Cables Directly buried

In trenching installations, a trench is dug in the earth to an appropriate depth, usually 75 cm or more, and the cable or cables placed into it. The minimum depth of the trench used depends on a number of factors, but deeper is generally better from a protection standpoint. However, deeper trenches are also proportionally more expensive to make. The trench is then backfilled with a variety of materials to protect the cable, such as clean sand or soil. Direct buried installations are typically marked with tapes laid parallel to the cable, but placed closer to the surface, and markers located visibly above the surface, in an effort to prevent dig-ups. Buried cables are typically connected to one another via hermetically sealed closures located in buried hand holes or manholes along the route. These closures house and protect the splices or connectors used to connect sections of cable. 

All-dielectric (duct) cables are used in applications where the additional protection afforded by metallic armoring is not needed or not wanted. Duct cable refers to all-dielectric cable designed primarily for use in a conduit, although such cable may be used in other apphcations as well Some such uses are in aerial lashed installations, or in limited direct buried apphcations where the below-grade environment is more benign and rodent attacks are not a concern. AU-dielectric cables may also be... 

Direct buried cable experiences fewer environmental extremes than aerial cable. This cable is usually laid into a trench or potentially plowed into the soil. Primary dangers to underground cable are dig ups and rodent damage. Rodent resistance of the cable is provided by a coated steel tape armor. Installation in a duct offers additional protection from rodents, as well as dig ups. Some guidelines follow ... 

When planning a direct buried cable route, locate and avoid existing utilities. 

The total length of the cable is assumed to be 12 km. Figure 3.11 shows the layout of the cables. It is assumed that the cables are directly buried at a depth of 1.3 m with a separation of 0.5 m between the phases. Earth resistivity is set to 100 Q m. 

Typical pipe products installed by the HDD method include steel. High Density Polyethylene (HDPE), Polyethylene (PE), and Polyvinyl Chloride (PVC) conduits, as well as direct buried cables. During the HDD installation, the pipe product will experience a combination of tensile, bending, and compressive stresses. The magnitude of these stresses is a function of the approach angle, bending radius, product diameter, length of the borehole, and the soil properties at the site. 

Bus bars of a transformer substation must not be directly grounded. They must be connected with rails by at least two insulated cables. Metal sheathing of feeder and return current cables must only then be connected with the rails or bus bar if an increase in anodic corrosion on other buried installations is absolutely excluded. The insulation of all return cables must therefore be monitored regularly. 

Conductive Polymers Anodes currently available consist of a conductive-polymer graphite material coated on to a multistrand copper conductor. The polymer provides an active surface but shields the conductor from chemical attack. A non-conductive outer braid may be used to give abrasion resistance and avoid direct contact with the cathode. The finished anode has the appearance of an electric cable and is claimed to have applications for buried/immersed structures and for internal protection of tanks, etc. Anode current densities are typically given as 14-30mAm ... 

The results of these experiments have been considered by the Joint Committee for the Co-ordination of the Cathodic Protection of Buried Structures and, in view of the various types of buried structures concerned and the circumstances in which field tests are conducted, the Committee decided not to amend its provisional recommendation that when cathodic protection is applied to a buried structure the maximum permissible potential change in the positive direction on a nearby pipe or cable should be 20 mV. If there is a history of corrosion on the unprotected installation no detectable positive change in structure/soil potential should be permitted. These criteria of interaction have been adopted in the British Standard Code of Practice for Cathodic Protection . 

Lead is used as a sheathing material for protecting the cable from chemical attack whilst it is buried directly in hostile ground conditions, e.g. in chemical and refinery plants. 

Protection of cables in walls - section 522 has been rearranged and modified to make it clearer. Amendment 3 at 522.6.202 states that all installations buried at a depth of 50mm or less, which includes cables installed in partitions constructed of metal parts irrespective of depth, shall have 30 mA RCD protection, unless other methods detailed in 522.6.203 have been applied. Compatibility - Regulation 512.1.5 tells us that the installation designer must ensure that all the installed fixed equipment is designed and manufactured in accordance with the Electromagnetic Compatibility (EMC) Directive. Installations composed only of CE marked apparatus will conform to this directive. 

Test coupons of steel of a specified size are buried near the pipeline and connected by cable at the test point with the cathodically protected pipeline. They simulate artificial defects in the coating. The protection current taken from the test coupon can be measured via the cable connection and the true potential determined from a reference electrode in front of the test coupon by momentarily interrupting the cable connection [28]. Ohmic potential drops between the reference electrode and the test coupon are obtained from a measuring test probe that has a built-in reference electrode on the back of it to measure the 7/ -free potential directly [see Eq. (2-34) with 5 0] without having to switch off the protection... 

A number of different arrangements for a supply system can be specified concerning the relationship between neutral and earth. A discussion on these various options is beyond the scope of this handbook. Instead we shall simply evoke the basic arrangement of separate protective conductors, a main earth terminal in the substation directly connected to buried electrodes. Protective conductors connect normally nonconducting metalwork, such as electric motor frames, switchgear panel metalwork, metal cable trunking and conduits, mechanical equipment and structural steelwork back to the substation main earth terminal. Protective conductors... 

For example, in the rapidly expanding trunk optical communication system in Australia, the requirements to directly plough and bury optical cables in transcontinental lines of 2,500 and 2,000 km over a wide variety of terrain has placed a heavy commitment on polymer material developments for strength members, insulation and outer jacketing. Similarly, the 2,500km oceanic optical cable link between Australia and New Zealand, which decends to depths of 2.5 kms, has created a similar demand for specialist materials and fabrication technologies. 

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