Materials
and
their applications
in the
electrical industry
49
CIRCUIT DAMAGE DUE
TO OVERCURRENTS
/line of force through P
r metres Length of magnetic
/-"" ^^^ /line of force thr
/ s "V^v - 2 ir r metres
&ιγ
Conductor carrying I
x
amperes
Figure 4.20
Consider a single conductor carrying a current of
I
x
amperes and the
resulting magnetic field as shown in Figure 4.20.
The magnetising force at point P is given by: H
=
-y- = ^- A
l\ 2nr
where l
x
is the length of the magnetic line of force through P.
Since the magnetic flux density B = μ^Η tesla (strictly in vacuum,
but considered true for air and solid insulators),
B =
4TT
x 10-
7
x^- = 2x 10-
7
x tesla
2nr r
A second conductor carrying I
2
amperes running parallel to the first
and through point P will suffer a force given by F= BI
2
l
2
newtons.
The arrangement is show