OK, this is going to get complicated. At higher frequencies, you really cannot think of the source in the power station is ZERO. You have to look at the whole thing in Transmission Line Theory. The line is very long from where you are to the station, instead of looking at the source as ZERO ohm, you really see the characteristics of the High and Low line ( the black and white wire in the cable). You have to look at the two lines as PARALLEL LINE transmission line. This is all in Electromagnetic and Transmission Line Theory. In short, the impedance between those Parallel Line Transmission Line is around 90 to 150ohm. Here is the online calculator where you put in the

https://cecas.clemson.edu/cvel/emc/c...tor/index.html
You can put in the size of the wire and the wires separation and get the characteristic impedance. So it's not a short circuit at all even the source in the power station is Zero ohm.

I draw two figure representing this.

**1) Fig 1** shows the power station, the noise source and the input of a typical amplifier without the EMI filter. As long as the length of the line to the power station is longer than a few wavelength of the noise frequency, you look at the line as transmission line and is typically between 90 to say 150ohm( roughly). Say the noise frequency is 100KHz, the speed of the signal is 186,000 mile/sec, the wave length is 1.86mile. So if you are more than say 3mile from the station, you can count on the line impedance is 90 to 150ohm or so. In practice, even if you are more than a mile away, you use transmission line theory already.

If you introduce noise of 10V into the line, and say the AMP presents a load of 50ohm, even if I use transmission line theory, by the voltage divider shown, you have 3.3V of noise at point

**A** injected into the amp. In real life, the noise source is close to the AMP, the impedance is almost ZERO from the noise source to the AMP, so you have the full 10V of noise into the AMP.

**2) Fig 2** shows the same situation except I put an EMI filter in between the line and the AMP. I drew the internal circuit of the typical EMI filter, which is two INDUCTOR(L1 and L2) in SERIES with the power line and capacitor C1 across the two line AFTER the L1 and L2. The impedance of the inductor at the noise frequency is say 200ohm each, so it's 400ohm across the two line on the C1 side. The impedance of C1 (0.1uF) is 16ohm at 100KHz. Using that, the noise voltage at point

**A** is only 0.385V. this show how the EMI filter works.

There is no easy way to explain this, these are all Transmission Line Theory and electromagnetic. You don't look at the source impedance is Zero and it is Zero ohm at the noise source. Hope this helps.