医疗器械漏电流测试示意图.doc

Safety Testing of Medical Electrical Equipment1 Hazards of Medical Electrical EquipmentMedical 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. Listed below are various types of common hazards. 1.1 Mechanical HazardsAll 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 while more exotic problems are addressed. 1.2 Risk of fire or explosionAll 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 explosive anaesthetic gases is not common today, it should be recognised that many of the medical gases in use vigorously support combustion. 1.3 Absence of FunctionSince 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. 1.4 Excessive or insufficient outputIn 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. 1.5 InfectionMedical 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. 1.6 MisuseMisuse 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. 1.7 Risk of exposure to spurious electric currentsAll 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 regimes which can be applied to all types of medical electrical equipment. For these reasons, it is the electrical hazards that are the main topic of this session. 2 Physiological effects of electricity2.1 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. 2.2 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. 2.3 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. 2.4 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. 2.5 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. 2.6 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 20A 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 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. 2.8 Natural protection factorsMany 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 are 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. 3 Leakage currents3.1 Causes of leakage currentsIf 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 currentEarth 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, then a shock hazard exists. It is a fundamental safety requirement that in the event of a single fault occurring, such as the earth becoming open circuit, no hazard should exist. It is clear that in order for this to be the case in the above example, the impedance between the transformer primary 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 reduced. 3.3 Enclosure leakage currentEnclosure 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. However, 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. 3.4 Patient leakage currentPatient 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 currentThe 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. 6 Electrical Safety Tests6.1 Normal condition and single fault conditionsA 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 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 single fault conditions since these 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. 6.2 Protective Earth ContinuityThe 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 0.2 O 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 recommend 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 toClass I, all types Limit:0.2DB9801 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 TestsIEC 60601-1, 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 recommend that for class I equipment the insulation resistance is measured at the mains plug between the live and neutral pins connected together and the earth pin. Whereas HEI 95 recommends using a 500V DC insulation tester, DB 9801 recommends the use of 350V DC as the test voltage. In practice this last requirement could prove difficult and it is 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 50M but may be less in exceptional circumstances. For example, equipment containing mineral insulated heaters may have an insulation resistance as low as 1M with no fault present. The test should be conducted with all fuses intact and equipment switched on (see figure 9). Applicable toClass I, all typesLimits:Not less than 50MDB9801 recommended?: YesHEI 95 recommended?: YesNotes:Equipment containing mineral insulated heaters may give values down to 1M. Check equipment is switched on. Figure 9. Measurement of insulation resistance for class I equipmentHEI 95 further recommends 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 les