Isolation transformer: Potential difference to the earth

[greypilgrim]

Hi.

In a normal power plug, neutral is on the same potential as the Earth and therefore, in theory, safe to touch. Line carries the full voltage difference and is dangerous if the body provides a conducting connection to the earth, since this closes the circuit to neutral.

Isolation transformers essentially remove the connection of neutral to the earth, so one can only close the circuit (and get shocked) by touching both connections.

I don't quite understand how this works in terms of voltage differences. If the voltage difference between the connections is 240V, at least on of the connections has a voltage difference of at least 120V to the earth. Why is there still no (potentially dangerous) current if one touches one connection (and is sufficiently connected to the earth)?

 

[Dale]

If the voltage difference between the connections is 240V, at least on of the connections has a voltage difference of at least 120V to the earth.

Not really. The voltage with respect to the two connections is fixed, but the voltage with respect to ground is not. So if you ground one terminal then the other will be at 240V, but it you ground the other then the first will be at -240V.

 

[greypilgrim]

I see. If I ground one terminal, there must be some initial charge exchange to bring this terminal to the same potential as earth. I assume the caused current is very low, but how could I calculate it?

Why do we even use ground as the potential reference for the line? Wouldn't it be a lot safer to have circuits that are completely disconnected from the ground? So electric shocks could only occur if both connections in a power plug are touched simultaneously.

 

[Dale]

I see. If I ground one terminal, there must be some initial charge exchange to bring this terminal to the same potential as earth. I assume the caused current is very low, but how could I calculate it?

You would have to know the stray capacitance of the circuit and then have some way of determining the voltage wrt ground without any resistive leakage. Then to a first approximation it would just be discharging that small capacitance and voltage.

Why do we even use ground as the potential reference for the line? Wouldn't it be a lot safer to have circuits that are completely disconnected from the ground? So electric shocks could only occur if both connections in a power plug are touched simultaneously.

I am sure there is a good reason, but I don't know it. Perhaps @anorlunda can shed some insight.

 

[anorlunda]

I see. If I ground one terminal, there must be some initial charge exchange to bring this terminal to the same potential as earth. I assume the caused current is very low, but how could I calculate it?

Why do we even use ground as the potential reference for the line? Wouldn't it be a lot safer to have circuits that are completely disconnected from the ground? So electric shocks could only occur if both connections in a power plug are touched simultaneously.

@Dale was right about stray capacitance. It is really small.

Safety is a tricky subject because it is needed when things go wrong in unexpected ways rather than the immediate simplest case you imagine, like one finger touching one wire. For most accident or mis-wiring scenarios, you want to have a low resistance path to ground that does not go through your body.

[/PLAIN]
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.
Isolation transformers block transmission of the DC component in signals from one circuit to the other, but allow AC components in signals to pass. Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and Earth ground. Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers, medical devices, or laboratory instruments.