Dave,
If an X capacitor shorts (line-to-line or line-to-neutral), it does not
cause a safety hazard. If a Y capacitor shorts (line-to-ground or
neutral-to-ground), it does cause a safety hazard. Please refer to IEC
950 sections:
* 1.5.6, Capacitors in primary circuits.
* 1.5.7.1, Bridging capacitors.
Here is an excerpt taken from chapter 8, Capacitors, of Robust
Electronic Design Reference, which I am writing for Kluwer:
Capacitors that connect straight to primary power, such as ones
used in AC-line filters, may be subjected to high-voltage spikes
and surges, and thus require a surge-voltage rating. These
capacitors are considered safety-critical parts by safety
agencies, and usually must be certified to EN 60384-14, EN 132400,
UL 1414, and CSA C22.2 No. 1. An X-capacitor goes line-to-ground
or line-to-line, and can not cause an electric shock if it fails.
A Y capacitor goes line-to-ground or neutral-to-ground, and can
cause an electric shock if it fails. These capacitors have
several subcategories, with surge-test voltages of:
· 4,000V peak for X1 capacitors, connected line-to-line across
a 3-phase line.
· 2,500V peak for X2 capacitors, connected line-to-neutral,
for voltages up to 250VAC from ordinary wall outlets.
· 8,000V peak for Y1 capacitors, connected line-to-ground or
neutral-to-ground, for voltages up to 250VAC, with one
capacitor bridging double or reinforced insulation.
· 5,000V peak for Y2 capacitors, connected line-to-ground or
neutral-to-ground, for voltages up to 250VAC, with one
capacitor bridging basic or supplemental insulation, but two
capacitors in series bridging double or reinforced insulation.
(We sometimes use a Y-connection, with three Y2 capacitors wired
together, and having their other leads going to line, neutral,
and ground.)
John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE
dBi Corporation
http://www.dbicorporation.com/
[email protected] wrote:
>
> Here is a new question/comment that is sure to stir up some responses. It
seems that many use the term Y to refer to caps from line-to-line and the term
X to refer to caps from line-to-ground (or is it the other way 'round). While
researching caps I found that X basically designates a cap which is designed
to never fail shorted. This would be the one to use for line-to-ground
applications. And that Y refers to a cap which can fail shorted. This would be
acceptable for line-to-line.
>
> Thanks to all for the input on my hipot question. That cleared things up for
me.
>
> Dave Cuthbert
> Micron Technology
>
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