How do isolation transformers affect the leakage current through protective earth in a system?

Scenario 1: Mains is directly connected to circuit. Metal enclosure has a protective earth connection to building earth. Current passing through earth conductor = I1.

Scenario 2: Line and neutral from mains are isolated from circuitry via an isolation transformer. Earth connection from enclosure to building earth remains as earlier. Current through earth conductor = I2.

I2 < I1. Is this right? Why is this? The explanation I was given was that its a simple matter of KCL. The current leaving the isolation transformer's secondary line has to return(node current equation) via the secondary neutral and hence the leakage current is lesser(in ideal cases 0).

On the other hand, if the system is powered directly by the mains, the current has the option of returning via the earth wire or the neutral wire as they have a galvanic connection at the mains end.

Firstly, is this right? If it is, then my problem with this explanation is this:

I understand that the leakage current arises because of the coupling between the circuitry and the earth connection, in this case the metal enclosure(which has a earth conductor to the building earth). Now this coupling should solely be decided by the dielectric properties of the material(s) that is separating the circuitry from the enclosure(earth). How will an isolation transformer affect this coupling(and hence the leakage current)? All the isolation transformer can do is to block out common mode noise on the primary to the secondary. So will I1 - I2 = common mode noise? Or am I oversimplifying this?

This is a conceptual question and I'm just trying to understand the basics here.

  • \$\begingroup\$ I'm not sure what your point is. Transformers don't "source" energy at all, they merely convert it. It sounds like you're trying to talk about the common-mode noise that couples across the isolation barrier. That is controlled by minimizing the coupling capacitance across the isolation barrier, and is entirely AC in nature. \$\endgroup\$ Commented May 23, 2013 at 7:51
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    \$\begingroup\$ Incidentally, in basically every mains-powered piece of equipment out there, the protective earth does nothing the great majority of the time (and some equipment doesn't even have an earth connection!). Realistically, there should be only micro/nano amps of current on the protective earth line, if any. \$\endgroup\$ Commented May 23, 2013 at 7:53
  • \$\begingroup\$ @ConnorWolf No, I am not referring to the common-mode noise. What I meant was that when a system is connected directly to the mains, the wires being line, neutral and earth; at a given instant, the current travelling through line is equal to the current through netural + current through earth. The last one is what we define as leakage current for a system. This leakage current can be controlled by using an isolation transformer. I am trying to understand how. \$\endgroup\$
    – Rustin
    Commented May 23, 2013 at 7:57
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    \$\begingroup\$ Yes. In a perfect transformer. As it is, you can optimize transformers for isolation, or for power-efficency, or for price, but not all three. \$\endgroup\$ Commented May 23, 2013 at 8:06
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    \$\begingroup\$ @Rustin "Since the metal enclosure has a substantial voltage wrt earth..." How, and why? Something is wrong with this picture. Any enclosure that has substantial voltage with reference to earth, is a serious risk to human life. Are you sure you are not confusing circuit ground with building earth? Circuit ground of a line-isolated device should never be connected to the enclosure - and yes, I am referring to industrial electronics, which is what I work with. \$\endgroup\$ Commented May 23, 2013 at 10:56

1 Answer 1


The point of a isolation transformer is to break any common mode connection to the rest of the world. This therefore allows one connection to the rest of the world that won't draw any current. Usually you try to not have any such connection, so that a accidental connection (you touching a live wire with one hand while grabbing a metal drain pipe with the other, for example) is the first connection and therefore safe.

A common mistake people make when using a isolation tranformer is to forget that connecting a oscilloscope usually means connection some point to real earth ground. That by itself is OK since it is only a single connection. However, now the circuit is no longer isolated and significant current can flow between some part of the circuit and a metal pipe, for example.

  • \$\begingroup\$ I have edited the question to include details from the discussion in comments. I'm trying to understand a situation in which the earth conductor is not removed from the circuit/metal enclosure. Only the line, neutral are isolated between primary and secondary. The earth connection is directly from the metal enclosure to the building earth. \$\endgroup\$
    – Rustin
    Commented May 24, 2013 at 5:16

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