Electricity terminology

Expansion of low-voltage switchgear assemblies

Verification of heating by calculation

Theory and practice

Special cases: Photovoltaics and E-Mobility

Roles and their interfaces

Cooling capacity of enclosures


Electricity terminology

In accordance with IEC 61439, the documentation for low-voltage switchgear assemblies (LVSA) must contain a clear definition of the interfaces that connect them with their environment. These connections include, for example, the electrical system they are part of, as well as environmental conditions such as temperature and humidity, as well as the requirements for maintenance and operation.

In terms of capacity, these interfaces are defined by the following variables:


Operating currents

Short-circuit currents

Nominal current

The nominal current is a common characteristic of certain types of distributors, e.g. transformer cabinets. According to the international definition, the nominal current serves as an identification feature, for example as part of the product description. The nominal current is a rounded value which is clear, uniquely identifiable and comparable. In the past, the rated current of the input device was often used as the nominal current for the distributor.

The rated current of the LVSA that has to be stated as part of the item designation is any current that can actually cause the equipment to reach the heating limit. It is dependent on the type of connection (via NH switch disconnector fuse or direct), the type and size of enclosure chosen, as well as the type of installation and installation location.

rated current Inc of the circuits and rated diversity factor (RDF)

The rated current  Inc of a circuit in an LVSA is the current which a single circuit can carry permanently without exceeding specified limit temperatures (defined in various standards) if no further outgoing line is loaded.

In practice, an outgoing line is seldom loaded alone. There is a specific, common loading of multiple circuits which is dependent on the application case, and is taken into consideration in the installation regulations as a simultaneity factor for the design of the electrical system. If, for example, five flats are supplied by one main distribution system, then these flats are to be rated with a connected load of 18 kW each, in accordance with TAEV (as fully-electrified residential units). This corresponds to a current of about 26 A for three-phase balanced loads. Furthermore, with five residential units a simultaneity factor of 0.4 – 0.45 must be applied, through which the total expected current is reduced from 5 x 26 A = 130 A to a maximum of 130 A x 0.45 = 58.5 A.

In the classification of the LVSA, the RDF (Rated Diversity Factor) describes the simultaneity. Hence, in the example of the five residential units, a distributor must be dimensioned with circuits of Inc = min. 26 A and an RDF of min. 0.45.

When verifying the heating of the LVSA by testing, Inc is verified on the one hand for each outgoing circuit of different dimensioning, and on the other hand for the entire LVSA. In the second case, the outgoing lines are operated with an operating current that is calculated using Inc and the RDF: IB = Inc x RDF, i.e. in the above example, 26 A x 0.45 = 11.7 A.

rated current InA of the low-voltage switchgear assembly

The rated current InA of the LVSA is the total current in the supply – or in larger systems, in the sum of all existing supplies – that the LVSA can also distribute without exceeding the temperature limits. This rated current is limited by the rated current of the devices used in the supply and/or the current carrying capacity of the bus bars at the ambient temperature that sets in. However, at higher temperatures inside the cabinet, degradation factors must be taken into account for the equipment. In addition, the InA may also be limited by the limit temperatures. This is especially the case if the supply is housed along with many outgoing lines in a rather small enclosure, making it impossible for thermal reasons to distribute the full operating current in all outgoing circuits. In the application example mentioned earlier with the five residential units, the distributor must be designed for a rated current InA of at least 58.5 A and for the safe operation of the system must not exceed this value permanently.


Expansion of low-voltage switchgear assemblies

Conversely, the sum of all operating currents may also be smaller than InA which is then only defined by the supply units. In this case, the distributor may be supplemented with further outgoing lines to be able to distribute more energy. However, several points must be observed here:

InA itself cannot be increased by adding further outgoing lines.. If InA is fully consumed or exceeded by the sum of the theoretically possible operating currents, it must be ensured that the distributor cannot be overloaded when additional outgoing lines are added. The user is responsible for compliance with the specified rated currents of the switchgear assembly. The manner in which compliance is ensured is not covered by IEC 61439. However, it is recommended to use appropriately dimensioned overcurrent protection devices.


Verification of heating by calculation

If compliance with the temperature limits of a type of switchgear assembly is demonstrated by calculation, then a reduction of the rated current must be taken into consideration for all current-carrying equipment in order to prevent localised overheating. These so-called “hot spots” cannot be determined by the calculation. Therefore, a limit value is stipulated through the reduction of the rated currents of all equipment used to 80% of its original value, which allows the experience from the test results to be incorporated. If the equipment manufacturer specifies an even higher degradation for the ambient temperature to be expected (e.g. 72% at 60°C internal cabinet temperature), then this degradation should be used. A calculation is always performed with the rated current Inc. The specification of an RDF is then not required (or RDF = 1).


Theory and practice

Rated currents for switching devices are permissible continuous currents that also may occur continuously for 24 hours a day. In practice, there are rarely applications that create such a continuous load. It can be concluded from laboratory tests that short-term, high loads that occur for only a few minutes are not a problem due to the sluggish heating processes in conventional distributors. It is not easy to give general guide values, as these are highly dependent on the equipment that is used (e.g. circuit breakers or NH fuse panels), the enclosure size and the complexity of the inner partitions, such as contact protection covers, equipment covers, etc..

It is neither technically nor economically feasible to dimension a distributor so that the short peak loads could be conducted as a continuous current. For this reason, IEC 61439 also points to options permitting, for example, intermittent operation to be converted to an equivalent continuous current.

Loads at the level of the rated currents applied over one hour or longer already lead to heating in many products that corresponds to at least 70% of the expected final temperatures when permanently loaded with the rated current. But even this is only a rough guide, which in turn depends on the equipment, conductor cross-sections, enclosure dimensions etc. Therefore to be able to provide an optimal product, a great deal of detailed information must be available during the planning phase of the switchgear assembly. The more details that are known about the characteristics of the loads, the selected conductor cross-section for incoming and outgoing lines, and the planned fusing, the better the switchgear assembly can be tailored to the application case. Unknown parameters that are necessary for the dimensioning must be assumed during the planning, and will then produce the respective rated values that are shown in the technical documentation of the products and must eventually match the actual usage. Some types of LVSA, particularly those that are not very complex in terms of design, can be used in many different operating cases, which cannot all be fully depicted by means of the information in accordance with IEC 61439. A good example of this is cable distribution cabinets or main fuse boxes, which consist only of a copper rail system and NH fuse panels or fuse switch disconnectors. The rated currents of these products are only partially dependent on the dimensions of the copper rail used. The fuse inserts, for example, have much more influence. Depending on the type of connection, the use of separating blades or fuse inserts, the presence of a loop-through possibility, and the number and type of outgoing lines (this also includes the supply line and outgoing line cross-sections used and the related fusing), different rated currents can result for the switchgear assembly for the same enclosure with identical copper rails. The maximum internal enclosure temperature at which the equipment used may be operated and the specified environmental conditions also play an influential role here, particularly in terms of an outdoor installation, which can and must differ from the “normal environmental conditions” in accordance with IEC 61439.


Special cases: Photovoltaics and E-Mobility

Current developments in the area of electric energy supplies are leading to load cases that are not “normal” and must also be observed in the design of LVSAs. The two most common cases are:

decentralised feeders such as photovoltaic systems, wind turbines, CHP plants
supply of charging infrastructure for electric vehicles

The energy turnaround in Europe is leading to decentralisation of the production of electrical energy. This means that power plants are no longer connected in the higher network levels, but rather increasingly in the distribution network level and hence in the low-voltage distribution. More and more, larger systems contain switchgear assemblies which connect both outgoing lines for consumers and feed-ins from generating systems to a grid connection and energy metering. When dimensioning these products, it must be ensured especially for photovoltaic systems that the decentralised feed-in can continuously supply the full rated capacity over an extended period of time (several hours) and deliver it to the distributor. Therefore the dimensioning of RDF = 1 must be provided for such products. Likewise, consideration must be given to the additional power source in the event of a short circuit.

Dimensioning of the final electric circuits with a simultaneity of 1 is already stipulated for the supply of electric vehicles. This is also valid for the upstream energy distribution, unless appropriate measures for load control are available. Moreover, for outgoing circuits that are designed for charging electric vehicles, the special feature applies that due to possible manufacturer specifications, overcurrent protection must be used with a nominal current in the amount of the expected rated current (e.g. 32 A for a 3-phase charging point with 22 kW). This means that the equipment will be operated at the limit of its nominal capacity and therefore the heating of the component is at a maximum. After consultation with the manufacturer(s), it is recommended to provide higher dimensioning in order to improve the heating behaviour. If this is not possible, then the continuous load can be addressed with design measures, for example.

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Roles and their interfaces

One of the main content elements in IEC 61439 is the division of roles with clearly defined duties for each role. The following examples are intended to clarify the role distribution and thus describe the obligations in each case.


The 61439 series of standards does not recognise this term. However, the component manufacturer is an important role in a business interaction. The manufacturer provides the components that can be assembled into an LVSA. A component is a part with its own product base standard, for example a single switching device (IEC 60947, IEC 60269, etc.), an empty enclosure (IEC 62208), a meter plate, etc. These components must be distinguished from system parts that are already manufactured in accordance with IEC 61439 (or parts thereof), or have been tested for their use, but are still not finished LVSAs.


The original manufacturer of an LVSA manufactures precisely these system parts which may in themselves already constitute an LVSA, but are not yet finished, that is they must not to simply be connected just as they are. This actually refers to mass production parts, such as, for example, a tested rail system1, an assembly on a mounting plate as an “insert”, and so on. The original manufacturer does not have to actually supply this mass-produced part at all, but can also outsource the manufacturing (e.g. to the manufacturer of the LVSA). But the original manufacturer is responsible for the planning (design) of the system part, and must confirm conformity with the standard by means of the points of the design certificate.

1 A rail system is understood to be a tested system made of copper rails and rail holders. Requirements from IEC 61439 may already be defined for such a system, such as the insulation strength and resistance to short-circuiting.


The manufacturer is the player that takes over responsibility for the finished LVSA, i.e. the last person to make a change to it before it is put into operation. If the manufacturer of the low-voltage switchgear assembly only puts system parts together whose type has already been verified and for which the verification also includes the assembly (including correct wiring, enclosure etc.), then the manufacturer of the LVSA only has to test by means of the item verification that no mistakes were made during assembly, and all the requirements from the type design have been complied with (e.g. air gap and creepage distances, IP protection rating, etc). If in addition to the system parts from original manufacturers other components are added to the finished LVSA (these can also be several different ones), then the manufacturer for this part of the LVSA must provide the design certificate. If the manufacturer of the (finished) LVSA does not use a single system component (module) from an original manufacturer, but only uses components, it then also assumes all the obligations of the original manufacturer, but still “formally” remains the manufacturer of the LVSA.

The manufacturer of the LVSA is responsible for the labelling of the product. This also includes the rating plate and the necessary data on this, and also the manufacturer’s mark and the relevant standard. Similarly, the manufacturer is responsible for the conformity of the finished product with the relevant EU directives. Only the manufacturer performs the conformity assessment procedure for the finished LVSA and affixes the CE mark on the product.


According to the standard definition, the users are the parties that “specify, buy, use and/or operate” the low-voltage switchgear assembly, or “someone acting on their behalf.” Thus, this definition does not comprise a particular player, but rather a group of possible players: an operator, an installer, a planner (call for tenders = specification). All these players take on certain obligations of the user role. However the correct and sufficient specification is particularly important. It enables the manufacturer of the LVSA to assemble the appropriate low-voltage switchgear assembly from system parts from the original manufacturers and/or components. Only when all the necessary data are specified can this selection of the parts be made correctly. If certain data are not specified, then the manufacturer of the LVSA can define these and they are then considered to have been agreed with the user (this also applies to information in sales documents such as catalogues, for example). The agreed specifications must be adhered to by the user, because otherwise the low-voltage switchgear assembly will be operated in an inadmissible operating range, which can lead to damage or failure.



Installation and environmental conditions

Operation and maintenance

Connection to the electrical grid

Electric circuits and consumers


Document handover and archiving


The component manufacturer is obliged to manufacture the respective components in accordance with their product standard and provide the corresponding documentation. This is added by the manufacturer of the LVSA to its design certificate. The operating manual, maintenance instructions or parts thereof can be made available to the user as part of the documentation, where this makes sense. The documentation also includes correct and complete marking on the device as well as the declaration of conformity with the applicable European EU directives (CE).


The manufacturer of the low-voltage switchgear assembly uses the installation or assembly instructions of system components from an LVSA system for the production of the complete product and as a specification for the item verification. The manufacturer prepares a design certificate for all subsequent manufacturing steps which are not carried out in accordance with the assembly instructions of these system parts. This design certificate also remains with the manufacturer of the LVSA. After completion of the item verification, the LVSA is labelled (rating plate, CE marking, etc.).

The manufacturer of the LVSA is also responsible for preparing the documentation. In addition to declaring the identifying features of the interfaces (e.g. as a datasheet), this documentation also consists of instructions on the handling (transport), installation, operation and maintenance of the LVSA and the equipment contained inside it. Circuit diagrams and terminal diagrams only have to be part of the documentation if the circuitry is not clearly evident from the structural arrangement of the installed devices. This documentation must be made available to the user. The identifying features (datasheet) must be supplied with the LVSA as minimum documentation.


Cooling capacity of enclosures

Only a certain limited amount of power dissipation can be incorporated in an enclosure so that the temperature inside the enclosure remains below the permissible limit. As an internal definition, the cooling capacity of the enclosures from ELSTA is specified as the capacity that can be introduced in order to limit the difference between the outdoor temperature and indoor temperature (ΔT) to ΔT = 20°C. This means that at an outside temperature of 35 °C, the internal temperature is a maximum of 35°C + 20°C = 55°C. Support by means of passive or active cooling measures (e.g. ventilation grilles, fans) is not taken into account.

 The level of the maximum internal temperature is determined by means of the allowable operating temperature range of the installed devices. Typical values are 40, 55 or 70°C, depending on the type and robustness of the installed devices. Likewise, in outdoor areas particular attention must be paid to the lower permissible temperature limit of the installed devices (-5°C or -25°C respectively).

 The heat dissipation from the enclosure to the outside depends on many parameters. But it generally applies that with a temperature difference of ΔT = 20 °C, neither the difference between the protection ratings IP 44 and IP 54, nor the material of the enclosure, plays a significant role. There is no disadvantage in the heat dissipation behaviour from using plastic as a material for the enclosure. At higher temperature differences between the internal cabinet temperature and outdoor temperature, the protection rating and the effect of passive ventilation measures become more noticeable.

 The temperature difference can also be approximately adapted for other ambient temperatures. At an outdoor temperature of 25°C, an internal temperature of about 45°C would therefore set in with the given cooling capacity.