Principles of safety

Having reached this stage in the book the reader may well be wondering about the factors that need to be considered in overcoming problems arising from static electricity and the principles on which safety precautions are based. From the systematic investigation of the case histories described in the following chapters various requirements for safety and the methods used in their implementation have been dealt with.

Although static charges are almost everywhere, they are, for the most part, benign in terms of the danger and nuisance they can cause because of their small quantity. It is when charges of different polarities become separated and then accumulate in more substantial quantities that problems can arise.

Charge separation can occur in a number of ways but the most frequent means are by the contact and separation of materials and by electrostatic induction. In Fig. 4.1 is shown a schematic diagram of the development of different types of incendiary gas discharge arising from accumulated static charges.

When the materials concerned are of a conductive (or dissipa-tive) nature the residual charges on their surfaces after separation are minimal, if not zero, and present no danger. However, when at least one of the materials is an insulator it will become charged and will retain the charge for a more or less long period of time. The other material, whether it is a conductor or an insulator, will acquire an equal and opposite charge to that on the insulator.

Figure 4.1 Development of different types of incendiary gas discharge

Should the charges on these surfaces be large enough, gaseous discharges can occur which, in the presence of a combustible atmosphere, might cause an ignition. It is, therefore, important to know the procedures that should be followed to avoid such discharges from insulating materials and from conductors.

Depending on the circumstances, the grounding of conductors can be an easy means of overcoming a potential danger providing it is done reliably and permanently. However, should such materials not be grounded they will behave as capacitors and when charged will be capable of releasing spark discharges. Grounding a charged insulator would be practically ineffective in getting rid of the charge because the latter has a low mobility. However, a charged insulator may, again depending on the circumstances, be partially or wholly neutralized by means of a corona discharge. Such discharges operate by depositing charge on the insulator of opposite polarity to that which is already there. When the charges are equal in magnitude the charge on the insulator is, effectively, neutralized. Even the partial neutralization of charge by this means is usually sufficient in preventing incendiary gas discharges from the insulator.

The accumulation of charge on an insulator may be on the surface, within the bulk, or in the form of a double layer. Charge accumulated on surfaces can lead to brush and super brush discharges. Charge within the bulk may give rise to cone discharges and charge accumulated in the form of a double layer can produce propagating brush discharges. The incendiary behaviour of these different types of discharge, as well as of spark discharges, is given in 3.4.2 (Table 3.1).

As would be expected, the size of a charged material is one of the factors which determine the quantity of charge that it can hold. It follows, therefore, that the probability of an incendiary discharge falls as the area of the insulating surface decreases. Brush discharges from insulating surfaces of area less than 2000 mm2 are not able to cause the ignition of sensitive mixtures of hydrocarbon vapours and gases with air. For cone discharges to be incendive the volume of the charged material needs to be, at least, a few cubic metres (see 3.6, (5)). Double layers of charge on continuous sheets of insulating plastic are safe from causing propagating brush discharges providing the sheets are of thicknesses greater than 10 mm or their breakdown potentials are less than 4.0kV (see 3.6, (6)). Spark discharges from small conductors of capacitance below 3 pF are not able to ignite hydrocarbon vapours and gases (see 3.6, (3)).

As the safety procedures referred to above are those applied to many of the case histories to follow, it might be tempting, when faced with a safety problem in a plant, to identify it with one of the case histories and act accordingly. However, the reader should beware of this possible trap as the facts of the problem may differ slightly from the case history, thereby requiring a different approach to its solution. It could also lead to the imposition of restrictions which are not necessary.

Studying the case histories will afford a useful training in becoming aware of the dangers that can arise from static electricity. However, in addition to this knowledge it is advisable always to consult the relevant guidelines as they appear in British Standards, German Berufsgenossenschaft and the American NFPA. As most guides are usually several years old it is worth noting that a CENELEC report entitled, 'Safety of machinery -Avoidance of hazards arising from static electricity' is at present in preparation. The final form of the work is, as yet, unknown but undoubtedly it will become one of the most comprehensive documents on electrostatic hazards.

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