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Infallible capacitor arrangement

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pj:
In the standards for intrinsic safety (60079-11) it talks about using capacitors in an infallible arrangement. Does anyone know what an infallible arrangement looks like, would it be two capacitors in series? Quote from standard below.

Thanks



--- Quote ---Either of the two series capacitors in an infallible arrangement of blocking capacitors shall be considered as being capable of failing to short or open circuit.
--- End quote ---

xrunner:
See if this document helps -

floobydust:
The infallible capacitor is two in series, so that you have coverage if one shorts. Typically used going to an antenna/bias-tee or fuel tank probe to block hazardous DC from the IS section. A fuel tank can have large static electricity charges so a dissipating resistor would also be used.
The caps must pass the hi-pot test 6.3.13 of 500VRMS, be low enough capacitance value (one part) for spark energy with safety factor 1.5, and meet the separation distances across one capacitor. If the product is mains-powered the hi-pot requirement goes up.
So they are a large parts. I've used 500V ceramic caps but these are a bear for cellphone modems in antenna circuits, you have to use a microwave-spec'd part I find.
Certifier will apply 500VAC across one cap for 1 minute I recall, and 1,500VAC to GND.

coppercone2:
Fuel tanks certainly pose interesting problems.

floobydust:
I didn't know static charges build up on refined fuels (surface) such as gasoline, diesel, LNG. Thought they were non-polar molecules and not triboelectric but this is not the case. An electrode in a fuel tank, say a capacitance probe does get zaps like ESD during tank filling or  slosh so clamp diodes are needed. A 1MEG bleed resistor dissipates any charge. I did see another commercial design use GDT but thought that's a bad idea to hard discharge the electrode.

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