Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 127)

4.1.3.3.3 Selectivity
From the point of view of the operational safety and reliability of an entire low-voltage installa-
tion, it is usually desirable to specifically isolate the part of a system affected by a short-circuit in
order to prevent spreading of the fault. Selectivity is intended to ensure that the protective
shutdown is as close as possible to the location of the fault so that unaffected installation
components can continue to operate normally. This is often also desired for safety reasons and
in IEC 60439-1 (low-voltage switchgear assemblies) addressed for installations that require a
high level of continuity in current supply.
In buildings and industrial plants, radial distribution networks are the norm. In radial distribution
systems there are several protective devices in series, usually with decreasing rated currents
from the supply end to the load end. While the operational currents decrease from the supply
end to the load end, in the event of a short-circuit the same fault current will flow through all the
protective devices connected in series. By a cascading of the trip characteristics it must be
ensured that only the respective protective device that is closest to the location of the fault is
activated and hence the fault is selectively limited to the smallest possible part of the installa-
tion.
The basic prerequisite for selectivity of protective devices connected in series is that the trip
characteristic of the downstream (closer to the load) protective device is faster than that of the
upstream device. And all this taking into account all tolerances and over the entire current range
up to largest prospective short-circuit current.
Special attention should be paid to the area of high overcurrents, where the effects of current
limitation and breaking times are significant. Thus an upstream fuse does not operate if the
entire I
2
t of the downstream protective device (fuse, circuit breaker) is smaller than the melting
I 2 t the fuse. An upstream circuit breaker on the other hand does not operate if the maximum cut-
off current I
D
of the downstream protective device is smaller than the activation value of its
magnetic release.
In individual cases, reference to manufacture documents and frequently the technical support of
the manufacturer is required for the correct selection of devices. The basic facts are presented
below.
Selectivity between fuses connected in series
Fuses connected in series act selectively if their time current-characteristic curves have
sufficient mutual spacing and their tolerance bands do not touch (
Fig. 4.1-19 ).
Time
t
Current I (r.m.s.)
Fig. 4.1-19
Selectivity between fuses connected in series
LVSAM-WP001A-EN-P - April 2009 4-19