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Operating low-voltage halogen lamps depends on operating devices that transform the usual mains voltage of 230 V to under 24 V. Safety transformers, of either electromagnetic or electronic (converter) design, have been in almost exclusive use for several years now. The type plate of electromagnetic transformers bears the symbol for safety transformers in accordance with VDE 0570, corresponding to EN 61558. Electronic converters are marked with the sign for Safety Extra-Low Voltage (SELV), which indicates that the product is an isolating converter whose secondary output is safe to touch even during no-load operation.
All Vossloh-Schwabe transformers are safety transformers, i.e. 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 (smoothed) between the conductors or a conductor and the earth conductor of a circuit that is separated from the mains by a safety transformer. The specified values apply for protected (non-touchable) voltages; 25 V AC and 60 V DC (smoothed) apply for exposed (touchable) voltages.
Depending on their design features to protect against touchable live parts, transformers and converters fall into one of two protection classes. Operating devices of protection class I are base-insulated and have a protective earth conductor connection terminal that must be connected to the protective earth conductor for safety reasons. Isolating transformers and converters of protection class II are equipped with double or reinforced insulation that protects against dangerous casing currents; these operating devices are solely available as independent operating devices (also see page 508; Protection Classes of Luminaires and Operating Devices).
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Electronic converters can also be fitted with a functional earth terminal that must be connected to a functional earth to ensure compliance with EMC requirements. In addition, some electronic converters are designed in such a way that neither a protective earth conductor nor a functional earth needs to be connected.
Operating devices can also be differentiated according to the way they are used. Built-in transformers have to be installed in a permanent casing, e.g. a luminaire. In contrast, so-called independent transformers and converters can be operated independently of a luminaire. These are often found in ceiling installations; in order to prevent possible noise development, isolation transformers must be mounted in such a way as to avoid vibration transmission.
Transformers or converters bearing the MM mark can be mounted on surfaces of unknown flammability, which can be the case when mounting these devices on wooden furniture elements. Such devices comply with the temperature requirements of VDE 0710, part 14, of < 95°C during normal and < 115°C during abnormal operation.
Converters are labelled with a tc point. The stipulated temperature (e.g. 75°C) must not be exceeded when installed so that the service life of the converter is not shortened. The temperature quoted in the triangle (e.g. 110) denotes that the surface of the converter must never (even in the event of a defect) exceed this temperature.
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