Why the 21 lighting transformer must double winding type safety isolating transformers, prohibited the use of autotransformer.

Answer:

A set of coils in a autotransformer having one tap that does not function as a barrier. Or, the two sets of coils are attached.

Double winding type safety isolating transformer load end and the power supply terminal is not connected, is safe, and autotransformer primary winding of a share, it is directly connected with the power supply end of the power, not safety.

So, safety regulations: lighting transformer must double winding type safety isolating transformer, prohibited the use of autotransformer.

The self winding transformer is connected by all its windings, and depends on the position of the sliding contacts to change the magnitude of the voltage. Therefore, there is not only a magnetic connection between the primary winding of the autotransformer (the power circuit) and the two winding (the power supply circuit), but also the direct connection of the circuit. It forms a circuit with the earth, so it can not be used as the power source of the safety voltage

Double winding type safety isolating transformer is provided with two windings respectively as input and output (primary) winding (secondary) winding, and between the two windings are insulated from each other, only through the magnetic connection to the power supply, which is the secondary winding and the earth does not constitute a loop, if the use of additional electrical insulation, it should be safety isolating transformer.