EN 61558-2-4, -2-2, -2-1
The German standard EN 61558 with the VDE classification VDE 0570 is the German edition of the international standard IEC 61558 (Safety of power transformers, power supply units and similar) and has fully replaced the previous standards VDE 0550 and VDE 0551.
Some transformers are subjected to more stringent manufacturing and testing conditions in view of these changes.
Transformers for general applications always have double or strengthened isolations for SELV voltages (exposed conductive parts, maximum AC 50 V or DC 120 V) i.e. these transformers are exclusively safety transformers.
In addition, all transformers have been marked with information about safety elements which can be used to protect them against short-circuits and overloads.
The SITAS transformer series includes the combined characteristics of safety, isolating, and control or mains transformers i.e. the transformers are suitable for (virtually) all applications. SITAS transformers fulfill the highest requirements (and with regard to safety the most stringent requirements) of the transformer designs contained in this catalog. A SITAS transformer is always the right one for all applications.
Rated power Pn for high ambient air temperatures – the characteristic for thermal rating
Reference conditions used under which the transformers can be subjected to the rated power Pn listed in the selection tables:
- Continuous operation Pn
- Frequency AC 50 Hz ... 60 Hz
- Degree of protection IP00
- Site altitude up to 1000 m above sea level and
- Ambient temperature ta, type dependent 40 °C or 55 °C.
Other installation and operating conditions than this will affect the permissible continuous load capacity. If the ambient temperature is 30 °C, the 4AP transformers, for example, can operate with 8 % more load (see load characteristic).
Short-time rating Pn(S6) of control transformers – the characteristic variable for the dynamic capacity
The most important selection criterion for control transformers is their short-time rating Pn(S6).
This is required for switching on electromagnetic loads, e. g. contactors with high making current in relation to the holding current. According to EN 61558-2-2 "Special requirements for control transformers“ the output voltage with this load should not drop more than 5 % in relation to the rated voltage in order to ensure safe switching.
Depending on their application, 4AP and 4AU control transformers ≤ 16 kVA are especially optimized for high short-time ratings with comparatively low ratings and thus small size.
Low inrush current primary side short-circuit and overload protection with standard circuit-breakers
The 4AP and 4AU three-phase transformers for the power range ≤ 16 kVA have been designed for protective devices which reliably protect the transformers against short-circuits or overloads.
Standard 3RV and 3VF circuit-breakers offer optimum protection. In this way, the transformers are protected on the primary side against both short-circuits and overload, without the possibility of nuisance tripping on startup. The low inrush current, the short-circuit current and the thermal load capacity on overload are matched to the tripping characteristics of the circuit-breakers.
It is also possible to protect the transformers on the secondary side against short-circuits or overloads with circuit-breakers or miniature circuit-breakers with C characteristics.
Note: The specified primary side circuit-breakers are for protecting the primary side of transformers in the event of short-circuits and overload on the secondary side. In the event of a possible short-circuit on the feeder lines between the protective device and the primary side of the transformer, the rated short-circuit breaking capacity of the circuit-breaker must be taken into account with regard to the maximum possible prospective short-circuit current at the place of installation. For device assignments, see table in the "Technical data".
All three-phase transformers 4AP and 4AU are supplied for screw mounting on a mounting plate.
The 4AP transformers are supplied for rated currents up to 60 A, the 4AU transformers for rated currents up to 43 A, and the 4BU transformers up to 81 A in the standard design with screw-type terminals.
For higher currents, the transformers are supplied with flat-type terminals or threaded pins.
4AP and 4AU SIDAC-T transformers can be supplied on request in protective enclosures to degree of protection IP23 and IP54.