Safety transformer

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A safety transformer (also known as a protective transformer ) is a transformer that supplies low voltages on the secondary side for SELV and PELV circuits.

Symbol of a short-circuit-proof, closed safety transformer

Basics

The performance of a safety transformer is generally single phase not more than 10 k VA and multi-phase 16 kVA at a maximum frequency of 500 Hz. After consultation between the manufacturer and the customer, the standard also applies to transformers higher performance. The output voltage may also be a maximum of 50  V eff alternating voltage (AC) or 120  V with smoothed direct voltage (DC). This serves to fulfill the protective measure safety extra-low voltage . The input voltage must not be higher than 1100 V.

Safety transformers can be short-circuit proof or conditionally short-circuit proof. Conditionally short-circuit-proof transformers are usually equipped with fuses or overcurrent releases. A safety transformer requires reinforced or double insulation between the input and output winding and more stringent test conditions apply.

The high-voltage winding (primary circuit) is usually separated from the low-voltage winding (secondary circuit) with an additional insulating material partition, so that there is no connection between the primary and secondary side even in the event of a fault ("safe electrical separation"). Portable safety transformers must not be designed for operation on different primary voltages. Fixed safety transformers may only be switchable to other voltages with the aid of tools. The output side must not be earthed.

Safety transformers are identified by the symbol shown on the right for closed transformers. In the case of open transformers, the symbol is missing the upper line. In addition, the maximum ambient temperature and the insulation class are given. For example, a transformer with the designation T40B can be operated in an ambient temperature of up to 40 ° C and corresponds to insulation class B.

For isolation transformers are similar requirements (DIN EN 61558-2-4), but the output voltage between 50V is here eff 1000 V eff and the maximum power allowed max. 25 kVA single-phase or 40 kVA multi-phase. Upon agreement between the manufacturer and the customer, the standard also applies to transformers with a higher output.

Special safety transformers

  • Toy transformers (DIN EN 61558-2-7) may have a maximum output voltage of 24 V. Themaximum power is 200 VA. They arebuilt according to protection class III .
  • Bell transformers (DIN EN 61558-2-8) must be short-circuit proof and the output voltage must not exceed 24 V.
  • Hand lamp transformers (DIN EN 61558-2-9) must be insulated and splash-proof or watertight .
  • Thawing transformers are used to thaw frozen metallic water pipes . The input and output voltages may not exceed 250 V to 24 V. They have to be short-circuit-proof and fully insulated.
  • Transformers for medical devices may have a maximum output voltage of 24 V as soon as the device to be connected could come into contact with the human body. In the case of a device that is inserted into the body, for example an endoscope for gastroscopy, the voltage must not exceed 6 V. These transformers must be fully insulated.

Norms

The previous standards were VDE 0550 and VDE 0551, which were initially replaced by EN 60742 (VDE 0551). Meanwhile, the DIN EN 61558-1 (VDE 0570-1) applies to transformers <= 1000 V. Part DIN EN 61558-2-6 also applies to safety transformers. (VDE 0570-2-6).

literature

Individual evidence

  1. ↑ Transformer Basics from Transtech GmbH, accessed in February 2016