¿How to protect a transformer ?

The transformer must be protected against possible overcurrents or short circuits that may occur in the electrical grid, thus guaranteeing the correct operation of the installation, avoiding failures and their destruction.

The effectiveness of protections in transformers is essential to minimize the damage caused by faults of the transformer itself, and to avoid short circuits and overloads of external origin.  

Primary -> Against short circuits: The protections must be dimensioned according to the inrush current

Secundario -> Against overloads: The protections are dimensioned at the nominal current (In) as maximum 

Inrush current




1.5÷2xIn curve D o fuse gL

In curve C or fast fuse gR


In curve D o fuse gL

In curve C or fast fuse gR


In curve C o fuse gL

In curve C or fast fuse gR

*In = Rated current (A)

The nominal value must be less than or  equal to that obtained from the calculation of the nominal current of the transformer

SINGLE-PHASE: In = Nominal power / Nominal voltage

THREE-PHASE: In= Nominal power / ( √3 *Nominal voltage)

All transformers manufactured by POLYLUX have an inrush current <=12In since high quality magnetic cores are used and the degree of compaction is high. 

The standardized 8In products are:  

Clinical use transformers |  TH, THX, TTH, TTHX, TTHW 

Ecological transformers  | TTGZ, TTGW, TTDZ

At POLYLUX we have developed a tool that allows you to calculate the recommended protections for single-phase transformers and for three-phase transformers and autotransformers. 

Discover the tool here