1. The standard value of the resistance voltage of the transformer (now called "short circuit resistance" in the standard) is expressed as percentage (standard mono value). The standard values of different transformers are specified in the national standards of transformers. But in the transformer manufacture will produce the error. The margin of error is between +7.5% and -7.5% (up from +-10%). If the standard value of your transformer's short circuit resistance is 4%, it is calculated as the previous 10%. The transformer's resistance range should be 3.6% to 4.4%. And this data should be typed on the transformer name plate. If, as you said, only 3.4% or so (the standard value for small power distribution transformers is 4% or 4.5%), it may be unqualified products, I suggest you check carefully.

2. The short circuit resistance of the transformer is related to many factors of the transformer. Such as transformer capacity, loss, internal coil structure and so on. But once the transformer is finished, it remains the same.

3. This short circuit resistance is an important technical indicator for users. For example, the stability of power supply system, the quality of power supply to load, the safety and reliability of transformer after parallel connection are all related.

The short circuit resistance of the transformer is its own resistance. Resistance, transformer need resistance short-circuit current multiple of small, relatively anti-short circuit ability, but outside the characteristics of the transformer (volt-ampere characteristic) soft (along with the increase of output current transformer, the output voltage, voltage of the transformer down badly). On the contrary, the short circuit resistance is small, and the transformer needs to resist the short circuit current multiple is large, which requires the transformer to have strong short circuit resistance. Of course, the external characteristics of the transformer are good.