Chapter A General rules of electrical installation design

H14
© Schneider Electric - all rights reserved
H - LV switchgear: functions & selection
Short-circuit relay trip-current setting (Im)
Fig. H31 : Tripping-current ranges of overload and short-circuit protective devices for LV circuit-breakers
t (s )
Ir Im Icu
I(A
Fig. H32 : Performance curve of a circuit-breaker thermalmagnetic
protective scheme
4 Circuit-breaker
Type of Overload Short-circuit protection
protective protection
relay
Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to
trip the circuit-breaker rapidly on the occurrence of high values of fault current. Their
tripping threshold Im is:
b Either fixed by standards for domestic type CBs, e.g. IEC 60898, or,
b Indicated by the manufacturer for industrial type CBs according to related
standards, notably IEC 60947-2.
For the latter circuit-breakers there exists a wide variety of tripping devices which
allow a user to adapt the protective performance of the circuit-breaker to the
particular requirements of a load (see Fig. H31, Fig. H32 and Fig. H33).
Domestic Thermal- Ir = In Low setting Standard setting High setting circuit
breakers magnetic type B type C type D
IEC 60898 3 In y Im y 5 In 5 In y Im y 10 In 10 In y Im y 20 In (1)
Modular Thermal- Ir = In Low setting Standard setting High setting
industrial (2) magnetic fixed type B or Z type C type D or K
circuit-breakers 3.2 In y fixed y 4.8 In 7 In y fixed y 10 In 10 In y fixed y 14 In
Industrial (2) Thermal- Ir = In fixed Fixed: Im = 7 to 10 In
circuit-breakers magnetic Adjustable: Adjustable:
IEC 60947-2 0.7 In y Ir y In - Low setting : 2 to 5 In
- Standard setting: 5 to 10 In
Electronic Long delay Short-delay, adjustable
0.4 In y Ir y In 1.5 Ir y Im y 10 Ir
Instantaneous (I) fixed
I = 12 to 15 In
(1) 50 In in IEC 60898, which is considered to be unrealistically high by most European manufacturers (Merlin Gerin = 10 to 14 In).
(2) For industrial use, IEC standards do not specify values. The above values are given only as being those in common use.
t (s )
Schneider Electric - Electrical installation guide 2008
Ir Im Ii Icu
Fig. H33 : Performance curve of a circuit-breaker electronic protective scheme
I(A
Ir: Overload (thermal or long-delay) relay trip-current
setting
Im: Short-circuit (magnetic or short-delay) relay tripcurrent
setting
Ii: Short-circuit instantaneous relay trip-current setting.
Icu: Breaking capacity