Earthing System 19.1 Basis of design

The basis of design for dynamic and static earthing systems is described in detail in [ES.2.03.0001, section 10.4]. The differences in approach for land-based and platform-based installations are emphasized in this reference.

In this document, [ES.2.06.0001], only those aspects that particularly relate to the installation site practices are described, so as to avoid conflicting information with [ES.2.03.0001]. However, the following subsection (19.1.1) is taken from [ES.2.03.0001] in order to emphasize the concepts involved, but not the detail design.

The effects of passing electric current through the human body are described in detail in [IEC 479, Part 1].

19.1.1 Extract from [ES.2.03.0001]

Danger from Electric Shock and /or ignition from flammable atmospheres by electrical sparks may occur if a surface can attain a voltage which is different from its surroundings. Where the

electrical resistance from one surface to another is too high this voltage difference can be caused by : -

i) Faults on electrical equipment and systems. ii) Lightning strikes.

iii) Build up of Static Electric Charges.

Low values of resistance are required to protect against faults and lightning strikes but relatively high values will afford protection against static charge. The intention is to ensure that voltage difference between adjacent metalwork (and for land based installations metalwork to ground) under electrical fault, lightning strike or static discharge conditions is kept to a level that is not dangerous to human life nor likely to cause sparks in a flammable atmosphere or otherwise cause damage.

Normally, petrochemical plants, oil refineries and offshore platforms are constructed of significant quantities of steel which inherently provides low resistance paths sufficient to ensure dangerous voltages do not arise. However, where this cannot be guaranteed, the conductive surfaces must be purposely bonded together and connected to earth to achieve a suitably low resistance.

To achieve the required degree of protection it is normal practice to follow the guidance of the following national codes : -

• BS code of practice [BS 7430].

• IEE regulations for electrical installations latest Edition. (The equivalent IEC standard is [IEC 364]).

• [BS 6651] (Formerly CP 326). The protection of structures against lightning.

• [IP model code of safe practice part 15 electrical (1991)]. 19.1.1.1 Faults on electrical equipment and systems

[BS 7430] gives overall guidance on the provision of earthing systems for power generation and distribution networks. The IEE Regulations provide detailed guidance on all aspects of the earthing and bonding of electrical apparatus and equipment. A key requirement is to ensure that the impedance between exposed conductive surfaces of the electrical apparatus (including any conductive surfaces nearby and within the proximity of human touch) and the fault current earth return path to the source of supply, are low enough for the fault current to operate the supply cut off device quickly and without giving rise to dangerous voltages. To provide the necessary degree of protection, relatively low impedance earth return paths are required (maybe as low as 0.2 ohm). In practice these are usually readily achievable via the steel decks of offshore installations where the inherently low resistance of metal to metal construction provides a low impedance path and largely eliminates the need for earthing and bonding conductors. However, if the earth return path is not sufficiently low, there is a possibility that a fault on a piece of electrical equipment could produce a dangerous voltage on itself or adjacent metalwork and therefore under these conditions low impedance bonding conductors must be provided.

In addition bonding conductors are often required between modules and support frames, especially skiddable drilling modules, portable and transportable equipment and similar situations where a permanent earth return path cannot be guaranteed.

For onshore installations reliable low values of impedance can normally only be achieved by supplementing any inherent earth return paths (e.g. the metallic armour of cables) with a network of copper earthing and bonding conductors. The earthing network is formed by conductors bolted between the exposed surfaces of the equipment and its surroundings (including the earth) and additionally connected back to the source of supply and its cut off device.

In addition to being of low impedance the earth return paths for electrical equipment must be electrically and mechanically robust and capable of carrying fault currents for the duration required to operate fault protection equipment without deterioration.

Metal work that is not part of the earth return path and which would become live due to a fault on associated electrical equipment must also have a rebust and sufficiently low impedance path to the earth return path. Where practical or economic factors prevent robust or sufficiently low impedance being achieved or guaranteed, e.g. metal cladding of thermal insulation on an electrically heat traced pipeline, fault protection equipment of the sensitive earth leakage current detection type must be used.

The correct impedance, current carrying capacity and robustness of the earth paths must be proven by testing before plant and equipment is commissioned and thereafter must be regularly and tested to ensure that unacceptable deterioration has not taken place.

Thus for faults on electrical equipment, low impedance earth return paths to the source of supply and between associated metal work and earth, supported by earthing and bonding conductors where necessary, are essential to protect the equipment against electric shock hazard and prevent arcing and sparking.

19.1.1.2 Lightning strikes

[BS 6651] provides detailed guidance on protection from the effects of lightning strikes. Lightning discharges to the earth or sea are attracted to any protruding metalwork and give rise to a very high electrical current and severe magnetic forces. [BS 6651] requires the resistance of the lightning conduction path or paths from the top of the structure to the earth to be a maximum of 10 ohms and nearby metal work to have low resistance to the main lighting conductor path to prevent side flashing.

The intention is to conduct the high current of a lightning strike to the earth without building up dangerously high voltages between elements of the structure or allowing the strike to cause sparks within the frame work of the structure. In practice offshore platforms and most land based oil and petrochemical plants are constructed of large steel beams and steel reinforced concrete that are of sufficient strength to resist the magnetic forces of the strike and have inherent resistance to earth much lower than the 10 ohm maximum. The highest electrically conducting point of the structure produces a conical zone of protection beneath it. Usually no special conductors are required for offshore or onshore steel structures (Apart from extra earth electrodes for some 'Dry Ground' installations onshore).

The normal metal to metal contact of welded and bolted structures is of sufficiently low resistance to prevent side flashing. However, care must be taken to ensure good electrical continuity across painted surfaces and any conductive surface likely to be insulated from the main structure that is in the vicinity of the expected current paths e.g. Acoustic Cladding.

Resistance values of up to 0.1 ohm are acceptable between adjacent surfaces and should be obtained by permanent metal to metal welding, riveting, clamping or screwing, that becomes an integral part of the arrangement, in preference to copper bonding straps etc.

19.1.1.3 Build-up of static electric charge

The [IP Code] covers in detail how static electricity can be generated and what precautions should be taken to prevent static charge build up. The main hazard in the oil and gas industry is from static discharge that can cause a gas ignition and explosion. The energy levels of a static charge are not sufficient to cause danger to life but an unexpected static shock may cause a more serious accident.

The code recommends that the resistance between conductive surfaces including the earth should not exceed 1 Meg ohm (1,000,000 ohms). This is sufficiently low to prevent an electric charge accumulating. In practice it is extremely unlikely for any part of the metallic structure of an offshore or onshore plant to be insulated to such a degree, bearing in mind the provisions made for faults on electrical equipment and lightning protection as outlined above. A survey of the electrical conductivity of pipe cladding has shown that even on relatively old installations resistances remain sufficiently low.

However, increasing use is being made of plastic sealants and paints having inherently high electrical insulating properties in equipment construction and corrosion prevention coatings. Consequently, in areas where these are to be used and where there is the likelihood of a static charge being generated, the designer must ensure that a continuity of 1 Meg ohm or less is achieved by permanent metal to metal welding, riveting, clamping or screwing, that becomes an integral part of the arrangement, in preference to copper bonding straps. This includes metallic thermal and acoustic cladding, especially that on heavily painted surfaces and any isolated conductive surfaces where subsequently, during construction or maintenance, a resistance greater than 1 Meg ohm is found then the continuity to earth should be improved in a permanent manner as detailed above.

Where the plant, equipment and supporting structures comply with the earthing and bonding requirements for electrical protection and lightning protection there should be no instances where further earthing against static charge is required. However, there may be some areas where static charges are likely to be generated e.g. by a high pressure gas or steam leak or water jet, they must be identified and addressed to ensure continuities or 1 Meg ohm or less are achieved.