Instrument Cable Specification
During detailed design project, the cable shall be selected with suitable specification for the intended application. In the beginning of the project, the specification for each cable part shall be carefully specified to satisfy the requirement of application and environmental condition. Normally the instrument cable consists of conductors, insulation, screen/shield, armor and outer jacket therefore the specification should covers and details each of them. The following information only describes the structure and its example, however the specification for each project may differ and shall be evaluated case by case hence details specified herein is not recommended to be copied and used directly.
Conductor (to transfer the electrical current from higher voltage to lower voltage point)
- Conductor material, commonly used material is copper. Tinned coated copper may also be selected to prevent copper oxides which provides more durable and corrosion resistance
- Design of the conductor, whether solid, stranded, flexible. Stranded conductor means the conductor is not single, instead it consists of several smaller size conductors which allow more flexibility.
- Size of the conductor cross section. Determining the size of the conductor requires information of the device operating voltage and current, length and resistance of the cable as they will contribute to the voltage drop across the cable. Cable shall be sized and ensured that the device at the end of the cable will function properly at a given voltage.
Insulation (to physically and electrically separate each conductors)
- Insulation material
The choice of insulation material is driven by several requirements such as electrical transmission properties, minimum and maximum temperature rating, burning behavior, abrasion and corrosion resistant
- Insulation thickness
The selection of insulation thickness, together with conductor specification, will be determined from the requirement of the voltage rating and cable strength. Note that the insulation thickness also contributes to the the flexibility of the cable.
Screen (to prevent interference)
Screen is constructed from aluminized polyester film/tape which provide protection from external interference. The aluminum tape is spirally wrapped with 25% overlap to guarantee 100% coverage of cable element including in bending area. Screen also consists of copper drain wire of which shall be electrically in contact with the screen along the cable.
Armor (to protect the cable against mechanical stress/load during construction and operation)
The selection of cable armor mainly depends on the cable installation and required mechanical properties such as maximum tensile loads, pressure loads, protection against rodent, minimum bending radius, direct burial installation. There are several types of cable such as galvanized steel wire braid, galvanized steel wire tape and galvanize steel wire round armor which each has advantages over each others.
Outher Sheath / Jacket (to physically protects the internal components of a cable)
Material of outer cable sheath/jacket shall be selected with regards to the following consideration:
- Environmental condition (humidity, temperature, solar radiation)
- Method of installation (indoor, outdoor, direct buried, on trays, etc.
- Possibility of oil, chemical spills or abrasion
- Behavior in fire (low smoke, zero halogen to avoid toxicant)
- Flame retardant or fire resistant
Determining the number of wire in multipair or multicore cable shall consider future expansion and maintenance.
Twisted pair shall be specified in 4-20mA analog signal cable to reduce the effect of interference. As recommended by API 552 Transmission System, twisted wire shall have minimum of six crossover per foot. Eight crossover per foot is a typical specification