DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 101

1. Ground leakage current: current flowing from all mains parts through or across the

insulation into the protective ground conductor of the grounded power cord
2. Enclosure leakage current: total current

fl

flowing from the enclosure and all accessi-
ble parts (but excluding applied parts) through an external conductive connection
other than the protective ground conductor to ground or another part of the enclosure
3. Patient leakage current: current

fl

fl flowing from the applied part by way of the patient

to ground, or flowing from the patient via an F-type applied part to ground; origi-

nates from the unintended appearance of voltage from an external source on the
patient

fl

It must be noted, however, that these leakage currents must not be confused with cur-
rents generated intentionally by the medical device to produce a physiological e ect on the

patient, or used by the applied part to facilitate measurement without producing a physio-
logical e

ff

ect. Examples of patient intentional currents are those used for the stimulation
of nerves and muscle, cardiac pacing and de

ff

brillation, and cutting and cauterization
with radio frequency. Patient auxiliary currents are used to bias the front-end ampli

fi

ers
designed 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 by
de

fi

nition, the RMS value of an alternating voltage V across a resistor R equals the direct
voltage, causing the same dissipation level in R . For a sinusoidal waveform, the RMS volt-
age V RMS is related to the peak-to-peak voltage V p-p by

fi

V p - p V RMS 2 1.414 V RMS

A similarly corresponding de fi nition applies to the value of an RMS current. In the case of

composite (ac dc) signals, the RMS value is calculated from

V RMS V

2
dc

V

ac

RM

S

ned for
both normal and single-fault conditions, assuming that the equipment is operating at max-
imum load and that the supply is set at 110% of the maximum rated supply voltage. Single-
fault conditions are de

As shown in Table 3.2, allowable patient leakage and auxiliary currents are de fi

fi ned as conditions in which a single means of protection against a

TABLE 3.2 Some Allowable Values of Continuous Leakage and Patient Auxiliary Currents under Normal and
Single-Fault Conditions (Milliamperes)

Equipment Type

B BF CF

Condition Normal Single fault Normal Single fault Normal Single fault

Ground leakage current 0.5 1 0.5 1 0.5 1

Enclosure leakage current 0.1 0.5 0.1 0.5 0.1 0.5

Patient leakage current 0.1 0.5 0.1 0.5 0.01 0.05

Patient leakage current — — — 5 — 0.05

(with power line voltage
on the applied part)
Dc patient auxiliary current 0.01 0.05 0.01 0.05 0.01 0.05

Ac patient auxiliary current 0.1 0.5 0.1 0.5 0.01 0.05