You don’t need shields on a power cord UNLESS you have a VFD, variable frequency drive, type environment that EMITS EGRESS RF, and would fail the FCC emissions requirements. Some power cords on PC monitors as an example use special alloy RF ferrite chokes to also block emitted RF. Not any alloy choke will work for the RF being blocked, every device is different. If the AC line molded-in chokes are necessary to pass FCC limits, the cords are CAPTIVE to the device. DO NOT remove the cord as it is part of the “solution" for emissions. An IEC socket on a device with a UL label indicated the device under test passed to tests with a “worst case" standard power cord. Thus, it again doesn’t need shielding. The internal design is high quality enough to blocK RF egress.
Few if any power cords shield RF INGRESS as the power supply is designed to filter incoming RF to ground. Also, if the shield is SPG, single point grounded, it is NOT a shield at all but a resonator that induces noise INTO the cord as well as radiating emissions OUT of the cord. UL will never pass a SPG “shielded” power cord. Simply put, BOTH ENDS have to be grounded to be a shield. That is not a debate, it is a fact.
If you lift the ground, and noise is reduced you have a high ground differential and the problem is worse than the improper lifter ground leaves you with. You have TWO problms concurrently; a resonance transfer function and a ground differential, both. Lifting the ground removed one of the two problems, but still a problem remains, fix the grounds high ground reference differential. Now you can use a REAL shield if you feel the need as it acts like one now.
For AWG requirements, it is based on the current in the cord to mitigate voltage drop (current times the cords DC resistance end to end). For a turntable, you don‘t need a 10 AWG cord! An amplifer, yes. Better materials are selected to also remove RF passively as well as provide a durable cord.
One of the main issues is keeping the DC path a low measured value which keeps “hum" and ground loop noise low. The AWG requirement is defined by the current draw. More current is potentially more hum (voltage or the hum we hear = current times DCR). Since we are adding wire between point A and B, we ALWAYS will have a potential difference and current flow. This differential between ground points is a reality, there is no “perfect" ground. Hum is there, but we keep it below audible levels. All our equipment sees the DCR path from the green GROUND reference. Low impedance high current draw device sources are more a problem than high impedance voltage transfer function devices.
Don’t go overboard on big AWG cords where you have low current draw devices, and you don’t need a shield for audio devices that have “open" IEC sockets. They pass emissions to a high standard already. A shielded cord with a proper ground isn’t going to help, but it will cost more and won’t improve things.
Best, Galen
