Hi Kris:
I'm also interested to understand the process of
the breakdown of 2 isolations in series, like how
can 1 insulation break down when the other did not
yet.
Think of two capacitors in series. Even though
there is no conductor at the center point of the
two capacitors, the voltage divides between the
two capacitors inversely proportional to the
capacitance. That is, the smaller voltage is
across the larger capacitor and the larger voltage
is across the smaller capacitor.
If one of the capacitor's dielectric is air, most
of the voltage will appear across the air
capacitor, while a small amount of voltage will
appear across the solid dielectric. This is
because the dielectric constant of air is 1, while
the dielectric constant of a solid dielectric is
more than 1 (assuming the area of the capacitor
plates is the same).
Thus, the air dielectric capacitor has more than
half of the total voltage and can break down while
the solid dielectric capacitor has less than half
of the total voltage and does not break down.
This same phenomenon can occur when the
capacitance is determined by the area of the
plates rather than different dielectric constants.
Consider a worst-case of one capacitor comprised
of two points and the other capacitor comprised of
two planes. In this case, the smaller capacitor
is the one comprised of two points, and will break
down while the other capacitor comprised of two
planes will not break down.
Hope this explanation helps.
Best regards,
Rich
From: Carpentier Kristiaan
[mailto:[email protected]]
Sent: Friday, June 12, 2015 8:06 AM
To: [email protected]
Subject: [PSES] Series Isolation
Hi group,
When 2 isolation components are put in series, my
experience is that the total isolation breakdown
voltage measured with an impulse voltage (ex.
1.2/50us) is always smaller than the sum of both.
An example could be the isolation between 2
transformers, for example between the AC mains
(transformer AC-SELV) and an Ethernet transformer
(SELV-ETH). Another example could be between 2
open relay contact, with each contact 2.5kV but
the sum "only" 3.5kV.
Does anyone know of literature that explains this
phenomenon?
I'm also interested to understand the process of
the breakdown of 2 isolations in series, like how
can 1 insulation break down when the other did not
yet.
Thanks & best regards
Kris Carpentier,
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