Specifications

Chapter 4 - Transient Protection of RS-422 and RS-485 Systems RS-422 And RS-485 Applications Ebook
Page 59 of 137 Manual Documentation Number: <DocNumber>
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protected. This ground connection is crucial for proper operation of the
shunting device. The ground connection should be made with heavy
gauge wire and kept as short as possible. If the cable must be longer
than one meter, copper strap or braided cable intended for grounding
purposes must be used for the protection device to be effective. In
addition to the high frequency nature of transients, an enormous
amount of current may flow. Several thousand amps typically result
from applications of the combination wave test in the ANSI and IEC
specification.
Connecting Signal Grounds
Since a local ground connection is required at each node implementing
shunt type protection, the consequences of connecting remote grounds
together must be considered. During transient events a high voltage
potential may exist between the remote grounds. Only the impedance
of the wire connecting the grounds limits the current that results from
this voltage potential. The RS-422 and RS-485 specifications both
recommend using 100 ohm resistors in series with the signal ground
path in order to limit ground currents. The following diagram illustrates
the ground connection recommended in the specification.
Figure 44: Signal Ground connection between two nodes with 100 ohm resistor
Note: If isolated devices are used between nodes on a network resistors are not required in the
ground connection.