WCA May 2016

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51
Wire & Cable ASIA – May/June 2016
Abstract
Flame retardant fillers are used in insulation and jacket
coatings to produce building and industrial cables that
meet critical fire safety standards and codes. These flame
retardant additives mitigate the inherent fuel properties of
the base resins used to provide electrical insulation to the
underlying conductor, thereby slowing the spread of fire
and providing critical escape time should a fire start.
VW-1 (UL 44 and UL 2556) is an industry specification
describing the flame retardancy of a polymer coated wire
and its likelihood to propagate a fire after the removal of
the initial heat source.
With the advent of moisture cross-linked formulations
in wire and cable systems, and the use of a draft-free
burn chamber, compound manufacturers and cable
producers have found it more and more challenging to
pass VW-1. Beyond the critical parameter of polymer
compound formulation, a further understanding of the
cable construction including insulation wall thickness
and conductor core (solid vs stranded) is necessary in
the design of systems that will meet this level of burn
performance.
In this work, the effects of wire construction, including
insulation thickness and conductor type, are investigated
on the burn performance of various flame retardant
formulations.
Moisture-cured, low voltage wire insulations are made by
mixing ethylene-vinylsilane copolymer, dibutyltin dilaurate
catalyst masterbatch and varying levels of flame retardant
masterbatch, and extruding on wires. Burn performance is
described by burn time and char length of wires passing
VW-1 as tested in a UL certified burn chamber.
1 Introduction
Underwriters Laboratories Inc (UL®) has established
the specification UL-44 (Thermoset-Insulated Wires
and Cables) for type XHH, XHHW, XHHW-2 RHH,
RHW, RHW-2, RH and SIS insulated wires. In addition
to dictating maximum operating voltages, conductor
properties (size, metal type, solid vs stranded, etc) and
insulation thicknesses, UL-44 specifies performance
requirements for the insulation materials.
These performance criteria are defined in terms of
physical and electrical properties, fluid resistance, and
thermomechanical performance. In addition, designations
of flame retardancy are defined with FV-2/VW-1,
vertical specimen, having one of the most rigorous burn
compliance criteria.
To achieve a marking of VW-1, a finished wire, either a 14
AWG (2.08mm2) copper or 12 AWG (3.31mm2) aluminium
conductor with a 30 mil (0.76mm) insulation layer must
not be capable of conveying flame along its length or
in its vicinity in accordance with the test[1]. While the
specification is not explicit about the use of solid versus
stranded conductor, insulation formulators typically test
specimens using solid conductors.
This is largely due to the fact that other physical and
electrical testing beyond flame performance does require
the use of solid conductors. However, because of their
flexibility and relative ease in handling during installation,
cable manufacturers by and large produce only stranded
conductors, even at smaller gauge sizes.
Therefore samples submitted by cable producers for VW-1
testing, either for new product introductions or existing
product compliance, are typically made using stranded
conductors.
It is therefore imperative that compound manufacturers
understand and are able to predict the burn performance
of their flame retardant compounds on a given conductor
type. The conductor size is known to be a key parameter
in the burn performance of wire and cable constructions,
with larger conductors providing a greater heat-sink, and
thus a disruption to the ‘fire triangle’ (heat/oxygen/fuel)[2].
There has been no effort in either the industry or the
literature to determine if burn performance is significantly
impacted by the type of conductor, stranded vs solid.
The Effect of Cable
Construction on Flame
Retardancy in Moisture-
cure Compounds
By Peter C Dreux, Abhijit Ghosh-Dastidar, Kurt A Bolz, The Dow Chemical Company