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Every EMC filter I've seen has capacitors between the AC lines and earth, configured like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Why are the caps not configured like this?

schematic

simulate this circuit

What is the advantage of having the extra cap from neutral to ground? It seems like it would reduce the capacitance to ground, and thus the effectiveness of the filter. Is it a safety issue, in case the capacitors fail short? But isn't avoiding that the point of using Y-rated capacitors?

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  • \$\begingroup\$ I have seen lots of EMC filters with 3 Y caps.On the face of it the 4 cap circuit is better and more economic BUT if the X caps are too big which is likely if you want to make EMC conducted then their lower self resonant frequencies could make you FAIL radiated EMC because powercords can and do radiate .In my neck of the woods people have much more trouble with radiated than conducted EMC .SO maybe its not always more economical to use just 1 Y cap. \$\endgroup\$ – Autistic Sep 10 '15 at 4:09
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Safety standards (UL, CE, etc.) place a limit on the amount of leakage current allowed to return on the earth ground. By using the 4 capacitor arrangement, the line-to-line filtering is made independent of the line-to-ground filtering. i.e. you can make the line-to-line capacitors a larger value without increasing the ground leakage current. Then the value of the single capacitor to ground can be set to not exceed the leakage current limit.

Clarification: By leakage I do not mean dielectric leakage. I mean AC current flowing through the capacitor because it is a capacitor. In the three capacitor topology, unless the three capacitors have exactly the same value (unlikely) and the AC line voltages are perfectly balanced relative to ground (unlikely) there will be AC current flowing in the ground wire which must be limited.

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So you have 3-phase power. There is noise that is line-to-line and noise that is common mode. The caps across the lines are in the right position to filter the line-to-line noise, and the single cap to ground can filter the common mode noise. If you used your method, the capacitance between phases would be less.

There may also be a consideration that the X capacitors are cheaper and smaller for the same capacitance (if you can even buy that capacitance) as compared to the Y capacitor (because the requirements for Y caps are so much more stringent), so it makes sense to minimize the number of Y capacitors (it also reduces the likelihood of an unsafe failure by 3:1).

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