2 provides equation
for calculating the maximum operating time the protective device to
prevent the permitted conductor temperature rise being exceeded as
follows:
t
k S
I
2 2
2
(s)
where
t duration time seconds
S cross-sectional area conductor square millimetres
I short-circuit rms current amperes
k constant dependent upon the conductor metal and type insula-
tion (see Table 43A the IEE Regulations).5. reduction
may occur because change the size type conductor because
of change the method installation change the environmental
conditions.
.Basic Electrical Installation Work
244
MCB Type 60898 will trip instantly between five and ten times
its rated current. However, this condition is
not met there some doubt, must verified that fault currents will
be interrupted quickly before they can cause dangerously high tempera-
ture rise the circuit conductors.
Installing overcurrent protective devices
The general principle followed that protective device must be
placed point where reduction occurs the current carrying capac-
ity the circuit conductors (IEE Regulations 433. This because the cable rating and the
overload rating the device are compatible. suitable for welding and X-ray machines where large
inrush currents may occur.
The IEE Regulations permit assume that where overload protec-
tive device also intended provide short-circuit protection, and has a
rated breaking capacity greater than the prospective short-circuit current
at the point its installation, the conductors the load side the pro-
tective device are considered adequately protected against short-cir-
cuit currents without further proof. Regulation 434.2).
Fault protection
The overcurrent protection device protecting circuits not exceeding 32A
shall have disconnection time not exceeding 0. The only exceptions this rule are where overload protec-
tive device opening circuit might cause greater danger than the over-
load itself for example, circuit feeding overhead electromagnet a
scrapyard.4s (IEE Regulations
411.2.3.2). more suitable for highly inductive commercial and
industrial loads.2 and 434.
MCB Type 69898 will trip instantly between and times
its rated current
