Typically one requirement is very high dielectric strength from the secondary to ground and the primary to secondary. The only size restriction is a 100VA minimum rating. Inverter transformer Used in UPS systems with a PWM input where no regulation or filtering is required. The only size restriction is a 100VA minimum. Potential transformer Used primarily in a step down environment to monitor voltage.
Potential transformers differ from ordinary step down transformers in two ways.
First, they typically are designed to handle the burden of a volt meter, therefore they have no real VA rating. Second, since they are used in a monitoring capacity they generally require much greater accuracy in when manufacturing. There are no minimum size restrictions. They are very similar to isolation transformers with one important exception. In an auto transformer the primary and secondary windings are electrically common to each other.