Hi Kris:
if the smaller capacitor with 2 points breaks
down, then a current should flow
Yes.
I think that's the definition or result of
"breaking down"
An insulator has two states: (1) an insulator,
usually having a resistance greater than 100
megohms, and (2) a conductor or resistor having a
resistance of very, very much less than 100
megohms. Breakdown is when the insulation
resistance drops to a low value and its insulation
properties (required by the circuit) are
destroyed.
if the larger capacitor with 2 planes still did
not break down, then there can be no current
flowing
Not true. The circuit is now a capacitor in
series with a resistor of low value. If dc, the
resistor will discharge the capacitor. If ac, the
impedance of the capacitor in series with the
resistor will provide a current path.
Most of the voltage now appears across the larger
capacitor. If this capacitor is solid insulation
(which has an electric strength many times that of
air) the solid insulation is not likely to fail.
Hope this helps to understand the situation.
Best regards,
Rich
From: Carpentier Kristiaan
[mailto:[email protected]]
Sent: Friday, June 12, 2015 2:34 PM
To: [email protected]; [email protected]
Subject: RE: [PSES] Series Isolation
Hi Rich and all others,
Thanks for your explanation, make things more
clear.
I still don't understand if the smaller capacitor
with 2 points breaks down, then a current should
flow (I think that's the definition or result of
"breaking down") but if the larger capacitor with
2 planes still did not break down, then there can
be no current flowing. Unless the explanation is
like Joe said:
2) In the example you used of an AC mains
barrier in series with an Ethernet barrier, the AC
mains supply typically has far more capacitance
across its barrier than the Ethernet barrier.
Note that in the case of Ethernet, the Ethernet
transceiver can be damaged even if the AC mains
barrier does not suffer catastrophic breakdown.
Even the small amount of current that couples
through the capacitance of the AC mains barrier
can be sufficient to damage some Ethernet port
designs
Do you have an idea about the question that the
breakdown of 2 insulations does not equal the sum
of both?
Best regards
Kris Carpentier,
From: Richard Nute [ <mailto:[email protected]>
mailto:[email protected]]
Sent: vrijdag 12 juni 2015 21:48
To: Carpentier Kristiaan;
<mailto:[email protected]>
[email protected]
Subject: RE: [PSES] Series Isolation
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]
<mailto:[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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