Instrument Installation – Cabling Guidelines

Instrument installation with the associated cable installation/electrical signal and control wiring should be carried out by skilled personnel who are acquainted with the safety requirements and regulations for the plant site for that specific project. Generally instrument cabling is usually run in multicore cables from the control room to the plant area (either below or above the ground) and then from field junction boxes in single pairs to the field measurement or actuating devices.

For distributed microprocessor-based systems, the inter-connection between the field and the control room is typically via duplicated data highways from remote located multiplexers or process interface units. Such duplicate highways would take totally independent routes from each other for plant security reasons.

The junction boxes must meet the hazardous area requirements applicable to their intended location and should be carefully positioned in order to minimize the lengths of individual run cables, always keeping in mind the potential hazards that could be created by fire.

Cable Routes

Cable routes should be selected to meet the following requirements:

• They should be kept as short as possible.
• They should not cause any obstruction that would prohibit personnel or traffic access.
• They should not interface with the accessibility for maintenance of other items of equipment.
• They should avoid hot environments or potential fire-risk areas.
• They should avoid areas where spillage is liable to occur or where escaping vapors or gases could present a hazard.

Cables should be supported for their whole run length by a cable tray or similar supporting steel work or any other allowed method of support for that particular application; supporting electrical cables help relieve mechanical stresses on the conductors and protects the cables from harsh conditions such as abrasion which might degrade the insulation.

The traditional and rugged method for cable routing is conduit, either metal or plastic (PVC). Metal conduit naturally forms a continuously-grounded enclosure for conductors which provides a measure of protection against electrical shock i.e. all devices & enclosures attached the conduit become safely grounded through the conduit and also shields against electrostatic interference which is key for power wiring to and from devices like VFD, rectifiers, etc. that disseminate electromagnetic noise. Plastic conduit, while it provides no electrical grounding or shielding, it is superior to metal conduit with regard to chemical corrosion resistance, thus, it is used to route wires in areas containing water, acids, caustics, and so forth.

Cable Trays

Cable trays may be made of solid steel wire for light-duty applications such as instrument signal cabling or computer network cabling, or they may be made of steel or aluminium channel for heavy-duty applications such as electrical power wiring. Unlike conduit, cable trays are open, leaving the cables exposed to the environment. Usually for the cable trays, it is a requirement to have a special cable insulation rated for exposure to ultraviolet light, moisture and other environmental wear factors. Cable trays offers ease of cable installation when compared with electrical conduit; however, they don’t provide a continuously-grounded surface for electrical safety like the metal conduit, additionally, cable trays don’t naturally provide shielding for the conductors because it does not completely enclose the conductors the way metal conduit does.

General Installation Guidelines for Cable Trays

Cable trays should preferably be installed with their breadth in a vertical plane.

The layout of cable trays on a plant should be carefully selected so that the minimum number of instruments in the immediate vicinity would be affected in the case of a local fire.

Cable joints should be avoided other than in approved junction boxes or termination points. Cables entering junction boxes from below ground should be specially protected by fire-resistant ducting or something same.

Cable Types

There are normally three types of signal cabling under consideration as follows:

• Instrument power supplies (above 50 V).
• High-level signals (between 6 and 50 V), this includes alarm signals, digital signals, and high-level analog signals such as 4-20 mA DC.
• Low-level analog signals, below 5 V. This is typically covers thermocouple compensating leads and resistance element leads.

General Installation Guidelines – Cables

Signal wiring should be made up in twisted pairs. Solid conductors are preferable so that there is no degradation of signal due to broken strands that may occur in stranded conductors. Where stranded conductors are used, crimped connectors should be fitted.

Cable screens should be provided for instrument signals, especially low-level analog signals, unless the electronic system being used is deemed to have sufficient build in “noise” rejection.

Additional mechanical protection should be provided in the form of single-wire armour and PVC outer sheath, especially if the cables are installed in exposed areas such as on open cable trays.

Cables routed below ground in sand-filled trenches should also have an overall lead sheath if the area is prone to hydrocarbon or chemical spillage.

Cable Segregation

General Installation Guidelines on Cable Segregation

Only signals of same type should be contained within any one multicore cable. Furthermore, conductors forming part of intrinsically safe circuits should be contained in a multicore reserved solely for such circuits.

When installing cables above or below ground they should be separated into groups as per the signal level and segregated with positive spacing between the cables. As a general rule, low-level signals should be installed furthest apart from instrument power supply cables with the high-level signal cables in between. Long parallel runs of dissimilar signals should be avoided as far as possible, as this is the situation where interference is mostly likely to occur.

Cables employed for high-integrity systems such as emergency shutdown systems or data highways should take totally independent routes or should be positively segregated from other cables.

Instrument cables should be run well clear of electrical power cables and should also, as far as possible, avoid noise-generating equipment such as motors.

Cable crossings should always be made at right angles.

When cables are run in trenches, the routing of such trenches should be clearly marked with concrete cable markers on both sides of the trench and the cables should be protected by earthenware or concrete covers.

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