I have seen this note in several datasheets from Kemet AVX and others
with no explanation.
Does anyone know the reason for this?
piezo effects causing mechanical cracking? failure mode is a short
circuits? not self healing? .
Your answer - the X7Rs in question are not X or Y rated:
CAPACITORS FOR RFI
SUPPRESSION OF THE AC LINE:
Symmetric interference can be envisioned as a source connected between
the main and neutral wires. A line-to-line capacitor (designated as type
X) properly applied is effective for this type. Asymmetric interference
is represented by a source between either main or neutral and chassis
ground. A line-to-ground capacitor (type Y) provides filtering in this case.
A European study by Unipede (a consortium of power producers) revealed
the types of transients found on the line:
80% of all transients have a duration between 1 and 10Î¼S and amplitudes
up to 1.2kV. These occur more than 10 times per day. Assuming a lifetime
of 10 years the RFI capacitors must withstand over 30,000 such surges,
as well as lesser numbers with amplitudes up to 6000 volts. The Unipede
data agrees with IEEE Std. 587 which recommends equipment be tested with
a 6kV damped sinusoid. IEEE 587 is discussed on page 7.
There are three types of RFI capacitors commonly in use on the AC line.
These are impregnated metallized paper, metallized film and ceramic. The
first two are categorized as â€œself healingâ€ or â€œclearingâ€ while ceramic
is not. The property of self healing, properly designed into a
capacitor, can extend its life while maintaining a small size and safe
See page 2 note 4 and 6.
Capacitors with X7R Dielectrics are not intended for AC line filtering
Contact plant for recommendations. Capacitors may require protective
surface coating to prevent external arcing.
Note: Capacitors with X7R dielectrics are not intended for applications
across AC supply mains or AC line filtering with polarity reversal.
Contact plant for recommendations.
Extremely low ESR and ESL
X7R dielectric is not recommended for AC line filtering or pulse
X7R dielectrics are not designed for AC line filtering applications. An
insulating coating may be required to prevent surface arcing.