EuroWire March 2016

Technical article

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 ).

30 mil 60 mil

Uncharred length (mm)

▲ ▲ Figure 2 : Effect of insulation thickness on uncharred length in VW-1 type test for different FR formulations (solid conductors)

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.

no burn time recorded for the 60 mil insulation with VB-1 formulation. These results are in line with the expectation that for insulation or articles made with flame-retardant materials, it is increasingly difficult to start a sustainable fire. The data also shows that for the less flame-retardant material, ie, for HB-1 sample in this case, the opposite may be true. This observation can be easily explained by the fact that below a minimum level of flame-retardance when thicker materials do catch and support a sustaining flame, they will just burn longer because of the larger mass of available flammable material. For both cases with HB-1 formulation, the wire samples burn all the way through the flag leaving no uncharred length. The effect of insulation thickness on the burn behaviour can also be expressed by the uncharred length of the samples as shown in Figure 2 . The results are shown only for the VW-1 rated samples as the horizontal burn rated sample burns through the entire length of the wire leaving no uncharred length. It is seen that for both the formulations, the uncharred length is higher for the thicker sample, indicating a greater flame-retardance with increase in thickness. The data also suggests that VB-1 is better than VB-2 in flame-retardance as evidenced by the higher uncharred length and shorter burn duration. 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

▼ ▼ Figure 3 : Effect of conductor type on burn duration for different formulations for 30 mil insulation thickness

Solid Stranded

Burn duration (sec)

▼ ▼ Figure 4 : Effect of conductor type on burn duration for different formulations for 60 mil insulation thickness

Solid Stranded

Burn duration (sec)

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March 2016