Engineering

1. UNIT-4
STUDY OF DIFFERENT TYPES OF COMPONENTS
IN ELECTRICAL DISTRIBUTION SYSTEM
PRESENTED BY
MOHSIN MULLA
ASST. PROF, DEPT OF EE.

2. STUDY OF DIFFERENT TYPES OF COMPONENTS IN ELECTRICAL DISTRIBUTION SYSTEM
Classification, general construction, types of cables, jointing of
cables, measurement of insulation resistance,
Requirements, materials used, types (Pin, Suspension,
Strain, Stay)
Different types, classification, design consideration,
various symbols, complete arrangement of substation (Single and
double bus bar), key diagrams for typical substations.

3. CONT...
Types: Rubber covered taped and
compounded or VIR, Lead alloy sheathed, Tough rubber sheathed,
Weather proof,
Flexible wire splicing, Termination (Twist splicing, Married
joint, Tap joint, Pig tail joint)
Tumbler, flush, pull, grid, architrave, rotary
snap, Push button, Ironclad water proof, Quick break knife switch.
Ceiling roses, Mounting blocks, Socket outlets plugs,
Mainswitches, Distribution fuse boards, MCB (Miniature Circuit Breakers)

4. By Voltage
• Low-tension (L.T.) cables – upto 1000 V.
• High-tension (H.T.) cables – upto 11,000 V.
• Super-tension (S.T.) cables – from 22 kV to 33 kV.
• Extra high-tension (E.H.T.) cables – from 33 kV to 66 kV.
• Extra super voltage cables – beyond 132 kV.
By Construction
• Belted cables: Maximum voltage of 11KVA
• Screened cables: Maximum voltage of 66 KVA
• Pressure cables: Maximum voltage of more than 66KVA

6. 1. Cores or Conductors
2. Insulation
3. Metallic sheath
4. Bedding
5. Armoring
6. Serving

7. • A cable may have one or more than one core (conductor) depending upon the
type of service for which it is intended. For instance, the 3-conductor cable
shown in the figure is used for 3-phase service.
• The conductors are made of tinned copper or aluminium and are usually
stranded in order to provide flexibility to the cable. The larger the conductor,
the greater the amount of current can flow through the conductor.
• The aluminium or copper conductor carries the electrical current. Aluminium
conductors are usually used for medium voltage distribution networks
requiring long distances and extensive cabling. Copper cables are used for
short links in substations and industrial installations where smaller cables or
higher power transmitting properties are required.

8. • Around the conductor, it is necessary to provide insulation to prevent
electrical short circuits.
• Each core or conductor is provided with a suitable thickness of
insulation, the thickness of layer depending upon the voltage to be
withstood by the cable.
• The insulation must be of sufficient thickness to withstand the
electric field under the rated and transient operating

9. 2. Cont....
• he commonly used materials for insulation are impregnated paper, varnished
cambric, rubber, mineral compound.
• In medium-voltage underground cables there are two main types of insulation
material:
• XLPE (Cross-linked Polyethylene)
• The most common material for MV cables today.
• EPR – Ethylene Propene Rubber
• More flexible than XLPE but not as efficient at reducing losses in circuits as
XLPE. Cables used in marine and offshore applications are normally
constructed with EPR insulation.

10. • 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 aluminium is provided over the insulation
as shown in Figure.
• The main function of the metallic screen is to nullify the electric field outside of the cable – it
acts as a second electrode of the capacitor formed by the cable. The screen needs to connect to
earth at least at one point along the route.
• The second function of the metallic screen is to form a radial barrier to prevent humidity from
penetrating the cable insulation system.
• When humidity and a strong electric field are present together, the insulation deteriorates by
what is called water-treeing, which can eventually cause the insulation to fail.

11. • 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.

12. • Over the bedding, armoring 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. Armoring may not be done in the case of some
cables.

13. • 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.
• Armoring and serving are only applied to the cables for the protection
of conductor insulation and to protect metallic sheath from
mechanical injury.

14. • Belted cables: Maximum voltage of 11KVA
• Screened cables: Maximum voltage of 66 KVA
• H- Type cables
• SL- Type cables
• Pressure cables: Maximum voltage of more than 66KVA

16. • The cores in the belted underground cables are not circular and are
insulated by impregnated paper. The cores are generally stranded and
may be of non-circular shape to make better use of available space. In a 3
phase cable, the three cores are grouped together and then belted with
the paper belt.
• The gaps between the conductors and the paper insulation are filled with
fibrous material such as the jute. This makes the cable to have a circular
cross-sectional shape. A lead sheath is used to cover the belt hence
protect it from moisture and provide mechanical strength. The lead
sheath is then covered with a single or multiple layers of an armoring
material and finally an outer cover.

17. • Since the electrical field in the three core cables is tangential, the paper
insulation and the fibrous materials are subjected to the tangential electrical
stresses. This stresses weakens the fibrous material as well as the resistance and
dielectric strength for the insulation along the tangential path.
• The weakening of the insulation may lead to the formation of air spaces in the
insulation. Under high voltages the air may be ionized and cause deterioration
and breakdown of insulation. For this reason, the belted cables are only suitable
for voltages up to 11KVa and not higher.
• Due to the large diameter of the paper belt, bending the cable may lead to the
formation of wrinkles and gaps.

19. • In a typical 3-core cable, each of the three cores is insulated by
impregnated paper and covered by perforated aluminium foil or other
metallic screen. The arrangement of the cores is designed to allow
each of the three metallic screens to make contact with each other.
The three cores are then wrapped around using a conduction belt
made of copper woven fabric tape.
• The H-type screened cable does not have an insulating belt; however,
it has the lead sheath, followed by bedding, armoring and a then a
serving. The core screens in the cable are all electrically connected to
both the conducting belt and the lead sheath. This ensures that they
are at the earth potential and all the electrical stresses are therefore
purely radial, hence reduced dielectric losses.

20. • Metallic screens improve the heat dissipation of the cable
• No formation of air pockets and voids in the dielectric, hence a high
breakdown strength and less dielectric losses
the cables are only suitable for low and medium voltages of up to 33KV,
but can reach 66KVA at times.

22. • The S.L type cables construction is almost like that of the H-type, however,
each of the insulated cores is covered separately with its own lead sheath.
Unlike the H-type, the S.L screened cable does not have an overall sheath,
however it has the armoring and serving.

23. • The use of separate sheaths reduces chances of core-to-core breakdown
• Easy to bend the cable
• Thinner lead sheaths are used hence need for greater care in manufacturing
and handling
• Only suitable for low and medium voltages of up to 33KV

25. • Suitable for congested urban areas where overhead lines may be difficult or
impossible to install
• Low maintenance
• Small voltage drops
• Fewer faults
• Not susceptible to shaking and shorting due to vibrations, wind, accidents,
etc.
• Not easy to steal, make illegal connections or sabotage
• Poses no danger to wildlife or low flying aircraft.

26. • More expensive
• Difficult in identifying and repairing broken cables
• Damage to cables or electrocution may occur to people digging the
ground and if they are unaware of the cable’s existence