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9. Cable Failures and Their Analysis
Cables can fail due to many reasons. The cable failure modes, stressors and effects were discussed in sub-section 188.8.131.52 in section 1 to provide some insights on cable failures. Some of the major causes for cable failures are discussed next.
Mechanical failures can be due to breaks and defects of sheath material, mechanical punctures by people or machines, or cracks due to sharp bending or vibration. Whenever mechanical damage occurs in the cable sheath, the entrance of moisture will produce slow deterioration of insulation material, resulting in eventual failure of the cable. It is important therefore to take every precaution that either direct or indirect mechanical damage be eliminated or minimized by correct selection, installation, and maintenance of cable systems.
Corrosion of sheath
Sheath corrosion can occur due to the following factors:
1. Dissimilar soil effects
2. Galvanic action
3. Acidity and alkali in conduits
4. Chemical contamination in the soil
Corrosion of sheath will eventually allow moisture to penetrate into the insulation system and cause an eventual failure. Sheath corrosion can be minimized by correct application of cathodic protection, application of insulating paints, providing adequate drainage, and removing the source of chemical contamination.
Moisture in the insulation
Because of mechanical damage or for other reasons, entrance of moisture into the insulation system will deteriorate the cable, and all precautions should be taken to prevent such entrance. Damage due to moisture can be indicated by the following:
1. Bleached or soggy paper
2. Resistance to tearing of tapes
3. Stain on the inside surface of the sheath
4. Visible water
5. Whitish powder on aluminum conductor
Heating of cables
As explained in the Section 6.5 on cable rating, increased heat rise in the cable results in insulation degradation. Heat can be due to overloading, high ambient temperatures, insufficient ventilation, manual heating due to cables being installed too close to each other, or external sources of heat. Care must be taken not to exceed the temperature rise of the cable insulation system. This can be done by first identifying the various environmental and operating factors that will determine the correct selection of the cable insulation and conductor size.
Once correct selection and installation are made, routine maintenance and inspection of cable will ensure safe and long operating life of the cable.
Fire and lightning surges
Fire in conduit or manholes can cause cable failure in adjacent manholes and junction boxes. Barriers can be installed between large groups of conductors to prevent i re damage. Lightning arresters should be installed to protect the cable where it is connected to overhead lines to minimize failures of cable due to lightning surges.
Once the insulation is weakened owing to any of the reasons already discussed, it may fail electrically. That is, the insulation system cannot con fine the flow of electrical current to the conductor inside the insulation system. Failure may be line-to-ground or three line-to-ground or line-to-line faults. Obviously, if the failure is a short-circuit due to defective conductors, it will be detected by the circuit protective device. Some of the not-so-easy-to- detect electrical failures can be indicated by the following:
1. Bulging of the sheath
2. Tree design marking (dendritic)
3. Polymerized compound (wax)
4. Lack of compound in the insulation The cable failures discussed in Section 6.9 can be further classified into two classes as follows: (1) inherent causes and (2) non-inherent causes.
Inherent causes can be classified as follows:
1. Sheath or jacket defects
2. Insulation defects
3. Conductor defects
Sheath or jacket defects
Sheath defects are due to the following:
1. Thin lead (splits under pressure)
2. Eccentric lead thickness less than 85%
3. Structural defects: radial splits, laminations, gas pockets, and others
4. Cracked, embrittled, soft sports, bulge, cuts, bruises, or gauges
These defects are due to the following:
Defects in workmanship: These can be indicated by the following:
1. Wrinkling or creasing of tapes
2. Torn tapes
3. Excessive registrations
4. Knotted or misplaced fillers
5. Soft walls
High dielectric loss: This can be indicated by the following:
1. Scorching or carbonizing of paper
2. Happens in one or more spots
3. Can be determined by PF at 60°C or higher
Incomplete saturation: This can be indicated by the following:
1. Scarcity of the compound in spaces between adjacent tape edges and surfaces
2. Paper is void of the compound
Unstable compound: This can be indicated by the following:
1. Visible change in the compound
2. Wax, in case of mineral oil Ionization: This can be indicated by the following:
1. Carbonized paths (tree design)
2. Strings or flakes of darkened wax containing carbon
Conductor defects can be indicated by the following:
1. Irregular strands
2. Sharp corners
3. Missing strands
4. Burrs on the strands
5. Poor brazing
Corrosion of sheath: Corrosion usually proceeds either to complete penetration of the sheath or weakness of the sheath, so that the sheath breaks open.
Electrical breakdown takes place owing to admission of moisture. Corrosion of the sheath can be due to the following:
1. Positive potential (anodic), indicated by rough, pitted surface and very thin deposits of white crystals
2. Negative potential (cathodic), indicated by heavy deposit of lead oxides colored red, yellow, or orange
3. Local galvanic action
4. Chemical action
5. Other causes
Local galvanic action
Galvanic corrosion may occur in the presence of an electrolyte and some other metal that is connected electrically to the sheath elsewhere. Such failures are indicated by corroded sheath, which may be identical with either type of corrosion depending on whether the sheath is anode or cathode.
Chemicals such as alkali attack cable insulation, which comes about from incompletely cured concrete; acetic acid, rotting wood, jute, and other materials. Usually, these can be identified by the chemical known to be present for a particular installation.
External fire and HV surges
These are due to fire in cable circuits and lightning strikes and surges.
This is mainly due to heating of a cable that is overloaded or external heat and high temperature.
Mechanical damage can be due to the following:
2. Expansion and contraction
3. External causes
4. Injury during installation
These can be classified as follows:
1. High internal pressure
2. Migration of compound on a slope or riser
3. Moisture admitted through defective joints, terminations, and bonds
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