QA/QC Questionnaires FIELD INSTRUMENT/JUNCTION BOX/COLOR CODING Grounding (J902)
CONNECTIONS at FIELD INSTRUMENTS and JUNCTION BOX
a. Connections at field instruments shall be screw type terminal blocks. Wire nuts and spring
type terminals shall not be used. Instruments’ with integral terminal blocks shall be
connected directly to the field cable.
b. The outer jacket of shielded twisted single pair/triad cables shall be left intact up to the
point of termination. Drain wires and Mylar shields on shielded cables shall be cut and
insulated with heat shrink sleeve at the field instrument unless otherwise specified by the
c. Twisted multi‐pair triad cables shall be cut to the appropriate length to minimize looping
and flexing of the cable within the junction box.
Power and signal wiring shall be color coded as follows:
17.1.1 Electrical systems must be connected to ground for the protection of personnel and
equipment from fault currents (AC safety ground) and to minimize electrical
interference in signal transmission circuits (Instrument circuit ground).
17.1.2 Two grounding systems are required for instrumentation systems:
a) safety ground for personal safety.
b) Instrumentation Safety Ground.
17.2 Safety Ground
17.2.1 All exposed non‐current‐carrying metallic parts that could become energized
with hazardous potentials must be reliably connected to the equipment
grounding circuits. This assures that hazardous potential differences do not exist
between individual instrument cases or between an instrument case and ground.
Therefore, all metal equipment and enclosures within a panel or series of panels
(i.e., instrument cases, hinged doors, racks, etc.) shall be bonded with bonding
jumpers and connected to a safety ground bus with a minimum copper wire size
of 4 mm² cross‐sectional area. Two copper conductors, 25 mm² minimum, shall
be connected from the safety ground bus to a single tie point on the safety
ground grid in a closed loop configuration. Safety ground connections must be
made such that when a case‐grounded instrument is removed, the integrity of
the rest of the safety ground system is maintained.
17.2.2 Enclosure for field instruments shall be grounded as follows: (J‐902)
220.127.116.11 Instruments Operating at greater than 50 Volts
The enclosure for Instrument devices operating at 120 V AC or
125 VDC shall be grounded per SAES‐P‐111.
Instrument Circuit Ground (J902)
17.3.1 The purpose of instrument circuit grounding is to reduce the effect of electrical
interference upon the signal being transmitted. An instrument circuit ground
bus shall be provided for consolidating instrument signal commons and cable
shield drain wires. This ground bus shall be isolated from the safety ground
system except at a single tie point as described below.
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