Hi Richard,
There are many thousands of initiating events. The EFT test runs at 5
kHz to 100 kHz for 15 ms bursts every 300 ms for a minute. In real life,
I have measured the EFT pulses coming in at 10 MHz, so the initiating
events will extend for many cycles of the power and at all relative
phase angles. If coming from a motor commutator, the EFT pulses would be
coming continuously for a long period of time.
I can regularly cause breakdown of various supplies that are UL
certified using 800 Volt ESD or EFT, even lower to 500 Volts, and that
is the generator setting. Less would be impressed across the barrier
except for resonance. Obviously there is a resonance happening that
increases the peak voltage and explains why some of the breakdowns are
in the opposite current direction from the applied stress.
So my question really is, given all these miniature breakdowns, within a
cycle of 60 Hz, would the ionized path allow more 60 Hz current to flow
even though there is not enough 60 Hz voltage itself to cause a
breakdown? If so, a number of 60 Hz cycles would be involved, not just
one, and in the case of a motor commutator, continuously for a long
period of time.
Doug
University of Oxford Tutor
Department for Continuing Education
Oxford, Oxfordshire, United Kingdom
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On 12/6/14 1:07 PM, Richard Nute wrote:
Hi Doug:
I assume you are assuming the initiating event is an air breakdown as opposed
to a solid insulation breakdown.
Note that, according to Paschen's Law, air does not break down below about 323
volts peak (at sea level).
The mains voltage does not exceed 170 volts peak (for 120 volts r.m.s.). So,
when the initiating event is over, and the mains voltage goes through zero, the
arc is extinguished and cannot start again.
For two-wire equipment, the current depends on the two capacitances to ground.
The first capacitor (transformer primary-to-secondary) breaks down, and the
second capacitor (secondary circuit to ground) controls the current.
Safety standards specify minimum clearances so that breakdown due to transient
overvoltages is not likely.
Hope this helps.
Best regards and happy holidays,
Rich
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Saturday, December 06, 2014 10:01 AM
To: [email protected]
Subject: [PSES] dielectric breakdown
Hi Everyone,
Is it possible that a fast breakdown of the dielectric barrier, by a fast
event, like ESD or EFT, in a two wire mains power supply, cause a larger
follow-on current from the AC mains and therefore a safety problem. How about
follow-on current from a simultaneous lightning surge that would not by itself
cause a breakdown? Anyone know of an incident of either type? I could set up an
experiment but don't want to set my lab bench on fire. -:)
Doug
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