Cables and Connectors Multiple choice Questions and Answers

Answer.3. 66 kV

For voltages beyond 66 kV, solid-type cables are unreliable because there is a danger of breakdown of insulation due to the presence of voids. When the operating voltages are greater than 66 kV, pressure cables are used. In such cables, voids are eliminated by increasing the pressure of the compound and for this reason, they are called pressure cables. Two types of pressure cables viz. oil-filled cables and gas pressure cables are commonly used.

Answer.2. Voltage

The thickness of the layer of insulation on the conductor in cables depends upon voltage. Insulation covers the base conductor. The material used in insulation have greater dielectric strength and absorbs all the voltage gradient just within a few mm or cm distance. As voltage increases the thickness of insulation increases.

Answer.1. There is a danger of breakdown of insulation due to the presence of voids

The voltage limit for solid type cables is 66 kV due to the following reasons:

1. As a solid cable carries the load, its conductor temperature increases and the cable compound (i.e., insulating compound over paper) expands. This action stretches the lead sheath which may be damaged.
2. When the load on the cable decreases, the conductor cools and a partial vacuum is formed within the cable sheath. If the pinholes are present in the lead sheath, moist air may be drawn into the cable. The moisture reduces the dielectric strength of insulation and may eventually cause the breakdown of the cable.
3. In practice, voids are always present in the insulation of a cable. Modern techniques of manufacturing have resulted in void free cables. However, under operating conditions, the voids are formed as a result of the differential expansion and contraction of the sheath and impregnated compound. The breakdown strength of voids is considerably less than that of the insulation. If the void is small enough, the electrostatic stress across it may cause its breakdown. The voids nearest to the conductor are the first to break down, the chemical and thermal effects of ionization causing permanent damage to the paper insulation.

Answer.1. The increase in length of the insulation

Insulation resistance is the resistance offered by the cable insulation to the leakage current flowing from the conductor to the sheath. The insulation resistance of a cable is inversely proportional to the length of the cable. As the length increases, the leakage current flowing through the insulation from the conductor to the earthed sheath increases, and hence, the effective insulation resistance decreases.

Answer.2. 50 cm

It is possible for 50 Hz currents to be induced in the metallic conductors of communications cables where they run parallel to high-voltage power lines carrying low-frequency (50 Hz) currents.

The voltage induced in the telephone line from the power cables happens in a manner similar to the operation of a step-down transformer. A higher potential on the primary circuit results in a higher potential on the secondary circuit. Hence, low-voltage ac power lines (less than 1000 volts) pose a threat for direct contact, but less of a threat for dangerous voltage being induced on the telephone line.

It is important to maintain spatial separation between conductive telecommunications cables and HV power cables to ensure that the possibility of induced voltage on the communications cables does not exceed 430 V ac.

The minimum distance between power and communications lines for parallel pole routes exceeding 330 kV is 50 cm.

Answer.2. Provide proper stress distribution

Intersheath grading:- The process of achieving uniform distribution in dielectric stress by providing a metallic inner sheath between successive layers of the same dielectric materials and maintaining the appropriate potential level at the inner sheath.

Answer.3. Varnished cambric

Varnished cambric (or empire tape):- This is a cotton cloth impregnated and heated with varnish. The cambric is lapped onto the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent.

• Its dielectric strength is about 4 kV/mm.
• Its permittivity is 2.5 to 3.8.
• Such cables do not require sealing.

Answer.3. Unsheathed cables

Conduit wiring system

In conduit wiring, cables and Unsheathed cables wires are carried through plastic giving good protection from mechanical injury or fire risks. This system is the best and most desirable system of wiring for workshops and public buildings.

It gives a very good appearance when concealed. In this wiring, the pipes are cut with the hacksaw and are threaded tee junction box, etc., and are then fixed on the walls on wooden gutties or plugs with saddles. Then wires are drawn with the help of fish wire (steel wire).

Nowadays P.V.C. conduit pipes are also available which do not require threading. Jointing is done with a special mode solution. These are flexible and can be bent easily. For concealed wiring, the pipes are directly buried in the wall and roofs, and then wires are drawn through them.

Advantages of Conduit Wiring Systems

• It is the safest wiring system (Concealed conduit wring)
• Appearance is very beautiful (in case of concealed conduit wiring)
• No risk of mechanical wear & tear and fire in case of metallic pipes.
• Customization can be easily done according to the future needs.
• Repairing and maintenance are easy.
• There is no risk of damage the cable’s insulation.
• it is safe from corrosion (in case of PVC conduit) and risk of fire.
• It can be used even in humidity, chemical effect and smoky areas.
• No risk of electric shock (In case of proper earthing and grounding of metallic pipes).
• It is reliable and popular wiring system.
• sustainable and long-lasting wiring system.

Disadvantages of Conduit Wiring Systems

• It is expensive wiring system (Due to PVC and Metallic pipes, Additional earthing for metallic pipes Tee(s) and elbows etc.
• Very hard to find the defects in the wiring.
• installation is not easy and simple.
• Risk of Electric shock (In case of metallic pipes without proper earthing system)
• Very complicated to manage additional connection in the future.

Depending on the operating voltage, the cables are classified as follows:

• L.T. (Low tension) cable – up to 1 kV
• H.T (High tension) cable- up to 11kV
• S.T. (Super tension)cable- from 22 kV to 33 kV
• E.H.T (Extra high tension) cable- from 33 kV to 66 kV
• Extra super voltage cable- beyond 132 kV

Depending on the operating voltage, the cables are classified as follows:

• L.T. (Low tension) cable – up to 1 kV
• H.T (High tension) cable- up to 11kV
• S.T. (Super tension)cable- from 22 kV to 33 kV
• E.H.T (Extra high tension) cable- from 33 kV to 66 kV
• Extra super voltage cable- beyond 132 kV