Pilot Little Maestro.

[Nick]

Over on another popular  forum the question of using X type caps as droppers has been raised again.
I queried this practice way back on this thread and appear to have been vindicated in my opinion that conventional high voltage capacitors were more suitable.
I understand that X types are made to fail o/c but the self healing properties will lead to a short operational life as droppers before the capacity is reduced to a low figure.

There's actually no requirement for either X or Y capacitors to fail in any specific way - both are used for EMI filtering - X capacitors are certified for use across the mains where their failure would not cause a shock hazard, but may prevent a fire risk - thus if they fail s/c, the hope is that some upstream safety device would fail, i.e. a fuse or breaker.

Y class devices are used where there is a shock hazard, i.e. from live to earth. The specifications for Y are tougher than for X, i.e. they should not fail, or at the very least, self heal. EN/IEC 60384-14-2 (2016) is the relevant standard document.

So, you can use Y capacitors in place of X (with suitable ratings), but not vice versa.