Protective earthing is a vital part of the safety of almost all electrical systems where potentially explosive gases may constitute a hazard. Correct earthing is important for all protection techniques, including intrinsic safety.

Adequate earthing, though not spelled out in the particular installation codes, is absolutely vital to the safety of all mains-powered electrical equipment, whatever the protection technique. Section 22 of the BS 5345 Part 1 lists no special requirements for the earthing of hazardous area systems but is a convenient cross reference to other documents.

BS 5958: Part 1: 1980, which is a code of practice on the avoidance of static hazards and contains some sound practical advice, should be read as good background information on which to build.

Consider a conventional site with a local distribution transformer and with the neutral star point connected to the standard earth mat. The primary purpose of this mat is to provide an earth return path for any faults that may develop in the distribution system, where the conductivity of the soil may provide a possible current path. Conventionally, each electrical installation provides a return path to the neutral star point in its wiring, via either the cable armour or a specific conductor which is capable of operating its protective network. This is supported by the interconnection of the equipment to the metallic structure which generates a web of structural interconnections to the various electrical equipment return paths. Further support is usually provided by the structure and the antistatic bonds which are normally present. There is nearly always a third parallel path through the soil, but this indeterminate path is not usually relied upon for first line safety connections.

The more probable source of direct invasion of intrinsically safe circiuts is within the safe area shown in Fig. 9.2. The first essential of the dafety earth on the barrier busbar is to provide a return path of low impedance so as to prevent any significant proportion of the faule current entering the hazardous area. This fault current is returned to the neutral star point bond and hence back to the distribution transformer.

The current flowing though the bonding conductor generates a potential difference between the IS earth point at the barriers and the neutral star point. The outside of the field-mounted instrument is bonded to the neutral star point, and the internal circiuts of the instrument are connected to the barrier busbar. The potential difference between IS earth point at the barriers and the neutral star point is therefore transferred to the hazardous area. It is normally safe because this internal circuits are isolated from the instrument housing, but this potential difference should be minimised so that if there is a local insulation failure no danger can arise. The installation conditions of barriers, screened transformers etc. nearly all call for a return path impedance less than 1 ohm. A figure of 0.1 ohm is normally achievable and is more desirable. It is important to remember that the resistance involved is that of the return conductor between points IS earth at the barriers and the neutral star point and the resistance of the earth mat is not important for this purpose. These principals are normally applied within the UK.