# Safety Capacitor Selection

When selecting an X2 safety capacitor what must be taken into consideration. There are only a few types of capacitance ranges and seem to be chosen based on voltage rather than capacitance.

Take mains AC, the cut off frequency is difficult to determine because we don't know the impedance so which capacitance do we pick? Can we assume a low resistance. Also, if the source is an alternator it will change the resistance to.

Is it common to put multiple of them in parallel to get the required range??

Also, will current draw in the circuit affect the value of capacitor? If we have more current flow do we need a bigger cap?

Thanks.

Edit 1:

The Kemet R463R422050M2K is a possible device that could be used.

mouser.ie/datasheet/2/212/KEM_F3095_R46_X2_310_110C-1104274.pdf

• X (and Y) capacitor values are selected to attenuate specific frequency ranges when paired with a specific common-mode inductor as part of an EMI filter, usually for frequencies in excess of 150kHz. Each capacitor will have its own frequency vs. attenuation curve and the precise selection is usually a mix of experience and empirical experimentation (measure, tweak, measure, tweak) – Adam Lawrence Oct 11 '18 at 16:13
• @Adam Lawrence if the source is inductive is it best to use that as inductance or should more be added? – MXG123 Oct 12 '18 at 7:20
• A possible part has been added however I am more concerned in picking a suitable capacitance at the moment rather than a specific device. – MXG123 Oct 12 '18 at 13:15
• I wouldn't say current "drawn" will change the capacitance, however if you go by safety MLCC capacitors which usually is voltage load dependent it will alter with voltage load. – Anton Ingemarson Jan 9 at 14:39
• This is a EMI suppression capacitor. The amount of capacitance/inductance required to suppress EMI depends on the power/current in the EMI, but not the power/current in the load. The impedance of the capacitor at the suppression frequency only has to be less than that of the device: it doesn't matter as much what the line impedance is. – david Jan 18 at 0:05