71277350 classes and types of medical electrical equipment

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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 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 arrest

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 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 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 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, 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 currentThe 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.

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


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