Doug,
My previous message gave the IEC rulings - terms
clamping voltage and nominal varistor voltage - for two
specific points on the MOV clamping (clipping) characteristic.
For completeness IEC 61010-1, ed. 3.0 (2010-06) defines the
following:
*working voltage *
highest r.m.s. value of the a.c. or d.c. voltage across any
particular insulation which can occur when the equipment is
supplied at rated voltage
NOTE 1 Transients and voltage fluctuations are not
considered to be part of the working voltage.
NOTE 2 Both open-circuit conditions and normal operating
conditions are taken into account.
I'm now giving an opinion.
Your (nominal) AC supply is 230V rms. The phase "not less
than twice the working voltage." means the hipot test is
done with an AC value of at least 460 V rms or 650 V pk.
The hipot test voltage of 0.9 times the clamping voltage of
the MOV is clearly crazy, as from your figures of 710 V
clamping voltage and 473 V nominal varistor voltage, testing
at 0.9x710 = 630 V would cause substantial current in the MOV.
One could skate round this and say that the 1 mA nominal
varistor voltage is the clamping/threshold/ voltage - a new
term - and use an MOV or combination of MOVs to have a
nominal voltage of 650/0.9 = 720 V.
Sounds like a letter to the Chair and Secretary of TC 66 is
needed to resolve this matter.
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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