INSTRUMENTATION CABLES UNDERSTANDING

Instrumentation cables covered by the NF M 87-202 standard are used in the oil industry to transmit AC or DC analogue signals. They are of the PVC/PVC type and can be non armoured, armoured or lead-sheathed armoured.

• Non armoured cables are used when there is no risk of mechanical deterioration.
• Armoured cables are used when there is a risk of mechanical deterioration.
• Lead-sheathed armoured cables are used when there is a risk of contact with aromatic hydrocarbons.

Meaning of the code consisting of 5 series of 2 figures or letters:

• 1st series = number of pairs, triplets or quads: 01 to 27
• 2nd series = pair (IP), triplet (IT), quad (IQ)
• 3rd series = conductive core 05 (1 wire 0.8 mm) or 09 (7 wires 0.4 mm) or 15 (7 wires 0.52 mm)
• 4th series = general screen (RG), individual screen + general screen (EI)
• 5th series = mechanical protection: non armoured (SF), with armour (FA), with lead armour (PF)

Conductor standard colours

• 1 pair: white – red
• 1 triplet: white – red – blue
• 1 quad: white – red – blue – yellow

Construction of an instrumentation cable

Core: Central metal part of a conductor (copper core) which can be:

• solid: a single wire
• stranded core: formed of several strands twisted together into one or more larger strands. Depending on the number of strands, the core is said to be rigid or flexible.

Screen: Individual screen (if applicable) and general screen (polyester tape + aluminium screen): Al/polyester tape with tinned copper drain strand (7×0.20mm)

Sheath: Lead in the presence of aromatic hydrocarbons (PVC outer sheath)

Insulation: Insulating material (PVC insulation) surrounding the core of a conductor and designed to insulate it.

Armour: Central part providing the mechanical protection for the conductors (steel tapes). Consisting of steel tapes or steel wires spirally wound around the cable, above the sheath and generally with a protective layer (paraffinimpregnated paper) between them.

Drain wires: For electrical continuity.

Mechanical protection: Grey (where applicable) PVC, PVC-HR oversheath (PVC inner sheath)

Conductor core

The core must satisfy the following conditions:

Good conductivity to reduce losses when transporting energy. The materials must therefore be carefully chosen (maximum values of ρ)

• copper: ρ = 18.51 mΩ.mm²/m at 20°C
• aluminium: ρ = 29.41 mΩ.mm²/m at 20°C

Mechanical strength sufficient to prevent the conductor breaking under the
forces applied during installation, attachment and tightening of the
conductors.

Good flexibility to simplify the transit of the conductors in the conduits, to
keep to the piping route, and to supply the mobile equipment.

Good corrosion resistance due to atmospheric agents and chemical agents.

Good reliability of the connections due to a good resistance to the physicochemical effects of the contacts.

The standard defines a range of nominal cross-sectional areas for the conductor cores and divides them into four classes in order of flexibility.

1. Class 1: solid and rigid
2. Class 2: rigid, stranded
3. Class 5: flexible
4. Class 6: flexible (or flexible ‘+’, e.g. used for arc welding cables, or cables on coiling units)

Insulation

This insulation must provide a good insulation for the conductor core and have the following properties:

General properties for good insulation

• high resistivity
• excellent dielectric strength
• low electric losses

Specific properties for the use of conductors and cables

• Good resistance to aging
• Good resistance to cold, heat and fire
• Not sensitive to vibrations and impacts
• Good reactions in case of attack by chemicals

Instrumentation Cables Main materials:

Thermoplastic materials

The temperature causes a reversible variation in plasticity. This applies for:

• Polyvinyl chloride (PVC) which is frequently used due to its good electrical and mechanical properties and its resistance to cold, heat aging, water and commonly-used chemicals and to flame spread. However, the combustion of this substance involves the emission of toxic and corrosive products.
• Polythene (PE), whose remarkable properties make it a preferred insulation material (particularly for HV). The combustion of this material does not involve the emission of toxic or corrosive products.

Cross-linked elastomers and polymers

They are elastic, i.e. able to accept major deformations. This applies for:

Cross-linked polythene (PR), mainly used for temporary overloads and unfavourable heat environments. It must also be noted that this material has a good resistance to cold and does not give off corrosive gases during combustion.

Ethylene-propylene copolymers, for rigid cables and particularly for flexible
cables. This material offers poor resistance to oils and little resistance to the
spreading of flames, but does not emit toxic products during combustion.
Also used for HV.

Silicone rubber, which has excellent resistance to extreme temperatures (between -80°C and + 250°C) and to external agents, giving it remarkable aging properties.

Maximum operating temperatures for the insulations

Protective sheaths

When selecting the materials for protective sheaths, you must take the following points into consideration:

• External constraints exerted on the cable
• Operating conditions, maximum temperature
• Installation conditions, minimum temperature
• Type of insulation used, particularly in terms of heat resistance.

The materials used are:

• Insulation materials, such as those used for the insulation (see above paragraph)
• Lead or lead alloys
• Lead sheaths have:
  • Perfect sealing
  • Excellent chemical inertness
  • Sensitivity to vibrations and repeated deformation
• Poor mechanical properties which require protection in the form of an armour or laying in conduits or in a cable tray.
• Vulnerability to certain forms of electrochemical or electrolytic corrosion

The screen

The screen protects the low current circuits against disturbances produced by nearby cables.

Individual screen (EI)

Individual screen per pair or per triplet, generally consisting of a spirally wound polyester
tape covering, the screen’s continuity is provided by a tinned copper drain wire strand laid
along the cable.

General screen (EG)

Screen applied over the whole of the assembled conductors, its construction is identical to
the EI screen, however the drain can be provided by 2 or 3 copper wire strands, according
to the cable diameter.

The choice of the type of screen depends on the used of the cable in general:

• DC power supply cable: no screen
• cable for high level analogue and digital signals (4-20 mA, 24V, 48V): a general screen (EG),
• cable for low level analogue signals, compensation cable: individual screen (EI) general screen (EG)
• cables for digital signals: individual screen (EI) + general screen (EG).

Mechanical protection

Provided by an armour either consisting of:

• Two mild steel tapes, sometimes galvanised or PVC coated, spirally wound at the joints. Disadvantage: rigidity, sensitivity to corrosion.
• One or two layers of spirally wound steel wires, generally galvanised. These wires can have a PVC sheath.

Advantage: improved flexibility, good resistance to longitudinal forces.

Disadvantage: high price.

Examples of instrumentation cables

As you can see, they are also available with multiconductors which range from 3 to 27
pairs and from 1 to 12 triplets. These multiconductor cables are called “multis” in
maintenance jargon.