Hi John and Richard:
The transformer screen/shield between primary
and secondary windings is intended to carry
the fault current in the event of a failure
of basic insulation between the primary and
the screen/shield.
The screen/shield must be capable of carrying
the full fault current and is often tested
using the 25-amp test.
The object of the test is to connect to the
screen/shield, and then pass the 25 amps
through the shield to its transformer terminal
and then to the PE terminal.
The difficult part is how to connect the
tester to the screen/shield. When the screen/
shield is a copper foil, a special transformer
must be wound with a wire connected (soldered)
to the foil screen/shield and brought out for
connection to the 25-amp source. If this
cannot be done, then a wire is soldered to the
edge of the foil screen/shield by forcefully
separating the sheet insulation that extends
beyond the edge of the shield, inserting the
soldering iron tip, and soldering the wire.
(A constructional problem is that the screen/
shield must extend BEYOND the primary winding
so that no part of the primary winding is
"exposed" to the secondary winding.)
(Another constructional problem is that the
ends of the foil must be overlapped, but with
insulation inserted between the overlaps;
otherwise, the screen/shield would constitute
a shorted turn and cause all kinds of
electrical, magnetic, and thermal problems.)
The connection of the tester to the foil
screen/shield typically has a very large
contact area. Any thickness of foil screen/
shield passes the test.
The conundrum is that the failure of basic
insulation could result in a point contact
between the primary winding and the shield.
The point-contact, because of its small area,
has very high current density, and will blow
a hole in thin foil (that otherwise passes
the 25-amp test).
With the hole in the screen/sheild, the fault
to the foil opens, and the fault no longer
exists -- even though there is a failure of
basic insulation. The connection of the foil
to the PE remains intact, and there is no
shock hazard.
(Of course, the heat of the point-contact
fault may very well cause failure of the foil
screen/shield-to-secondary functional
insulation, which might very well result in a
shock hazard from the secondary SELV circuit.)
The point is that the test does not
necessarily test what can occur within the
transformer, yet will pass the transformer
screen/shield construction.
Nevertheless, when the basic insulation fault
occurs, the hole in the screen/shield may very
well end the event safely.
Best regards,
Rich
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