John,
As lots of people have already said, and as is described in IEC 60384-14, Y
caps are placed where there is a risk of electric shock were the capacitor to
fail short-circuit. If we consider a 230V ac mains supply with 2.5 kV
transient voltages on it (i.e. Installation Category II per IEC 60664-1) then
a Y2 Capacitor would provide the equivalent of Basic Insulation whereas a Y1
Capacitor would provide the equivalent of Reinforced Insulation {or so IEC
60950 says}.
Therefore, for ac mains powered products designed for an Installation Category
II environment (e.g. designed to be plugged into a standard wall outlet) for
Europe, Australia, and the like, a Y2 capacitor is placed from phase to ground
and neutral to ground. A Y1 capacitor may be used per IEC 60950-1 between a
Primary Circuit (e.g. L & N) and a floating SELV Circuit.
Probably because there are restrictions on the upper limit of protective
conductor current (or leakage current as it used to be called pre IEC 60990)
that flows from the ac mains through the ground lead to the building ground,
the maximum value of capacitance for a Y capacitor is only a few nF.
Now for X Capacitors. These are connected from phase to phase or phase to
neutral. If you want to comply with IEC 60950-1 then DO NOT use them from
Phase or Neutral to ground. These capacitors can be used in EMI filters that
are located at the power entry to the equipment: as such there may be no
equipment fuse in series with them. Failure of these components could
therefore potentially lead to a fire. Since these devices are connected
between phases, and/or from phase to neutral, the maximum capacitance value
obtainable is much higher than for Y types.
So, both Y caps and X caps can be used in applications where a failure could
present a safety hazard (electric shock or fire respectively). In order to
ensure that the chances of either a Y capacitor or an X capacitor going short
circuit is extremely small, they are subject to a series of rigorous tests, as
defined in IEC 60384-14 and the other standards you mention.
It is possible to obtain capacitors that have both a X and a Y rating, but
obviously the capacitance rating of these devices is quite low.
The next edition of IEC 60950-1 will be dealing with this subject in a lot
more detail.
Although your excerpt contains some useful information, it also contains a
number of errors and you may like to consider a revision before publication.
Your choice, of course ;-)
Regards,
Richard Hughes
Safety Answers Limited.
