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The problem with PLC is that it was, is, and will be unreliable. There is no way to qualify the impedance and noise environment, and as a result, no way to design a product that can be guaranteed to work. So maybe one outlet works, while the one next to it does not. Unless you have CFL bulbs in your house, in which case pretty much none of the outlets will work :(
I did not take the time to look it up, but I can't imagine that the US NEC will allow this any more than the EN code in Europe does. If these signals are more than a few ma, they will screw up ground fault detection in GFI breakers and outlets, which looks for small currents on the ground leg as an indication of fault. And the high frequency in particular may disrupt sensing in AFI (arc fault interrupting) breakers that are now required for all new construction in the US.
Do you think the boys and girls at Qualcomm have tested these scenarios? Sure...
Clause 9 of EN 50065-1:2011 prevent use of line to ground communication in Europe. It states;
"Common-mode signalling devices may disturb the normal operation of other signalling equipment or certain residual current protection devices and may cause serious safety hazards to the user. Therefore, on common-mode signalling devices a clearly written warning (either on the product or on the leaflet) shall indicate:
“Due to safety reasons the use of this product is not allowed in the residential
environment. Use of common-mode signalling devices in commercial or industrial
premises is allowed, provided that adequate safety checks are made in advance of
installing this equipment and regularly thereafter.”
Many older American homes do not have a safety ground wired throughout the house, just line and neutral. Houses built more than 40 years ago did not have the requirement, and retrofits are prohibitively expensive.
One could always use the CSCJ (Carrier Sense Carrier Jam) method to discourage this. Shouldn't be too hard to breadboard a circuit that detects the first RF burst and fills subsequent AC zero crossing with a few watts of garbage. All the new HomePlug units in your neighbourhood will be quickly returned to Frys or BestBuy or wherever else they came from.
More pollution for the ether.
Kiss your AM broadcast radio goodbye.
Kiss your HF amateur radio goodbye.
The manufactures of such devices claim that the signal is contained within the powerlines.
It is not. Any disruption in the impedance will cause radiation from that point.