DIYbanter - View Single Post - Radial X2-AC Safety Capacitors (Question)

Dimitrij Klingbeil

On 24.08.2017 23:58, wrote:
On Thursday, 24 August 2017 19:37:11 UTC+1, rickman wrote:
tabbypurr wrote on 8/24/2017 2:22 PM:
On Thursday, 24 August 2017 18:53:09 UTC+1, rickman wrote:
John-Del wrote on 8/24/2017 1:38 PM:

Across the line use a type X cap, and 250V is fine. You
could (should) add a fuse in between the cap and the line
for an extra margin of safety.

I thought the X and Y capacitors were used when a short would
be a safety problem as in a shock hazard. Why would there be
a shock hazard if a cap across the power line shorts? If you
use a fuse any concern about a fire is eliminated. Then why
would you need the X cap?

Y is for shock risk, X is for fire risk only.

Ok, I stand corrected. But the question remains. Fuse and X cap
is like wearing a belt and suspenders.

You need belt & suspenders since both fail. Just don't put the pics
online.

Besides, using an X2 capacitor does call for an additional means of
protection. Capacitors rated for being connected across the line "X"
come in different sub-categories: "X1" and "X2".

"X1" capacitors are rated for a 4 kV transient overvoltage and they are
often found where no other means of protection (fuse) is installed in
the equipment "upstream" of the capacitor (that is directly across the
power line).

"X2" capacitors are only rated for a 2.5 kV transient overvoltage and
the circuits they are installed in need to be fused as an additional
means of protection (additional besides the dielectric in the cap).

Both X1 (rated to 4 kV pulse) and X2 (rated to 2.5 kV pulse), when used
within their specifications and unless their safety certifications are
fake, tend to be better protected than "plain" 600 V type capacitors.

The voltage ratings indicate the maximum AC line voltage that the X or Y
capacitors may safely be connected to (not their peak pulse ratings).

So, it's correct (and would be required in a repair) to replace the old
600 V capacitor with a 250 V safety rated "X" capacitor (assuming the
nominal mains voltage in your country is not higher than 250 V), but if
the capacitor is an "X2" (rather than "X1") type, the device also needs
to be fused "upstream" (on the mains side) of the X capacitor.

Regards
Dimitrij