Electricity can flow from an overhead line to an underground cable and vice versa. For transmission voltages, this happens via a cable sealing end, housed within a sealing end compound. All voltages of overhead lines can be diverted underground in this way but the size of the compound will vary. Depending on the voltage of the overhead line these compounds can be as small as the footprint of a single pylon or as large as a football pitch. For lower voltage lines the underground cables may join onto the pylon or pole itself.

The EMFs produced by a sealing end compound come from the overhead line and cable. There is no other equipment within the compound that produces significant EMFs.