calculating total capacitance on shielded cables

[Desreft]

this might be a simple calc but I am confused. If I have a shielded cable that is rated at (C1) 30 picofarads / ft between conductors AND (C2) 54 picofarads / ft between conductor to shield. To calculate total capacitance (C) per ft on this cable, should I :

a) add C1 and C2 as if the capacitances were in series:
1/C = 1/C1 + 1/C2

or
b) add them as if they were in parallel: C = C1 + C2

:-?

 

[steve66]

It kind of depends on what you are trying to calculate. Since one capacitance is between the two wires in the twisted pair, and the other is between each line and the shield, they aren't exactly in parallel.

But they aren't in series either.

The parallel calculation would seem to make more sense in most cases, like trying to determine the lost amount of signal lost. Not to say that is exactly correct.

 

[Desreft]

One Fire Alarm manufacturer has literature that states that their initiation circuits are designed for a maximum line capacitance of 0.5 microfarads. If I use a shielded cable, then the cable literature will show me a capacitance / ft value between conductors, and another capacitance / ft value between conductors and shield. To make sure that my design does not exceed the maximum limits, I am trying to estimate the capacitance for my initiation circuits.

I think that maximum line capacitance refers to the capacitance / ft between conductors. But if this was the case, then why would there be a separate value for the capacitance between conductors and the shield? And will this other number affect the preformance?

I am trying to not have this come back and bite me.

 

[steve66]

If the fire alarm signal is ungrounded, I would assume that the line-line capacitance is what you want to use.

If it is a grounded signal, then I would probably assume the line to shield capacitance is the one that matters. However, I couldn't be sure without asking the manufacturer what they meant by "max. capacitance".

Even if you use C= C1+C2 (this should be the worst case), that gives you over 5000 ft of cable you can use. Do you really have more than that on one circuit??

Steve

 

[Desreft]

some of my runs are pretty long, but I don't think that any of them are over 5000'. Going with worst case scenario is what I should be using. Thanks for the help.