OVERHEAD Vs UNDERGROUND SYSTEM

COMPARISON OF OVERHEAD AND UNDERGROUND CABLE

·         The distribution system can be over head or underground. The choice between overhead and underground system depends upon a number of widely differing factors. Therefore,  it is desirable to make a comparison between the two.

S.NO	OVERHEAD LINE	UNDERGROUND CABLE
1.	Only bare conductors are used in open place	Insulated conductors are placed in underground
2.	Not very safe	More safe
3.	Conductor size is small	Conductor size is large
4.	Fault occurs due to atmospheric conditions such as snow, severe rain, storms, lightning, fires, tree touching, etc.	Chances of fault are very rare
5.	Insulation cost is less	Insulation cost is more
6.	Conductor is less expensive	Conductor is costlier
7.	Easier expansion and replacement is possible	Expansion is difficult
8.	Low initial cost	High initial cost
9.	Fault detection is easy	Fault detection is difficult
10.	Causes interference with communication lines	No such problems
11.	it is used for long transmission	Due to charging current it is not used for long transmission
12.	Appearance is not good	It gives better appearance
13.	Useful life time is less	Useful life time is two times that of O.H lines

  

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What is the difference between self and mutual induction ?

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What is the difference between static and dynamically induced EMF ?

Statically induced EMF:

It is an emf induced without a relative motion between a conductor and magnetic field

Example: EMF induced in transformer

Dynamically induced EMF:

It is an emf induced with a relative motion between a conductor and magnetic field

Example: EMF induced in generator

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what is the difference between core and shell type transformer ?

S.No	Core type transformer	Shell type transformer
1	Core is surrounded by winding	Winding is surrounded bycore
2	It has two limbs	It has three limbs
3	It is made up of two L shaped strips	It is made up of E and I shaped strips
4	Easy to dismantle the windings for repair work	Difficult to dismantle the windings for repair work
5	Better heat dissipation	Poor heat dissipation as the winding is surrounded by core.

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what are the biasing conditions for a transistor to act as an amplifier?

Region	Emitter base junction	Collector-base junction	function
Active	Forward biased	Reverse biased	Amplifier
Cut-off	Reverse biased	Reverse biased	Off-switch
saturation	Forward biased	Forward biased	On-switch

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What is the difference between fuse and circuit breaker?

(i) Fuse performs both detection and interruption functions. Circuit breaker performs interruption function only. The detection of fault is made by relay system.

(ii) The breaking capacity of fuse is very small compare to that of circuit breaker.

(iii) The operating time of the fuse is smaller than operating time of CB.

(iv) After every operation, replacement is required in the fuse. In case of circuit breaker, replacement is not required.

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What is the necessity of starter in DC motor ?

What is the necessity of starter in DC motor ?

The voltage equation of dc motor is

V = Eb + IaRa

at start Eb = 0, V= IaRa

Ia = V / Ra.  (Ra alone limits the Ia)

generally motor is switched on with normal voltage and Ra is very small, the Ia at start is very high.This will blow out the fuses and may cause permanent mechanical damage to the motor.To limit this starting current to a safe value a variable resistance is connected in series with armature.This resistance is called as starter.

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What is the difference between Electrical and Electronics ?

What is the difference between Electrical and Electronics ?

Electrical --- study of current flow in conductors.

Electronics ---study of current flow in semiconductors

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What happens when a d.c motor is connected across an ac supply?

What happens when a d.c motor is connected across an ac supply?

• Since on ac supply, reactance will come in to play,the a.c supply will be offered impedance(not resistance) by the armature winding.consequently,with a.c supply,current will be much less.The motor will run but it would not carry the same load as it would on d.c. supply.
• There would be more sparking at the brushes
• Though motor armature is laminated as a rule, the field poles are not. consequently,eddy currents will cause the motor to heat up eventually burn on a.c. supply.

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Why transformers are rated in KVA and motors are rated in KW ?

The power factor of transformer is dependent on load we only define Volt-ampere rating and does not include power factor  but in case of motors, power factor depend on construction and hence rating of motors is in KW and include power factor.

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Why we can’t store AC in Batteries instead of DC?

Why we can’t store AC in Batteries instead of DC?

We cannot store AC in batteries because AC changes their polarity upto 50 (When frequency = 50 Hz) or 60 (When frequency = 60 Hz) times in a second. Therefore the battery terminals keep changing Positive (+ve) becomes Negative (-Ve) and vice versa, but the battery cannot change their terminals with the same speed so that’s why we can’t store AC in Batteries.
Also when we connect a battery with AC Supply, then It will charge during positive half cycle and discharge during negative half cycle because the Positive (+ve) half cycle cancel the negative (-Ve) half cycle, so the average voltage or current in a complete cycle is Zero. So there is no chance to store AC in the Batteries.

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Does zero watts bulb consume no power ?

There is no bulb which consumes zero watt, it is not possible to light a bulb without drawing an current and consuming greater than zero watts .

Actually a zero watt bulb consumes about 12 to 15 watts, the inaccurate electric meters of the early days could not measure such a negligible amount of consumption as during those days,the energy meter had a rotating disc and when all other lights switched off,the disc would have minimum rotation,which would not record even a 10 watts consumption.This led to the term Zero-watt. 

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What is hunting ?

What is hunting ?

• The oscillations of rotor due to sudden application or removal of load is called swinging or hunting.

• During hunting there are changes the current drawn by the motor which may cause problem to other appliances connected to the same line.Hence hunting is not desirable phenomenon from motor point of view and must be prevented.

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What are the disadvantages of low power factor?

What are the disadvantages of low power factor?

Low power factor increases the current drawn from the supply to supply same power to the load.The high current due to low p.f has the following disadvantages:

• Large conductor size and  results in high cost
• More copper losses and poor efficiency.
• Large voltage drop,results in poor regulation.

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If the supply frequency increases, then skin effect is ...........

Answer: increases

The skin effect depends upon the following factors:

• Nature of material
• Diameter of wire - increases with the diameter of wire
• Frequency -    increases with the increases in frequency.
• Shape of wire - less for stranded conductor than the solid conductor.

Skin effect is negligible for Frequency < 50 Hz

Conductor diameter < 1 Cm

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How does a bird does not get electric shock when they sit on electric wire?

How does a bird does not get electric shock when they sit on electric wire?


There is no voltage difference along a single wire the birds sitting on.since the birds are touching only one wire,the circuit is incomplete and the birds are safe.

To get shocked,electricity has to travel through its body;this requires two conductors.

Large birds can be shocked because they can make contact with two conductors,usually at a metal power pole.

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What is the difference between unilateral and bilateral elements?

What is the difference between unilateral and bilateral elements?

Bilateral element: Voltage-current relation is the same for current flowing in either direction.

example: resistor,inductor and capacitor.

Unilateral element: different relations between Voltage and current for the two possible directions of current.

example: Vacuum tubes, silicon diodes, metal rectifiers ect.

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What is the difference between active and passive elements ?

What is the difference between active and passive elements ?

An active element is one, which is capable of generating the energy on its own. Examples,transistor,current source , FET, ect.

A passive element is one, which is incapable of generating energy on its own. But it is capable of storing and dissipating energy.It always need some external source of power.Examples, resistor, capacitor, inductor, ect.

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Why power factor is not more than unity?

• Power factor is the cosine angle between voltage and current.The magnitute of cosine is never greater than one.

• The power factor is equal to watts/ Volt-Amps.Watts can never exceed Volt-amps

                                          

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A lamp draws 0.5 A at 230 V. If the voltage is reduced to 115 V, will the current be halved ?

A  lamp draws 0.5 A at 230 V. If the voltage is reduced to 115 V, will the current be halved ?

No.because ohms law is true for constantant temperature.If the voltage is reduced from 230 to 115 volt,results in decrease in resistance. Consequently ohms law cannot be applied.

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