Sparky,
When there is a danger of flash point being reached in the performance of your duties. OSHA 1910, and some in 1926 (I think) states that linemen have to wear it when switching or working on energized circuits because if they "mess up" the arc flash will be hot enough to melt skin, set clothes and hair on fire. Nomex is not the only thing you can wear, but it does meet the spec. You can wear a lot of fire resistant clothing, all of it stiff and pretty miserable. Carhartt makes a line of it that meets the criteria. You can, in some instances, wear coveralls over your normal clothing.
www.carhartt.com [This message has been edited by George Corron (edited 01-09-2002).]