EMC Question of the Week: March 26, 2018
Which of the following is a common reason for connecting the shield of a long cable to ground through a capacitor?
- to prevent unsafe currents from flowing in the cable shield
- to reduce radiated emissions by keeping high-frequency currents off the shield
- to create a bandpass filter
- to offset the inductance of a pigtail connection
Answer
The best answer is "a." When the shields of long cables are grounded in different buildings, or different locations in a large building with a poor ground structure, it is possible for relatively small differences in the ground potential to cause large currents to flow on the shield. For example, a few volts of potential difference could cause tens of amperes of current to flow in a shield with an end-to-end resistance on the order of 100 mΩ. While tens of amperes of current at power frequencies is not inherently dangerous, anybody breaking that connection would experience a very large L(dI/dt) voltage backed by enough energy (½LI2) to be hazardous. Connecting the shield directly to ground at one end and through a capacitor to ground at the other end prevents large power-frequency currents from flowing in the shield.
The answer "b" is incorrect, because the reason the capacitor is there is to make a connection at high frequencies. To reduce radiated emissions, it is generally necessary to ensure that there is a good high-frequency connection between a cable shield and a device's ground structure. The answers "c" and "d" are also incorrect. Creating a bandpass filter (e.g. by offsetting an inductance with a series capacitance) is rarely a good EMC design strategy. At best, this would only work for a very narrow band of frequencies. Well-designed products with shielded cables generally depend on the cable shields to significantly reduce radiated emissions at all frequencies of interest.
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