Control panels need to be designed and built in a way that enables them to withstand thermal and dynamic loads caused by potential short-circuit currents, up to the specified limit values and under acceptable conditions.
Suitable protection must therefore be taken against short-circuit currents. Upstream overcurrent protective devices can be located both inside and outside control panels.
Tips for you
Does the short-circuit strength of control panels have to be verified?
There are two key standards for control panels within IEC standardization:
- IEC 60204-1 Electrical equipment of machines
- IEC 61439-1 Low-voltage switchgear and controlgear assemblies
Whereas IEC 61439-1 describes the options for verifying short-circuit strength in detail, IEC 60204-1 does not include any verification procedure. Other standards such as IEC 61439-1 are referenced as an option; this includes a note to the effect that the planner is free to decide whether, in addition to the requirements of IEC 60204-1, they take into account the relevant parts of the IEC 61439 series for particular elements of the electrical equipment, depending on how the machine is used and what electrical equipment it has.
What options are there for verifying short-circuit strength?
As far as the IEC 61439-1 standard is concerned, there are two methods for verifying short-circuit strength:
- Verification by testing
- Verification by comparison with a proven reference design
An inspection is therefore a prerequisite in both cases, either of parts of the control panel or the control panel in its entirety.
Note: Siemens offers comprehensive, proven reference designs for carrying out verification by comparison, depending on the structure of the circuits of the switchgear and controlgear assembly in question. You can find these in the "Siemens Industry Online Support" (SIOS) by searching for "Short-circuit ratings as a function of minimum enclosure size in accordance with IEC 61439-1".
Is there a way of preventing the complex task of verifying short-circuit strength in accordance with IEC 61439-1?
Yes, the standard describes 3 scenarios in which the verification procedure may be prevented:
- Switchgear and controlgear assemblies with rated short-time withstand current Icw or rated conditional short-circuit current Icc with maximum 10 kA rms value.
- Switchgear and controlgear assemblies or circuits of switchgear and controlgear assemblies protected by current-limiting devices whose let-through current at the highest permissible, uninfluenced short-circuit current at the terminals of the supply to the controlgear assembly does not exceed 17 kA. In practice, this is a common scenario. In order to be able to verify this, the let-through characteristics of the individual protective devices must be analyzed.
- For auxiliary circuits:
UN ≥ 110V, PN ≤ 10kVA and uk ≥ 4%
UN ≥ 110V, PN ≤ 1.6kVA and uk ≥ 4%
Note: The free "SIMARIS curves" software tool from Siemens contains a wide range of overcurrent protective devices including all key characteristics. The tool can be used to not only determine, but also to easily document the let-through value for the relevant short-circuit current.