# What is conductor to conductor to shield capacitance?

I am using 6.5 ft (2 meters) BELDEN 9154 shielded twisted pair cable for a sub 1 MHz application and I wanted to make a model of the cable for some simulations. The datasheet of the cable lists two capacitance as shown below: How to interpret these capacitance? For a 1 ft segment what values of capacitance is observed across the conductors and across shield and one of the conductors?

The specification you're refering to must be interpreted in the following way:

1. You take one conductor and connect it to shield.
2. Then you take the other conductor and measure the capacitance across it and the other one.

Now take a look at this diagram: Source of image: this paper. Ignore capacitances $$\C_1\$$, $$\C_2\$$, $$\C_E\$$, and assume that $$\C_{S1}=C_{S2}\$$.

From the diagram we can see that, because you're shorting one of the conductor to shield, the capacitance measured must be then the parallel of the conductor-to-conductor capacitance $$\C_{12}\$$ and the conductor-to-shield capacitance $$\C_{S1}\$$.

Capacitances in parallel add together, so we can calculate the conductor-to-shield capacitance $$\C_{S1}\$$ from the specification, just subtracting. In this case, the conductor-to shield capacitance $$\C_{S1}\$$ is just 100 pF/ft - 60 pF/ft = 40 pF/ft. That's a 40 pF/ft from each conductor to shield (two capacitances) in your model. Exactly like analogsystemsrf has outlined in his answer.

This is only for the capacitance. If you want a complete lumped model don't forget the rest of parameters (inductance and resistance per length unit) that you can find in the datasheet.

I'd draw the circuit like this simulate this circuit – Schematic created using CircuitLab