[vcf-midatlantic] Safety capacitors, was Re: The RIFA Cap Says: PFFFFFT!

[Dave McGuire]

On 12/14/2015 01:07 PM, Systems Glitch via vcf-midatlantic wrote:
> Just had one blow in an old ECL monitor, releasing tons of stinky
> capacitor smoke. Had to take the monitor outside to let it finish
> smoking.

  Yuck.

> I know these are "X2 safety rated" devices, but what does that really
> mean? "Keep two handy cos they fail a lot?" Seems to me that not
> blowing up would be a great safety feature, something you wouldn't
> relegate to cheap rice paper in epoxy.

  These are called "safety capacitors".  The important part is that
they're designed to fail only in very specific failure modes.  An X2
capacitor is designed to be placed across the line ("X") and is impulse
rated to 2.5kV in most cases.  Here is my entire set of notes on safety
capacitor types and ratings:

--------------------------------------------------------
Type X: across the line (X1, X2 and X3)
Type Y: line to ground (Y1, Y2, Y3 and Y4)

     impulse test V   peak service V
------------------------------------
X1:     4.0kV[1]      2.5kV - 4.0kV
X2:     2.5kV[2]      <= 2.5kV
X3:     -             <= 1.2kV

Notes:
[1]: If C<=1.0uF.  For C>1.0uF, V(kV)=4/sqrt(C).
[2]: If C<=1.0uF.  For C>1.0uF, V(kV)=2.5/sqrt(C).


     impulse test V   rated V
----------------------------------
Y1:     8.0kV          <= 500V
Y2:     5.0kV          150V - 300V
Y3:     -              <= 250V
Y4:     2.5kV          <= 150V
--------------------------------------------------------

                -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA