Cable Capacitance In Intrinsically Safe Systems
I'm trying to understand some of the requirements for choosing an intrinsically safe diode barrier to be used with an intrinsically safe sensor. I think I understand the concepts pretty well with the exception of one key item. The thing I'm struggling with is how to determine the cable capacitance (and inductance) to be added to the sensor parameters to make sure the requirements of the barrier are met.
Specifically, I am having trouble understanding what cable capacitance to use. Manufacturers typically give two capacitances on their spec:
CC : Conductor to conductor CS : Conductor to all other conductors and shield
I'm tempted to say that it would make the most sense to interpret the cable capacitance as: CT = CC + CS
Where CT : Cable Capacitance Total
However, I can't find a single reference that nails this down. I was wondering if you could either give me some guidance on how this is typically calculated or estimated in industry or point me to a reference that might explain how to approach this? I do understand that in absence of other information, a capacitance of 200 pF/m can be used, but I'd really like to better understand the application when the two standard cable capacitance parameters are known.
Also, any in-depth literature or standard which covers this would be welcome as well.
The application I'm using this for is a residential propane tank liquid level sensor. (Rochester R3D type.)
1 answer
The reason for the maximum capacitance spec is to limit stored energy that could cause a spark when that energy is released all at once. There are tables and equations that show how much capacitance is allowed for any given voltage.
You should therefore use your CS value. According to your description, it includes all possible capacitance that the cable adds to any conductor. The conductor to conductor capacitance is already included. I'm basing this on what you said. It's your job to read the cable spec carefully to make sure this is really true.
The way to find the maximum allowed cable length is to look at the Co parameter of your barrier, and subtract the Ci parameter of your sensor in the propane tank. That's how much capacitance the cable can add before violating the IS rules.
For example, let's say your barrier limits its output voltage to 10 V, which means its Co spec is probably 2.75 µF. If your sensor adds 2.0 µF (high for a passive sensor, but possible for a powered sensor), then the cable has a budget of 750 nF max. That divided by your 200 pF/m spec means the cable can't exceed 3.75 km. You'll have other issues with a cable that long.
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