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One option are telephone coupling transformers ( **broken link removed** ) but you have to be aware that these transformers are in general not rated for mains voltages ..
This issue can be addressed by proper selection of series capacitors as high pass filter + MOV (overvltage protection) at the HV side, and diodes/zener diodes/transient voltage suppressor at the LV side ..
i don't know how we got from power transformer to phone decoupling transformers. nevertheless the answer to your original question is a power tranformers [switching] means that DC is switched on the input to transfer power to the ouput by cutting flux lines. the switching is usualy done by devices that turn ON-OFF