ABSTRACT
Large professional studios often have ‘technical earth’ systems, where a dedicated earth is sunk into the ground, and to which no equipment is connected other than the audio equipment. Sometimes this earth is isolated from other earths, and sometimes it is bonded to the earth which is used by the electricity supply company. The regulations about this vary greatly from country to country, and even from region to region, so in a book such as this, designed for an international readership, it is only possible to discuss things in general terms. However, it is always imperative to discuss earthing matters with a local, licensed electrician who is fully aquainted with the local regulations. There is an inherent problem in this, though, because many qualified electricians are totally unfamiliar with the concepts of technical earths, and even professional studios sometimes encounter problems with such negotiations. Nonetheless, the object of the exercise is to provide a ground reference
plane with a low impedance to earth, which acts as a common reference plane for all electrical signals. As the impedance of the surface of the planet is so exceptionally low, and its mass is so great, if one can get as low impedance a path as possible to earth, then whether the technical earth is, or is not, connected to an electricity company safety earth is usually not of too much purely technical relevance. The task is to get the audio system earth connected to any other earths in such a way that any common impedances are minimal. Figure 25.1 shows two different, right and wrong approaches which should be more or less self-explanatory. Incidentally, the terms earthing and grounding, which frequently get inter-
changed, may also have different meanings in different countries. Often, they are one and the same thing, but, strictly speaking, the chassis of an aeroplane can be the ground for all the electronic systems, even though in flight the aeroplane is not connected to the earth. The ground, in general, is the common reference plane for the electrical/electronic circuits, whether or not it is physically connected to the earth. In fact, in some audio installations, the audio ground may be separated from earth by a suitable impedance.
Normally, however, in order to overwhelm the effects of any differences in ground potentials due to leakage inductances and capacitances to earth, a low impedance connection is made from the ground plane to the physical earth of the planet. Furthermore, if all metal chassis are earth grounded, then they cannot become lethal in the event of a loose live wire touching them – a fuse or a breaker merely blows, and not a human being. Geography, or rather local geology, can also play a great part in the
effectiveness of earthing systems, and if a studio is sited in a region of poor ground conductivity then little can usually be done about it. However, when the audio system interfacing is carefully and correctly terminated, the necessity for an excellent earth is usually significantly reduced. On the other hand, when the earthing system is excellent, an audio system may stand a better chance of tolerating less than optimum audio interfacing without the manifestation of too many problems. Obviously, though, one should strive to get both aspects as good as one can. The general level of electromagnetic interference has risen in recent years
to a point where we now seem to be swamped with it. The amount of radio traffic is now enormous, which has polluted the air with electromagnetic signals of all sorts. Even in the earth, surprisingly large noise currents can flow near to railway lines where digital signalling and control equipment is used. In industrial areas, there can also now be enormous amounts of electromagnetic
pollution in the air, in the ground, and in the mains electricity supply. Staying clear of all this is not an easy task, but by paying due attention to each potential source, the effects can usually be reduced to insignificant levels. Unfortunately, the types of equipment often found in low budget studios tend to be more prone to external interference than top-of-the-line equipment. This problem is compounded by the fact that the level of knowledge about how to deal with the noises is less readily found in the smaller studios. This now often leads to situations where the studios which are least equipped to deal with these problems are the ones which most need to do so. Good audio interfacing practices and good earthing practices can go a
long way to reducing interference problems, but they may be able to do little about mains-borne interference. The European Union and other authorities around the world have already introduced legislation which restricts both the amount of interference which equipment can generate, and its sensitivity to it, but this will take years to take real effect. In fact, due to the seemingly endless growth of the use of electrical and electronic equipment, much of the legislation may only serve to slow down the rate of growth of electromagnetic pollution, rather than to actually reduce it from present levels. The two cardinal rules are to ensure that studio equipment is supplied with the lowest practicably achievable source impedance for its electricity supply, and to keep all the interconnected equipment on the same phase. Figure 25.2 illustrates a right and a wrong way of installing power wiring if interference problems are to be avoided.
