Biomedical Engineering Reference
In-Depth Information
4.1.17.5 Electrical safety
of electromedical equipment
In
Out
Signal
Power lin
e
Special safety precautions are taken for electromedical
equipment. Both patient and operator safety is considered
(as well as damage of property).
Electromedical equipment
is equipment which is situated in the patient environment
and is in physical contact with the patient, or which can
deliver energy (electrical, mechanical or radiation) to the
patient froma distance. Equipment for in vitro diagnosis is
also important for patient safety with respect to correct
diagnostic answers, but as long as it is not in the patient
environment the safety aspects are different.
The basis for the national or international standards
(IEC, UL, VDE, MDD (the European Medical Device
Directive), etc.) is to reduce the risk of hazardous currents
reaching the patient under normal conditions. Even under
a
single fault condition
patient safety shall be secured.
The part of the equipment in physical contact with
the patient is called the
applied part.
It may ground the
patient (type B applied part), or keep the patient floating
with respect to ground (BF - body floating, or CF -
cardiac floating) by
galvanic separation
circuitry (mag-
netic or optical coupling, battery-operated equipment).
In most situations higher safety is obtained by keeping
the patient floating. If the patient by accident comes in
contact with a live conductor the whole patient will be
live, but
little current
will flow.
Figure 4.1-32
shows the most important parts of an
electromedical device. The power line and earth con-
nection are shown to the right. The signal connections to
the outside world are shown at the upper part. Important
safety aspects are linked with these signal input and
output parts: they may be connected to recorders,
printers, dataloggers, data networks, coaxial video cables,
synchronization devices, etc. These devices may be re-
motely situated and outside the electrical control of the
patient room. With a floating applied part hazardous
Electromedical
equipment
(device)
Patient
(applied part)
Protective earth (PE)
?
Figure 4.1-32 Basic parts of a grounded (class I) electromedical
device.
currents from the outside do not reach the patient, the
galvanic separation protects both ways.
The device may be grounded for safety reasons (safety
class I, as shown in
Fig. 4.1-32
, maximum resistance in
the
protective earth
(PE) conductor between power plug
and chassis 0.2
U
), or double insulated (safety class II).
Leakage currents
are currents at power line frequency
(50 or 60 Hz), they may be due to capacitive currents
even with perfect insulation, and are thus difficult to
avoid completely.
Patient leakage currents
are the leakage
currents flowing to the patient via the applied part. Pa-
tient
auxiliary currents
are the functional currents
flowing
between
leads of the applied part (e.g. for
bioimpedance measurement). They are not leakage cur-
rents and therefore usually not at power line frequency.
Earth leakage current
is the current through the ground
wire in the power line cord (not applicable for double
insulated devices). According to IEC it shall be
<
500
m
A
during normal conditions for all types B, BF or CF.
Enclosure leakage current
is a possible current from
a conductive accessible part of the device to earth.
Grounded small devices have zero enclosure current
during normal conditions, but if the ground wire is
broken the enclosure leakage current is equal to the earth
leakage current found under normal conditions.
The current limits according to IEC-60601-1 (2005)
is shown in
Table 4.1-4
.
Table 4.1-4 Allowable values of continuous leakage and patient auxiliary currents (mA) according to IEC-60601 (N.C. ΒΌ normal conditions)
Currents
Type B
Type BF
Type CF
N.C.
Single fault
N.C.
Single fault
N.C.
Single fault
Earth leakage
500
1000
500
1000
500
1000
Higher values, for example,
stationary equipment
Patient leakage
100
500
100
500
10
50
Patient leakage
5000
50
Mains on applied part
Patient leakage
5000
Mains on signal part
Patient auxiliary
100
500
100
500
10
50
AC
Patient auxiliary
10
50
10
50
10
50
DC
