The plot thickens: look at claim 1 of
1. A hybrid electrical cable providing for power transmission or distribution and low power signal communications, the hybrid cable comprising: a) a power cable including a first group of one or more conductors for conducting power; b) a power cable insulation jacket overlying the first group of one or more power conductors, the power cable insulation jacket including a first layer comprising a soft magnetic material
having a coercivity of 1 oersted or less; c) a group of one or more low power signal conductors disposed exterior of the power cable insulation jacket; and d) an outer jacket overlying the power cable, the power cable insulation jacket and the group of one or more low power signal conductors, the outer jacket comprising a metallic material."
Apparently Ulectra Corporation (North Miami Beach, FL, US), the assignee, is producing this thing.
So here's your ferrous metal for mag field shielding (use BX cable!) and the patent does mention shielding later on as a design constraint and so drewdin may have to resort to extraordinary measures to keep his Ethernet functioning to full capacity.
Or, he may luck out.
"by absorbing radio frequency (RF) electromagnetic emissions generated by high voltage, high frequency electrical transients which may be present on one or more power conductors of the power cables due to external high frequency electrical disturbances."
implies that the problem is not so much the 60 Hz but that the Romex acts as an antenna to pick up, or conduct, all kinds of electrical noise.
In this case the Ethernet may work better late at night when the PoCo customers are asleep.
BTW, when I left the USPTO the patents were just beginning with the 5,100,000 numbers. IIRC, they had 20% turnover each year. . .and I lasted a few years.