This demonstration video illustrates the potential ignition of cotton undergarments using a cotton bra. OSHA, NFPA 70E and CSA Z462 all require that workers not wear melting materials, even in underwear, excluding "incidental elastic".
e-Hazard has investigated two accidents in which bras melted or ignited under flame resistant clothing. It is recommended to never use a melting undergarment. Instead, use the arc rated shirt no more than its rating or use an arc rated bra, which can be purchased at ArcStore.
This testing is performed by ArcWear.