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As far as I know, all RCDs / GFCIs sense leakage current by differential measurement between active and neutral with a current transformer. Both the active and neutral wires will pass through the transformer.

I have tried without success to find references on why we cannot simply pass the earth wire through the current transformer instead. Is this because some other kind of leakage or noise can occur on this wire that does not represent a fault condition?

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  • \$\begingroup\$ The method you are describing is an ELCB and is often used as a plugin adapter for safety in situations where cable damage is a possibility, such as a lawn mower. \$\endgroup\$
    – Majenko
    Jan 7, 2017 at 9:35

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I feel the GFCI (Ground Fault Circuit Interrupter) is slightly mis-named, as it is intended to trip if any current leaks in or out of the circuit, regardless of the path it takes.

If there is a fault, the "missing" current need not flow through the ground wire that goes by the GFCI - it might flow from a kettle, through you, to the water pipe, and then to ground, with no current flowing through the ground pin in the outlet.

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    \$\begingroup\$ Thanks for your answer, and to others who answered. All the answers were helpful and it was difficult to choose which to accept. \$\endgroup\$
    – user133493
    Jan 7, 2017 at 2:16
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why we cannot simply pass the earth wire through the current transformer
instead [?]

There are 3 wires: hot, neutral, earth. Hot and neutral are intended to power the load. Earth is untended as a safety ground.
So far, so good.

Let's imagine that you are measuring current only though the earth lead, and you are not measuring anything else. A victim accidentally touches the hot and the cold water pipe. The residual current will go through the victim, but will not happen to go through the earth lead on which you are sensing current.

By the way, there are plenty of appliances that don't have the 3rd prong for earthing at all.

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Sensing current on the earth/ground wire does not detect leakage current to ground not to the ground pin of the outlet. For example, a hairdryer used in a bathroom by someone sitting in a bathtub and it gets dropped into the water. The current leakage path is through the water and/or body to the plumbing.

Also, the used arrangement allows a GFCI to safely replace a two-prong outlet where there is no ground connected.

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It's because if there is a fault, the current may not be passing through the ground line. It may be passing through you, or something else. The current through the line and neutral should always be equal, and when they are not, there is some kind of fault, which may or may not involve the ground line.

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