DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

LEAKAGE CURRENTS 1011. Ground leakage current: current flowing from all mains parts through or across theinsulation into the protective ground conductor of the grounded power cord2. Enclosure leakage current: total current flowing from the enclosure and all accessibleparts (but excluding applied parts) through an external conductive connectionother than the protective ground conductor to ground or another part of the enclosure3. Patient leakage current: current flowing from the applied part by way of the patientto ground, or flowing from the patient via an F-type applied part to ground; originatesfrom the unintended appearance of voltage from an external source on thepatientIt must be noted, however, that these leakage currents must not be confused with currentsgenerated intentionally by the medical device to produce a physiological effect on thepatient, or used by the applied part to facilitate measurement without producing a physiologicaleffect. Examples of patient intentional currents are those used for the stimulationof nerves and muscle, cardiac pacing and defibrillation, and cutting and cauterizationwith radio frequency. Patient auxiliary currents are used to bias the front-end amplifiersdesigned to detect biopotentials, to enable the measurement of impedance of living tissues,and so on.In the standards, the terms voltage and current refer to the root-mean-square (RMS)values of an alternating, direct, or composite voltage or current. Remember that bydefinition, the RMS value of an alternating voltage V across a resistor R equals the directvoltage, causing the same dissipation level in R. For a sinusoidal waveform, the RMS voltageV RMS is related to the peak-to-peak voltage V p-p byV p-p V RMS 2 1.414V RMSA similarly corresponding definition applies to the value of an RMS current. In the case ofcomposite (ac dc) signals, the RMS value is calculated fromV RMS V 2 dcV 2acRMSAs shown in Table 3.2, allowable patient leakage and auxiliary currents are defined forboth normal and single-fault conditions, assuming that the equipment is operating at maximumload and that the supply is set at 110% of the maximum rated supply voltage. Singlefaultconditions are defined as conditions in which a single means of protection against aTABLE 3.2 Some Allowable Values of Continuous Leakage and Patient Auxiliary Currents under Normal andSingle-Fault Conditions (Milliamperes)Equipment TypeB BF CFCondition Normal Single fault Normal Single fault Normal Single faultGround leakage current 0.5 1 0.5 1 0.5 1Enclosure leakage current 0.1 0.5 0.1 0.5 0.1 0.5Patient leakage current 0.1 0.5 0.1 0.5 0.01 0.05Patient leakage current — — — 5 — 0.05(with power line voltageon the applied part)Dc patient auxiliary current 0.01 0.05 0.01 0.05 0.01 0.05Ac patient auxiliary current 0.1 0.5 0.1 0.5 0.01 0.05