Screened and Shielded Cabling - Noise Immunity, Grounding, and the Antenna Myth
The Ground Loop Myth
It is a common myth that ground loops only appear on screened and shielded cabling systems. The fear is that ground loops resulting from a difference in voltage potential between a screen/shielded cabling system's ground connections cause excessive common mode currents that can adversely affect data transmission. The fact is that both screens and shields and the balanced twisted-pairs in a UTP cable are affected by differences in voltage potential at the ends of the channel.
The difference in the transformer common mode termination impedance at the NIC and the network equipment naturally results in common mode noise current being induced on each twisted-pair. Grounding of the screened/shielded system in multiple locations can also result in common mode noise current induced on the screen/shield. However, these common mode noise currents do not affect data transmission because, regardless of their voltage magnitude, their waveform is always associated with the profile of the building AC power (i.e. 50 Hz or 60 Hz). Due to the excellent balance of the cabling at low frequencies, common mode currents induced onto the twisted-pair either directly from equipment impedance differentials or coupled from a screen/shield are simply subtracted out by the transceiver as part of the differential transmission algorithm.