Table of Contents
- Network
- Network Composition
- Ring Conductor System
- Earthing Points
- Dividing Points
- Earthing Conductors
- Connections
- Installations
- Safety Earthing
Details See BN-DS-E7
An earthing network has to be installed throughout the plant(s) to provide for: safety earthing, protection against lightning and static charges.
The earthing network shall consist of a ring conductor system or systems with earthing points, dividing points, equipment earthing conductors and the requirements for safety earthing.
The ring conductor system shall be of the un-interrupted design.
When required by the area the earthing network can be divided into several ring conductor systems.
These systems have to be elec. interconnected.
Earthing points shall be made by means of earth-electrodes and/or concrete piles.
Each earthing point shall be connected with the earthing network via a dividing point to enable resistance measurements in an easy and simple way.
Each earthing point shall have a resistance against earth not exceeding a value of 2 Ohms.
The number of parallel electrodes and/or piles per point shall suit this requirement.
The horizontal distances between earth electrodes or piles shall be at least equal to the length of the electrodes/piles.
-
Vertical copper earth electrodes may be:
-
Copper earth rods with a min. diameter of 12.5 mm (“Copperweld” or equal)
An uninterrupted bare copper wire or tape with a min. cross-section of 25 mm² and thickness of min. 2.5 mm driven into the ground with a steel tube.
Copper earth electrode heads shall be accessible for inspection and testing, only for Mainstations, Substations, Transformer yards and in case the earthing point is not directly connected to a dividing point. (see detail 9 of BN-DS-E7)
The mainlead from an earthing point to a dividing point shall be marked red at the dividing point.
Concrete piles used as earth electrode shall have min. 3 reinforcing bars welded together each with a diameter of min. 15 mm (see detail 8 of BN-DS-E7).
Earth electrodes used for Lightning protection shall have a min. vertical length below ground of 5,5 metres.
Pre-cast concrete piles may not be used for lightning protection.
Dividing points shall be provided where necessary, but each earthing point shall be terminated at a dividing point.
Dividing points shall normally be 50 x 6 mm copper bar and made suitable to receive the connections: from the earthing point, the main loop and the possible earthing conductors to equipment (see detail 10 of BN-DS-E7)
Note : Disconnection the earthing point may not disturb the other connections.
Underground earthing conductors shall normally be stranded copper wire (medium hard drawn), PVC covered, coloured green/yellow.
PVC cover in order to avoid electrolytic corrosion of underground metal equipment.
Above grade earthing conductors shall normally be bare tinned copper wire or PVC covered copper wire or PVC covered copper wire coloured green/yellow
Copper conductor size in mm2 (not for safety earthing)
|
System |
Above |
Under |
|
Ring system general Ring system around Subst. * Prot. against Lightning Prot. against static charges |
25 70 50 10 |
50 70 50 25 |
* Including connections at main elec. equipment in the Substations.
Use of structural steel (when elec. continuous) as conductor is allowed only for the protection installation against static charges.
All connections between copper earth wires, except connections as mentioned in next points, shall be made by means of “Crimpit” type connectors.
In case the conductor is PVC covered, the “Crimpit” connector has to be insulated with PVC self-sealing tape.
All above ground connections of the Lightning protection system shall be hard soldered with the exception of the down-comer which shall be connected to the underground earthing installation by means of screwed couplings.
Above ground connections between Lightning- and any part of the Electrical installation are not allowed.
Connections at equipment or structure shall be made by earthing bosses acc. detail II of BN-DS-E7 or compression type cable lugs, bolted to the frame work (see details 12 thru 17 of BN-DS-E7).
Connections with movable equipment like trucks etc. will be made as per detail 18 of BN-DS-E7.
All outdoor main equipment and structures shall be connected to the earthing system for protection against lightning and static charges.
All indoor main equipment shall have an earth connection to avoid static charges.
All cable trays shall be electrical continuous and at several locations connected to earth via a dividing point.
Underground or overhead earth conductors have to run in elec. cable trenches or -trays where possible.
Underground earth conductors have to be laid at a depth of min. 60 cm or at the same depth as the elec. cables if both are installed in the same trench.
Above grade earth conductors shall be mounted with saddles or bandages suitable for use in a corrosive and chemical atmosphere and resistant against galvanic corrosion.
For practical reasons, the number of joints in earthing conductors should be kept as low as possible.
The earth conductors shall be protected by non metallic conduits or other means when they emerge from underground to above ground level and when mechanical damage is likely.
The conduits must be scaled with a plastic adhesive material or with a suitable end cap after pulling in of the earth conductor.
Min. required number of connections to earth for equipment protected against lightning and static charges are:
|
Equipment |
Number of connections |
|
Towers Boilers/Heaters Structures Drums/Vessels Heat exchangers Tanks-Stacks Pumps/Compressors A/G Pipelines Pipe racks Pipe sleepers Portable equipment |
2x 2x see also point 8.8 2x 1x 2x as required by National or local standards 1x via pipe racks and -sleepers 1x every 10-12 metres 1x each 6th one (up to a max. of 50 metres) 1x as per detail 18 of BN-DS-E7 |
Equipment of considerable height, such as towers, stacks, structures and buildings, so equipment what is most likely to attract lightning strikes, shall have connections via the shortest way to earth.
Preferable each connection to its own earthing point.
Where stacks have a metal ladder or lining they shall be connected to the lightning protection system at their upper and lower ends
Underground pipelines shall not be used for any earthing purpose.
Each elec. consumer shall be earthed by an earth conductor within its feeder cable to provide for a direct connection to the network earthing system.
The earth conductor size is related to the phase conductors as follows:
-
The same size as the phase conductors up to 25 mm2
-
Half size for phase conductors over 25 mm2., provided that the half sizes requirement is uprated to the next higher standard cross-section.
The earth conductor is either an extra core in the cable or, below 25 mm2, the armouring of the cable with additional copper wires to make up for the required equivalent copper section.
In case a High voltage cable is not provided with an extra core for equipment earthing, the consumer will be earthed by means of a separate insulated copper earth conductor.
This conductor must follow exact the same route as the cable itself.
The lead sheet of cables (regardless of its thickness) and armouring of the cables when not suitable for equipment earthing shall be earthed at the switchgear side only. This means also that the cable screen must be insulated from the enclosure of the consumers.