Emerson Process Management Bristol Series 9110 - Manual (Page 61)

Supplement S1400 / 3
GENERAL RECOMMENDATIONS
When wiring equipment into a system, the electrical conduit must have a diameter that will
accommodate the desired number of wires. The cross- sectional area of the conduit should be
large enough to allow the wires to be pulled through without excessive tightness or binding. A
conduit that is too tight can shred insulation, damage wiring, and result in possible opens,
shorts, or intermittent effects. Such conditions are often difficult to trace because the defect is
concealed inside the conduit.
Noise and Signal Errors
Noise and signal errors are often the result of poor wiring and grounding practices. Some
common problem areas are listed as follows:
o Shielding AIs and AOs. Very often analog DC signal leads must run parallel to wires
radiating AC fields, pulse information, or switching transients. Due to inductive and
capacitive pickup, some of this information can leak into an analog I/O and cause
peculiar effects in the control systems. To minimize or eliminate this problem, the use of
insulated and shielded, twisted lead pairs is recommended between the external devices
(transmitters, sensors, etc.) and the instrument inputs (controllers, recorders, etc.).
The shields of each analog signal source should only be grounded at the input of the in-
strument. In some equipment, the shield will connect to the instrument chassis. In other
equipment, a "shield" terminal will be provided with several grounding options. The user
should refer to the instrument manual and follow grounding recom-mendations.
o Common Returns. The use of a single "common" return wire for two or more input
signals is not recommended. This approach may introduce system ground loops that
cause erroneous readings at the instrument. Shielded transmitter or sensor wires should
be grounded at the input of the instrument, or connected to a shield terminal (where
provided) to prevent "sneak" ground paths.
o Discrete Outputs. Instruments provided with bi-state discrete outputs perform functions
such as control switching, alarm switching or pulse duration com-munications. These
outputs are furnished as either open collector or relay contact outputs that operate at
low power levels. While these levels are sufficient to operate many devices, some will
require much higher power levels. The use of external amplifiers or repeating relays to
drive end devices will prevent output overload and add to the reliability of the system.
o Compatible System Wiring. In a distributed system, the designer should use the same
wiring identification and color codes throughout to maintain system uniformity. By
observing compatibility at all levels, wiring mistakes are minimized and troubleshooting
tasks are simplified.
o Placement of Wiring. The dressing or physical placement of wiring requires close
scrutiny. Cables inside cabinets should be neatly secured at regular intervals. Cables
running between cabinets at different locations should be placed in conduits. The cable
length should allow sufficient slack for routine operational checks and maintenance of