Welwyn W20 Series Wirewound Resistors - Voltage Limits

[Rontone]

I have some W21 3W and W22 7W resistors and was planning to use them as HT node droppers and power tube screen resistors, I have seen the W22 larger used many times in old amps, on Marshalls as screen resistors and droppers and the W21 3W are sold as screen resistors online...

But now looking at the datasheet they have quite a low "Limiting element voltage", reading around some other topics and forums it seems this is the maximum voltage?

Or does this mean the voltage ACROSS the resistor? As those W22's pop up everywhere but are only rated for 200V

[SEL49]

Or does this mean the voltage ACROSS the resistor?

Yes, that's what I believe.

[Rontone]

Thanks, an educated guess told me they must be doing alright in those positions otherwise they would all be toasted! but wasn't quite sure,

The use of the word element made me wonder if they were referring to the actual resistor body so therefore 'across' as opposed to 'what its being hit with'

I worded my search differently online and found someone asking exactly the same on ampgarage

[acheld]

Interesting question, since I've seen quite of few of those over the years, and never gave it much thought. 

The achilles heel of this resistor is its vitreous enamel coating/insulator.  While fireproof, the insulating properties are poor when at high temps, and these can be reached whilst operating at or just above their max power rating.  See p3 of the data sheet.

"Therefore, resistors operated at near maximum temperature cannot be classed as insulated
and should not be used in contact with any conducting material." 

On the other hand, I haven't seen one of these fail, or cause problems. 

I've tended to use Vishay wirewounds because I liked the way they looked when I first started this hobby (LOL, not knowing any better!).  When I looked up the data sheet, their max "working voltage" is sqrt(PxR).  That approach makes sense to me, and fits with Steve's thought.