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Ok so this is gonna be a lot of mini-questions.

  1. What exactly is the neutral wire?

  2. How does earth conduct electricity for a live AC mains even though its just dirt and stones (it should be an insulator)?

  3. How come earthing can complete circuit for AC mains but not for a DC battery? say if I take a positive terminal of battery and use earth as negative terminal , no current flows

  4. How does Earthing or Grounding as a protection work? i.e how is a Earthing terminal different (why not just use neutral again)?

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  • $\begingroup$ Look at the diagrams here: en.wikipedia.org/wiki/Earthing_system . The earth is not just dirt and stone - there are moisture and ions. The powerplant is grounded and nothing prevents you from doing that with a battery. en.wikipedia.org/wiki/Residual-current_device $\endgroup$
    – hpekristiansen
    Jun 23, 2016 at 19:52
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    $\begingroup$ neutral wire. Earth is a conductor. For point 3, you need to insert the negative terminal of the battery into the earth to complete the circuit. Earthing keeps conductors at earth potential so that when you touch any exposed part you don't get a shock. Inside electronic devices a ground is a conductor which is taken as the reference potential for the whole circuit inside the device. $\endgroup$
    – Tea is life
    Jun 23, 2016 at 20:59

2 Answers 2

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  1. A neutral wire is one of the two, three or four wires used to deliver electricity. The neutral wire is grounded at the entrance/main panel and therefore its potential is close to zero. Other wires (line wires) are hot or live.

If the neutral wire was not grounded, the electricity would be still delivered to the devices, but the standard earth protection system would not work.

  1. With some exceptions, the distribution of electricity does not rely on the conductivity of the earth: it is distributed through wires - at least two wires for each destination.

There are systems like SWER (Single-Wire Earth Return) where the electricity is delivered by one wire with the return current flowing through the earth. They are not very efficient and they are mostly used to power some remote loads, in many cases consuming relatively low power.

SWER transmission lines, as all long haul power transmission lines, operate at high voltages (tens of kilovolts) and therefore carry relatively low currents, so the losses due to the earth resistance are not very high. Besides, the connections to the ground are done with long rods reaching ground waters, so it is not just dirt and stones. After the step down transformer, where the current increases, the electricity is carried by two wires.

  1. The conductivity of the earth is roughly the same for DC and low frequency AC currents. In both cases through, you need to go deep under ground to achieve low resistance.

  2. Grounding of the equipment housing keeps its potential at near zero level even when it comes in contact with a live (hot) wire. This system is well described on numerous websites. The illustration below will hopefully make it easier to understand.

enter image description here

Here red is a line wire, blue - neutral and green - ground. For more details, you can check out this post.

Under ideal conditions, connecting the housing to the neutral wire (grounded at the main panel) would work fine as well. But in the real world it is not such a good proposition: besides some more subtle reasons, an accidental break of the neutral wire, leading to a device grounded in such fashion, will bring the housing of the device to the line voltage, which could be lethal. Grounding the housing by a separate ground wire adds redundancy and therefore is more safe.

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  • $\begingroup$ Very good answer. I was recently confused about this and found two points that helped clarify #4 for me which may be worth addressing. First, my understanding is that the purpose of equipment grounding the case is to cause large current to flow in the event of a fault and importantly trip the circuit breaker. If the breaker is not tripped then in the diagram some (probably small) fraction of the total fault current will flow through the person and this could potentially be dangerous . For this reason the circuit breaker is a critical part of the circuit. Please correct if I am misunderstood. $\endgroup$
    – Jagerber48
    Apr 25, 2018 at 4:25
  • $\begingroup$ @jgerber Your understanding is correct. There are more details regarding the circuit breaker in this post: physics.stackexchange.com/questions/291148/… $\endgroup$
    – V.F.
    Apr 25, 2018 at 4:38
  • $\begingroup$ You caught me in the middle of my next point! My next point is that if you look at picture three one might think it could be made safer by removing the neutral-ground connection at the service entrance, thus having an isolated system. The question then is why we ground neutral at all. I guess the answer is that then you can more unpredictable problems in the event that two faults occur. I've also hear there are advantages for lightning protection when you have an actual earth connection. $\endgroup$
    – Jagerber48
    Apr 25, 2018 at 4:43
  • $\begingroup$ Yes, that is my understanding as well. $\endgroup$
    – V.F.
    Apr 25, 2018 at 11:38
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Neutral is a circuit conductor that normally carries current back to the source, and is connected to ground (earth) at the main electrical panel. In the electrical trade, the conductor of a 2-wire circuit connected to the supply neutral point and earth ground is referred to as the "neutral".

A difference can occur when either current is flowing down the earth wire or through someone's body to ground (and back to the neutral point through the earthing system). Electricity flows only when the circuit is complete. And our body is a great conductor of Current.

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