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Wire & Cable ASIA – May/June 2016
The Dow Chemical Company
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Collegeville, Pennsylvania,
USA
Tel: +1 610 644 2075
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The likely explanation for this phenomenon is the higher
thermal mass provided by a thicker article thus acting as
a bigger heat sink. Moreover, as the outer layer burns and
creates a protective char layer, it provides a barrier for the
air to reach the inner material, thus starving the system of
necessary oxygen.
At the same time the heat continues to get dissipated into
the polymer and the conductor, further helping the cause
of fire retardance. The results comparing solid vs stranded
conductor also reveal a very significant impact of the
construction in their relative burn behaviour.
The inferior burn performance exhibited by the stranded
wire is probably due to the presence of voids between
insulation interior and the conductor. The presence of
voids increases resistance to heat transfer between the
insulation and the conductor and does not carry away heat
as efficiently as that with a solid conductor.
UL burn tests specify use of solid 14 AWG conductors with
a 30 mil insulation thickness for VW-1 burn test under UL
44 protocol. However, material qualified for VW-1 rating is
routinely used for 14 AWG stranded Cu conductor.
The results clearly demonstrate that the use of stranded
conductor adversely affects the burn behaviour and may
lead to failure in the VW-1 test for marginal compounds.
5 References
[1] “UL Standard for Safety and Thermoset-Insulated Wires and Cables,
UL44,” 18th edition, 28th March 2014
[2] M M Hirschler “Survey of Fire Testing of Electrical Cables” Fire and
Materials, 16, p107-118 (1992)
[3] Elliot, P J Whiteley, R H, “A cone calorimeter test for the measurement
of flammability properties of insulated wire,” Polymer Degradation and
Stability, 64, p577-584 (1999)
[4] “UL Standard for Safety for Wire and Cable Test Methods, UL 2556,”
third edition, 22nd March 2013
[5] J Fan and L Hunter “Engineering Apparel Fabrics and Garments,”
p271, first published 2009, Woodhead Publishing Ltd and CRC Press
LLC
Paper courtesy of the 64th IWCS Technical Symposium,
Atlanta, Georgia, USA, November 2015.