TECA's toroidal transformers produced for electromedical equipment fully respect
the norms of quality and safety specific to that sector, according to the following
standards:
• EN61-558-2-15 regarding specifications
and particular characteristcs of insulation for transformers
which supply medical sites. Essential electrical characteristics
are: maximum supply voltage of 1000Vac, maximum exit voltage
below 250Vac, frequency below 500Hz, power between 3kVA and 10kVA,
monophase or multiphase.
• EN60601 regarding general safety
norms for electromedical equipment. In particular supply
transformers must be adequately protected against overload (protected
through construction, protected but not through construction
or protected against faults), the coils must be adequately insulated
so as to limit dispersion current within the values established
by the norms. Electrostatic screens may be placed between the
insulation of the various coils to increase galvanic insulation.
• UL544 (norm which refers to the
North American market) regarding medical and dental equipment. Particular
attention is paid to the differences between pieces of equipment
which come into contact with the patient in different ways (patient
care equipment or non-patient equipment). Here too the maximum
values of dispersion current, galvanic insulation, safety protection,
self-extinguishing capacity and the circuits necessary to carry
out these checks are all established precisely.
Since transformers for electromedical equipment have characteristics which differ
greatly from transformers in general use, it may be useful to underline some
important points which demonstrate the attention with which TECA produces and
tests these objects which are indispensable in equipment for check-ups, diagnosis
and care, all fundamental for our health.
Dispersion
current
This type of non-functional current crosses or slightly touches
the insulation and can be divided into:
• Dispersion current towards earth when it flows from the network
connection towards the protection conductor or along the insulation
(with a maximum value of 3.5mA for the norm EN61-558-2-15 and a value
of 2/1/0.5 mA for the norm EN60601 appendix k, depending on the class
of equipment;
• Dispersion current on the protective cover when it flows from
the protective cover or part of it towards earth or towards a different
part of the protective cover itself across an external conductor
(which should not be the protection conductor, maximum value 0.1mA
for the norm EN60601);
• Dispersion current in the patient when it flows from the part
applied to the patient towards earth or from the patient towards
earth through an insulated applied part (0.1mA norm EN60601).
It is easy to understand that the maximum values of the currents
established by the norms are extremely low, given the dangers for
the safety of the patients. This is why TECA is extremely careful
both in the design phase and in the successive production phases
to produce coils with particular characteristics and also to increase
the thickness of insulation, all of which helps to limit the dispersion
current.
Equipment
Any electrical or electronic device destined for use near
to and/or attached to the patient in such a way that his safety
can be affected may be defined as electromedical equipment. For
example: diagnostic apparatus (like electrocardiographs, X-rays,
ultrasound devices for nuclear medicine etc), equipment for therapy
and surgery (defibrillators, muscle stimulators, dentists' drills
etc), general equipment (dentists' chairs, operating tables
etc).
Equipment used in the medical sphere can be categorised on the
basis of how far it is from the patient:
• Patient equipment: equipment used in sanitary structures
near the patient, for example paramedical applications, dialysis
rooms, dentists' surgeries etc;
• Non-patient equipment: equipment used in sanitary structures
but not used near the patient.
The fact that TECA knows the final use of the equipment allows
the company to design and test the transformers together with the
client according to the specific application; this will certainly
be less expensive and less complex in the case of non-patient equipment,
compared with the cost for patient equipment.
Screens
"Separation between two coils or protection of one
part or of the whole transformer by means of a conducting material
for functional reasons"
This is the official definition of screens supplied by the technical
norms, where the aim is to increase safety conditions and the performance
of the transformers, reducing the dispersion current and increasing
the level of insulation between the coils with high voltage and
the other coils.
Thanks to the production of
electrostatic
screens between primary and secondary coils connected
to
earth, produced with binding tape, with insulated copper film,
wound over all the surface, the galvanic insulation value is significantly
increased (insert detail of electrostatic screen).
A qualitative and functional aspect to be considered in the presence
of electronic components is the need to limit the magnetic flow
lost; this is because magnetic disturbances can interfere with
electronic details of particularly sensitive medical equipment.
The particular configuration of a continuous core without butt-gaps
or discontinuity typical of the toroidal transformer already in
itself guarantees a significantly reduced dispersion flow compared
with a traditional transformer. But for some versions particular
electrostatic screens are produced which prevent all dispersion
of magnetic flow.
Protections
A further classification for electromedical transformers
can be made according to the type of electrical protection against
overloads and short circuits, in three groups:
• Resistent to short circuits due to the way it is constructed: design and construction methods guarantee that the temperatures
do not exceed the limits specified in the norms during overload
and/or short circuit and that when the fault has been removed,
the transformer goes back to its regular function without damage
or deterioration in the coils and insulation;
• Resistent to short circuits but not through construction: the transformer has a device that is an integral part of it which
prevents the temperatures established by the norms being exceeded
in cases of overload and/or short circuit, through the interruption
of the supply to the circuit, with a return to regular function
when reset or when the protective device has been replaced;
• Not resistent to short circuits: in this case the part relative
to the protection of the transformer is the responsibility of the
user, who should provide his equipment with protective devices
which will guarantee conditions of protection and safeguard all
the electrical in cases of overload and/or short circuit. All the
protection values to use for correct protection, according to tests
and trials indicated by the norms of reference, are shown on the
transformer's label.
As for all the other technical and construction aspects, TECA recommends
the client to use forms of protection suitable for the type and
function of the equipment.
Tests
As we have seen, the aspect of safety is paramount for electromedical
transformers, thus the tests that these transformers must pass
are very strict. In particular:
• The test of the dialectric seal of the insulation establishes
a resistance that is more than doubled, with a single voltage test
which is for double or reinforced insulation
• The dispersion current between the input and output circuit
when the input is supplied at nominal voltage and at nominal frequency
must not exceed 3.5mA (norm EN61-558-2-15), measured with the circuit
shown in the diagram:
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