About insulation resistance of Cable.

1. EE504.53 1
OBJECTIVE
• Derive an equation for the insulation resistance of a cable

2. EE504.53 2
Known to Unknown
Resistance of a conductor in terms of its specific resistance,
length and cross-sectional area is
a
l


R

3. EE504.53 3
Insulation Resistance of a Cable
• The cable conductor is provided with a suitable thickness
of insulation material.
• Insulating material prevents leakage current.
• The path for the leakage current is radial through the
insulation.
• The opposition offered by insulation to leakage current is
known as insulation resistance of the cable.
• The insulation resistance should be very high.

4. EE504.53 4
Insulation Resistance of a Cable
Let
• r1 = conductor radius in metres
• r2 = internal sheath radius in metres
•  = resistivity of the insulation in  - metres
• l = length of the cable
• Consider a single core cable
Fig.1

5. EE504.53 5
Insulation Resistance of a Cable
• Consider a very small layer of insulation of thickness dx at a
radius x.
• The length through which leakage current flows is dx.
• Area of cross-section offered to leakage current is
Fig.2
2 xl


6. EE504.53 6
Insulation Resistance of a Cable
• Insulation resistance of considered layer is,
ix
length
R
area
resistivity
 
  
 
ix
dx
R
2 x l


 
  
 

7. EE504.53 7
Insulation Resistance of a Cable
• Insulation resistance of the whole cable is,
2
i
1
dx
R
2
r
r x l


 
  
 


 2
1
2
r
r x
dx
l



8. EE504.53 8
Insulation Resistance of a Cable
2
1
i log
2
R
r
r
x
l
e



1
2
i log
2
R
r
r
l
e




Insulation resistance of a cable is inversely
proportional to its length.
