# Connection of Cx capacitors to neutral in 3-Phase EMI filter

I'm working with a circuit which uses a common schematic for EMI filtering. I simulated the schematic and I added a 200 kHz AC signal to the main 50 Hz line, to model the differential mode noise.

In most of EMI filtering topologies, Cx capacitor are somehow connected to the neutral:

Also there are some topologies (maybe IT systems), in which there is no connection between CX capacitors and the neutral:

My question is how this CX capacitors filter the differential noise without having path to the neutral?

As you can see, from my simulation result, the 200 kHz noise which I added to the main signal, is filtered by the LC filter (only one line is displayed)

But when I remove the connection of Cx capacitors to the Neutral, no low pass filtering occurs:

Thanks

• The noise source should be on the LOAD side as the EMI filter is meant to attenuate the load noise for others on the PCC
– user16222
Nov 23, 2021 at 14:38
• Yes, you are right, I did it just to simplify the simulation
– Nima
Nov 24, 2021 at 5:42

Referencing to N is only required for Common-Mode filtering. The Cx capacitors are not providing any Common-Mode filtering, they are creating a 2 stage Differential-Mode filter.

Cy are providing the single stage Common-Mode filtering by providing a "low impedance" circulatory path back to the LOAD, for any CM noise that may have circulate outside the unit

If you re-arrange the drawing you can see the purpose:

simulate this circuit – Schematic created using CircuitLab

CX capacitors' purpose is to provide filtering for "differential mode noise". Normally, the NEUTRAL is a current carrying conductor. So a filter for differential mode noise can be placed across each phase and neutral. CY capacitors (i.e. the ones having their one end connected to EARTH) are for common-mode noise filtering because common mode noise exists on each phase conductor and is EARTH-referenced. Remember that ground conductor is assumed to carry the fault current only, not the load current.

Now, looking at this one:

It's obvious that the CX capacitors are for differential mode noise. However, CY capacitors are only placed across L3-E and N-E. This might be an indication of that this product can be used for a single phase system (using only L3, N and E).

• Please refer to this link, then you will see a topology with Cx capacitors connected to the neutral: apitech.com/globalassets/images/products/…
– Nima
Nov 24, 2021 at 5:26
• I edited the question with this link's image
– Nima
Nov 24, 2021 at 5:55
• @Nima It appears that you don't have the basic knowledge about differential mode noise and common mode noise. This may help. Nov 24, 2021 at 6:49
• Thanks rohat, But I really studied so much about that and I just simplified the question to be understandable for everyone. I am almost done with the definition of common mode and differential mode noise, their frequencies, and also calculation of the filter components in SINGLE phase system. but stuff is more complicated in 3-phase system. you said the ones having their one end connected to EARTH or NEUTRAL, are for common-mode noise filtering. but I gave you a link that the differential mode filtering capacitors are connected to the Neutral. That's my question.
– Nima
Nov 24, 2021 at 7:16
• @Nima see my edit. Nov 24, 2021 at 7:38