[Amps] 100 Watt WW resistor Voltages

[jtml at lanl.gov (John T. M. Lyles)]

What is the maximum safe end to end DC voltage you can reliabily use 
a 100 Watt wirewound resistor for? In a bleeder application. It 
dawned on me that I remember seeing one flash over and self destruct 
one time, at some unreasonably low voltage. Ohmite catalog says 500 V 
RMS per inch of winding length in air, but this seems high from 
experience.