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FAQ power transformers

Is the protection of power transformers standardized?

Partly. IEC 60 787 gives guidance HOW to protect, but does not advice WHAT current rating should be taken. Recommendation lists of the fuse manufacturers are helpful..

What do I have to consider when looking for the proper fuse?

The high-voltage fuse-link has to withstand the transformer inrush current. This means, that the time/current-value at 100 ms has to be more or less 12 times the transformer rated current. The interrupting current of the fuse-link must be smaller and the fuse breaking performance higher than the maximum short circuit current of the circuit. The fuse rating has to be higher (generally twice) than the transformer maximum loading current. Selectivity must be given between the low-voltage and the high-voltage fuse-link.

What is the technical background of the manufacturers recommendation lists?

The recommendation lists of the fuse manufacturers recognize the standardized guidance and advises the proper fuse rating for the selected transformer. There are often two or three given ratings per transformer size in order to reduce the number of different ratings for all the installed transformers. Care should be taken if low-voltage fuse-links for transformer overload protection are fitted. In this case a comparison between both curves (LV- and HV- fuse) is useful.

Why is the recommended fuse rating generally twice the transformer rating?

It is quite normal to load the transformer (often at night) with 150 or 200 % of its rated power. The fuse has to withstand that higher load. As the loading current should be smaller than the fuse rated current, a higher fuse rating has to be used.

Is there a need for discrimination distance between upstream and downstream fuse-links?

Yes, the upstream fuse has to have a current rating of twice the downstream fuse.

What is the power loss of the fuse-link at transformer rated current?

The power loss of the fuse at transformer rated current is about 20 % of the value given in the manufacturers catalogues.

How warm may a fuse be at transformer rated current?

It depends on the fuse rated current and the transformer being protected. Temperatures may reach 100 °C on the fuse barrel at transformer full load.

Do HV fuse-links age or is there a need for maintenance?

No, if the fuse is taken according to the manufacturers recommendation list, ageing is not expected. There is no need for maintenance.

Any questions?