1. The insulating material for a cable should have
A. low cost
B. high dielectric strength
C. high mechanical strength
D. all of the above
Answer: D. all of the above
Explanation:
Insulating Materials for Cables:
- The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used.
- Therefore, the proper choice of insulating material for cables is of considerable importance.
The insulating materials used in cables should have the following properties:
- High insulation resistance to avoid leakage current.
- High dielectric strength to avoid electrical breakdown of the cable.
- High mechanical strength to withstand the mechanical handling of cables.
- Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like a lead sheath.
- Non-inflammable.
- Low thermal coefficient
- Low cost so as to make the underground system a viable proposition.
- Unaffected by acids and alkalies to avoid any chemical action.
2. Which of the following protects a cable against mechanical injury?
A. Bedding
B. Sheath
C. Armoring
D. None of the above
Answer: C. Armoring
Explanation:
Lead sheath:
- In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in figure.
- It has a minimum dielectric stress in a cable.
Bedding:
- Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape.
- It is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.
Armoring:
- Over the bedding, armouring is provided which consist of one or two layers of galvanized steel wire or steel tape.
- Its purpose is to protect the cable from mechanical injuries while laying it or handling it.
3. Which of the following insulation is used in cables?
A. Varnishes] cambric
B. Rubber
C. Paper
D. Any of the above
Answer: D. Any of the above
Explanation:
- The various insulating materials used in manufacturing of cables are rubber, VIR, paper, Polyvinyl Chloride (PVC), Varnished Cambric, Polyethylene, Gutta-Percha, Silk, Cotton, enamel etc.
- The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. In general, the insulating materials used in cables should have the following properties.
- High insulation resistance to avoid leakage current
- High dielectric strength to avoid electrical breakdown of the cable
- High mechanical strength to withstand the mechanical handling of cables
- Non-hygroscopic i.e., it should not absorb moisture from air or soil; The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like a lead sheath
- Non-Inflammable
- Low cost so as to make the underground system a viable proposition
- Unaffected by acids and alkalies to avoid any chemical action
4. Empire tape is
A. varnished cambric
B. vulcanized rubber
C. impregnated paper
B. none of the above
Answer: A. varnished cambric
Explanation:
- Empire tape is usually made of varnished cambric.
- Empire tape or Electrical tape (or insulating tape) is a type of pressure-sensitive tape used to insulate electrical wires and other materials that conduct electricity.
- It can be made of many plastics, but vinyl & varnished cambric most popular, as it stretches well and gives an effective and long-lasting insulation.
- Electrical tape for class H insulation is made of fiberglass cloth.
5. The thickness of the layer of insulation on the conductor, in cables, depends upon
A. reactive power
B. power factor
C. voltage
D. current carrying capacity
Answer: C. voltage
Explanation:
- The thickness of the insulation layer on the conductor in cables dependent upon voltage.
- In a conductor, the voltage gradient is developed along the length of the conductor which decreases radially outwards from the conductor. If no insulation is being used than air behaves as dielectric to absorb that voltage gradient and may take a few meters to completely absorb the voltage gradient. For example, overhead bare conductors, that's why they have separated apart with a certain distance. In distance between the bare conductor increases with the increase in voltage.
- But in the case of underground cables insulation covers the bare conductor. The material used in insulation has greater dielectric strength and absorbs all the voltage gradient just within a few mm or cm distance. As the voltage increases the thickness of insulation increases.
- Thus, insulation absorbs the voltage gradient. Also, insulation is made of such material so that it can dissipate the heat generated in the conductor inside. As the thickness of the conductor depends on current, so proportional thickness of the conductor allows optimal heating in a conductor, and heat generated dissipates through the insulation although this is very small.
- Thus, insulation is independent of current and depends only on voltage.
6. The bedding on a cable consists of
A. hessian cloth
B. jute
C. any of the above
D. none of the above
Answer: C. any of the above
Explanation:
- Bedding is provided for the protection of metallic sheath against corrosion and mechanical injury, a layer of “bedding” consisting of paper tape compounded with a fibrous material like jute or Hessian tape is used.
- This protects the metallic sheath from corrosion and mechanical injury due to armoring.
- The compound used should be such that it does not react with the armoring material and lead sheath but at the same time it should be adhesive enough so that it sticks on, both to the lead sheath and armoring. Bedding is used in paper-insulated lead-covered cables but not in polyvinyl chloride (PVC) cables.
7. The insulating material for cables should
A. be acid proof
B. be non-inflammable
C. be non-hygroscopic
D. have all above properties
Answer: D. have all above properties
Explanation:
Insulating Materials for Cables:
- The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used.
- Therefore, the proper choice of insulating material for cables is of considerable importance.
The insulating materials used in cables should have the following properties:
- High insulation resistance to avoid leakage current.
- High dielectric strength to avoid electrical breakdown of the cable.
- High mechanical strength to withstand the mechanical handling of cables.
- Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like a lead sheath.
- Non-inflammable.
- Low thermal coefficient
- Low cost so as to make the underground system a viable proposition.
- Unaffected by acids and alkalies to avoid any chemical action.
8. In a cable ...... immediately above metallic sheath is provided.
A. earthing connection
B. bedding
C. armoring
D. none of the above
Answer: C. armoring
Explanation:
Construction of Cables:
The various parts are:
(i) Cores or Conductors:
- A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended.
(ii) Insulation:
- Each core or conductor is provided with a suitable thickness of insulation, the thickness of the layer depending upon the voltage to be withstood by the cable.
- The commonly used materials for insulation are impregnated paper, varnished cambric, or rubber mineral compound.
(iii) Metallic sheath:
- In order to protect the cable from moisture, gases, or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation.
(iv) Bedding:
- Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armoring.
(v) Armoring:
- Over the bedding, armoring is provided which consists of one or two layers of galvanized steel wire or steel tape.
- Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling.
- Armoring may not be done in the case of some cables such as low voltage to avoid excessive sheath losses.
(vi) Serving.
- In order to protect armoring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armoring. This is known as serving.
9. The current carrying capacity of cables in D.C. is more than that in A.C. mainly due to
A. absence of harmonics
B. non-existence of any stability limit
C. smaller dielectric loss
D. absence of ripples
E. none of the above
Answer: C. smaller dielectric loss
Explanation:
- Compared with the AC cable, the power (dielectric loss) during the transmission of the DC cable is small.
- The power loss of the DC cable is mainly the DC resistance loss of the conductor, and the insulation loss is small (the size depends on the current fluctuation after rectification).
- The AC resistance of the low-voltage AC cable is slightly larger than the DC resistance, the high-voltage cable is obvious, mainly because the proximity effect and the skin effect, the loss of insulation resistance accounts for a large proportion, mainly the impedance generated by the capacitor and the inductor.
- Therefore, the current carrying capacity of cables in DC is more than that in AC.
10. In case of three core flexible cable the colour of the neutral is
A. blue
B. black
C. brown
D. none of the above
Answer: A. blue
Explanation:
Colour of different types of Wire or Cable:
Type | Colour code |
Single-phase line | Red/Brown |
Single-phase neutral | Black/Blue |
Ground wire | Green |
Three-phase line 1 | Red |
Three-phase line 2 | Yellow |
Three-phase line 3 | Blue |
Three-phase neutral | Black |
Three-phase protective ground or earth | Green (or) Green - Yellow |
| Neutral wire (3-core flexible cable) | Blue |