Transformers double wound

Isolating transformers - such as separated extra-low voltage (SELV) transformers. If the primary winding and the secondary winding of a double wound transformer have a separate connection to earth, then the output of the transformer is effectively isolated from the input since the only connection between the primary and secondary windings is the magnetic flux in the core. 

Regulation 607-05-01 covers TT systems where the protective conductor current is so high that its product with the earth electrode resistance is 25 V or greater. That being the case, the installation must be supphed through a double-wound transformer or similar. 

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