Brochure/Catalogue

Electrical data
The derating curve shows which
currents may ow continuously and
simultaneously via all possible
connections when the component
is subjected to various ambient
temperatures below its upper limit
temperature.
The upper limit temperature of a
component is the rated value
determined by the materials used. The
total of the ambient temperature plus
the temperature rise caused by the
current load (power loss at volume
resistance) may not exceed the upper
limit temperature of the component,
otherwise it will be damaged or even
completely ruined.
The current-carrying capacity is hence
not a constant value, but rather
decreases as the component ambient
temperature increases. Furthermore,
the current-carrying capacity is
inuenced by the geometry of the
component, the number of poles and
the conductor(s) connected to it.
The current-carrying capacity is
determined empirically according
to DIN IEC 60512-3. To do this, the
resulting component temperatures
t
b1
, t
b2
and the ambient temperatures
t
u1
, t
u2
are measured for three different
currents I
1
, I
2
.
The values are entered on a graph
with a system of linear coordinates to
illustrate the relationships between the
currents, the ambient temperatures and
the temperature rise in the component.
Derating curve (current-carrying capacity curve)
The loading currents are plotted on
the y-axis, the component ambient
temperatures on the x-axis.
A line drawn perpendicular to the x-axis
at the upper limit temperature t
g
of the
component completes the system of
coordinates.
The associated average values of the
temperature rise in the component, Δ t
1
= t
b1
– t
u1
, Δt
2
= t
b2
– t
u2
, are plotted
for every current I
1
, I
2
to the left of the
perpendicular line.
The points generated in this way are
joined to form a roughly parabolic
curve.
As it is practically impossible to choose
components with the maximum
permissible volume resistances for the
measurements, the base curve must be
reduced.
Reducing the currents to 80 % results
in the “derating curve” in which
the maximum permissible volume
resistances and the measuring
uncertainties in the temperature
measurements are taken into account
in such a way that they are suitable for
practical applications, as experience
has shown. If the derating curve
exceeds the currents in the low
ambient temperature zone, which is
given by the current-carrying capacity
of the conductor cross-sections to be
connected, then the derating curve
should be limited to the smaller current
in this zone.
t
g
= maximum temperature of component
t
u
= ambient temperature
I
n
= current
t
g
= maximum temperature of component
t
u
= ambient temperature
I
n
= current
a = base curve
b = reduced base curve (derating curve)
Base curve
max. temperature
of component
Derating curve
W
Technical appendix/Glossary
W.212729490000