Attach the link to the Hum FAQ?
|...the cable shielding, and consequently also the splitter housing, is bonded to the buildingâ€™s "grounding electrode" as specified by the National Electrical Code (NEC) or other requirements? The shield and splitter housing are placed at the same ground potential as the buildingâ€™s electrical power neutral conductor.|
This is done to minimize potential differences between cable and power grounds. It also means that power company or in-house neutral fault currents will be shared between the neutral conductor and cable shield.
Neutral fault currents that exist on the outer surface of cable shielding can enter a subscriberâ€™s drop wiring via the toroidal transformers found in drop splitters.
This will, under some circumstances, cause an electrical current to flow on the cableâ€™s center conductor. If the electrical current is large enough, it will saturate the splitterâ€™s toroidal transformer ferrite core and cause hum modulation.
The initial fix was to install a voltage blocking coupler (VBC) on the splitterâ€™s input port, which prevented the unwanted electrical current from flowing in the drop. This success led to the use of drop splitters with built-in blocking capacitors on all of the splitter ports, a low-cost and effective fix to this potentially serious problem.