Survey of fire testing of electrical cables

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Abstract
The National Electrical Code (NEC) is the document which regulates electrical cables in the United States. It addresses two fire properties only for which it requires testing: flame spread and smoke obscuration. Thus, a hierarchy of tests exists which cables need to pass to be allowed in occupancies regulated by the NEC. On a flame spread basis they are, in increasing severity: UL VW‐1, UL 1581, UL 1666 and UL 910. For smoke obscuration only one test is mentioned in the NEC: UL 910. The /LS category (limited smoke) introduced in NEC '90, as a voluntary label, will probably be addressed in NEC '93 under the new standard UL 1685, a modification of UL 1581. Rate of heat release is measured for cables only for R&D purposes. However, it is by now well established that rate of heat release is the one most important fire property to assess fire hazard. Cable flame spread tests (except for VW‐1) have all been modified, in non‐standard ways, to measure rate of heat release, which gives much more useful results than char‐length determinations. Moreover, small‐scale RHR test instruments (cone calorimeter, OSU calorimeter) have also been used extensively to test cables. The results of such tests have been correlated with those of UL 1581 (and of similar cable tray tests) in several cases, with excellent outcome. Work is underway to develop algorithms to predict largescale cable test results from small‐scale compound tests. This area of research is very promising, and, once completed, would decrease product development costs considerably and allow faster introduction of advanced materials into the market. However, such work can only be completed by using rate of heat release techniques in both large‐ and small‐scale tests.