Electrical Engineering ⇒ Topic : Insulation Resistance of a Cable Capacitor

Insulation Resistance of a Cable Capacitor

The cable conductor is provided with a suitable thickness of insulating material in order to prevent leakage current. The path for leakage current is radial through the insulation. The opposition offereed by insulation to leakage current is known as insulation resistance of the cable. For satisfactory operation, the insulation resistance of the cable should be very high.

Consider a single-core cable of conductor radius r₁ and internal sheath radius r₂ as shown in Fig. (a). Let I be the length of the cable and p be the resistivity of the insulation. 

                                                              FIGURE (A)

Consider a very small layer of insulation of thickness dx at a radius x. The length through which leakage current tends to flow is dx and the area of X-section offered to this flow is 2πxL

        Insulation resistance of considered layer

Insulation resistance of the whole cable is

This shows that insulation resistance of a cable is inversely proportional to its length. In other words, if the cable length increases, its insulation resistance decreases and vice-versa.