I Isolation transformer: Potential difference to the earth

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1. Oct 27, 2016

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)?

2. Oct 27, 2016

Staff: Mentor

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.

3. Oct 27, 2016

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.

4. Oct 27, 2016

Staff: Mentor

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.

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

5. Oct 27, 2016

Staff: Mentor

@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.

Last edited by a moderator: May 8, 2017