Echelon LONWORKS PLT-22, Руководство пользователя

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interface lines in either direction and a separate crystal may be used for the Neuron
Chip clock.
Ground Leakage Currents
There are both safety and practical limits on the level of ground leakage currents that
are permitted in power line systems that use line-to-earth coupling. In the case of
products intended for use in commercial buildings and homes, many safety agency
standards set a maximum limit of 3.5mA of ground leakage current. This leakage limit
determines the maximum value of C1 (in figures 4.8 and 4.9) for line-to-earth coupling
circuits. In the application schematics that follow, the values for these capacitors have
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 PLT-22 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 may not be suitable for some applications with low-current GFIs, and line-to-
neutral coupling should be used instead. Local regulations also may prohibit to the use
of earth as the return path for a signaling system.
Capacitor Charge Storage
The coupling capacitors depicted in the earlier figures can retain substantial charge
even after a PLT-22 transceiver-based device has been disconnected from the power
mains. This can be of significant concern in applications where a line cord could be
touched by a user after being disconnected from the power mains. To minimize
potential shock hazard, coupling circuits should include a large value bleeder resistor
to discharge the capacitors following disconnection from the mains. Even in
applications where the connection to the mains is permanently wired, it is good
practice to include the resistor to protect service personnel. The coupling circuit
schematics in this document include appropriate bleeder resistors.
Line Surge Protection
Coupling circuits that connect the PLT-22 transceiver to the power mains require the
addition of component(s) to provide protection of the PLT-22 transceiver from the
high-voltage surges that occur on all power distribution systems. Primarily lightning
induced, these surges can present voltages of up to 6kV at very high current levels
for brief periods to the coupling circuits inside buildings and homes. Even higher
voltages can be seen on mains wiring outside of buildings. The recommendations
that follow are based on testing performed on a particular PCB
L
ON
W
ORKS
PLT-22 Transceiver User’s Guide 4-13