The shielding principle of the shielded cable is different from the balanced cancellation principle of the twisted pair. The shielded cable is to add one or two layers of aluminum foil on the outside of the four pairs of twisted pairs, and use the metal to reflect, absorb and skin effect principles of electromagnetic waves (the so-called trend). The skin effect means that the distribution of the current in the conductor cross-section tends to be distributed on the surface of the conductor with the increase of the frequency. Electromagnetic interference enters the cable, but also prevents the internal signal from radiating out, interfering with the work of other equipment.
Experiments show that electromagnetic waves with frequencies exceeding 5MHz can only pass through 38μm thick aluminum foil. If the thickness of the shielding layer exceeds 38 μm, the frequency of electromagnetic interference that can enter the cable through the shielding layer is mainly below 5MHz. For low-frequency interference below 5MHz, the balance principle of twisted pair can be applied to effectively cancel.