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Entry type: FAQ Entry ID: 109744226, Entry date: 07/23/2019
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Differing current specifications for 3RF23 and 3RF24 solid-state contactors

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This entry contains information about the different current specifications for 3RF23 and 3RF24 solid-state contactors

QUESTION:

Different current specifications are given for 3RF23 and 3RF24 solid-state contactors. What are the differences and how should these current values be taken into account?



ANSWER:

The maximum currents that can be fed to solid-state contactors depending on the applicable mounting instructions.

As a general rule:
The most important current specifications for 3RF23 and 3RF24 solid-state contactors refer to the switching of resistive loads, which corresponds to utilization category AC-51 as laid out in EN 60947-4-3.
The permissible currents for solid-state contactors are highly dependent on the ambient temperature. The specifications given on the rating plate or data sheet refer to an ambient temperature of up to 40 °C for applications subject to IEC standards and up to 50 °C when used in accordance with UL standards. The corresponding current reductions required at higher ambient temperatures (derating) can be found in the data sheets.
The highest power losses, and therefore, the highest levels of heat generation, occur during continuous operation, such as in the heating-up phase of a heating unit. In the case of intermittent operation, i.e. the frequent switching on and off of the heating unit, significantly lower power losses occur, leading to a much lower level of heat generation.
The power losses for specific currents can be found in the technical data of the solid-state switching devices. They must then be taken into account during the thermal configuration of the control panel.


Maximum thermal configuration at 40 °C
The combination of the heat sink and the high performance of the installed semiconductor allows for very high currents to be switched and permanently carried in the solid-state contactors. If the capacity of the solid-state unit is utilized to its full extent, the heat sink also operates at very high temperatures. Some of the heating of the solid-state unit also results from the supply cables.
The very high temperatures at the heat sink (up to 100 °C) must be taken into account when positioning the solid-state switching devices. Even if the prescribed minimum distances for correct cooling are maintained, the immediate environment can be subject to considerable heat generation. The cables must also be designed to cope with high temperatures.
The maximum permissible currents should also not be exceeded during intermittent operation.


Rated current acc. to EN 60947-4-3 at 40 °C
Such high levels of heat generation do not occur at these current values. However, high ambient temperatures can also lead to proportionally higher levels of heat generation at the heat sink, something which must be taken into account when positioning the solid-state switching devices.


Rated current acc. to UL/CSA at 50 °C
This current value stems from the maximum thermal configuration, but is based on a higher ambient temperature. According to the UL/CSA approval, there are no restrictions regarding the overtemperatures on the solid-state contactors.



Key words
Imax; type current; IIEC