This document discusses safety classifications for medical electrical equipment. There are three main classes - Class I uses protective earth grounding, Class II uses double insulation, and Class III operates at low voltages but is no longer recognized. There are also types B, BF, and CF that define the degree of protection against electric shock for different applications, with CF providing the highest protection. Proper classification and testing of equipment is important to ensure safety for patients.

1. TESTING EQUIPMENTFOR CHECKINGSAFETY
PARAMETERS

2. Hospitals are confronted with the difficult problem of
creating a safe electric environment for the care and
comfort of the patients. Electric shock burns and fire
hazards result from the careless use of electricity. When
electricity is relied upon to support life with devices like
external pacemakers, respirators, etc. power failure is a
continuous threat. Shock resulting from electric power is
a common experience. Disruption of physiologic function
by leakage current applied internally remains sometimes
hidden and mysterious. While faulty electric cords and
appliances contribute to the former, lack of concept and
faulty design are responsible for the latter.

3. Electric current can flow through the human body either
accidentally or intentionally. Electrical currents are
administered intentionally in the following cases:
For the measurement of respiration rate by impedance
method , a small current at high frequency is made to flow
between the electrodes applied on the surface of the body,
High frequency currents are also passed through the body
for therapeutic and surgical purposes,
When recording signals like ECG and EEG, the amplifiers
used in the preamplifier stage may deliver small currents
themselves to the patient. There are due to bias currents .
accidental transmission of electrical current can take place
because of a defect in the equipment; excessive leakage
currents due to defect in design; operational error (human
error) and simultaneous use of other equipment on the
patient which may produce potentials on the patient circuit.

4. All electrical equipment is categorized into classes
according to the method of protection against electric
shock that is used. For mains powered electrical equipment
there are usually two levels of protection used, called
"basic" and "supplementary" protection. The supplementary
protection is intended to come into play in the event of
failure of the basic protection.
• CLASS I EQUIPMENT:
• CLASS II EQUIPMENT:
• CLASS III EQUIPMENT:

5. Class I equipment has a protective earth. The basic means of protection is the
insulation between live parts and exposed conductive parts such as the metal
enclosure. In the event of a fault that would otherwise cause an exposed
conductive part to become live, the supplementary protection (i.e. the protective
earth) comes into effect. A large fault current flows from the mains part to earth
via the protective earth conductor, which causes a protective device (usually a
fuse) in the mains circuit to disconnect the equipment from the supply.
It is important to realize that not all equipment having an earth connection is
necessarily class I. The earth conductor may be for functional purposes only
such as screening. In this case the size of the conductor may not be large enough
to safely carry a fault current that would flow in the event of mains short to earth
for the length of time required for the fuse to disconnect the supply.
The symbols below may be seen on medical electrical equipment adjacent to
terminals.

6. The method of protection against electric shock in the case of class II
equipment is either double insulation or reinforced insulation. In double
insulated equipment the basic protection is afforded by the first layer of
insulation. If the basic protection fails then supplementary protection is
provided by a second layer of insulation preventing contact with live
parts.
In practice, the basic insulation may be afforded by physical separation of
live conductors from the equipment enclosure, so that the basic insulation
material is air. The enclosure material then forms the supplementary
insulation.
Reinforced insulation is defined in standards as being a single layer of
insulation offering the same degree of protection against electric shock as
double insulation.
Class II medical electrical equipment should be fused at the equipment
end of the supply lead in either mains conductor or in both conductors if
the equipment has a functional earth.
The symbol for class II equipment is two concentric squares illustrating
double insulation as shown below.

7. Class III equipment is defined in some equipment standards as that in
which protection against electric shock relies on the fact that no
voltages higher than safety extra low voltage (SELV) are present. SELV is
defined in turn in the relevant standard as a voltage not exceeding 25V
ac or 60V dc.
In practice such equipment is either battery operated or supplied by a
SELV transformer.
If battery operated equipment is capable of being operated when
connected to the mains (for example, for battery charging) then it must
be safety tested as either class I or class II equipment. Similarly,
equipment powered from a SELV transformer should be tested in
conjunction with the transformer as class I or class II equipment as
appropriate.
It is interesting to note that the current IEC standards relating to safety
of medical electrical equipment do not recognize Class III equipment
since limitation of voltage is not deemed sufficient to ensure safety of
the patient. All medical electrical equipment that is capable of mains
connection must be classified as class I or class II. Medical electrical
equipment having no mains connection is simply referred to as
"internally powered".

8. As described above, the class of equipment defines the method of
protection against electric shock. The degree of protection for medical
electrical equipment is defined by the type designation. The reason for
the existence of type designations is that different pieces of medical
electrical equipment have different areas of application and therefore
different electrical safety requirements. For example, it would not be
necessary to make a particular piece medical electrical equipment safe
enough for direct cardiac connection if there is no possibility of this
situation arising.
The symbols and definitions for each type classification of medical
electrical equipment.

9. TYPE SYMBOL DEFINITION
B
Equipment providing a particular
degree of protection against electric
shock, particularly regarding
allowable leakage currents and
reliability of the protective earth
connection (if present).
BF
As type B but with isolated or floating (F -
type) applied part or parts.
CF
Equipment providing a higher degree of
protection against electric shock than
type BF, particularly with regard to
allowable leakage currents, and having
floating applied parts.