Table 17.5.1.A Life Expectancy
19.2 Practical requirements
The following clauses state the general practical requirements for making robust high continuity earthing and bonding connections and gives specific instances of where, in practice, an earthing or bonding cable or alternative arrangement is required.
19.2.1 Earthing and bonding connections
An earthing and bonding cable shall be terminated or connected using crimped lugs, serrated spring washers (‘star’ washers), nut and locknuts and should be protected at the connection points by the use of corrosion preventive grease (chemodex or approved equivalent). All
connections shall be mechanically and electrically effective with precautions being taken to prevent electrolytic action between dissimilar materials in wet and salt laden conditions. Where earthing or bonding is achieved by means of equipment fixing or holding down bolts the metal surfaces shall be cleaned and a ‘star’ washer inserted under the nut or head of the bolt so that the equipment is effectively earthed.
Exothermic reaction welding methods approved by the principal may be used for the connection of earthing conductors including Tee joints, earthbar through joints, and connections to cable trays.
Each earthing and bonding cable shall be identified with a label and recorded on a cable schedule for maintenance records.
Bonding or earthing conductors shall be single core stranded annealed copper conductors as specified by M.E.S.C. listings, and [ES.2.14.0070]. The size of conductor selected shall be sufficient to remain below the maximum permissible conductor temperature under fault conditions and sufficient to resist physical damage at its location.
19.2.2 Plant and equipment earthing and bonding
Equipment that is bolted, clamped, screwed riveted or welded to structural part of the earthing system equipotential plane, in an electrically and robust manner does not need additional earthing and bonding cables.
Free standing equipment which is neither bolted, clamped, screwed, riveted nor welded to a structure which is part of the earthing, system shall be bonded by means of a cable connection. This includes : -
• Equipment that is electrically isolated from the structure by its mountings e.g. Rubber anti-vibration mountings
• Skid mounted equipment. e.g. Generators and transformers
Tanks and vessels which are directly connected via their supports, either welded or bolted to the platform structure do not require bonding cables.
Each main switchboard, motor control centre or control panel shall be furnished with a continuous earth bar running the entire length of the equipment to which all sections of the metal enclosure including cable boxes, gland plates, etc. shall be bonded.
The equipment earth bar shall have separate bolted earth connections having isometric thread size (at least M10) at each end of the bar for connection to earth.
The frames of electrical motors for use in flammable areas shall be suitably constructed to avoid sparking due to circulating currents in their frames and shall be provided with an earth stud for bonding to the main earth. Where terminal boxes on large motors have gaskets fitted the box should be earthed to the frame earth.
Large H.V motors of unit construction may require bonding across bolted faces of the frame to eliminate circulating currents.
Where cable rack and tray is not sufficiently connected to earth by metal contact as an integral part of the construction it shall be bonded to earth at about 25 meter intervals, at each end and where it is not electrically bonded together. Bonding is not required across bolted fish plate metal to metal joints.
Where HVAC ducting is not sufficiently connected to earth by metal to metal contact as an integral part of the construction it shall be bonded together and connected to earth at about 25 meter intervals and at each end.
The resistance to earth of this common earthing resistance grid shall be as low as practicable, but at least such that in the case of earth faults the electrical protective devices shall prevent shock hazard potentials on any part of the equipment which is not part of the electrical circuits.
For portable and mobile equipment, with exposed conductive parts likely to be held in the hand, the following correlation between duration and voltage shall not be exceeded: -
Table 19.2.2A Endurance time for electric shock
For AC For DC Continuous Less than 50 V 110 V
5 seconds 50 V 110 V 1 second 75 V 110 V 0.5 seconds 90 V 110 V 0.2 seconds 110 V 110 V 0.1 seconds 150 V 110 V 0.05 seconds 220 V 110 V 0.03 seconds 280 V 110 V
Earth wires shall be of the standard copper green/yellow PVC covered type, suitably sized to cope with fault current level and fault clearing time. For mechanical reasons main earth wires shall be at least 50 sq. mm., branch earth wires 10 sq. mm., unless adequate mechanical protection is provided by other means, e.g. wire installations in conduit, or earth conductors forming part of a cable, allowing a smaller size.
The connections between earth electrode and conductors shall be so executed that easy inspection and testing of the earth resistance of each individual electrode, without disconnecting any part of the earthing system, is possible.
The earth bar in the distribution board shall be connected with two earth wires each suitable for 100% duty to the above-mentioned common earthing grid.
19.2.3 Selection of components
The components shall be suitable for their particular application as regards their rated voltages, rated currents, service life, making and breaking capacities, short circuit strength, etc.
The components having a short circuit strength and/or a breaking capacity insufficient to withstand the stresses likely to occur at the place of installation shall be protected by means if current-limiting devices, for example fuses or circuit breakers. When selecting current-limiting deviced for built-in switching devices, account shall be taken of the maximum admissible values specified by the manufacturer of the apparatus, having due regard to coordination. Components incorporated in the distribution board shall, in their design and construction, comply with the corresponding IEC standards.
Plugs of different rated currents or voltages shall not be interchangeable, so as to avoid errors in connection.
Connections for three-phase plugs shall be made in such a way as to retain the same order of phases.
The socket outlets and plugs used outdoors shall have a degree of protection as given in [ES.2.03.0001, Appendix H, Table H.3.A], both when the plug is removed or fully inserted. Where metallic cladding is not sufficiently connected to earth by metal to metal contact as an integral part of construction it shall be bonded together and connected to earth at about 25 meter intervals and at each end.
All metallic wall cladding shall be adequately earthed where required via the fixing arrangement or by earth bonding straps.
19.2.4 Packaged equipment
Earthing and bonding of electrical equipment to packaged skids shall be the responsibility of the package vendor. However, the skid shall be provided with two (minimum size M10) earth studs, diagonally opposite to each other for connection to the earthing system only where the skid holding bolts etc. do not provide a satisfactory inherent metal to metal connection.
For offshore installations the bonding of all vessel or tank internals such as floating covers, stirrers etc. shall be the responsibility of the package vendor.
19.2.5 Cable armour termination
The armour of single core cables must be earthed at one end only to avoid induced circulating currents flowing longitudinally in the cable armour. Also to avoid standing voltages in hazardous areas the earthing should be at the hazardous area or load end. To achieve this either insulated ‘Island Type’ mechanical cable glands or insulated gland plates shall be used at the supply end.
All multicore power cable armour shall be connected to earth at both ends of the cable. This will be achieved using conventional mechanical cable glands complying with [BS 6121]. Gland entry threads screwed into enclosure gland plates satisfy this requirement.
If the cable, or the circuit it is feeding, is protected by a core balance current transformer arrangement an insulated gland must be used at the supply end of the cable and the armour connected to earth. This cancels the electromagnetic effects of earth currents in the cable armour on the current transformer.
Metallic cable glands complying with [BS 6121] entering metallic termination boxes or enclosures shall be considered as adequately bonded by virtue of their direct contact through the entry threads. However, where glands enter through clearance holes the inner face of the enclosure shall be cleaned and a ‘star’ washer shall be fitted under the back nut.
For larger size glands installed in untapped gland entries where star washers may not be available the earthing shall be obtained by the use of gland earth tags. Bonding of gland earth tags shall be completed with 16sq. mm. minimum stranded copper single core cable with green and yellow sheath.
Non metallic enclosures such as polypropylene junction boxes, push button stations etc., shall be fitted with internal brass or equivalent metal inserts to provide cable armour continuity. Where internal inserts are not available and an earth return path or equipotential bond is needed, earth tags shall be installed externally and connected together.
The use of tapped entries into sheet metal enclosures to terminate cable glands is not permitted.
Where armoured flexible cables are used, an earth continuity conductor of nominally half the cross sectional area to the largest associated current carrying conductor shall be provided where it is necessary to bond the armour to earth.
Where cables are glanded into devices which are activated from within a vessel or tank the armouring of such cables shall be bonded to the vessel or tank close to the point where the device enters the vessel or tank.
A means of bonding all internal metalwork within non-metallic electrical enclosures shall be provided. This bonding may be achieved by an earth wire run within the cable, or by local bonding to the structure. All metallic glands on each enclosure shall be connected to the earthing system.
19.2.6 Cable dielectric screens
High voltage cables contain screens to reduce the effects of electric stress for both single and three core H.V. cables. The dielectric screen shall be connected to earth at one end only. The point of the bond shall be at the load only. The screen at the supply end shall be carefully ‘dressed’ and kept well clear of earthed parts of the switchboard. The cable termination kit manufacturer shall provide an appropriate set of instructions to cover this aspect.
19.2.7 Instrument system earthing for platform-based installations
Measurements taken at offshore installations have revealed typical resistance across the platform in the order of 100 micro-ohms and because of this the complex earthing arrangements found on land based installations are not required. The platform deck can therefore be regarded as an equi-potential plane. Power system and instrumentation system earth connections to the deck need only have a minimum 5 metre separation. Earth bar supports shall be welded to the deck at each geographical location, e.g. the installation control center, to which all instrument earths shall be connected.
19.2.7.1 Instrument room power and instrumentation earthing
A power earth bar shall be installed in the instrument room and connected to local deck steel work by two 70 sq. mm cables. All equipment and cabinet frames shall be connected to this bar to provide power system earthing and bonding.
An instrument earth bar shall be installed and connected to the deck steel work by two 70 sq. mm cable not less than 5 meter away from the power earth connection. Instrument earthing and bonding shall be connected to this bar which may be regarded as the local deck earth bar referenced.
19.2.7.2 Earthing to provide a reference potential for computing, telecommunications and instrument equipment
All earthing to provide reference potentials for instrumentation shall be made by connection to a deck earth bar local to the equipment. (The deck provides a far lower conductive path to other associated equipment than a cable). Earth bars shall be provided in the relevant cabinets for this purpose and shall be connected to the local deck earth bar by two 35 sq. mm cables. 19.2.7.3 Intrinsically safe barrier earthing
All instrumentation cabinets containing intrinsically safe barriers, where barrier earthing outside a hazardous area is a requirement of the certification, shall contain an earth bar which shall be connected to the local deck earth bar by two 35 sq. mm cables. The QGPC Instrumentation Engineering department shall be consulted for the details pertaining to the earthing of instruments and intrinsically safe systems.
19.2.7.4 Instrument screen earthing
Screens with associated drain wires are built into instrument cables to reduce interference from other equipment, mainly large power equipment. The drain wires shall be connected to earth at one end so that pick up on the screen will be limited to electromagnetic radiation and not induction or conduction e.g. circulating currents will not flow.
As radiation effects will be low, energy associated with standing voltages at the non-earthed end will be small. Earth connections will therefore be made to the local deck earth bar in the instrument room where significant fluctuations in the earth potential due to large currents are less likely. Screen drain wires shall be earthed in cabinets at an earth bar provided for this purpose. It will be connected to the local deck earth bar by two 35 sq. mm cables.
19.2.8 Pipe flanges
In general pipe flange bonding is not required since an effective contact is attained through flange faces and bolts. However, in the case of insulated flanges braided earth straps shall be provided on the pipe work for bonding each section in accordance with specific requirements of the design. (Bonding across insulated flanges may affect impressed current cathodic protection or may induce corrosion of dissimilar metals).