WIRING REGULATIONS AND EARTHING REQUIREMENTS
Depending on the situation it may also be necessary to follow the IEC standards, e.g. IEC 60364 - Electrical Installation in Buildings. Outside of the IEC arena, both the standards and the terminology which they use may differ considerably from the norm. In North America, for example "earthing" would normally be referred to as "grounding".
The earthing of both the electrical systems, and also the metallic structures that they may come into contact with is a safety critical activity for obvious reasons. In the United Kingdom the BS 7430:1998 the code of practice for earthing, provides guidance on earthing of electrical supply systems, electrical installations and connected equipment, for the proper operation of systems and the protection of life. This covers basic principles, earthing methods and most general applications.
When using armoured cables (or conduit), both in onshore and offshore or marine situations, any metallic parts of the cable (or conduit) must be earthed, and this also applies to the lead covering used in lead sheathed cables. Wherever armoured cables are used it is normal practice to terminate the metallic armour, and / or lead cover (lead sheath) in the body of a metallic cable gland.
In accordance with BS 7671:2001, the IEE Wiring Regulations 16th Edition, both the cable and the accessories used for its connection must be tested to ensure compliance with the required safety levels. Cable glands and their fixing accessories and earth tags must be selected correctly to ensure that any risk of electric shock to personnel from coming into contact with live parts due to inferior earth connection is avoided. These earthing components must therefore be able to meet the minimum short circuit fault current withstand tests of the associated cable, and also be installed by competent personnel in line with good engineering practices
The cable armour is primarily for mechanical protection, and as already stated, this metallic armour must be effectively earthed. In general, the armour wire current carrying capacity must be equal to 50% of that of the largest current carrying conductor in the cable. The cable must be tested to determine its short circuit earth fault current rating, with the earth fault current being carried by the cable armour wires. Users should refer to the cable manufacturers design data for the short circuit fault current carrying capacity of the armour wires of each cable.
Cable glands connected to SWA cables must be able to provide earth continuity from the termination point of the armour in the cable gland body through to the equipment, if the enclosure is metallic, or via a metallic gland plate that is bonded to an external earth point, and / or directly to an external earth point via an earth tag. In the interests of safety most earthing systems associated with armoured cables will utilise a number of directly grounded external earth link cables connected to the earth tag which is in contact with the cable gland. Usually the cable would be earthed at one of its two ends as a minimum, and this approach ensures that in the event of a short circuit or earth fault in the cable, the quickest and most direct route to ground will be achieved as a result of the design philosophy adopted. When multiple cable entries are required in non-metallic enclosures (e.g. GRP terminal box) that do not have an external earth point, the user may prefer to engage an external earth cable between each metallic cable gland via an earth tag. At least one of the earth tags would also be used to connect an earth cable directly to ground. This method of providing earth continuity is sometimes referred to as a "daisy-chain" arrangement. The earth tag described above must also be tested to ensure that it can withstand the equivalent short circuit earth fault current test rating of the cable and cable gland, otherwise the system will not have adequate (safety) earth protection.
LC / SWA or LC / PVC / SWA cables
In addition to the standard requirements for SWA cables, there is a set of special guidelines for lead covered cables intended for direct burial in the ground of hydrocarbons processing and refinery sites. These guidelines were introduced in the UK by the Oil Companies Material Association (OCMA) and responsibility for these was subsequently transferred to EEMUA, the Engineering Equipment and Material Users Association. The EEMUA Publication 133 "Specification for Underground Armoured Cable Protected Against Solvent Penetration and Corrosive Attack" defines the requirements of the petroleum industry for underground lead-sheathed cables for use where protection against solvent penetration or corrosive attack is required.
When the cable is LC / PVC / SWA the addition of the lead sheath introduces another metallic part of the cable that could effectively become live in the event of an earth fault or induced voltage and this must also be earthed. Any metallic parts used to terminate the lead sheath as part of the earth path must also be tested and documented to demonstrate that adequate levels of protection are maintained. In the case of LC / SWA cables the earth fault short circuit levels will be increased due to the parallel earth path existing between the armour wires and the lead sheath. It is important therefore that the whole cable gland and earth tag arrangement is tested on lead sheathed cable to ensure that the arrangement can withstand the overall affects resulting from short circuit in the cable armour / lead sheath.
If the cable glands and accessories chosen have not been tested, and are incorrect the connection of the earth circuit via the cable armour will become the weakest link in the system, and could result in potential fatality, lost time incident (LTI) or other accident.