System grounding refers to conductors with a common potential. The external metal on audio equipment is connected to 'ground'. Usually if two pieces of audio equipment are connnected together a continuity test between the two external chassis will result in a positive outcome. In this instance the gear is said to have a 'system ground', essentially they are tied together.
'Earth ground' is referenced to moist earth, conductors are buried within the earth and a reference is made. Water supply pipes in bathrooms usually provide an excellent earth.
In a power cord the earth line approximates an earth ground. However, currents flowing through the line (finite resistance) confuse the system and the result is that no two points are at exactly the same potential. Therefore, it is important for an audio system to be connected to only one earth ground. i.e all the mains plugs should go to the same wall socket, this is sometimes called a 'star' configuration as all the mains leads come out from the socket in a star manner.
If a star system is not used a hum can be heard in the audio usually at an annoying 50Hz. In audio jargon this is called a 'ground loop' or 'pain in the arse!'
The reason for the ground line in electrical equipment is for safety reasons. Should part of the internal electronics break free and touch the outer casing, a lethal amount of voltage and current could be present, the ground route provides a path of least resistance. The lethal current flows to ground and not through us! Our bodies provide a higher resistance path to the current. It is the current flow that kills, not the voltage.
One poor engineering technique is to drop the earth (ground) in the mains plug of some gear. This might remove the hum, but it might also remove a life! Only complete village idiots would use this technique.
To sum up, when connecting an audio system always plug everything up from the same wall socket, most studios don't draw huge amounts of current. In a live situation the same rule applies. The same supply (ground) should be used for the engineer and the band (assuming they are connected electrically with DI's etc. If lighting rigs are used, a separate ring main should be used is posible as lights are electrically nosiy. Always insist the lighting engineer find his/her own supply.
