ELECTRICAL AND INSTRUMENT CABLES REQUIREMENT AS PER INTERNATIONAL STANDARD

This article is about  ELECTRICAL AND INSTRUMENT CABLES REQUIREMENT as per international  standard. For facilities of petrochemical and industrial plants, cables which are installed should be as per international standard.

ELECTRICAL AND INSTRUMENT CABLES REQUIREMENT 

(The general requirements of cables are as given below. Also refer relevant Electrical Specification for further details on cables.)

Cable Types:

The inner and outer jacket of the cables shall be made of extruded flame retardant 90 C PVC to IS 5831 type ST2. The O2index of PVC shall be over 30. The temperature index shall be over 250C.
The cables used shall in general be of the following types:

a) Fire Resistant Cables:

Cables of this type shall be used for all circuits that have a safety-related function, or are required to remain operational under emergency conditions. This includes all parts of F & G Systems, ESD systems, PA Systems, warning beacons and platform status lamps, fire control systems, and power supplies to those systems. These cables shall comply with the requirements of IEC 60331 and IEC 60332-3 (Category C) or better.

b) Fire Retardant Cables:

Cables that are not Fire Resistant shall be Fire Retardant type and shall comply with the requirements of IEC 60332-3 (Category – C). The properties of fire retardant cables tobe used for different applications shall in general be as follows:

i) Signal Cables:

Twisted single pair/triad cables shall be 600 V rated and shall have 1.5 sq. mm. PVC covered, 7 stranded Copper conductor, 100% aluminum Mylar shielding shall be provided with 25% overlap on either side & 18 AWG 7 strands copper drain conductor. These shall have PVC inner jacket, galvanized steel armoring and overall PVC outer
jacket. Ripcord shall also be provided. Multi pair cables shall have following additional features:
• Pair identification by color coding/numbering.
• Individual pair shielding apart from overall shielding.
• One pair of 22 AWG 7 strand copper wire with PVC insulation, for communication.
• Maximum of 12 pairs per cable

ii) Control / Alarm / Status Cables:

These shall be same as signal cables except there shall be no individual pair shielding in case of multipair cables.

iii) Compensating Cables for Thermocouples:

These cables shall be generally same as signal cables except conductor shall be solid 16 AWG in single pair and solid 20 AWG in case of multi pair cables. Color coding, thermoelectric characteristics and limits of errors shall be as per ANSI MC 96.1.

iv) Power Cables:

Power cables shall be 3-core, minimum 2.5 mm2copper conductors with PVC insulation, galvanized steel armoring and overall PVC sheathing.

c) Special Cables:

Cables for special instruments like analyzer, turbine meter etc. shall be as per manufacturer‘s recommendation.

Cable Sizes:

Cables with individually screened pairs shall be used for very low-level signals such as thermocouple wiring or similar, or for pulsed signals. Types and sizes shall be as follows:
• Analogue multi pair cables: Overall screen 1.5 mm2conductors
• Digital multi pair cables: Overall screen 1.5 mm2conductors
• Digital cored cables: For 24 VDC power, no screen, 2.5 mm2conductors
Larger sizes shall be used where required to keep cable voltage drop below 2% of the supply voltage.

Cable Shielding:

All instrument signal cables shall be continuously shielded with the shield grounded at the same point as the signal circuit, generally at the control instrument (panel) end.

Cable Runs:

All instrumentation cables shall be supported / layed in trays. Power cables shall not be run in trays carrying signal cables. The physical separation of power and signal cables shall be as per API-550 Part –I SECTION VII. Cables in intrinsically safe circuits shall preferably be not run in the tray for cables in non-intrinsically safe circuits. If run in the same tray a metallic earthed separator shall be provided. Conduits carrying intrinsically safe cables shall be painted with the blue color bands at each end. Where cables arerun through conduit, the entry and exit shall be smooth and free from burrs. The use of
conduits shall be kept to a minimum, as far as possible. Cables must be pulled into conduit in a way that ensures there is not damage to the cable. Splices shall be made only at terminals, in instruments or approved equipment/junction boxes in any cable path shall be limited to one only.

Please read also: QC Notes Lecture_1 for Cable Specifications and Cable Installation ARAMCO STANDARD

Cable Marking:

All cables shall be identified with stainless steel cable markers securely fixed to the cable with cable tie wraps at the following locations:
• All cable glands
• Entering and leaving cable ladders, ducts and supports
• Both sides of walls or bulkheads Cable markers shall be fitted during or immediately after cable installation.

Cable Identification:

All wiring, cables, tubes, multi-tube bundles, junction boxes and auxiliary equipment shall be suitably identified as per applicable codes and practices. Plastic adhesive tapes shall not be used for identification. Identification and details of all such equipment and wiring/tubing shall also be clearly marked on appropriate drawings. All wiring shall be tagged with slip on or clip-on wire marker at both ends with the wire number specified on the drawings. Terminals for electrical
connections including thermocouple extension wires shall be numbered and identified to indicate polarity, ground Connections, test connections, and any other pertinent information.
Cable cores shall be coloured as follows:
• 24 V DC Digital Circuits: Positive – White, Negative – Black
• Analogue Circuits: Positive – White, Negative – Black
Cable outer sheaths shall be coloured as follows:
• Intrinsically Safe: Light Blue
• Non-IS Cables: Black

Cable Segregation:

The Contractor shall ensure that there is no interference between cables having different voltage or signal types. Instrument cables shall be segregated from antenna cables and others carrying RF signals so as to avoid interference. Very low-level
signals, such as thermocouple wiring, or of pulsed signals or any other signal types, which may be liable to affect or be affected by any other cables, shall have individual and overall screens and shall be installed on a separate tray or ladder adequately separated from other cable systems. Intrinsically safe signal cables may be run on the same tray as non-IS signal cables if they are segregated by metal barrier.

Cable Installation:

All instrument signal cables shall be continuously screened, with screens grounded at the equipment room end only, and securely insulated at the field end. All cables exiting equipment rooms shall be armored, including serial link and data highway cables. For ease of instrument disconnection, an adjustable elbow or union shall be provided
between the terminating gland and the instrument. A neat loop of cable 250 mm diameter shall be left immediately adjacent to all instrument devices in the field. Cabling to locally mounted instruments shall not be routed across normal or emergency walkways nor interfere with access to equipment. Cable transits shall be used to provide gas-tight cable penetrations through decks or firewalls.

Cable Termination:

All cables shall be terminated using double compression type glands on explosion proof equipment. These glands shall be explosion proof, certified. All cable glands in outdoor service and on equipment subjected to salt water spray shall be weather proof also. All terminations shall be screw clamp type terminals for 2.5-mm2 conductors minimum (no flying leads) and correctly certified where necessary. Insulated crimp lugs shall be used for all cable core connections, with only one conductor per terminal side.
No wire shall be terminated/left with mechanical strain on it in any conduit or trunking. Cables shall be terminated in 316 SS glands, suitably certified where necessary. The inner sheath shall be left on the cable after the gland, and removed at the point of entry into wiring duct. Cable cores shall not be ‗pig tail‘ finish. Cable screens and drain wires shall be securely insulated at the final field termination.
The communications conductor of a cable shall be terminated in the bottom terminal(s) of a row of terminals. Printed sleeve-type ferrules shall be fitted to all cable cores on both sides of every core. ‗C‘ type and adhesive type shall not be used. All cables shall be individually loomed at maximum fixing centers of 60 mm and where cores leave the
loom. Only one conductor shall be used per terminal side. Cable entry to control room/other rooms shall be through listed multi cable transits.
Terminal blocks shall be non-hygroscopic, Vibration proof, stack on type and shall use captive screws for terminals. Terminals shall be tinned or Silver plated.
All terminations shall be through use of wire lugs. Terminal blocks shall be flame retardant type. Cable Termination for Skid Items: Manufacturer or fabricator supplying skid mounted equipment or vessels with instrumentation which provides or utilizes off skid alarms / shutdowns / control functions, shall terminate signals on a central junction box, near skid boundary, available for hook-up by Contractor. The location of this junction box shall be noted on vendor drawings.