Micromod RetroPAK: SLC Installation Manual - Manual (Page 50)

SLC RetroPAK Controller
POWER, GROUNDING, AND BUILT-IN I/O CONNECTIONS
26
3.4.4 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 availability 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).
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.
• For best noise reduction performance, the microprocessor-based device's ground
terminal should be connected to a nearby grounded large metal structure, using the
shortest length wire possible. If a three-wire cordset is used to power the
microprocessor-based device through a receptacle, the ground wire is generally too long
and too noisy to be a good ground.