Earthing the Secondary Phase of PTs

I hope you are talking about PT secondary 'V' connection? In this case how the 110/sqrt 3 will be obtained if the v point is not earthed?

A double wound transformer only gives you a ratio of the primary voltage on the secondary winding depending upon the number of turns on the windings.

The secondary voltage can float across the range of the primary - in theory an 11000 -110v transformer could show 10890 - 11000 on the secondary winding, which still gives 110v but at very hazardous limits.

By tying one end of secondary winding to earth then the 110v secondary voltage ranges between 0 -110v - a much safer situation and as previously stated if a 4 wire secondary output is required the star centre requires to be earthed

The secondary winding is wrapped adjacent to one end of the primary winding. Therefore there will be capacitve coupling from that end of the primary to the secondary. This capacitance can be several hundred pF or more, so there can be several miliamps of current coupled through this capacitance. Grounding the secondary shunts this current to ground and away from the measuring circuit.

Don

Hi M.,

Many electronic instruments require outputs that are floating from ground (earth), therefore the secondary of the power transformer is not earthed. I think most, if not all of the "brick" supplies for common household items have secondaries that are not earthed.

S