The IEE Wiring Regulations Explained and Illustrated, 2nd Edition

Protection

All of these foregoing requirements ensure that conductor insu-

lation

is

undamaged when

an

overload occurs.

Under short-circuit conditions

it is

the conductor itself that

is

susceptible to damage and must be protected. Figure

17

shows one

half-cycle of short-circuit current

if

there were no protection. The

RMS value (0.7071 x maximum value)

is

called

the

prospective

short-circuit current. The cut-off point

is

where

the

short-circuit

current

is

interrupted

and an arc is

formed;

the

time

t

x

taken

to

reach this point is called the pre-arcing

time.

After the current has

been

cut off, it

falls

to

zero

as the arc is

being extinguished