医疗器械漏电流测试示意图.docx

1、医疗器械漏电流测试示意图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

2、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 troll

3、eys. 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 o

4、verlooked 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 t

5、he 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 s

6、uch 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 haza

7、rdous. Equally, too lowan 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 c

8、ause 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 medic

9、al 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 r

10、isk 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

11、 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 equipme

12、nt 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

13、polarities in opposite directions through a medium is called electrolysis and can be made to occur bypassing 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 pa

14、inful 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

15、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 electric

16、al 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 involunt

17、ary 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

18、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 muscula

19、ture 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, uncoordinatedtwitching resulting in no

20、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 20 A directly through the heart may be sufficient. It is for

21、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 tha

22、t 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 factor

23、sMany 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 int

24、entional 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

25、 the clinical environment who may have electrical equipmentintentionally 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 c

26、ompared 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 monitor

27、ing 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

28、 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 ea

29、rth, 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 betw

30、een 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 medi

31、cal 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 equipmen

32、t, 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 wit