The subject answers your question about the constraints
on such testing:
Equipment insulation testing.
For the purposes of this discussion, there are two
applications (or kinds) of insulation:
1) Insulation between the poles of a circuit or the
insulation within a circuit (also known as
"functional" or "operational" insulation).
2) Insulation between two circuits that are electrically
isolated from each other (e.g., insulation between
primary and secondary circuits; insulation between
primary circuits and ground).
In the field of safety, insulation is the principal means
of providing protection against electric shock. When the
voltage of a circuit exceeds an arbitrary value (e.g.,
30 V rms), insulation must be interposed between the body
and the circuit conductors. This interposed insulation
protects the body from electric shock from the circuit.
(In the field of safety, the body is presumed to be
electrically connected to ground.)
Insulation that is interposed between the body and the
circuit conductors, by definition, is an insulation
between two circuits that are electrically isolated from
each other, (2). The functional circuit is one circuit,
and the body, by virtue of being connected to ground, is
another circuit. The insulation between the body and the
circuit is an insulation that provides isolation between
two circuits.
Or, consider the consequences of a short-circuit of the
insulation. If the short-circuit would result in a high
current through the body, then the insulation is an
isolating insulation. If the short-circuit would result
in an equipment malfunction (and no current through the
body), then the insulation is not an isolating insulation
but a functional insulation.
Consider the overhead power line. The air between you and
the wires is an isolating insulation, and provides
protection against electric shock. However, the air
between any two wires is a functional insulation.
As a general rule, isolating insulations are not bridged
by components (because the circuits would no longer be
isolated). Functional insulations are necessarily
bridged by components (because they require voltage or
current for operation).
There are two insulation tests commonly required for
isolating insulations:
a) Insulation resistance test.
b) Insulation voltage withstand (hi-pot) test.
Neither test can be applied to functional insulation
because the insulation is bridged by various components
which would either limit the voltage or be destroyed by
the voltage.
Writers of early safety standards did not fully understand
the difference between the two insulations. Manufacturers
complained that the voltage test would damage components.
So, since they were only concerned with the integrity of
insulation, all parties involved readily accepted a
proviso that pole-to-pole components could be disconnected
during the voltage test.
Another conclusion to the question of what insulations
require testing (sorting out isolating insulations from
functional insulations) is simply that those circuits
that would be damaged by the voltage test are not
isolating insulations.
Best regards,
Rich
ps: Yes, some isolating insulations do have components
across them: Y capacitors and high-value resistors.
A capacitor is two conductors separated by an
insulator. Y capacitors are especially designed to
withstand the voltage test.
An insulation is a high resistance component, where
the value of the resistance is sufficiently high as
to be largely inconsequential to the function.
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