According to the standard EN/IEC 61558-1, low-voltage transformers are categorized – among others – in isolating transformers, where output voltage is above 50V (part EN/IEC 61558-2-4) and safety isolating transformers when output voltage is under 50V (part EN/IEC 61558-2-6).
The very notion of the isolating transformer lies on the fact that when the primary winding is galvanicly separated from the secondary, there is no danger of electocution for the end user when the user touches one of the two output lines that are connected to the loads; this is because since the voltage that is produced in the secondary winding is based on the phenomena of mutual induction and there is no “phase”. As *known, when the end user of an installation comes in touch with a phase of the network, then the circuit between him and earth closes, thus electocuting him. These transformers are also called galvanic isolation or galvanic protection transformers.
Safety isolating transformers provide both the galvanic isolation as mentioned above and the safety voltage SELV (Safety Extra Low Voltage), up to 50V.
Another category, similar to the above mentioned, are control transformers which are used in applications like auxiliary circuits, electical panels etc. The difference between these transformers and the ones mentioned above is, that according to the standard that are based on (part EN/IEC 61558-2-2), safety requirements are reduced due to the fact that they are incorporated in a general electrical circuit so there is no danger for the end user to touch them.
In low voltage transformers, are also included the naval transformers which are used to transform the ship’s generator voltage and supply loads such as lighting (e.g. 440V of the generator to 230V).
Another special category of low-voltage transformers is the isolating transformers for medical locations (part EN/IEC 61558-2-15) which are installed mostly in operating rooms, ICU etc. The most important feature of these installations is the uninterrupted supply of the loads. For this reason, they are combined with an insulation and temperature monitor on order to have an alert in case of first fault.
In another special category we find transformers for construction sites (isolating and safety isolating) which are portable in order to provide the necessary security in the electrical supply of construction site loads, such as electrical tools.
From all the above, it is made clear that most of the applications relate to cases where the electrical load may come in touch with the end user through a transformer. The quality of manufacture should satisfy the required safety levels as they are defined in the relative standards. This requirement is only met when selecting certified transformers with a recognized quality label (e.g. TUV, VDE, KEMA, etc.). This applies for everyone, including the electricians who reasonably do not have specialized knowledge of manufacturing transformers.
The production lines of certified transformers are checked regularly by the certification body according to strict standard procedures and this ensures that the produced transformers always meet the standard safety and other requirements.