While designing your radio equipment and then connecting the circuits together, there are times when the mains supply voltage frequency is experienced to cause a noise mains hum interference.
The reasoning behind an earth grounding is to clean the ground voltage. With a full wave diode rectifier, the -ve supply side is usually used as the zero voltage supply line, while the +ve supply line is used as the positive voltage line.
The mains hums comes from the -ve supply side of the full wave rectifier, as while the rectifier smoothing capacitor does its job to smooth out the full wave rectified AC supply, into a DC supply, the 0∙6Volt floating -ve supply line, is what causes the mains hum noise interference.
The DC PSU smoothing capacitor, depending upon its capacitor size relative to the coulomb charge current demand required for your circuit use, will eventually decide whether the smoothing capacitor prevents sufficient enough of the noise hum voltage upon the floating ground supply line.
The principle behind the mains supply earthing, is to provide a ground AC supply return to the electrical supply distribution supply. Most houses and premises use an earthing system referred to as a PME “phase multiple earthing”.
The idea behind a PME earthing system is to give a premises earthing from a single phase supply tap of a 3 phase electrical supply.
With a UK mains electrical supply, a 400 Volt 3 phase supply, when tapped using a 3phase star connection supply, is to tap the outside 3 phase supply between the star grounding and one of the single phases, in the UK will give a 230 Voltage supply to the house or premises.
The principle of a 3 phase star connected supply distribution network, is commonly used. But the grounding earth rod of your house premises is important. The grounding rod needs to see a low impedance back to the 3phase star connected power network supply transformer.
The grounding resistance back to the distribution transformer, is referred to as the “ground loop impedance”. Should this loop impedance be high, then if an electrical fault occurs with a piece of electrical equipment, then the ground voltage because of the high loop impedance, will show a raised voltage on the ground pin of your electrical equipment.
Do remember that the “residual current circuit breaker, RCCB”, requires to see an unbalanced current load to trigger the “RCCB” circuit breaker to function. While the ground could raise during a fault condition, the unbalanced load may be seen, but the over current circuit breaker will partially fail due to the raised ground loop impedance.
This is because the over current circuit breaker would only see the higher trigger current level once the current ground loop impedance is used. The higher the ground loop impedance, this will offset the trigger point of the over current relay, and hence also a wire fuse. Even a high speed fuse, also known as a “high rupture capacity, HRC” fuse, will false trigger, although a high ruption fuse will blow in just a half cycle of the mains supply. In short, a high mains loop impedance will cause your earth terminal mains supply voltage to float.
The floating ground mains voltage would also contribute to a mains hum interference.
However, if the mains pin of the equipment causes a main hum upon the audio, then chances are that some where is an electrical short being experienced by that equipment PSU, bearing in mind that the full wave rectifier -ve supply side, is a floating ground supply line, to that of 0∙6volt above zero volts. Hence thus, the 0∙6 volt short circuit to the ground pin of the electrical mains plug.
However, also bear in mind, that a full wave rectifier -ve and +ve is above and below the mains earth ground supply, because the full wave rectifier will produce a -ve and a +ve supply, say -6ve and +6ve, giving 12 volts overall. So to tie the zero voltage of a full wave rectifier PSU to ground, will the short out the -6ve to the ground earth pin of the mains plug, causing the main hums.
To tie a digital ground to a mains ground, can cause the similar problem, but with digital noise on the audio and RF signal.
The principle of a RF earth, is similar in reason to the mains ground return to the power transformer, but in this case to the transmitting transmitter from the radio receiver.