Wednesday, October 28, 2009

Electrical earthing system

Almost all electrical systems need an earthing system in order to operate safely. The following is an example of simple earthing specifications for electrical substations. This is just a simple specification and you may need to expand it longer depending on the type and complexity of the project you are going to use it in.

This section covers the earthing of all equipment and the provision of a complete earthing system including materials, electrodes and connections. The complete installation of the system shall be in accordance with the recommendations of the British Standard Code of Practice CP 1013:1985.

Electrical earthing Drawings
All the quantities of electrical earthing materials that have been detailed in the material schedules shall be treated as estimated quantities for tendering purposes only.

The contractor shall be required to prepare installation shop drawings and accurate schedules of materials shall be submitted as part of the shop drawings. The drawings and schedules shall be submitted to the Engineer for approval.

Electrical earthing System
A grid of hard drawn high conductivity copper tapes of cross-sectional area not less than 150, shall be installed by the contractor at each site in excavated trenches at 300 mm depth or, where permitted, in formed concrete cable trenches.

The frame of all electrical apparatus and structural steelwork shall be connected by branch conductors of the same cross sectional area to the substation main bar or where applicable, to subsidiary bars running to a group of equipment.

All isolator bases, post insulator bases, tension isolator strings, earth terminals and earthing switches, neutral current transformers, power transformers, surge diverter bases and where agreed with the Engineer, fences shall be connected to the grounding earth system.

Isolator and earthing switch operating mechanisms and circuit breaker control kiosks not integral with the circuit breakers shall be connected to the earth system by a branch conductor entirely separate from that employed for grounding the isolator, the grounding switch or the circuit breaker structure.

Electrical earthing points
The earthing point shall be of 16mm diameter copper-jacketed steel rod, supplied in 1500mm length and shall have provision for screw coupling with another standard length.

The copper jacket shall be molecularly bonded to the steel core to ensure that the steel core and the copper jacket are permanently bonded.

All rods shall only be driven into undisturbed soil.

Each electrode shall be provided with a complete set of approved non-ferrous clamps for connecting the earth rod to the earthing conductor.

 The rods shall also be provided with a hardened steel tip and cap for driving by means of a power hammer.

The number of earthing points required shall be determined at site after the letting of the contract.

Earthing link chambers and covers shall be provided and installed under this contract. The drawings showing the proposed arrangement shall be submitted by the Contractor for the approval of the Engineer.

The prices quoted in this contract shall include the driving of all earth rods, installation of link chambers, connection to earthing conductors, testing and commissioning to the satisfaction of the Engineer.

Precise locations for the link chambers and the interconnection arrangement will be decided by the Engineer when the complete test result of the electrical earthing system is known.

The combined resistance of all earth rods shall, if possible, be less than one ohm. The result shall not exceed three ohm under any climatic conditions.

Earthing Conductors

Conductors for interconnection between individual earth rods in any group and between groups shall have a cross-sectional area of 150

There shall be at least two such connections from each electrode group to a link chamber.

Conductors for connection between the link chambers and the substation main bars shall have a cross-sectional area of 150 and at least two such connections shall be provided.

There shall be two separate connections between the neutral points of the high voltage systems and the link chamber of the earthing rods. Each of these connections shall a have a cross-sectional area of not less than 50

The material of the earthing conductors shall be annealed high conductivity copper and shall be stranded. They shall be protected with an extruded PVC sheath of 1000 volts grade.

Earthing conductors shall be directly buried in the ground between the link chambers and buildings. Inside buildings they shall be fixed to walls and ceilings using appropriate copper saddles.

Alternative routes to run the conductors on cable racks or in trenches can also be allowed provided appropriate methods and accessories are to fix the earthing conductors are used.

Jointing and Bonding
Prior to jointing (soldering) of the earthing tapes, the surface of the tapes shall be cleaned and thinned. The area of the joint shall be riveted with copper rivets.

There shall not be less than four 3mm rivets used for each joint.

Only non-corrosive flux shall be used in all soldered joints.

Alternative methods employing chemical welding or high compression joints or clamps, may be permitted subject to the approval of the Engineer.

Any alternative method of joints shall only be considered after full details are officially submitted and no work employing them shall commence prior to the official approval from the Engineer.

Tests at Site
The contractor shall carry out all necessary tests to determine the soil resistivity at each area of the sites.

The contractor shall also be fully responsible for proving that the electrical earthing systems comply with the specifications.

It is the contractor’s responsibility to measure the resistance of each electrode installation and the resistance of each complete electrical earthing system without additional charge.

A complete method statement covering the installation procedures, the inspection checklists and the full testing procedures shall be submitted to the Engineer for approval prior to the commence of the actual installation work.

No work shall commence prior to the official approval of the method statement.

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