Question

An electric cable of aluminium conductor $[K=240W/mK]$ is to be insulated with rubber $[K=0.15W/mK]$. If cable is to be located in air $[h=6W/m^{2}K]$, the critical radius of insulation will be

• A. $25mm$
• B. $40mm$
• C. $80mm$
• D. $160mm$

Answer 1

The correct option is A $25mm$

For a cylindrical pipe, the critical radius of insulation is given by

$r_c=\frac{K}{h_0}=\frac{0.15}{6}=0.025$

$=25mm$