What are Control and Instrumentation Cables?

While power cables are what we can refer to the “arteries” of industry, control and instrumentation cables are its “nerves” and are used for the control of equipment and data collection. They range from switch-core cables utilized in the wiring of control panels and switchgear, to the complex control and instrumentation cables employed in power stations and petrochemical sites.

At one end of the scale are the single-core cables used within machine tools and switchgear. Where normal ambient temperatures are involved and PVC insulation is used. At the end of the scale is, for instance, at an oil terminal utilizing 450 km of cable and connecting as many as 2000 instruments measuring flow rates or liquid levels in storage tanks.

Control cables have copper conductors and are laid up in multicore or multi-pair formation, each core being separately identified.

Thermocouple cables for example, are used for connecting the thermocouple to its measuring instrument. The term ‘thermocouple cable’ is usually used to describe both extension and compensation cables. Extension cables utilize conductors of the same alloys or metals as the thermocouple itself, while compensating cables utilize conductors of a cheaper material although having similar thermoelectric characteristics. The typical conductor materials or alloys used are constantan, copper, iron, copper-nickel, nickel-chromium and nickel-aluminium.

We have a number of national standards that exist for the colour identification of insulation and sheath.

To prevent electrical interference in both control and thermocouple circuits within and between cables, metallic screens (normally in the form of tapes) are applied over the individual pairs and/or the laid-up cores.

The finish of any type of cable to be buried in a petrochemical environment has to be given special consideration because of the presence of hydrocarbons in the soil. The steel wire armour which is usually applied as a mechanical protection on underground cables is not a barrier to the ingress of hydrocarbons into the core of the cable; this is best achieved by applying a lead sheath. Hence, such a control and instrumentation cables contain PVC bedding, lead sheath, another PVC bedding, single wire armour and PVC over-sheath.

Safety in hazardous areas has to be meticulously taken into consideration. Intrinsic safety is a protective technique which ensures that any electrical sparking which may occur is incapable of causing an ignition of gas or vapour. Even though a cable itself will rarely cause an explosion, it is possible for gas or vapor to percolate along the interstices of cable from a hazardous zone to a non-hazardous one. This problem can be fixed by use of a stopper box or sealing gland.

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