EMC Question of the Week: June 16, 2025
Many cables with foil shields have an extra uninsulated wire called a drain wire. The main purpose of the drain wire is to provide
- mechanical strength
- a path for low-frequency current
- an earth ground connection
- a connection to the connector shield
Answer
The best answer is “b.” Cable shields grounded at both ends can sometime carry relatively large low-frequency currents. These currents may not present an EMC problem, but they can damage thin foil shields. The drain wire has a relatively low and reliable DC resistance. It carries a large percentage of the low-frequency current without heating or arcing.
Drain wires are normally thinner than the signal wires, so they do not provide much additional mechanical strength. They generally make the same electrical connection as the shield and do not provide a separate grounding function of any kind.
In an unshielded connector, the drain wire is sometimes used to connect the cable shield to a connector pin. This "pigtail" connection can be sufficient at lower frequencies but does not make a good high-frequency connection due to its inductance. In a shielded connector, the connector shield must be bonded to the cable's foil shield.
If you did an internet search to try to answer this question, you might have found any of the answers above. As always, be very careful about how you use any information you find on the internet.
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