What types of transformer are available? What are they used for?

Isolating transformer

The primary and secondary windings of an isolating transformer are electrically isolated using basic insulation. The objective is to reduce risk in the circuit fed by the secondary winding in the event of accidental and simultaneous contact with the earth and the active components. An isolating transformer ensures that the primary and secondary windings are electrically isolated.

Applications for isolating transformers include:

Conversion of voltages (e.g. 230V to 400V).

Changing the earthing system/neutral conductor regime (for example from TN-C to IT).

Safety isolating transformer

A safety isolating transformer is a variant on the isolating transformer design, which has double or enhanced insulation between the input and output windings in addition to the basic insulation, for additional safety.

It is used in similar applications to an isolating transformer. 

Safety transformer

A safety transformer is a safety isolating transformer all of whose secondary voltages are lower than 50V. The secondary voltage is therefore a Safety Extra Low Voltage (SELV). That is, a voltage lower than 50V.

The applications for safety transformers are the same as for isolating transformers, but the SELV and the double insulation provides an optimal guarantee of personal safety. 


Control transformer

Control transformers are isolating transformers which are typically used in machine control circuits. They are more robustly constructed to allow them to briefly deliver a multiple of their nominal capacity with a limited voltage drop.

Control transformers are typically found in circuits feeding components with a high operating current (switches, interlocks etc). Control transformers can also be used as ‘normal’ isolating transformers in other applications.

They can also be supplied in the safety transformer or safety isolating transformer formats.


Part of the primary and secondary winding is shared in an autotransformer. This reduces both weight and cost! The downside is that such transformers lack  galvanic isolation.

Autotransformers are suitable in circumstances where only conversion of the voltage is required.  The lack of galvanic isolation makes it impossible to use an autotransformer to modify the distribution network.

Because of the manner in which its magnetic circuit operates, a three-phase autotransformer must always be symmetrically loaded. In other words, the currents in the three lines must be identical. This limits the field of application of three-phase autotransformers to the feeding of symmetrical loads such as motors, compressors and the like.

Applications for autotransformers are limited to:

Conversion of voltages (e.g. 230V to 400V)