Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 133
systems where a separate earth conductor is present and accessible (i.e., safety ground, which is not the same as a neutral
wire with an earth bond), and wiring systems where there is no earth conductor.
When an earth conductor is always present, a coupling method known as line-to-earth coupling provides the best
communications performance. With line-to-earth coupling, the communications signal is coupled to the line wire relative
to earth, and earth is used as the return path for the communications signal. This coupling technique is also referred to as
earth-return coupling. Local restrictions might apply to the use of line-to-earth coupling (see Ground Leakage Currents
in the Safety Issues section of this chapter for more information). As a general rule, line-to-earth coupling is only used in
commercial applications in North America and non-EU countries where local electrical codes require the presence of an
earth safety ground and permit the associated 50/60Hz leakage current. Figure 4.15 illustrates a simple example of a line-
to-earth coupling circuit.
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Figure 4.15 Line-to-Earth Coupling Method
To understand the advantage of line-to-earth coupling, recall that a major component of signal attenuation is due to the
loads presented by devices that are connected to the power mains between the line and neutral wires. These loads have
only minimal affect on signal attenuation when line-to-earth coupling is used. Field measurements have shown consistent
improvements in received signal-to-noise ratios of more than 15dB for transceivers using line-to-earth coupling, relative