Mains Electricity – Understanding Mains Return Current


Since the neutral wire is grounded at the circuit breaker panel, I am confused as to whether current actually flows along the neutral back to the service head?

If not, then what actually absorbs the current? (I realize that it is AC, but the current must be going somewhere during a single cycle).

Best Answer

Perhaps the below sketch will help. This is earthed according to Australian practice (AS 3000).

Note that the dirt ('general mass of the earth') is high impedance and no appreciable current flows between the earth electrodes. Therefore, any current flowing via the active conductor must return via the neutral conductor, since there is no other possible path for it to flow.

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Q: "If there is no appreciable current between the earth electrodes, then why is it dangerous to touch a live wire?"

A: Touching a live wire does not, itself, kill you. Note that birds are quite happy to sit on powerlines.

What kills you is completing a circuit between active and earth (or neutral). A very direct way of doing this is shown below. Note that many things are bonded to earth - the steel frame of your house, the reinforcement bars in concrete slabs, plumbing (metallic water services must be bonded to earth in Australia), and so on.

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You don't even have to touch two metallic conductors to be electrocuted. Ground has 'high' resistance but not infinite resistance. Therefore, when a live wire touches the ground, a current diffuses into the ground, eventually returning to the source. If you present a lower resistance path for this current, it will flow through you. This is the concept of 'touch voltage' and 'step voltage' which we must consider when designing high-voltage installations.

Touch voltage is very bad, because if you touch a live conductor with your hands, and the current flows to your feet, it will traverse across your chest i.e. your heart. The heart is very sensitive to electric shock.

Step voltage is not so bad, because the current flows into one leg and out the other. There are no really important organs between your legs.

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As an aside - we have had incidents where crane drivers have contacted HV overhead lines. The recommended action is to stay in the crane as the surrounding metal is all at the same voltage, so you won't be electrocuted. If you do have to run away, i.e. because the crane is on fire, the recommended action is to jump off (so you don't touch the metal of the crane, and the ground at the same time - they may be at different voltages) and then bunny hop away with your feet close together. Keeping your feet close together minimises the voltage between your feet.

Yes, I am serious.