Chapter A General rules of electrical installation design

G - Sizing and protection of conductors
0
General
Loads Circuit cabling
Maximum load current IB
IB
Nominal current In or
its regulated current Ir
I n
zone a
Fig. G6 : Current levels for determining circuir breaker or fuse characteristics
Criteria for circuit-breakers:
IB y In y Iz and ISCB u ISC.
Criteria for fuses:
IB y In y Iz/k3 and ISCF u ISC.
Iz
Maximum load current Iz
zone b
Protective device
Schneider Electric - Electrical installation guide 2008
I2
Conventional overcurrent
trip current I2
1.45 Iz
1.45 Iz Isc
ISCB
zone c
3-ph short-circuit
fault-current breaking rating
IB y In y Iz zone a
I2 y 1.45 Iz zone b
ISCB u ISC zone c
b Its 3-phase short-circuit fault-current breaking rating is greater than the 3-phase
short-circuit current existing at its point of installation. This corresponds to zone “c” in
Figure G6.
Applications
b Protection by circuit-breaker
By virtue of its high level of precision the current I2 is always less than 1.45 In (or
1.45 Ir) so that the condition I2 y 1.45 Iz (as noted in the “general rules” above) will
always be respected.
v Particular case
If the circuit-breaker itself does not protect against overloads, it is necessary to
ensure that, at a time of lowest value of short-circuit current, the overcurrent device
protecting the circuit will operate correctly. This particular case is examined in Subclause
5.1.
b Protection by fuses
The condition I2 y 1.45 Iz must be taken into account, where I2 is the fusing (melting
level) current, equal to k2 x In (k2 ranges from 1.6 to 1.9) depending on the particular
fuse concerned.
A further factor k3 has been introduced ( k = k2
3 ) such that I2 y 1.45 Iz
1.45
will be valid if In y Iz/k3.
For fuses type gG:
In < 16 A → k3 = 1.31
In u 16 A → k3 = 1.10
Moreover, the short-circuit current breaking capacity of the fuse ISCF must exceed
the level of 3-phase short-circuit current at the point of installation of the fuse(s).
b Association of different protective devices
The use of protective devices which have fault-current ratings lower than the fault
level existing at their point of installation are permitted by IEC and many national
standards in the following conditions:
v There exists upstream, another protective device which has the necessary shortcircuit
rating, and
v The amount of energy allowed to pass through the upstream device is less than
that which can be withstood without damage by the downstream device and all
associated cabling and appliances.
G5
© Schneider Electric - all rights reserved