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In normal functioning of household power supply, the current flows through the hot and the neutral wire. Why does it not take the path of the ground rod, even when the ground rod, neutral wire and the ground wire are all connected to the same neutral bar? I think this is because to complete the circuit the current would have to flow from the ground making it the path of very high resistance.

But, why then does the current flow through the ground rod when lightning strikes the breaker box? Shouldn't current follow a path of low resistance, and travel through the hot and neutral wires?

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  • $\begingroup$ Household power systems can differ by country. Which applies to your question? $\endgroup$
    – Bob D
    Aug 6, 2020 at 20:49
  • $\begingroup$ I'm talking about power systems in the US. $\endgroup$
    – Vasvi Sood
    Aug 7, 2020 at 4:38

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The normal rules of current flow don't always apply when it comes to lightning. This is because air gaps inside the box between the various conductors stop being insulators at the high voltages present in a strike, and the resulting arcs across those air gaps will carry current flow that does not always follow the wire (right there!) that you'd ordinarily expect it to.

A strike to the breaker box will follow the straightest path to the ground rod connection and then follow that into the ground. Inside the breaker box, it will arc across any gaps standing in its way and the bulk of the strike power will be expended along that path. Large voltage spikes will also propagate throughout the network, destroying appliances plugged into it, but the path that gets flashed into plasma is the most direct line from the breaker box housing to the ground rod.

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The ground and the neutral are typically tied to the earth at some point. At that specific point they have the same potential. Under nominal conditions, however, the ground is carrying no current and the neutral is carrying significant current due to the load on the electrical system. That means at arbitrary points in the system, ground and neutral will have different potentials due to the wires being imperfect conductors.

So there is probably less resistance in the ground for the lightening strike. That said, the current from a lightening strike would be signficant, so I don't know that all of the current from the strike would go along that path.

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Why does it not take the path of the ground rod, even when the ground rod, neutral wire and the ground wire are all connected to the same neutral bar?

Actually, some of the current may take the path of the ground rod. But depending on the system being used (which can vary between countries), the resistance in the path through ground is likely to be much greater than that returning from the neutral bus back to the utility supply.

But, why then does the current flow through the ground rod when lightning strikes the breaker box? Shouldn't current follow a path of low resistance, and travel through the hot and neutral wires?

When it comes to a lightning strike, the grounding of the high voltage utility transformer primary provides a relatively low impedance to the earth thereby suppressing the maximum voltage from the high voltage wires (highest point for lightning to strike) to ground. It would generally be unusual for the lightning to strike the breaker box first, as it generally seeks the highest point in the system.

Hope this helps.

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  • $\begingroup$ What do you exactly mean by: When it comes to a lightning strike, the grounding of the high voltage utility transformer primary provides a relatively low impedance to the earth thereby suppressing the maximum voltage from the high voltage wires (highest point for lightning to strike) to ground. $\endgroup$
    – Vasvi Sood
    Aug 7, 2020 at 4:35
  • $\begingroup$ I don't clearly understand that statement. Could please give a more detailed explanation? $\endgroup$
    – Vasvi Sood
    Aug 7, 2020 at 4:36
  • $\begingroup$ Without the utility earthing electrode a lightning strike hitting the high voltage wires would raise the entire electrical system hundreds of thousands of volts above ground resulting in catastrophic failures of electrical equipment and insulation. The electrode provides a path to more safely discharge the lightning to ground $\endgroup$
    – Bob D
    Aug 7, 2020 at 7:29

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