Electromagnetic transformers

Low-voltage halogen lamps operate at voltages of under 24 V. Owing to this low voltage, all Vossloh-Schwabe transformers are safety transformers. Safety transformers are isolation transformers for supplying SELV (safety extra low voltage) and PELV (protection extra low voltage) circuits. With such systems, the voltage must not exceed a value of 50 V AC or 120 V DC (smoot hed) between the conductors or a conductor and the earth conductor of a circuit that is separated from the mains by a safety transformer. The stated values apply for non-touchable parts only, for touchable parts values are 24 V AC and 34 V DC.
Owing to the low internal impedance of electromagnetic transformers, high currents can occur in the event of a short-circuit on the secondary side, which can lead to the transformer being destroyed. For this reason, IEC 61558-1 differentiates between three types of transformer:
Not short-circuit proof transformers
These transformers require external protection to prevent excessive temperatures being generated.
At Vossloh-Schwabe, these transformers are marked with the symbol "not short-circuit proof safety transformer". To protect against current overload during over load or short-circuit operation, Vossloh- Schwabe recommends installing a fuse on the primary side. As an aid to the user, the rating of this fuse is stated on the type plate in accordance with IEC 60127. The installed primary-side fuse should be easily accessible so that it can be readily replaced at any time.
Short-circuit proof transformers (limited short-circuit resistance)
These transformers feature a safety device that prevents excessive temperatures being generated.
Electromagnetic transformers with thermal cut-outs afford a limited degree of short-circuit resistance and do not need to be additionally fused. VS safety transformers of limited short-circuit resistance are designed to safely cut out in the event of overload or short-circuit, but not to restart automatically after cooling off. The transformer must first be disconnected from the mains (i.e. switched off and on) before it can be restarted. The thermal cut-outs are dimensioned to ensure that the maximum permissible winding temperature of 225 °C (transformers of thermal class B) or 240 °C (F) or 260 °C (H) is not exceeded in the event of overload or short-circuit.

Short-circuit proof transformers (unlimited short-circuit resistance)
These transformers are designed to ensure that fixed maximum temperatures are not exceeded in the event of overload or short-circuit.
This type of safety transformer is not in use within the lighting industry as it requires relatively large dimensions to meet the overload and short-circuit requirements.
All transformers will function perfectly and meet the requirements of the standard after the overload or short-circuit has been eliminated.
In addition to the above, there are also so-called fail-safe transformers that are rendered permanently in operative in the event of improper use, but do not pose a threat to the user or the surroundings. Vossloh-Schwabe does not provide this type of isolation transformer.
All Vossloh-Schwabe transformers are tested for compliance with the safety requirements of European standard EN 61558 regarding creepage and air clearance distances, the winding temperature and the maximum permissible ambient temperature (ta).
EN 61558 specifies five insulation classes for electromagnetic transformers; respective testing temperatures and times are assigned to this insulation system. Due to the quality of the insulation materials used by Vossloh-Schwabe, VS transformers are only available in the three highest insulation classes B (120 °C), F (140 °C) and H (165 °C). In this case, the quoted temperature refers to the maximum permissible winding temperature during permanent operation.
As luminaire casings made of plastic or sheet metal will discharge heat to varying degrees and because transformer installation conditions can differ, a transformer´s winding temperature must be tested within the luminaire. The measured values will show whether the maximum temperature complies with the transformer´s insulation class.
On request, Vossloh-Schwabe can carry out such luminaire tests to assess built-in components.