We will look overcurrent protection, fuses and MCBs more detail later
in this book Chapter 12.
Separated extra low-voltage supplies (SELV) provide protection against
both ‘basic’ and ‘fault’ protection.2) achieved protective equipotential bonding and
automatic disconnection the supply the event fault occurring.
Key Fact
Fuses
How does fuse work?
● under fault conditions excessive
current flows
● the fuse element gets hot
● the fuse element melts
● disconnecting the circuit it
protected. both cases, the basic requirement for pro-
tection that the circuit should interrupted before the fault causes a
temperature rise which might damage the insulation, terminations, joints
or the surroundings the conductors.
Part the IEE Regulations deals with the application protective meas-
ures for safety and Chapter with the regulations for switching devices or
switchgear required for protection, isolation and switching consumer’s
installation.
. short circuit is
an overcurrent resulting from fault negligible impedance between live
conductors having difference potential under normal operating condi-
tions.Basic electrical circuits and cables
135
In general, protection against touching live parts achieved insulat-
ing live parts and called ‘basic protection’.
The consumer’s main switchgear must readily accessible the con-
sumer and able to:
● isolate the complete installation from the supply
● protect against overcurrent
● cut off the current the event serious fault occurring.1. Overload currents usually occur circuit because abused by
the consumer because has been badly designed modified the
installer.
Safety First
Bonding
● bonding clamps must an
approved type
● must fitted clean pipework
● must tight and secure
● must have visible label
● IEE Regulation 514. the device used for overload pro-
tection also capable breaking prospective short circuit current safely,
then one device may used give protection from both faults (Regulation
432.13.
Definition
An overload overcurrent defined
as current which exceeds the rated
value otherwise healthy cir-
cuit.
Protection against overcurrent
Excessive current may flow circuit result overload a
short circuit. Protection against touching
something made live result fault, and called ‘fault protection’ (IEE
Regulation 131.2.1). Short circuits usually occur result accident which could
not have been predicted before the event. Devices which offer protection from overcurrent are:
● semi-enclosed fuses manufactured 3036
● cartridge fuses manufactured 1361 and 1362
● high breaking capacity fuses (HBC fuses) manufactured 88
● MCBs manufactured 60898. overload overcurrent defined current which
exceeds the rated value otherwise healthy circuit. short circuit overcurrent
resulting from fault negligible
impedance.
An overload may result currents two three times the rated current
flowing the circuit, while short circuit currents may hundreds times
greater than the rated current
