By knowing the capacitance and inductance (i.e. stored energy) in the circuit that the switch could interrupt; you then assume this stored energy gets dissipated by the arc. If there is no appreciable inductance, then the possibility of an arc is greatly diminished (no flyback voltage spike to initiate an arc). You could measure the arc dissipation with a suitable o-scope tracing current and voltage across the switch during an opening transient, you then need to convince people that the next switch won't be 2x or 10x worse...or test every switch and circuit combination.

FWIW, you might try posting this in the Electronic Engineering forum, where more sparkies are likely to lurk. If you do that, just post a link back to this question, don't repost your question there, that is considered rude.

thanks.
i do not see electronic group in this eng-tip. have a link?
if energy = 0.5 * L * I^2, L will be, for 100 uJ, 800 microH. if the system has no solenoids... is this L number too high for all the wiring?

Click on the link at the top of the page called "Eng Tips Forums" to get a list. Scroll down a bit and look for the "Electrical and Electronic Engineers" section.

Have you considered that the wrong fuel may be mistakenly put in the tank?
I ask because of a real incident at a small refinery.
The product was lubricating oil. On the line to the loading dock, there was a full flow paper filter to ensure that no particulate matter was shipped. Unfortunately the filter built up large static charges in the lube oil. As the oil was loaded into the tank trucks, it was common for the drivers to report seeing static discharge sparks in the oil.
The energy of the discharges was well below the ignition point of the lube oil.
Then one day a truck delivered a load of gasoline and then came to the refinery to load lube oil.
Unfortunately the static discharge energy was well able to ignite gasoline fumes.
The explosion blew the driver (who was on top of the tank watching the loading level) up into the air.
He may have died from the explosion, he may have died from the fall, he may have died from the burns.
Somewhat moot. He died.
Do a google search on intrinsically safe devices.
Consider methods to locate the pressure switch out of the hazardous area.
Another possibility may be an intrinsically safe transducer outside the hazardous area with an intrinsically safe pressure sensor in the tank.
This thread has an old tune running through my head:
To the tune of My Bonny Lies Over the Ocean.
My Bonny leaned over the gas tank.
To see what the level might be.
I lighted a match to assist her.
Oh bring back my Bonny to me!

Bill
--------------------
"Why not the best?"
Jimmy Carter

Using any voltage above about 8V guarantees you will have a spark. btrue's scope suggestion works for coming up with the energy.

I'm not sure how you guarantee someone doesn't come along and drop a huge inductor into your circuit because they're having a "noise problem" somewhere.

In industry the requirements to prevent explosive mixture ignition in a case like this always requires a 'barrier' which is a certified piece of equipment that actually does guarantee that enough energy can never get to an interruption inside the hazardous area.

Keith Cress
kcress - http://www.flaminsystems.com

Dho...

What Kind of Fuel: Gasoline [Octane X]? Jet Fuel [Y]? Diesel Fuel [Z]?

Perhaps You should consider stepping the sensor voltage and current waaay down... outside of the tank environment.

I have seen static electricity arc/spark and start detonations in tactical aircraft fuel tanks [JP-4... really bad]... only to be 'arrested' within a few inches by the polyurethane ballistic foam stuffed into the tanks and large tubing.

I am sure that fuel quantity sensor manufacturers have this problem and may be willing to share their expertise for $$.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

BS EN 60079-11:2012 provides data and the method of performing the required calculation. You will need to know inductance and capacitance values for the device and cabling.