SOLUTIONS TECHNICAL ARTICLE Specifying Armoured Instrument and Control Cable It’s sometimes difficult to wade through the myriad of options and features available when selecting a control or instrumentation cable for your application. Despite the variety, choosing the more popular attributes and understanding the reasons for doing so will make your task much easier. A review of the product offering of one of Canada’s leading suppliers of control and instrumentation cables will shed some light on this subject. Control or Instrumentation? There isn’t a precise definition as to how a control cable differs from an instrumentation cable, but there are some general guidelines that can help to categorize each, with the application being the ultimate defining factor. The most significant differentiating factor is whether the cable is an assembly of single conductors or multiple pairs (for triads). A cable composed of two or more pairs of twisted conductors tends to be used for instrumentation purposes and an assembly of three or more single conductors tends to be used for control applications. A far greater generalization is that instrumentation cables tend to incorporate metallic shielding and small conductors than control cables. Conductors With the exception of thermocouple extension cable, copper is the only conductor material offered. Compared to aluminum, copper provides better conductivity, requiring less area and reduced overall cable size and cost. Although solid conductors are sometimes specified, stranded conductors are more common as they offer improved flexibility, with seven strands being standard. Conductors 16 AWG and smaller are often tinned to provide a degree of corrosion protection when the cables are to be terminated in potentially harsh environments. Conductors used in thermocouple extension cables are used to transfer the signal from a heated thermocouple cable junction back to a monitoring device. Several different alloy types are used and are specified in the following table, along with their respective colour codes. Thermocouple Extension Alloys ANSI Type Temperature Range (°C) Positive Wire Alloy Negative Wire Alloy Positive Wire Colour Negative Wire Colour Jacket Colour EX 0 to 200 Trophel® Cupron® Purple Red Purple JX 0 to 200 Iron Cupron® White Red Black KK 0 to 200 Trophel Nial Yellow Red Yellow NX 0 to 200 Nicrosil Nisil Orange Red Orange RX 0 to 200 Copper Copper-Ally II Black Red Green SX 0 to 200 Copper Copper-Ally II Black Red Green TX -60 to 200 Copper Cupron® Blue Red Blue ® ® Trophel, Cupron and Nial are registered trademarks of CARPENTER Specialty Alloys Please note: Although the content used for this brochure has been produced and processed from sources believed to be reliable, no warranty, expressed or implied, is made regarding accuracy, adequacy, completeness, legality, reliability or usefulness of any information. V.2 SOLUTIONS TECHNICAL ARTICLE Insulation The ARMCON® Advantage The vast majority of insulation is comprised mainly of extruded crosslinked polyethylene (XLPE) or polyvinyl chloride (PVC) and usually rated at either 300 or 600 volts. XLPE insulation is designated as CSA Type RW90 with 90°C wet and 105°C dry ratings. PVC insulation is designated as CSA Type TW75 with 75°C wet and 105°C dry ratings. PVC is usually specified for instrumentation applications where smaller conductors and lower current requirements don’t necessitate the more robust characteristics of XLPE insulation. XLPE insulations undergo a cross-linking process that make the material tougher and able to withstand higher operating temperatures with less risk of deformation. Cross-linking is obtained through electron bombardment of the insulation during an irradiation process, or through other more conventional means. Often ampacity requirements are much less than that of the smallest available Teck cable with 14 AWG conductors. In this instance, ARMCON® 16 is offered as an alternative to Teck cables for low ampacity control cable applications. It features XLPE insulated 16 AWG conductors that result in a lighter, smaller cable with reduced bending radius, improved flexibility and reduced cable tray loading. Additionally, ARMCON® 16 includes all of the benefits of a standard ShawFlex armoured control cable, including an FT4 flame rating, -40°C cold impact/cold bend performance, and is suitable for installation in hazardous locations. Insulated conductors are colour and number coded to many different standards and unique customer requirements. Multiple conductors in a cable are typically coloured white with sequential alpha-numeric print Modern insulating extrusion to differentiate them. Pairsline are normally coloured black and white, with the white conductor of adjacent pairs marked sequentially with alpha-numeric print. Triads include an additional red conductor. The first conductor in an ARMCON® cable is white, with the balance coloured black. Alpha-numeric coding is used to identify individual conductors. The outer PVC jacket is coloured blue, although other colours are available upon request. Conductor Assembly Up to 60 components may be cabled (twisted) together in a bundle to form the cable core. The purpose of cabling is to produce a generally round cable profile, mechanically hold the components together, and provide some protection from electromagnetic interference. Most cables are ‘bunched’, with all components having the same direction of ‘lay’ or twist. The direction of lay is usually expressed as “left hand”, with the components falling to the left as a cable is viewed from an axial perspective away from the observer. The length of each twist or ‘lay length’ is determined by measuring the distance between each 360 degree rotation of a component in a particular orientation. Generally, individually shielded pairs and triads have a nominal two inch lay length after cabling, whereas multiple conductors have a lay length of 15 times the diameter of the assembled cable core. Shielding The main function of an electrostatic shield is to prevent the electrical signal in a cable from being negatively impacted by voltages induced by other electrical equipment located within the vicinity of the cable or between conductors of the cable itself. Electrical ‘noise’ external to the cable is dealt with by wrapping a polyester backed aluminum foil tape over the assembled cable core. Overlapping the tape ensures 100 percent shield coverage. Individual pairs are similarly shielded to reduce electrical interference from adjacent pairs. Copper tape wrapped helically over the cable core is another popular shielding method with electrical utilities as the relatively low resistance path readily dissipates large induced currents often occurring due to lightning strikes on the electrical grid. Alternatively, a braided copper shield may be used if flexibility is an important consideration in addition to low electrical resistance. ShawFlex ARMCON® Cable Armour Most control and instrumentation cable sold in Canada incorporates either an aluminum or steel interlocked armour. There are several advantages with an armoured cable including superior crush resistance, better cold impact and cold bend performance, and a broader range of acceptable installation practices. CSA certified interlocked armoured cables are suitable for use in hazardous locations. In Group B, C and D atmospheres, individually shielded pairs or triads may be extended through a sealing gland without removing the shielding tape, thus simplifying installation and maintaining shield integrity. Interlocked armour also improves the cable’s resistance to flame spread, allowing lower cost insulation and jacketing compounds which offsets, to some degree, the cost of the armour. The bulk of interlocked armour is comprised of aluminum. Although similar in cost to steel armoured cable, aluminum armour offers the benefit of reduced weight. Both meet, essentially, the same performance criteria, but some users prefer steel armour for certain applications as it provides slightly more protection to the cable core. Served steel wire armour is also available, and is particularly suited for vertical or small diameter/bend radius applications. Served wire armour consists of strands of galvanized steel wire wound concentrically over the jacketed core. This design offers superior flexibility, cut through resistance and pulling strength. Please note: Although the content used for this brochure has been produced and processed from sources believed to be reliable, no warranty, expressed or implied, is made regarding accuracy, adequacy, completeness, legality, reliability or usefulness of any information. V.2 SOLUTIONS TECHNICAL ARTICLE Jacket All jackets, including the inner jacket under the armour, consist of flame retardant PVC. As an added benefit, ShawFlex supplies, as a standard, an outer jacket that emits low levels of acid gas (<12% w/w) when burned. This feature reduces the opportunity for corrosive damage to metallic structures and electrical equipment caused by acid gas emissions during a fire. Inner jackets include a nylon ‘rip cord’ to facilitate easy removal of the jacket when preparing the cable end for terminating. Jackets are typically coloured grey or ‘intrinsically safe’ blue, but are available in many optional colours. In addition to the standard cable print legend, all cables are sequentially metre marked along their length to verify to the user the quantity of cable remaining on the shipping reel following cuts. The use of modern ink jet printers permit other markings to be placed on the cable relatively easily during the manufacturing process. For example, the end user’s purchasing part number can be applied to allow for easy identification of the cable while in inventory and after it is placed in service. Standards CSA C22.2 #239 Control and Instrumentation Cables. CSA C22.2 #174 Cables and Cable Glands for Use in Hazardous Locations (Class I, Zone 0, 1 & 2; Class II, Div. 1 & 2). Head Office ShawFlex A division of ShawCor Inc. 25 Bethridge Rd. Toronto, Ontario Canada M9W 1M7, Canada Tel: +1 416 743 7111 shawflex.com Please note: Although the content used for this brochure has been produced and processed from sources believed to be reliable, no warranty, expressed or implied, is made regarding accuracy, adequacy, completeness, legality, reliability or usefulness of any information. V.2