WCA May 2016

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] .

Formulation 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. LOI, % HB-1 VB-1 VB-2 24 27 27 ❍ ❍ Figure 5 : Effect of conductor type on uncharred length for different formulations ❍ ❍ Table 2 : Limiting Oxygen Index of the flame-retardant formulations studied

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Wire & Cable ASIA – May/June 2016