Most 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 currents
If 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:
- the voltage on the conductor.
- the capacitive reactance between the conductor and earth.
- 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 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.
3.3 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. The terms are further discussed in connection with the electrical test methods under paragraphs 6.6. 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 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
3.5 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