Cable Construction & Cable Selection- Part:3
May 2, 2016 9 Comments
- Code: IS: 7098 / IS: 3975 / IEC:60502 / BS:6622/BS:7835.
- Material: metallic or non-magnetic Alumimium, Steel wire/strip.
- Used for : LV, MV & HV Cables
- The armor provides mechanical protection against crushing forces.
- Armor also can serve as an Earth Continuity Conductor (ECC).
- The armoring type could be:
- Mechanical protection of the cable is provided by a single layer of wire / Strip strands laid over the bedding. Steel wire / Strip is used for 3-core or 4-core cables, but single-core cables have aluminum wire armoring.
- When an electric current passes through a cable it produces a magnetic field (the higher the voltage the bigger the field). The magnetic field will induce an electric current in steel armor (eddy currents), which can cause overheating in AC systems. The non-magnetic aluminum armor prevents this from happening.
- Magnetic Material’s armoring for 3Ph System: With 3-core or 4-core cables the vector sum of the currents in the conductors is zero, and there is virtually no resultant magnetic flux. In Multi-core armored cables have either single layer of Galvanized Steel wire Armor or Galvanized steel strip applied over inner sheath with left hand lay.
- Non Magnetic Material’s armoring for 1Ph System: This is not so however for a single-core cable, where eddy-current heating would occur if a magnetic material was used for the armoring. The material has to be non magnetic for armoring as in this case of return current is not passing through the same cable. Hence it will not cancel the magnetic lines produced by current. These magnetic lines which are oscillating in case of A.C. systems will give rise to eddy currents in magnetic armoring and hence armoring will become hot, and this may lead to failure of the cable. Hence Single core cables for use on A.C systems are armored with single layer of nonmagnetic (Aluminum) material.
- Armoring is Mostly following Type
- SWA – Steel wire armor, used in multi-core cables (magnetic),
- AWA – Aluminum wire armor, used in single-core cables (non-magnetic).
- Tinning or galvanizing is used for rust prevention. Phosphor bronze or tinned copper braid is also used when steel armor is not allowed.
- As strip construction is economical, the manufacture always provides steel strip armoring unless wire armoring is specified.
- As per IS: 1554 Round Wire armoring is provided in cable where calculated diameter under amour is upto13mm. Above this the amour is either steel wire or steel strip of size 4.00X0.80mm.
(10)Over Sheath (Outer Jacket):
- Code: IS: IS:7098 / IEC:60502 / BS:6622/BS:7835.
- Material: PVC Flame Retardant / Flame Retardant Low Smoke / Zero Halogen (LSOH), High density Polyethylene HDPE, Halogen Free Flame Retardant (HFFR)
- Used for : LV, MV & HV Cables
- It is the outer protection part of the cable against the surrounding environment.
- Protected against water ingress, Protection against termite, Protection against UV and Protection against differing soil compositions.
- It is applied over armoring in case of armored cable and over inner sheath in case of unarmored cable called as “Outer Sheath”.
- The standard sheath color is Black other colors such as Red , Light Blue can also be provided
- High-voltage cables are identified by outer sheaths colored red; a black sheath indicates a low-voltage cable
- The following are the electrical property may be consider while selecting a outer Sheath Materials
- Dielectric Strength: Cable Sheath may be semiconducting or insulating.
- Discharge and Tracking Resistance: When a non shielded cable rests upon or comes into contact with a ground plane, the ground plane acts as the outer plate of the capacitor, made up of the conductor, insulation and the ground plane. Discharges and tracking may cause erosion of the Outer Sheath material.
- Material: A major consideration in selecting Outer Sheath may be a thermoplastic or thermosetting material. Mostly a thermoplastic jacket is less expensive. However, thermoplastics will melt at some elevated temperature and, thus, could run or drip from the cable under extreme conditions.
- Thermoset materials will not melt and run or drip at elevated temperatures.
Comparison of Cable:
PVC Insulated Cable:
- PVC insulation becomes stiff making it difficult to fold and the soft PVC loosens its softening agent over years, making it brittle and prone to rip.
- Even at the time of disposing, burning PVC emits toxic dioxin, which is responsible for causing cancer and does, when dumped scantly dissolve
- PVC is thin insulation mainly used in LT side cables and XLPE is thick insulation used in MV & HT cables.
XLPE Insulated Cable:
- Higher Current Capacity: XLPE has higher current carrying capacity as
- Higher Temperature Withstand Capacity: It can withstand higher temperature compared to PVC cable.
- Higher Overload Capacity: XLPE have high overload capacity.
- Low Dielectric Constant: XLPE has lower dielectric and constant power factor.
- Light weight & Small Bending Radius :XLPE cables are lighter in weight, has smaller bend radius, and hence lesser installation cost.
- Higher Short Circuit Capacity: XLPE has higher short circuit rating. XLPE can withstand higher & lower temperatures insulation is usually thinner but the resistance is higher.
- Higher Moisture Resistance: XLPE also has a higher moisture & chemical resistance.
- Cable Installation Job for XLPE is easier than PVC insulated cables because of less Wt, less Diameter and Less Bending radius.
- The Volume Resistivity (ohm-cm) for XLPE is way higher than the PVC cables which are of the order of XLPE cables has insulation resistance of 1000 times compared to PVC cables.
Elastomer Insulated Cable:
- Elastomer cables are preferred for flexible application and in congested locations where the bending radius are very small. Elastomer cables are available from low voltage up to 33 kV grade.
- Elastomer cables are also available with rigid copper conductors and having properties like Fire Survival, Zero Halogen and Low toxicity FS properties.
Rubber Insulated Cable:
- Rubber insulation remains in the best condition after a long span of time,say,25-30 years and remain soft and pliable even when the temperature is low.
- Rubber Cables are predominantly used in special applications like, mining, ship wiring, transportation sector and Defense applications & earth moving machines.
- These materials have the potential to be recyclable since they can be molded , extruded and reused like plastics, but they have typical elastic properties of rubbers which are not recyclable owing to their thermosetting characteristics