Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 141
The receive-mode impedance of the transformer isolated coupling circuit dips near 10kHz due to the series resonant
effect between C101 and T101. This dip in out-of-band impedance does not have any adverse effect on communication
performance. If local regulations require a receive impedance at this resonant frequency of greater than 5 ohms, then an
optional series RLC circuit can be added as shown in Figure 4.21.
Figure 4.21 Transformer-Isolated Coupling Circuit with Optional RLC Circuit
Ground Leakage Currents
In power line systems that use line-to-earth coupling, there are both safety and practical limits on the level of ground
leakage currents that are permitted. In the case of products intended for use in commercial buildings and homes, many
safety agency standards set a ground leakage current limit of 3.5mA. This leakage limit determines the maximum value
of C101 (in Figures 4.19 and 4.20) for line-to-earth coupling circuits. In the line-to-earth coupling circuits documented at
the beginning of this chapter the value of C101 has been chosen to limit the ground leakage current to less than 3.5mA.
A practical limit on the use of line-to-earth coupling also exists. Many single circuit ground fault interrupters (GFIs), also
known as residual current devices (RCDs), can be triggered with ground currents as low as 4mA. If each PL Smart
Transceiver employing line-to-earth coupling generates about 3mA of ground current, then only one such transceiver can
be installed on each GFI-protected circuit. For this reason line-to-earth coupling might not be suitable for some
applications with low-current GFIs, and line-to-neutral coupling should be used instead. Local regulations also might
prohibit the use of earth as the return path for a signaling system.