Biomedical Engineering Reference
In-Depth Information
standards de
ne rigid and articulated probes that must be used to verify the acceptability
of enclosure openings.
The second type of part is the
live part
, a part that when contacted can cause the leakage
current to ground or to an accessible part of the equipment to exceed the limits established
by the standard. One form of live part is the mains part, de
fi
fi
ned as a circuit connected
directly to the power line.
The third type of part comprises
signal-input and signal-output parts
, referring to cir-
cuits used to interface a medical instrument to other instruments: for example, for the pur-
poses of displaying, recording, or processing data. The fourth and most critical part of a
medical instrument is that which deliberately comes into physical contact with the patient.
Such a part, called an
applied part
, may include a number of patient connections which
provide an electrical pathway between it and the patient. The patient circuit comprises all
patient connections as well as all other parts and circuits of the medical instrument that are
not electrically isolated from these connections.
The level of electrical shock protection provided to patients by the isolation of applied
parts classi
fi
es them as follows:
•
Type B
: applied parts that provide a direct ground connection to a patient
•
Type BF
(the F stands for “
“floating”): indicates that the applied part is isolated from all
other parts of the equipment to such a degree that the leakage current
fl
flowing through a
patient to ground does not exceed the allowable level even when a voltage equal to 110%
of the rated power line voltage is applied directly between the applied part and ground
•
Type CF
: similar to type BF, but refers to applied parts providing a higher degree of
protection, to allow direct connection to the heart
fl
The use of F-type applied parts is preferable in all cases to type B applied parts. This is
because patient environments often involve simultaneous use of multiple electronic instru-
ments connected to the patient. In any case, type B applied parts are prohibited whenever
patient connections provide either low-impedance or semipermanent connections to the
patient (e.g., through recording bioelectrodes as in ECG or EEG, or for stimulation of tissues,
such as TENS). Furthermore, all medical electrical equipment intended for direct cardiac
application (e.g., intracardiac electrophysiology catheters, invasive cardiac pacing) must con-
tain only CF-type applied parts. Additionally, the applied parts of instruments for cardiac
diagnosis and therapy are often designed to withstand the application of high-voltage high-
energy shocks, such as those used for cardiac cardioversion and de
fi
brillation.
cations have more than academic purpose. The standards provide the
designer with clear indications regarding the minimal level of circuit separation and the
application of insulation between these parts to accomplish acceptable levels of isola-
tion. As such,
insulation
is not only de
These classi
fi
ned as a solid insulating material applied to a
circuit, but also to spacings that establish creepage distances and air clearance between
parts. The separation of two conductive parts by air alone constitutes a
clearing distance
,
while the separation of conductive parts on a nonconductive plane (e.g., tracks on a
printed circuit board) is a
creepage distance
. The minimum separation distance between
elements of two parts is determined by the working voltage between parts as well as by
the insulation rating required to a
fi
ord protection against electrical shock.
A basic insulation barrier is applied to live parts to provide basic protection against
electrical shock. For example, its use applies to the separation between a live part and an
accessible conductive part that is protected by connection to ground. Supplementary insu-
lation is an independent insulation barrier applied in addition to basic insulation in order
to provide protection against electrical shock in the event of failure of the basic insulation.
Double insulation and reinforced insulation provide protection equivalent to the use of
both basic and supplementary insulation.
ff
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