Screened and Shielded Cabling - Noise Immunity, Grounding, and the Antenna Myth
Ground loops develop when there is more than one ground connection and the difference in common mode voltage potential at these ground connections introduces (generates) noise on the cabling as shown in figure 4. It is a misconception that common mode noise from ground loops can only appear on screens and shields; this noise regularly appears on the twisted-pairs as well. One key point about the voltage generated by ground loops is that its waveform is directly related to the profile of the building AC power. In the US, the primary noise frequency is 60 Hz and its related harmonic, which is often referred to as AC "hum". In other regions of the world, the primary noise frequency is 50 Hz and its related harmonic.
FIGURE 4: Introduction to Ground Loops
Since each twisted-pair is connected to a balun transformer and common mode noise rejection circuitry at both the NIC and network equipment ends, differences in the turns ratios and common mode ground impedances can result in common mode noise. The magnitude of the induced noise on the twisted-pairs can be reduced, but not eliminated, through the use of common mode terminations, chokes, and filters within the equipment.
Ground loops induced on the screen/shield typically occur because of a difference in potential between the ground connection at the telecommunications grounding busbar (TGB) and the building ground connection provided through the network equipment chassis at the work area end of the cabling. Note that it is not mandatory for equipment manufacturers to provide a low impedance building ground path from the shielded RJ45 jack through the equipment chassis. Sometimes the chassis is isolated from the building ground with a protective RC circuit and, in other cases, the shielded RJ45 jack is completely isolated from the chassis ground.
TIA and ISO standards identify the threshold when an excessive ground loop develops as when the difference in potential between the voltage measured at the shield at the work area end of the cabling and the voltage measured at the ground wire of the electrical outlet used to supply power to the workstation exceeds 1.0 Vrms. This difference in potential should be measured and corrected in the field to ensure proper network equipment operation, but values in excess of 1.0 Vrms are very rarely found in countries, such as the US, that have carefully designed and specified building and grounding systems. Furthermore, since the common mode voltage induced by ground loops is low frequency (i.e. 50 Hz or 60 Hz and their harmonic), the balance performance of the cabling plant by itself is sufficient to ensure immunity regardless of the actual voltage magnitude.