ELECTRICAL SAFETY OF MEDICAL EQUIPMENT

BIO-MEDICAL DEPARTMENTMAHKOTA MEDICAL CENTRE

(unedited..yet)

Hazards of Medical Electrical Equipment

• Medical electrical equipment can present a range of hazards to the patient, the user, or to service personnel. Many such hazards are common to many or all types of medical electrical equipment, whilst others are peculiar to particular categories of equipment. The hazard presented by electricity exists in all cases where medical electrical equipment is used, and there is therefore both a moral and legal obligation to take measures to minimise the risk. Because there is currently very little official guidance on precisely what measures should be in place in order to achieve this in respect to medical equipment, user organisations have developed procedures based on their own experience and risk assessments. The information in these notes is intended to assist in the development of suitable procedures to this end.

• Any test and inspection regime intended to minimise the electrical risks from medical electrical equipment should take into account the likely degree of risk from electrical hazards compared to other hazards of medical equipment. For this reason, various hazards associated with medical electrical equipment are discussed briefly below.

Mechanical Hazards

• All types of medical electrical equipment can present mechanical hazards. These can range from insecure fittings of controls to loose fixings of wheels on equipment trolleys. The former may prevent a piece of life supporting equipment from being operated properly, whilst the latter could cause serious accidents in the clinical environment. Such hazards may seem too obvious to warrant mentioning, but it is unfortunately all too common for such mundane problems to be overlooked whilst problems of a more technical nature are addressed.

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 certain environments such fires may cause explosions. Although the use of flammable anaesthetics is not common today, it should be recognised that many of the medical gases currently in use, such oxygen or nitrous oxide, vigorously support combustion. Wherever there is an elevated concentration of such gases, there is an increased risk of fire initiated by electrical faults.

Absence of Function

• Since many pieces of medical electrical equipment are life supporting or monitor vital functions, the absence of function of such a piece of equipment would not be merely inconvenient, but could threaten life.

Excessive or insufficient output

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

Infection

• Medical equipment that has been inadequately decontaminated after use may cause infection through the transmission of microorganisms to any person who subsequently comes into contact with it. Clearly, patients, nursing staff and service personnel are potentially at risk here.

Misuse

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

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 is potentially greater since patients are intentionally connected to such equipment and may not benefit from the same natural protection factors that apply to people in other circumstances. Whilst all of the hazards listed are important, the prevention of many of them require methods peculiar to the particular type of equipment under consideration. For example, in order to avoid the risk of excessive output of surgical diathermy units, knowledge of radio frequency power measurement techniques is required. However, the electrical hazards are common to all types of medical electrical equipment and can minimised by the use of safety testing and inspection regimes which can be applied to all types of medical electrical equipment.

Physiological Effects of Electricity• ElectrolysisThe 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.

• BurnsWhen 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.

• Muscle crampsWhen 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.

• Respiratory arrestThe 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. • Cardiac arrestThe 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.

• Ventricular fibrillationThe 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.

• Effect of frequency on neuro-muscular stimulationThe 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.

• 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 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.

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). 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.

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. 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.

Figure 3. Enclosure leakage current path

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

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 a physiological effect (see figure 5).

Figure 5. Patient auxiliary current path

Classes and types of medical electrical equipment

• All 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.

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.

Figure 6. Symbols seen on earthed equipment.

• 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.

• 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.

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

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.

• 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".

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.

CF

Equipment 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

Electrical Safety Tests

• The following paragraphs and diagrams describe the electrical safety tests commonly available on medical equipment safety testers.

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.

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.

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.

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).

Applicable to Class I, all typesLimits: Not less than 50MΩ

DB9801 recommended?: YesHEI 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?: NoHEI 95 recommended?: Yes

Notes: Move probe to find worst case.

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

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.

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.

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.

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.

• 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 SFCDB9801 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

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.

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

Patient auxiliary current

• 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.

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.

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

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.

* 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.

Leakage current Type B Type BF Type CF

NC SFC NC SFC 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

Test and Inspection ProtocolsWhen to test

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

• During routine planned preventative maintenance.• 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.

Example inspection and test 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.

• 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 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 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.