"What is an earth, anyway!"
Here we expose another contradiction causing massive confusion - the difference between Earth and Ground.
Ground, also referred to as True Earth, is the point at which a grounding system has the lowest impedance to Terra Firma i.e. the point of connection at the grounding system (ground rod or mat).
Earth is any portion of the system referenced (connected) to Ground or True Earth, regardless of its distance from the Ground.
What I find does not help is there appears to be a definition floating about stating the exact opposite. If this were the case, then electrical connections would be not be marked L-N-E. The 'E' denotes that it is to be connected to Earth, and not necessarily Ground (although there is no reason why it could not be).
Earth is often derived from the Neutral which is grounded at the transformer only. Should it be derived from Neutral at e.g. the meter point or switchboard in a TNC-S system, then it takes on the voltage of Neutral which is not 0V as the cables have resistance. Earth is therefore better regarded as a Safety Earth, and not a reference to or of True Earth or Ground.
'E' denotes the Safety Earth of a piece of equipment, and usually refers to any exposed portions such as a case or framework.
There are two reasons. The first is safety. If anything goes wrong inside a piece of equipment then it should, theoretically, go to earth and not endanger the person using it. This is why the earth on equipment should never be left off.
The second reason is if a piece of equipment is not earthed then the chassis (if exposed) can potentially have a voltage on it (with respect to earth) of anywhere between Neutral and the full Line voltage. This is dependent on the ratio of capacitance between the chassis and Neutral and chassis and Live. Typical voltages of half L-N are found i.e. 115V in 230V countries. Apart from being able to give the user a serious tingling feeling, this can also cause interference to data signals and the like.
The requirement for 'good' earthing on equipment has led to another cause rife with the human element - the belief that all Earths are Earth. Even if all connections to Earth were of one potential, the human, through the way most equipment is wired, manages to create the classic 'earth loop'.
When one starts defining what a loop is i.e. that it is an inductor (coil), then things start to change dramatically. The biggest problem with the ground loop is it is a shorted coil. A very small change in the magnetic field within the coil may only generate a very small voltage, but as the impedance imposed on the coil is effectively zero ohms, the current experienced within the coil is large.
In reality no earthing system is 'current free' meaning there is always a voltage potential between two separate power sockets. Using them to power a single workstation comprising more than one device (e.g. computer and monitor), then by simply connecting the devices together will offer a secondary path for the earth currents to flow.
The only reason such grounds believe they are ground is
There are situations when the chassis is not grounded (such as a TV). This is because it is direct rectified off the mains. However, such chassis are not open to the user (well insulated). There are others such as Hi-Fi amps etc. but these go through very tough tests to make sure the cases don't go live.
A referenced ground is not to be construed as Ground or even a Protective Earth. It is derived from the Neutral by some means of limited leakage, usually a small capacitor or high resistance. As well as economic reasons (saves on a 3-pin plug, earth wiring, etc.), this method, as can be seen in the diagram, eliminates high current earth loops simply because ground is never used.
The reason limited leakage is used is such systems are prone to being wired up wrong (many simply have a 2-pin plug which is easily reversed) and the reference lands up Live, rather than Neutral. Should one device be the wrong polarity, or the devices are some distance apart and fed from separate supplies, then these leakage currents flow along the signal ground.
Those who are electronically inclined will immediately notice an inherent danger; Yes, if the capacitor fails and the line it is connected to happens to be Live, then the chassis of the equipment becomes live. It, therefore, comes as no surprise there are countless reports of musicians, stage-hands, and others using sound equipment being electrocuted.
To make matters worse, should the one device with the failed capacitor bring its chassis live, any devices it is connected to will also have their chassis become live too. This means one failed capacitor can make an entire system dangerous.
At 50Hz this leakage tends to be relatively small, however, the mains is rarely 50Hz but a mix of harmonics as well as other clicks, whistles, and pops that come from switching. The default to achieve a reference ground is through a capacitor. With the reactance of a capacitor dropping as the frequency goes up means it is the harmonics and other high frequencies that are conducted. This explains why mains 'hum' is more a buzz than a hum.
This effect is even further aggravated if two devices are accidentally both wired wrong (references to Live) and are operating on two separate phases. First, there is now an average of 1.73 times the leakage current, as well as two phases' worth of noise to contend with. It is believed that this is why some audio-visual professionals have been coaxed into thinking all gear must run on a single phase.
This leads us neatly into the next human trait...