under this protocol to measured under single fault condition, earth open circuit (EOC). The rationale behind this is that any problems are likely to be evident under this condition and it is not improbable that the fault condition may arise when the equipment is in use.

At the foot of the form, it is recorded whether the equipment has passed or failed in the light of the visual inspection and the electrical safety test results. The date of the test and the identity of the person who performed the test must also be recorded.

The comments field below the table is a useful feature of any recording system. It allows any observations to be recorded, for example, of peculiarities of the equipment under test or concerns about test results. The record should be referred to by the person performing the next test and inspection on the equipment prior to carrying out the inspection and test.

General points on safetyMany electrical safety tests are performed under single fault conditions such that a means for protection against electric shock has been removed. In the case of patient leakage current with mains on applied parts, a hazard is actually introduced.

Even under normal condition, the equipment under test cannot be regarded as safe, since the supplementary protection may have been compromised by the test arrangement. For these reasons no equipment under test should be touched whilst tests are being undertaken, as parts of the equipment may be hazardous live. For similar reasons, tests should be conducted on suitable non-conductive surfaces and conductive objects should be kept well clear of the equipment.

The potential hazard is exacerbated by the use of automatic testers when running in automatic or semi-automatic modes since hazardous voltages may appear on the equipment under test at any time without any warning. Where it is not possible to remove equipment to a workshop facility for testing, particular care must be taken to ensure that there is no possibility of any other persons coming into contact with the equipment under test.

Many categories of medical electrical equipment can produce outputs for treatment purposes that, if applied incorrectly to a person can prove fatal, or at least cause serious injuries. Examples of these categories include surgical diathermy machines, nerve and muscle stimulators, short-wave therapy units and defibrillators. Persons who have not had specific training on such equipment sufficient to enable them to avoid the hazards should not be allowed to perform electrical safety testing on it.

The tests applied in the course of routine safety testing can cause damage to equipment if carried out incorrectly or inappropriately. Such damage may lead directly or indirectly to patient injuries or death if the equipment is put back into service in this condition. It is clear that only maintenance personnel who are sufficiently trained to avoid such occurrences arising should carry out electrical safety testing of medical equipment.