# When current must flow in a circuit (loop), why does it flow to ground?

I learned that current must flow in a circuit (loop).

When I swap the N-wire with the PE-wire on a light bulb in my house, the circuit breaker trips as soon as I turn on the light. This tells me that current is flowing from the L-wire (from the power plant) to the the PE-wire that is connected to a long metal rod that is put into the ground just outside my house (as far as I know). So there is no circuit, no loop that leads back to the source.

To check whether current really needs to flow in a circuit (loop), I took a battery and connected the positive pole to a light bulb, and the light bulb to a PE-wire of my house. The light bulb did not turn on. When I connected the light bulb to the negative pole of my battery, it turned on because the circuit is completed.

When current must flow in a circuit (loop), why does it flow to ground? And why does it not work with an electrical circuit where power comes from a battery?

• The circuit needs to be closed in order to be an ..uh.. circuit. "Ground" in most cases is merely an arbitrary point on a circuit that we chose as reference for measuring voltages. Commented Mar 12, 2021 at 14:38
• Did you really connect a battery to the mains power? Don't do that. Besides that you could get a shock, the battery could explode.
– PMF
Commented Mar 12, 2021 at 14:41
• Please be careful when using electricity in your house to run experiments. It is quite dangerous and you can kill yourself (or others) if you don't understand how it works. Commented Mar 12, 2021 at 14:41
• why does it flow to ground? It doesn't. You have add another ground symbol at the - connection of V3. Then current can flow and that net will be called "ground". You could call it "bimpel" or "Daniel" as well if you like. But using that ground symbol is a convention we EEs use to indicate that any voltage potential is referenced relative to that ground point. Commented Mar 12, 2021 at 14:41
• Does this answer your question? When does and when doesn't current flow to ground? Commented Mar 12, 2021 at 14:52

Your PE and N are connected together at somewhere in your building or at substation. N is used for normal return and PE as the safety return. So both N and PE are at the same earth potential. The PE would be quite useless if it simply was going to metal rod pushed to soil.

And since a breaker trips, it should then be a RCD which checks that both Live and Neutral have equal current or then it will trip.

And yes, the two battery terminals need to be connected to the two terminals of the lamp via some wiring for it to light up. Current does not normaly flow via an air gap (except whn arcing).

Don't mess around with house wiring, you don't want to damage the wiring.

• "you don't want to damage the wiring." Yes, we hate it when houses burn down and damage what would normally be a nice fully functional electrical system. Makes us EE's sad. /s Commented Mar 12, 2021 at 15:25
• When you say connected together at a substation, do you mean that the PE in my house is connected to the metal rod and also at the substation with a continuous wire? Commented Mar 12, 2021 at 17:59
• @ChrisKnudsen It makes the fire marshalls even sadder, then they come by and make sure that you don't have two outlet surge strips plugged in series Commented Mar 12, 2021 at 18:25
• @Daniel I cannot possibly know know the exact details how your grounding and your local regulations about it, but one place or another, the Neutral and PE have to be connected together at somewhere in the electrical system, or the PE would be useless for protection. Commented Mar 12, 2021 at 18:43

I learned that current must flow in a circuit (loop).

Yes.

So there is no circuit, no loop that leads back to the source.

No, the ground fault circuit breaker is triggered because there is current flowing in a loop where it shouldn't be.

I took a battery and connected the positive pole to a light bulb, and the light bulb to a PE-wire of my house. The light bulb did not turn on.

There needs to be a circuit. You have not described a circuit.

When current must flow in a circuit (loop), why does it flow to ground?

The term "ground" has two meanings. One refers to the

long metal rod that is put into the ground just outside my house

The other meaning of "ground" is simply a reference point in a circuit.

Current will only flow to an "earth" ground if the earth is part of a circuit. That is, there actually needs to be two connections to that earth ground.

why does it not work with an electrical circuit where power comes from a battery?

If a battery powered circuit has only one connection to earth, then no current (from that circuit) will flow through the earth. The current needs to return to its source, which is the battery, and there is no return path through the earth.

So there is no circuit, no loop that leads back to the source.

Incorrect.

Figure 1. There is a circuit from your earth rod back to the supply transformer. The neutral wire is "neutralised" there by connecting it to earth.

When current must flow in a circuit (loop), why does it flow to ground? And why does it not work with an electrical circuit where power comes from a battery?

Because in the first case the earth is closing the circuit. In the second there is no circuit.

See my answer to How does ground mains work? for more.

• Your figure is correct. However, my lamp has only 2 wires: L and N. I connected it to L and PE and current did flow nonetheless. The PE in your figure does not close the circuit. Hence my question, where the current flows to. Commented Mar 12, 2021 at 17:33
• If flows through the earth between the two earth / ground rods. That's the whole idea of them. Did you read the linked answer? It talks about the earthed chassis of the lamp but you've just done the same thing by earthing the lamp. Commented Mar 12, 2021 at 17:36
• I did read your answer, but I have a lot of trouble imagining that current really literally does flow through soil. Especially over long distances. That would mean that if I put two rods in my - garden 20 metres apart - and connect them to a car battery, current would flow? Commented Mar 12, 2021 at 17:49
• Have a look at test equipment kew-ltd.co.in/2019/05/02/a-guide-to-earthground-resistance-test and electrical-engineering-portal.com/…. Commented Mar 12, 2021 at 18:07

The simple answer is that the ground forms a part of the circuit; current flows to ground in one place and back out of the ground in another.

If you had connected both the -ve terminal of your battery and the light bulb at different points along the same ground wire, the light would have come on.

The difficulty is partly one of words. The wire in one direction of a circuit is known as the supply or signal line depending on its function, while the other is known variously as the return, 0V, earth, ground, neutral, etc. etc.

But in many systems, there is also a safety wire independent of the return, and that is universally known as the earth or ground.

So sometimes "ground" means the return wire and sometimes it means the safety wire - and sometimes it literally means the dirt under your feet. Often several of these are connected together at an "earth point" somewhere, so in that situation nobody really cares what they mean. But sometimes it matters a lot. And it does confuse the newcomer.

• I read that PE in a house is literally a metal rod pushed into soil. You write current flows to ground in one place and back out of the ground in another. Do you mean current flows through the rod into the soils and then finds its way to another metal rod connected to a power plant? Commented Mar 12, 2021 at 17:57
• @Daniel That depends on the standard to which the system is designed. They can vary widely. So sometimes yes, sometimes no. I just edited my answer a little, but this is hardly the place for a full discussion of earthing standards and practices. Commented Mar 12, 2021 at 18:13