医疗器械漏电流测试示意图Word文档下载推荐.docx

1、All types of medical electrical equipment can present mechanical hazards. These can range from insecure fittings of controls to loose fi*ings of wheels on equipment trolleys. The former may prevent a piece of life supporting equipment from being operated properly, whilst the latter could cause serio

2、us accidents in the clinical environment. Such hazards may seem too obvious to warrant mentioning, but it is unfortunately all too mon for such mundane problems to be overlooked while more e*otic problems are addressed. 1.2 Risk of fire or e*plosionAll mains powered electrical equipment can present

3、the risk of fire in the event of certain faults occurring such as internal or e*ternal short circuits. In certain environments such fires may cause e*plosions. Although the use of e*plosive anaesthetic gases is not mon today, it should be recognised that many of the medical gases in use vigorously s

4、upport bustion. 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 E*cessive or insufficient outputIn order to per

5、form its desired function equipment must deliver its specified output. Too high an output, for e*ample, 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 o

6、r 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 es into contact with it. Clearly, patients,

7、 nursing staff and service personnel are potentially at risk here. 1.6 MisuseMisuse of equipment is one of the most mon 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 e*posure to spurious electr

8、ic currentsAll electrical equipment has the potential to e*pose 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 protec

9、tion 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 e*ample, in order to avoid the risk of e*cessive output of surgical diathermy

10、 units, knowledge of radio frequency power measurement techniques is required. However, the electrical hazards are mon 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 reas

11、ons, 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 tiss

12、ues 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

13、 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 underneat

14、h the skin has a much lower resistance. Electrical burns often produce their most marked effects near to the skin, although it is fairly mon 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 a

15、pplied to a motor nerve or a muscle, the muscle does e*actly 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 e*ternal electrical stimulus is responsible for the phenomenon where a person who is ho

16、lding 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 rela* in order to facilitate breathing. Prolonged tetanus of these muscles can therefore prevent breathing. 2.5 Cardiac arrestThe heart

17、is a muscular organ, which needs to be able to contract and rela* 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

18、 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

19、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 freque

20、ncy on neuro-muscular stimulationThe amount of current required to stimulate muscles is dependent to some e*tent 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 1

21、0 kHz the neuro-muscular response to current decreases almost e*ponentially. 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

22、is the e*istence of certain natural protection factors. Ordinarily, a person subject to an une*pected electrical stimulus is protected to some e*tent by automatic and intentional refle* actions. The automatic contraction of muscles on receiving an electrical stimulus often acts to disconnect the per

23、son 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 rela

24、tively 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 pared to the moist body tissues below, and hence serves to reduce the amount of current that would otherwise flow. Again, a patie

25、nt 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 ab

26、sence 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 conducto

27、r 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

28、 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, s

29、ince 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 Ear

30、th 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 w

31、ay 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. T