6
\$\begingroup\$

I'm having trouble understanding why the earth is a viable fault path for electricity brought into my home. Isn't there really high resistance? I mean, it's dirt?

Also, let's say I connect the hot wire coming into my house directly to the ground wire. What is the path that the electricity takes to get back and "complete the circuit?"

\$\endgroup\$
  • \$\begingroup\$ What country do you live in? In my country (Australia), "the earth" might mean the protective earth conductor, which is a copper wire that runs all the way back to the supply transformer, where it joins up with the neutral conductor. Therefore, "the earth" is a continuous copper circuit which is quite capable of carrying earth fault current. \$\endgroup\$ – Li-aung Yip Jul 13 '14 at 14:48
3
\$\begingroup\$

I think of the ground as many parallel high resistances paths, which add up to a low resistance. Good earth connections (<50ohm) usually involve a decent bit of copper rod in the ground, for an average pole top tranny you would probably have 4+ metres buried depending on soil resistivity and local regulations. Really good earth connections such as in a district substation are usually less than 5ohm and as such are excellent return paths. From your house, the return path is back to the star (earthed) point on your service transformer, either via the earth or neutral wire depending on the earthing arrangement in your area.

\$\endgroup\$
  • \$\begingroup\$ +1 Do you have a link to that model explaining ground as many parallel high resistances paths? \$\endgroup\$ – VMMF May 2 at 17:54
1
\$\begingroup\$

It's not, really. What it is, is reliable in the sense that it doesn't go away. So the earth path is not as good as the wires it's connected to, but it forms a reliable backup path.

If you connect the hot to ground, you short out the circuit, since neutral is connected to ground as well. So current flows from hot through the ground wire to earth, and thence through a short earth path to neutral.

\$\endgroup\$
  • \$\begingroup\$ I see you talk about the case where one of the phases is shorted to ground on a solidly grounded wye system. Would any current flow on the neutral in such case? The current would flow from the shorted phase through soil back to the grounded star point of the transformer, but would the neutral see any current? \$\endgroup\$ – VMMF May 2 at 17:58
1
\$\begingroup\$

In my country (Australia) we use an earthing system that looks like this:

enter image description here

In case of an earth fault, the fault current path looks like:

enter image description here

Note the connection between the earth bar and the neutral bar at the "MEN link". The "earth fault path" doesn't actually go through the literal earth, i.e. the dirt - it goes through copper wires the entire way.

The above diagram shows what happens if your active wire faults to earth, but the same thing happens if you connect your active wire to earth directly.

NOTE: The above pictures show a "multiple earthed neutral" or TN-CS earthing system to AS/NZS 3000. The earthing arrangements in your country may differ significantly.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.