Cable And Device Grounding; General Grounding Guidelines; Ground Wire Specifications; Ground Source Specifications - Emerson E2 Einstein Manual

7 Cable and Device
Grounding
Proper grounding is an essential part of reducing
network noise. Failure to properly ground power and
network wiring is one of the most common causes of
critical noise problems. Follow these guidelines
when installing RS485/MODBUS and Echelon net-
works, and refer to them when troubleshooting prob-
lematic installations to ensure the grounding is
correct.
7.1. General Grounding
Guidelines
A good ground wire provides a low-DC-resis-
tance path between the cable's or device's ground
connection and the earth ground. However, because
of the presence of high frequency AC noise, the
ground wire itself must also be limited in length, or
else it may become an "antenna" and add noise to the
network.

7.1.1. Ground Wire Specifications

The length of all ground wires should be kept to
6 inches (15.3 cm) or less. Use a 14AWG stranded
wire or larger.
7.1.2. Ground Source Specifica-
tions
The best "good" earth ground sources to use are
listed below, in priority order:
• A large metal panel or plate that is connected to a
good electrical ground. A panel or plate that is at least
3 feet by 3 feet (0.9 m by 0.9 m) is desirable.
• Earth grounded steel rack.
If none of these types of earth ground sources are
available, as a last resort use one of the following
types of earth grounds. These are not ideal and
should not be used unless absolutely necessary:
• Metal electrical conduit, connected to an earth ground
• Water pipes (metal, assuming there are no plastic sec-
tions).
10 • E2 Controller Wiring Practices
• Electrical system earth ground.
Note: If grounding against a metal plate or
panel that is coated or painted, scrape away
the coating to expose the bare metal before
making the ground attachment, and use a tooth
washer.
7.2. Power Input and RS485/
MODBUS Grounding
For E2s networked with RS485 I/O Network
devices, such as MultiFlex, or MODBUS devices
such as Control Link RSCs or CDs, grounding must
be done on both the power inputs and the network
connector terminals for each device in the network
(including the E2).
7.2.1. E2
7.2.1.1. Grounding the EARTH
GROUND Terminal
The Power Interface Board (PIB) on the E2 has
an earth ground connector (J3) next to the power con-
nector on the lower right corner of the board. Con-
nect this connector to an earth ground source using a
ground wire at least 14AWG and no longer than six
inches. To further ensure the ground path is com-
plete:
• Scrape away any paint or coating (if grounding to a
panel or chassis)
• Use the shortest piece of 14AWG stranded wire avail-
able.
• If grounding to the door of a panel, make a second
14AWG wire connection between the panel door and
the chassis (do not rely on the hinges of the panel to
provide ground connection to the chassis.
7.2.1.2. Grounding the RS485 Network
Cable
Provided all power connectors are properly
grounded, the shield wire for all cables connecting to
an E2 network connector may be connected to the
center (0V) terminal of the network connector.
026-1903 Rev 4 19-APR-2010