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