DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

TESTING FOR COMPLIANCE 131The frequency-response characteristics of the AAMI load have been selected to approximatethe inverse of the risk current curve as a function of frequency. In turn, this risk currentcurve was derived from strength/frequency data for perceptible and lethal currents.These data showed that between 1 and 100 kHz, the current necessary to pose the samerisk increases proportionately to 100 times the risk current between dc and 1 kHz. Sinceinsufficient data exist above 100 kHz, AAMI decided not to extrapolate beyond 100 kHz,but rather, to maintain the same risk current level corresponding to 100 kHz.Actual current measurements should be conducted after preconditioning the deviceunder test in a humidity cabinet. For this treatment, all access covers that can be removedwithout the use of a tool must be opened and detached. Humidity-sensitive componentswhich in themselves do not contribute significantly to the risk of electrocution may also beremoved. Next, the equipment is placed in the humidity cabinet containing air with a relativehumidity of 91 to 95% and a temperature t within the range 20 to 32°C for 48 hours(or 7 days if the instrument is supposed to be drip-proof or splash-proof). Prior to placingit in the humidity cabinet, however, the equipment must be warmed to a temperaturebetween t and t 4°C.The measurement should then be carried out 1 hour after the end of the humidity preconditioningtreatment. Throughout this waiting period and during testing, the same temperaturet must be maintained, but the relative humidity of the environment must onlybe 45 to 65%. Testing should be performed with the equipment’s on–off switch in bothconditions while connected to a power supply set at 110% of the maximum rated supplyvoltage. When operational, the maximum rated load must be used. As mentioned in thefirst part of this chapter, allowable patient leakage and auxiliary currents are defined forboth normal and single-fault conditions. Single-fault conditions are defined as conditionsin which a single means of protection against a safety hazard in the equipment is defectiveor a single external abnormal condition is present. Specific single-fault conditions thatmust be simulated during testing include interruption of the supply by opening the neutralconductor as well as the interruption of the protective ground conductor. Patient leakagecurrent between an F-type applied part and ground assumes that an external voltage equalto 110% of the maximum rated supply voltage is in direct connection with the applied part.For battery-powered equipment, the external voltage that is assumed to be connected to theF-type applied part is 250 V.Leakage current tests are conducted as shown in Figure 3.24a–d, with the device’spower switch in the on and off conditions and creating the single-fault conditions specifiedin the figure. If the enclosure or a part thereof is made of insulating material, a piece ofmetal foil 20 cm 10 cm applied to the nonconductive part of the enclosure must be usedas the protectively grounded enclosure connection. The metal foil is wrapped on the surfaceof the insulating enclosure, simulating the way in which a human hand could act as acapacitively coupled electrode.The connections for measuring patient auxiliary currents are shown in Figure 3.24e.Here, the current flowing between each patient connection and every other patient connectionis measured under normal and single-fault conditions. For this test, the measuringinstrument should be capable of differentiating the ac components from the dc componentsof the RMS current reading. As you can see from Table 3.2, different ac and dc auxiliarycurrent levels are permitted to flow through the patient, depending on the use intended forthe equipment.Versatile MicroammeterFigures 3.25 to 3.28 present the schematic diagrams of a versatile instrument for the measurementof leakage and auxiliary currents. The core of the circuit is an AAMI load thatconverts a leakage or auxiliary current into a voltage waveform with a factor of 1 V/mA