Mains Decoupling Capacitor

[jonwilhelmjr]

I have an image of a mains plugging into a piece of equipment. The schematic shows decoupling caps to ground.  First question,  are these for ESD. Second how are they selected,  what calculations should I accommodate to select the correct value. At the very least please provide a circuit description for me to research.

[Andy Chee]

Not ESD.

More likely rudimentary EMI/RFI suppression.

https://recom-power.com/en/rec-n-class-x-and-class-y-safety-capacitors-142.html?0

[selcuk]

You may read 15.7 and 15.8 from this book:

https://link.springer.com/chapter/10.1007/978-3-031-14186-7_15

A short summary about that type of components:

https://www.we-online.com/components/media/o156212v446%20Catalogue_EMC%20Filters%20(rev1).pdf

[madires]

The two caps are for EMI/RFI suppression (low-pass filters shunting high frequencies to PE). Since they are also safety-critical (a shorted cap could be deadly) they have to be class Y safety caps. Theoretically you would choose the capacitance value based on the desired cutoff frequency of the filter. However, there are some important points to consider. The caps will pass a small AC current (50/60Hz mains). This increases the power draw and could also become a problem when the device loses its connection to PE (electric shock hazard). So the current needs to limited to a safe value (not to harm anyone touching a metal part of the device). Another point to consider is the trigger current of RCDs. If you have multiple devices with a high leakage current (via the class Y caps) they could trigger the RCD. And there could be also country specific regulations to limit that leakage current. Therefore the class Y filter caps have usually only a few nF.

[Vovk_Z]

how are they selected,  what calculations should I accommodate to select the correct value.

That's Y-class capacitors (for EMI/RFI suppression). Must be either Y1 or Y2 type. The calculation is quite easy (when simplified). Less then 1000 pF (1 nF) is too low and wan't help much. More then 4700 pF (4.7 nF) can give too much current through them and possibly can hurt people (if not grounded). So, if your device is not very noisy you chose 1.0-2.2 nF Y1/Y2. If your device is powerful and noisy then your choice is 3.3-4.7 nF Y1/Y2.