Clamping voltage is the MOV voltage under specified surge
conditions. Typically that will be an 8/20 impulse.
IEC 61643-331, ed. 1.0 (2003-05)
clamping voltage VC
peak voltage across the MOV measured under conditions of a
specified peak pulse current (IP) and specified waveform
Your 1 mA voltage is called the nominal voltage (not
clamping voltage) when the MOV draws a specified low-level
current.
IEC 61643-331, ed. 1.0 (2003-05)
nominal varistor voltage VN
voltage across the MOV measured at a specified pulsed
current (IN), of specific duration
NOTE The MOV manufacturer specifies the current. Otherwise,
1 mA is normally used. The pulse duration should be less
than 40 ms, unless otherwise specified. In general, nominal
value ±10 % is specified by the manufacturer.
Regards
Mick
On 10/05/2012 00:27, Doug Powell wrote:
A change was made in 3rd Ed. for routine mains hipot tests
while clamping devices are still in the circuit;
specifically clause F.3.2 (Ed. 3).
The standard states the test can be carried out at 0.9
times the clamping voltage of the device and "not less
than twice the working voltage." Edition 2 said "not less
than that of the working voltage."
My application is 230Vac and I initially selected 275Vrms
MOVs. Plugging in the F.3.2 equations, I get a hipot
failure.
These devices have a very soft voltage knee and as a
result of the dynamic resistance, the knee very time
dependent. The MOV supplier states the "maximum" clamping
voltage using the 8/20 mS surge is 710V and the DC
clamping voltage with a 1 mA current source is 473V. I
would assume the 473V is very similar to the peak of the
275 Vrms 50/60 Hz waveform, plus some headroom for
component tolerance (~18%).
If you try running the numbers to re-select
a different MOV value, it is very possible to get all tied
up in knots. So, which "clamping voltage" are we to use?
My thought is to use 2 x 230 = 460V and select the next
higher MOV from the catalog.
Opinions, rulings, decrees from on high?
--
Thanks, -doug
Douglas E Powell
doug...@gmail.com <mailto:doug...@gmail.com>
http://www.linkedin.com/in/dougp01
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