STRAIGHT THROUGH JOINT AND BONDING: EARTING OF HIGH VOLTAGE CABLE

Underground High Voltage Power Cable Jointing, Bonding, and Earthing

The cable joints are basically used to connect low voltage, medium voltage, high voltage, and extra high voltage cables. All joints must provide electrical insulation as well as mechanical strength and protection. 

Cable joint sizes, shapes, and configurations may vary according to the voltage range, structure, insulation, and the number of cores of the cable to be jointed.

Cable Joint & Link Box Connection Schematic Diagram

Voltage Range in Bangladesh Standard:

1. low voltage up to 11kV ;
2. medium voltage up to 33kV;
3. high voltages up to 132kV;
4. Extra high voltage 23kV and above.

Structure:

1. straight through connectors;
2. branch connectors.

Cores:
The cable joints are required to have the same number of cores as those of the cables to be joined.

Insulation:

1. heat or cold shrinkable insulation;
2. molded type of insulation;
3. use of tape insulation.

Straight through Jointing Procedure

Straight-through joints mainly two types according to cable sheath jointing:

1. NJ- Normal Joint, where cable sheath connection is, as usual, not insulated through link-box;
2. IJ- Insulated Joint, where cable sheath connection is as insulated through link-box. 

There are about four commonly used types of joints that differ in the mechanical arrangement and location were at used.

1. Straight through joint;
2. T or Y branch joint;
3. Pot end joint ;
4. Indoor/Outdoor/GIS termination.

Factors Should Consider Straight Through Cable Joint

Each cable is to be run in one continuous length except where the run of cable exceeds the normally manufactured lengths. No other straight joints will be permitted unless specifically authorized by the Engineer.

                                                              

The outer sheath of all joints buried in the ground shall be contained in a glass fiber or similar box filled with compound or other approved method.

The XLPE cable is provided with sectionalization on the sheaths on every sixth joint or 3 km whichever is less, with links and earthing provided for the earth screen. Normal operation will be with the sheath bonding links closed and the earth link open.

All XLPE joints and terminations shall be carried out under strictly controlled conditions by cable jointers for the XLPE cable manufacturer. The joint insulation shall be of the vulcanized type or other approved type.

The installation rates of the joints specified in Schedule IV are to include allowance for the complete joint bay installation including excavation civil work jointing earthing and ancillary equipment.

Where the armor wires of plastic insulated cables constitute the earthed metallic shield such wires shall be efficiently bonded across the joint in an approved manner. The electrical resistance of such bonds shall not exceed that of an equal length of the cable armor wires. Suitable care shall be taken to ensure that the bond contacts do not deteriorate in service.

Bonding and Earthing Procedure

Bonding and earthing connection formation are arranged in the earthing link box. The application of the link box is as below:

1. Direct grounding
2. single point bonding
3. Cross bonding
4. Cross-bonding and transposition
5. Sheath voltage limiters (SVL)

Earthing link box formation may be following five types:

1. Link Box for earthing in 1-1 way;
2. Link Box for earthing in 3-1 way ;
3. Link box for earthing in 3-1 way with cross bonding;
4. Link box for earthing in 3-1 way with SVL (sheath voltage limiter);
5. Link Box for earthing in 3-1 way for joint bay earthing.

Typical Cable Joint Link Box Connection Diagram

Factors Should Consider Cable Bonding and Earthing Procedure

Surge protection shall be provided at all joints and terminations not permanently connected to the earth to limit induced voltages too (specified limits). 

The surge limiting device shall limit the peak voltage as specified for a peak current of 10 kA. These devices shall also withstand the specified a c. r. m. s. maximum voltages without injurious heating.

All cable screens, bonding leads earth conductors earth links, etc. shall be capable of withstanding the prospective fault current appropriate to the relevant system voltage for 1.0 seconds without injurious heating, distortion, or damage.

The contractor shall install the minimum number of joints in the cable route, consistent with the specification requirements and sound practice.

The tenderer shall state, in the Tender, the philosophy of and details of the screen earthing leads and shall route these conductors in a manner such as to minimize or eliminate circulating currents. 

The contractor shall ensure that the voltage on exposed metalwork or earth conductor arising from ht passage of load or fault current does not constitute a hazard to personnel, livestock, or equipment (including building structures, etc.) and shall if required by the Engineer, furnish full information and calculations to demonstrate compliance with this clause.

The contractor design of the system shall take into account the requirement that regular testing of insulation integrity will take place. 

Connections to earth shall therefore be made through bolted links or by other means to the approval of the Engineer, permitting isolation of cable sheaths and all associated insulated conductors for testing purposes.

Cable sheaths joints and terminations and all associated conductors shall be insulated for voltage likely to arise in operation and shall in any case withstand the specified test voltages.

The contractor is to provide earthing and earthing electrodes for all equipment supplied under this contract in accordance with the requirements of the specification and the tender drawings.

Extensible solid copper earth rod electrodes 1220 mm long and 16 mm diameter shall be driven in sufficient numbers to ensure that the electrode resistance does not exceed 10 ohms for ancillary equipment and 2 ohms at cable sealing ends remote from the substation. 

All earth electrodes shall be electrically bonded together using copper taps 50 mm x 4 mm. The tops of all electrodes shall be at least 500 mm below the surface of the normal reinstated ground level and the joint between the electrode and the copper tape shall be immersed in the asphaltic coating.

The cable contractor is to provide and terminate the earthing 50 mm x 4 mm copper tape from the link box at the outdoor sealing ends or the splitter box to the switchyard or substation main earth. Ancillary equipment shall be earthed to the substation main earth by the cable contractor, earth. Ancillary equipment shall be earthed to the substation main earth by the cable contractor.