What is capacitance?
Two conductors which are separated by a distance can store an electric charge between them. A cable or harness with two or more wires can also store a charge and this can affect the way the cable performs. Capacitance describes the ability of two conductors, separated by an insulating material, to store charge.
Capacitance in cables is usually measured in pf/m (pico farads per meter) or pf/ ft (pico farads per foot). The lower the capacitance the better the cable performance.
Capacitance is a particular problem with data or signal cables. When a voltage signal is transmitted through a twisted pair or coaxial type cable, a charge builds up across the insulation between the conductors. The charge that builds up in the cable over a period of time is due to the inherent capacitance this results in a delay causing interference in the signal transmission. Digital data pulses which are square in shape are transformed to form a shape similar to “saw teeth” due to the ramp up and discharge, this may result in the circuitry failing to recognise the digital pulses.
There are a number of ways to reduce the capacitance in cable design including:
- Increase the insulation thickness
- Decrease the conductor diameter
- Use an insulation with a lower dielectric constant
Usually a combination of all three is used as either method has its limitations.
Having a metallic shield over the cable introduces a further capacitance, that of core to shield, which can significantly increase the overall capacitance of the cable.
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