Arc flash testing update: Effect of arc electrode geometry and distance on cotton shirt ignition

The results of experimental arc testing of a nontreated cotton T-shirt with three different geometrical arc electrode arrangements are presented and discussed. Different geometries of arc electrodes result in different electromagnetic interactions of an arc current with current flowing between arc electrodes that in turn cause very different arc behavior. Ignition threshold of a T-shirt was determined using: a) apparatus for standard mannequin arc test method with vertical open air arc electrodes; b) apparatus for standard arc-in-a-box test method IEC 61482-1-2 with vertical arc electrodes placed in a box; c) end-of-line, ejected arc apparatus with horizontal and parallel open air arc electrodes. Test parameters were included: a) three different test arc current levels for each test apparatus with fixed distance between arc electrodes and a tested T-shirt; b) variable distance between arc electrodes and tested T-shirt for the same level of test arc current. A strong dependence of ignition threshold on the electromagnetics was demonstrated by the tests. For end-of-line, ejected arc T-shirt ignition demonstrated a longer arc flash boundary distance than calculated by NFPA 70E or IEEE 1584 calculations raising a need for strong caution in using cotton in arc flash conditions. It is suggested that electrode configuration should be considered in risk analysis. Reported test results could provide a framework in developing arc flash boundary for ejected arc applications. It is shown that unlike most cases where the IEEE 1584 calculations and NFPA 70E tables were observed to be conservative, in ejected arc calculation results should be used with caution as they are less than conservative.