WCA May 2016

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54 Wire & Cable ASIA – May/June 2016
Formulation LOI, %
HB-1 24
VB-1 27
VB-2 27
❍
❍ Figure 5: Effect of conductor type on uncharred length for
different formulations
❍
❍ Table 2: Limiting Oxygen Index of the flame-retardant
formulations studied
Solid - 30 mil
Stranded - 30 mil
Solid - 60 mil
Stranded - 60 mil
Uncharred length (mm)
3.2 Effect of conductor type (Solid vs stranded)
Even though a systematic study of the effect of insulation
thickness on burn properties for low voltage wire is
not available in the literature, similar studies have been
conducted for other flame retardant articles, an example of
which is fabrics for upholstery or children’s clothing[3].
Comparison of such studies with flame retardant wires
needs to be made with some caution since the presence
of a metallic conductor with its high thermal conductivity
provides a thermal sink for the hot insulation and adds
to the complexity in understanding the effects of various
construction and geometric parameters of the polymeric
layer.
In this study, another aspect of the conductor, ie, solid vs
stranded copper, is investigated for its effect on the burn
behaviour of the wire.
Figure 3 shows the effect of the conductor type on burn
duration for all the formulations in a VW-1 burn test for
30 mil insulation thickness. For both the vertical burn
rated compositions, the flame extinguishes much sooner
for the solid conductor than that for the stranded ones,
suggesting that using solid conductor provides a better
flame-retardance for the wires.
One possible reason for the superior performance of the
system with solid conductor may be due to the intimate
contact it provides with the insulation, thereby acting as a
better dissipator of heat from the polymer.
For the stranded conductors, on the other hand, the
voids between the polymeric layer and solid copper act
as thermal insulation and thus trap more heat inside the
polymer. The difference is significant since passing VW-1
burn test requires the burn duration to be less than 60
seconds for the samples. For both the vertical burn rated
formulations, when a stranded conductor is used, the
samples exceed the maximum limit for burn duration, thus
failing the test.
In fact, for the VB-2 sample, the entire length of the wire is
consumed, leaving no uncharred length (shown in Figure 5 ).
On the other hand, the constructions with solid conductors
pass the VW-1 test with comfortable margins.
The burn duration data shown in Figure 3 for the HB-1
samples could also be somewhat misleading without
considering the fact that for both instances (solid and
stranded), the wires burn through the flag, leaving
no uncharred sample. Interestingly, even though the
VB-1 stranded sample burns for a long time (>60 sec),
it still leaves substantial uncharred length after flame
extinction.
The effect of solid vs stranded conductor for 60 mil
insulation thickness is illustrated in Figure 4. As seen for
30 mil thickness, both solid and stranded HB-1 samples
burn completely through their entire length. The same
is true for the VB-2 sample with stranded conductor.
The results again show that for identical formulation
and geometry, stranded conductors exhibit a poorer
burn performance.
For the same VW-1 burn tests conducted to compare the
effect of conductor type, the results are also expressed by
the uncharred length of the wire samples for the vertical
burn rated formulations in Figure 5.
As discussed earlier, irrespective of insulation thickness,
both the VB-2 stranded wires burned completely and did
not leave any uncharred length. Overall, the data again
shows the superior burn performance of wires made
with solid conductors as opposed to the stranded ones.
The results also confirm that the VB-1 formulation is
comparatively better in flame retardance than the VB-2
material.
In fact, for the 30-mil stranded wire, which is the most
difficult condition for a formulation to pass VW-1 test
among all the conditions studied, the VB-1 sample came
very close to passing with only the burn duration time
exceeding the maximum allowed time by a few seconds.
On the other hand, the VB-2 sample completely burned out
even for the 60 mil insulation thickness when a stranded
conductor was used.
As a side experiment, the Limiting Oxygen Index (LOI) for
the three FR formulations was measured and the results
are shown in Table 2.
The LOI data confirms that the VB-1 and 2 formulations
are superior to HB-1 composition in flame retardance.
However, the differences between the two vertical burn
rated materials cannot be distinguished by the LOI data
even though they showed marked difference in the VW-1
test.
4 Conclusions
Investigation into the effects of the two important cable
construction parameters, namely insulation thickness
and conductor type (solid vs stranded), for VW-1 burn
performance provides some important insights.
The effect of insulation thickness very much falls in line
with the behaviour seen for the other FR articles such as
textiles and home furnishings, where larger thickness
provides better flame resistance[5].