Low-Voltage Wiring Installation Guidelines

Generally use of shielded instrument and thermocouple wire. Individual shield drain wires and overall shield drain wires shall be tied to ground either at a central ground point or at the instrument (refer for exceptions when wiring thermocouples). Such wiring shall be installed without spurious grounding of the shield system to metal surfaces or exposed portions of conductors. Procedures for Low-Voltage Wiring Installation Guidelines to be followed include:

Low-Voltage Wiring Installation Guidelines

  • Overall and individual shields shall be stripped back only as far as necessary for installation, and the ends of the shields shall be secured with tape or heat-shrink tubing, as required. Overall shield drain wires shall be grounded only at one point, the programmable electronic system (PES) equipment (safety) ground bus or control panel safety ground bus. If the multi-cable does not extend to the PES or control panel, overall shield drain wires shall be grounded at the terminal box electrically closest to the PES or control panel. 
  • Outer jacket of single pair cable and single triad cable in terminal boxes, control panels, and PES cabinets shall be stripped back only as far as necessary for installation—generally the portion between the cable tag and the termination point. When wireways are provided for routing wiring, only the portion of the cable extending beyond the wireway between the cable tag and the termination point of the conductors shall be stripped back only as far as necessary for installation.
  • All terminated shield drain wires shall be insulated with clear heat-shrink sleeving for their entire exposed length, to prevent accidental contact with grounded surfaces or exposed portions of conductors. Cables shall not be stripped, exposing shields, until actual termination of cable conductors is being performed, particularly when electronic system testing is being performed before complete termination of all system wiring.
  • The ground wire from plant grounding electrode system, individual shield connections to grounded instruments, and overall shield/drain wire connections must be terminated before PES or control panel power is energized. (Low-Voltage Wiring Installation Guidelines)
  • The contractor shall report to the field representative any shield drain wire that is not grounded or is grounded at more than one point.
  • Generally uses grounded thermocouples; all thermocouple conductors shall be installed, terminated, and checked for spurious grounding before final connection to the grounded field element.
  • In the thermocouple head, the contractor shall strip back shield and drain wire only and allow for a small service loop. The shield shall be secured with heat shrink or tape, and the drain wire shall be connected to the grounding screw. Since the drain wire has been connected at this field device, make sure that the other end of this drain wire has not been tied to ground, cut back, and insulate shield and drain wire.
  • Wiring to control panel terminal strips shall be installed such that all wire markers are visible without removing wiring duct covers.
  • All thermocouple and instrument wiring shall be checked for continuity and point-to-point terminations. 
  • Thermocouple and instrument wire shall be terminated as follows:
Device Device Marking Wire Color
ISA Type T + Blue
Thermocouple Red
ISA Type J + White
Thermocouple Red
ISA Type K + Yellow
Thermocouple Red
Resistance Red (Black) Red
Temperature White (Yellow) White
Detectors (RTDs) Red (Black) Black
Instruments + Black
(mV and mA) White
Bentley Nevada -18 Vdc Red
Proximitors Common Output Black
White

  • Low voltage wiring to instrumentation should be done such that there is a service loop in cable or wiring runs inside junction boxes and control panels and inside instrumentation termination heads so that there is additional wire available for service/replacement/retrofit activities.

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