4Siemens · 10/2015
Introduction
Fuse Systems
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Overview
Rated voltage Un
The rated voltage is the designated voltage of the fuse and is
used to determine its test conditions and operational voltage
limits.
For LV HRC and SITOR fuse links, the rated voltage is always the
rms value of an AC voltage.
For wind power plants and some industrial applications, a higher
voltage tolerance is demanded of the LV HRC and SITOR fuses
than the tolerance of +5 % defined in the standard. On request,
you can obtain a manufacturer's declaration for the rated voltage
of 690 V +10 %.
In the case of NEOZED and DIAZED fuse links, a distinction is
made between AC and DC voltage values.
Rated current In
The rated current of a fuse link is the designated current of the
fuse link and is the current up to which it can be continuously
loaded under prescribed conditions without adverse affects.
Rated frequency
The rated frequency is the frequency for which the fuse link
is rated with regard to power dissipation, current, voltage,
characteristic curve and breaking capacity.
Selectivity
Several fuses are usually connected in series in a system. Selec-
tivity ensures that only the faulty electric circuit and not all oper-
ating processes are interrupted in a system in serious cases.
Siemens fuses of operational class gG, at an operational voltage
of up to 400 V AC and a ratio of 1:1.25, are interselective, i.e.
from rated current level to rated current level. This is achieved by
means of the considerably smaller band of scatter of ± 5 % of
the time/current characteristics, which far exceeds the demand
for a ratio of 1:1.6 specified in the standard.
It is therefore possible to use smaller conductor cross-sections
due to the lower rated currents.
Breaking capacity
The rated breaking capacity is the highest prospective short-
circuit current Ip that the fuse link can blow under prescribed
conditions.
A key feature of these fuses is their high rated breaking capacity
with the smallest footprint. The basic demands and circuit data
for tests – voltage, power factor, actuating angle, etc. – are
specified in both national (DIN VDE 0636) and international
(IEC 60269) regulations.
However, for a constant fail-safe breaking capacity, from the
smallest non-permissible overload current through to the highest
short-circuit current, a number of quality characteristics need to
be taken into account when designing and manufacturing fuse
links. These include the design of the fuse element with regard
to dimensions and punch dimension and its position in the
fuse body, as well as its compressive strength and the thermal
resistance of the body. The chemical purity, particle size and
the density of the quartz sand also play a key role.
The rated breaking capacity for AC voltage for NEOZED fuses –
and the majority of DIAZED fuses – is 50 kA, and in the case of
our LV HRC fuses (NH type), it is even 120 kA. The various type
ranges of SITOR semiconductor fuses have different switching
capacities ranging from 50 to 100 kA.
.
Faster arcing and precise arc quenching are the requirements for a
reliable breaking capacity.
Operational classes
Fuses are categorized according to function and operational
classes. The first letter defines the function class and the second
the object to be protected:
1st letter
a = Partial range protection
(accompanied fuses):
Fuse links that carry currents at least up to their specified rated
current and can switch currents above a specific multiple of their
rated current up to their rated breaking current.
g = Full range protection
(general purpose fuses):
Fuse links that can continuously carry currents up to at least their
specified rated current and can switch currents from the small-
est melting current through to the breaking current. Overload
and short-circuit protection.
2nd letter
G = Cable and line protection
(general applications)
M = Switching device protection in motor circuits
(for protection of motor circuits)
R, S = Semiconductor protection/thyristor protection
(for protection of rectifiers)
L = Cable and line protection
(in acc. with the old, no longer valid DIN VDE)
B = Mine equipment protection
Tr = Transformer protection
The designations "slow" and "quick" still apply to DIAZED fuses.
These are defined in IEC/CEE/DIN VDE.
In the case of "quick" characteristics, the fuse blows in the break-
ing range faster than those of operational class gG.
In the case of DIAZED fuse links for DC railway network protec-
tion, the "slow" characteristic is particularly suitable for switching
off direct currents with greater inductance. Both characteristics
are also suitable for the protection of cables and lines.
Full range fuses (gG, gR, quick, slow) reliably break the
current in the event of non-permissible overload and short-
circuit currents.
Partial range fuses (aM, aR) exclusively serve short-circuit
protection.
© Siemens AG 2016