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Safety first when playing around with AC power.

Posted by Tweekeng (A) on June 21, 2012 at 21:22:55

In Reply to: caps on AC - impressions and uf sizing posted by rhyno on June 21, 2012 at 18:05:42:

Line-filter capacitors come in two general classes (as defined by the International Electrotechnical Commission (IEC), the international standards organization for electrical and electronics matters) and must be approved by whichever safety agency holds sway where the equipment will be sold. In fact, you can always recognize a line-filter capacitor by the "EN132400" printed on it, plus agency acronyms (UL, VDE, CSA, etc.). Older capacitors would have the symbols for SEMKO, DEMKO, and others, but these no longer exist as governmental agencies and are, if they exist at all, commercial enterprises carrying out testing.

Class X is for applications where failure could not lead to electric shock (hot to neutral). Class X1 capacitors are intended to operate safely even in the presence of spikes on the mains supply of up to 4 kV (installation category 3 or overvoltage category 3 according to IEC60664), which are normally industrial supplies, but some standards call up class X1 capacitors if they are connected directly to the mains supply upstream of the equipment fuse, irrespective of the type of mains supply. Class X2 capacitors are intended to operate safely even in the presence of spikes on the mains supply of up to 2.5 kV (installation category 2 or overvoltage category 2 according to IEC60060), which are normally residential, commercial and light industrial supplies. X capacitors can be found from 0.001 uF to at least 10 uF and are only made in film.

Class Y capacitors are for applications where failure could lead to electric shock if the ground connection were lost. This includes hot/neutral to ground, and antenna isolation capacitors. Because Y capacitors shunt current to ground, leakage-current limitations limit their size to a maximum of about 4700 pF in many commercial and industrial applications (but refer to the relevant standard for definite information) and about 470 pF in medical applications.Cappic Larger ones are available however. Y caps are available from 1000 pF to 0.1 uF and are made in both film and ceramic.

Classes X1, X2, and Y were originally defined by the IEC in IEC 60384-14. CENELEC has adopted EN 132400 (technically equivalent to, but structurally different from IEC 384-14 2nd edition), which now defines seven classes of line-filter capacitors. Class X1 capacitors are impulse tested to 4 kV (higher for capacitors over 1.0 uF). Class X2 capacitors are impulse tested to 2.5 kV (higher for capacitors over 1.0 uF). Class Y1 capacitors are impulse tested to 8 kV, and Class Y2 are impulse tested to 5 kV. Classes X3, Y3, and Y4 are for lower-voltage capacitors, none of which are presently called up in safety standards. Other impulse tests also apply. These include a 1000 hour endurance test during which the capacitor is subjected to a continuous overvoltage condition, plus periodic 1000 VAC spikes, and a flammability test during which the capacitor is hit with a series of transients while under rated voltage. Capacitors conforming to IEC60384-14 normally also conform to EN132400, and vice versa, and should be accepted in all European countries.

Line-filter capacitor voltage ratings are based on what the agency considers the "nominal" line voltage in its jurisdiction to be. A part can have one voltage rating in the US and Canada (125 or 250 VAC), a different rating in Europe (perhaps 275 or 300 VAC), and the manufacturor might recommend its use at a higher voltage yet (a 250 VAC part on a 260 VAC measured line). This can make for a little confusion in application, and it may be best to ask the manufacturor for details.

These capacitors are also rated according to min/max temperature and allowed average annual humidity exposure. IEC 68-1 calls for an X/Y/Z marking system where X is the low-temperature limit, Y is the high-temperature limit, and Z is the number of days tested in a damp heat test (+40C at 93% RH). For example, 40/100/56 would mean the capacitor was rated from -40 to +100C and tested for 56 days in the damp heat test. This is printed on the capacitor.

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