Hello, i understand the basics of how transformers work but have not come accross them much until i started a new job recently. These may seem like stupid questions but i would appreciate it if you experts could clear up a few things-

As both coils are isolated is it normal to have the output voltage present between the output and input Neutral or Earth if the output 0v is not grounded. i.e i had a 240v to 24v tranny (output zero not grounded) and upon testing i had the correct voltage out between 24v and 0v but i also had 24v between the 24v and the input Neutral or ground. Is this right ? i thought i would only get i voltage between 24v and the output 0v or have i not grasped something? the tranny was working o.k.

Will an RCD on the input side protect against an earth fault on the output side?

This might sound the most stupid but i cant quite understand where the 0v current flows to on the output side, i.e on the input i know current comes from the live wire through the coil and out through the Neutral where it is dissipated th ground at source. But on the output it flows out of the 24v through the load and back to the 0v, but then where? how can it flow round the coil as i thought current only flowed from a greater to a smaller potential?

 

I know the questions might seem basic but if you could clear them up it would be a great help.

 

It's possible you are measuring a fraction of the primary voltage which is being capacitvely coupled to the secondary. What you are actually measuring may be a co-incidence with respect to the secondary voltage magnitude of 24V. Are you absolutely sure that the two windings aren't connected in any way? Have you done a resistance test between primary and secondary - with no power connected to the primary - to check?

A truly isolated secondary has no galvanic connection to earth. So a primary side RCD would not respond to a ground "fault" or short to ground on either end of the secondary winding. An RCD detects & responds to an imbalance in active and neutral currents. Simply grounding one side of the isolated secondary wouldn't create a primary side active-neutral current imbalance. Even if you intentionally ground both sides of the secondary you won't get a trip due to RCD function alone - but you would most likely get a trip due to primary side over-current detection.

The transformer simply transforms the voltage magnitude from primary to secondary. You can think of the secondary winding terminals corresponding to those of an equivalent voltage source having an emf commensurate with the primary to secondary turns ratio and the applied primary voltage. As long as there is an external continuous load path for the secondary current to flow through from one end of the secondary winding to the other, there will be a predictable current.

 

Thanks for the response, yes i have metered between primary and secondary and there is no connection between them. Does this mean even with the output 0v not grounded and i touched the 24v wire to earth would it blow a fuse as i am picking up a voltage between them?

 

Try placing a 1K resistor to ground from the 24 volt winding. I would bet that the voltage reading across the resistor will be very close to zero. I agree with T_N_K that you are seeing the effect of capacitive coupling. You are probably measuring using a very high impedance meter. Grounding either lead of the secondary should not cause any fuse blowing.