The IEE Wiring Regulations Explained and Illustrated, 2nd Edition

The IEE Wiring Regulations

Having found a disconnection time, we can now apply the

formula.

Example of use ofadiabatic equation

Suppose that in

a

design the protection

was

by

40

A

BS

88 fuse; we

had chosen a

4 mm

copper CPC running with our phase conduc-

tor; and the loop impedance Z

s

was 1.2 ohms. Would the chosen

CPC size be large enough to withstand damage in the event of

an

earth fault?

We have

/

f

=C/

0

/Z

s

= 240/1.2 =

200

A

From the appropriate

curve

for

the 40

A

BS 88

fuse (Figure

33),

we

obtain a disconnection time toils. From Table 54C of the Regu-

lations,

A:

= 115. Therefore the minimum size of

CPC

is given by

s

=

y/iti

01 k = V(200

2

x ...