Micromod MOD: MODCELL Multiloop Processor 2002N Model C and Associated Hardware - Manual (Page 33)

MODCELL Multiloop Processor
ELECTRICAL CONNECTIONS
3.4.5 Noise Prevention Measures
Primary power circuit distribution system:
•
Ideally, each microprocessor-based device should be provided with an independent
dedicated power source. Where this approach is not feasible due to space available
or cost per device an acceptable alternative is to install constant voltage, isolation
transformers in the branch circuit where the microprocessor-based device is installed.
• In addition to the above, install a combination transient surge suppressor and noise
filter in the instrument side of the power distribution system. The combination device
suppresses transients and effectively reduces other noise forms such as
electromagnetic (EMI) and radio frequency (RFI) interferences. These devices can be
connected to multiple units to reduce overall cost.
Input signals
•
Twisted wire pairs are essential. The wire type should be stranded not solid. The
largest wire gauge allowed is best and the more twists per foot the better. A 2-inch
lay (6 twists per foot) should be the minimum used.
• In addition to the above, signal wires should be physically isolated from all power
conductors (separate conduit, cable race, etc.).
• Shielded wire is also essential. Shields must be terminated at the instrument or in the
field in accordance with local regulations.
! CAUTION 1. Never terminate a shield at both ends. One end must always be left
"floating" or ground currents may be introduced.
2. Thermocouple shields should be terminated at the process
measurement end. Most thermocouples are constructed where the
sensor is electrically equivalent to the process connection (grounded
junction).
• Keep digital signal wires separate from analog wiring.
• Some analog inputs and outputs are isolated from the instrument circuitry so they can
be connected to external devices having different ground references. Such circuits
should never be left completely floating because they can pick up noise. If a remote
ground reference is not provided, then the circuit should be grounded at the
instrument to minimize noise pickup.
Equipment grounding:
•
Grounding practices defined by the National Fire Protection Agency (NFPA) in their
National Electrical Code (NEC) handbook or State agency amendments to this code
should be strictly observed.
• Existing ground conductors and ground paths should be periodically inspected and
tested to insure continuity and compliance with current code requirements.
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