The derating curve shows which currents may flow 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 influenced 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 tb1, tb2, … and the ambient temperatures tu1, tu2 are measured for three different currents I1, I2, I3, …
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. 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 tg of the component completes the system of coordinates. The associated average values of the temperature rise in the component, Δ t1 = tb1-tu1, Δ t2 = tb2-tu2, … are plotted for every current I1, I2, … 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.