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What
is so special about Medical Grade Isolation Transformers?
Medical Grade Transformers generally refer to the transformers
used in medical devices as well as hospital, biomedical and
patient care equipment. There are a number of strict safety
rules, guidelines and laws governing the design, construction
and the test of these transformers. These safety rules, guidelines
and laws include, but are not limited to:
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The
limit of maximum earth leakage current |
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The
maximum patient leakage current |
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The
maximum enclosure leakage current |
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The
maximum patient auxiliary current |
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The
values of test voltage (high potential voltage test between
windings) |
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The
minimum creepage distance |
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The
minimum air clearance |
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The
maximum temperature rise at load and at over load |
One
can find these rules and regulations in UL 2601-1, IEC 601-1,
CSA C22.2 No. 601 and EN 60601-1 standards.
The majority of medical grade transformers are isolation transformers.
There are generally two types of isolation, one that relies
on Safety Ground (Protective Earth), and one that relies on
Double or Reinforced Insulation. A transformer that relies on
safety ground uses a basic isolation between the primary and
the safety shield and shield to secondary. This shield has to
be thick to be able to meet required tests in the safety standard.
If the isolation breaks, the electrical path goes directly to
ground, providing safety. For a transformer that relies on double
or reinforced Insulation there is no "safety" shield,
but the insulation as indicated by the name, is much thicker.
It is designed so that all layers of the insulation can pass
the thickness and high potential voltage tests required by the
standard. If one insulation layer breaks, the next layer will
be able to provide the required safety. In both the design options,
the construction design must still meet the required creepage
and clearance requirements. If the transformer includes a static
shield for noise reduction, it is important to note that this
shield does not provide the transformer with Safety Ground but
it working as a functional earth. When the shield is grounded,
it attenuates the common mode noise and can reduce the leakage
current from the primary to the secondary winding (please note
that this leakage reduction is not the same as when measured
Primary to Ground).
Normally, without extra mechanical barriers between the primary
and shield and secondary, a transformer with double/reinforced
insulation has lower leakage current than the one with Safety
Shield.
A Safety Shield wire should be connected to the Protective Earth
terminal in the equipment while the Static Shield, if included,
should be connected to the Functional Earth terminal
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