1st unit notes of electrical and electronics engineering materials /aktu/uptu notes

Leakage Currents


The leakage current is that current which is set up in a dielectric due to its electrical conductivity.
in solid dielectrics the following two types of currents are distinguished :

(i)Volume Leakage current                                   (ii) Surface leakage current

volume leakage current passes through the bulk of the dielectric from one plate to another.

Surface leakage current passes over the surface of dielectric .

Dielectric losses    
while calculating the dielectric losses, usually, only the volume leakage current is taken in to

consideration .but there is also loss of certain amount of power due to surface leakage current

but this power is primarily dissipated into heat the surrounding atmosphere and this loss does not

influence the heating of the dielectric in most of the cases.

Breakdown Voltage and Dielectric strength
If the voltage across the plates is increased slowly, a stage is reached at which dielectric breakdown

 and short circuiting of the plates may be caused. The voltage at which this occurs is called Breakdown Voltage.

this is also called disruptive voltage.

the intensity of the electric field at which breakdown occurs is called dielectric or electric strength


                             

it is expressed in kv/cm or kv/mm.


Effects of breakdown

the effects of breakdown can be summed up as follows:

1.Electrical effects
the insulating resistance of the material starts deteriorating as the voltage approaches the breakdown value.the current indicates a rise with the time even though the voltage keep unchanged.
When the voltage exceeds this value, the breakdown takes place followed by production of spark and puncturing of medium. 

2.Mechanical effects
The breakdown is accompanied by the perforation of the insulation ,whose nature depends on the applied voltage and time of application .it is significantly pronounced in all insulting material

Stranded Conductors

                  Stranded Conductors
Stranded conductors are the conductors made of thin wires of small cross-section area and bunched together.they are flexible, not rigid and can be coiled very easily.
They eliminate to a large extent the dark risk of breaking through insulation.

The following points are worth noting about stranded conductors---

(1)   A stranded conductor is made by twisting wires, called the strands,together to form layers.
(2)  A wires of each layer are laid in helical fashion round the preceding layer.
     the process is called 'Stranding' .
(3)  Generally stranding is done in opposite direction for successive layers.This means if the wires of each layer twisted in left hand direction then the next layer of wires will be twisted in right hand direction , and so on.
(4)  A standard stranding consist of 6 wires around one wire ,then 12 wires around the previous 6 , 18 wires around the previous 12,then 24 wires around 18 and so on.
the number of layers to be provided depends upon the number of wires to be provided.
the center wire is not counted as layer. Instead of single, 3 or 4 stranded wires may also be put in the center and over them layers may be formed. If three stranded wires are put in the center, 9 wires will be in the first layer, 15 wires will be in the second layer and so on.
The increase in the no of wires in each successive layer is 6 in each of the above cases. 

Formulae for stranded conductors---

the stranded conductors are expressed as follows 7/2.24,19/2.50
where the first number 7,19 indicates total no of wires.
And the second number indicates diameter of each wire in cm.

Different types of stranded conductors are shown below------

Fig. ........ Typical three core cable with stranded conductor.

Fig....... Circular stranded conductor