1
\$\begingroup\$

What is the difference between neutral and ground? I do not understand the difference.

\$\endgroup\$
  • 1
    \$\begingroup\$ Neutral is the return path for AC. Assuming you mean earth ground, it is literally a wire connected to a stake that's pounded deep into the ground. Neutral is intended to carry current, earth ground generally is not. \$\endgroup\$ – DerStrom8 Aug 10 '15 at 16:16
4
\$\begingroup\$

This Wikipedia page covers it well - but in short:

Neutral is the return current conductor for AC power circuits. Ground (earth) is a safety connection provided as a separate return conductor for fault currents - i.e. it does not carry current during the normal operation of any connected equipment.

Neutral and earth are connected together at the point of power generation and at the output of each transformer in a transmission system. However, since current can flow in the neutral line during normal operation, voltage can develop between it and earth at the point of consumption. As a result, in your home (for example) neutral and earth will be similar in voltage, but not quite the same.

There are a few different connection methods for ensuring the earth voltage stays close to the actual potential of the surrounding ground, with three most common in the United Kingdom.

T-T (usually older systems, also used for temporary power provision) - a stake in the ground at the transformer connects neutral and earth, and a similar stake at the consumer premises provides a local earth connection - unconnected to the transformer.

TN-S (older systems - but more recent than T-T) - transformer connected as T-T, but a separate earth cable runs from the neutral-earth bond at the transformer supply all the way to the consumer premises, providing a dedicated separate path for fault currents.

TN-CS (newer installs - last 20 years) - the neutral conductor from the transformer acts as the return path for earth currents as well. Earth within the premises is provided as a connection to neutral at the point of entry to the premises - this connection is made at one point only. Further distributed earth connections are generally made between the ground and the neutral conductor along the length of the cable in order to minimise the voltage between the ground itself and neutral.

This video covers all three systems in detail, including pros and cons of each approach.

\$\endgroup\$
  • 1
    \$\begingroup\$ Neutral and earth are also connected together at the breaker panel in a typical home installation (in the U.S., anyway). \$\endgroup\$ – Pete Becker Aug 10 '15 at 17:36
  • \$\begingroup\$ Ahhh, interesting Pete. I guess that's down to your split-phase system. \$\endgroup\$ – stefandz Aug 10 '15 at 17:39
  • \$\begingroup\$ Okay, that confirms that we're talking about a regional difference. So let me add that in the U.S. there's an earth connection at the service entrance. Despite the differences, the answer is correct as to the possible difference in potential between neutral and earth. (I keep wanting to type "ground" instead of "earth", but that would just confuse things) \$\endgroup\$ – Pete Becker Aug 10 '15 at 19:23
  • \$\begingroup\$ I have just had a chat with a friend regarding this - he has sent me a YouTube video which apparently clarifies things and introduces some variability in the UK setup too - I shall report back / edit if this changes the validity of the answer given. \$\endgroup\$ – stefandz Aug 10 '15 at 20:15
0
\$\begingroup\$

The language isn't used consistently in all domains of electrical engineering. But when it comes to mains power, ground is tied to the earth, typically at a single point in a building. Since it is tied to the earth you're standing on, it is safe for an (otherwise isolated) human to touch; you can't get a voltage potential across your body if all you're touching is the ground.

Single-phase AC has two conductors, which are symmetrical relative to each other; neither is "high" or "low", they swap those roles rapidly enough that it doesn't matter. But both legs do have a voltage relative to earth; all things have a voltage relative to all other things, after all. In general, in the US, one of the legs is tied to earth at the same point that all the ground wires are tied to earth. This gives the AC line a fixed reference to earth.

(More accurately, a residence will have center-grounded 220V feed, with most outlets receiving only half the voltage.)

So the ground wire and the neutral wire are both tied to earth. But the neutral wire has current flow through it, so the potential from neutral to earth at a particular outlet will vary depending on the wire length, gauge, and level of current flow. The ground wire should never have current flow, so it is always at exactly earth potential. If there ever is current flow through the ground wire, that means something has gone horribly wrong; this is a ground fault.

Since it is safe to touch earth, it is also safe to connect (say) the metal shell of your toaster to it. In this way, you can be guaranteed that the shell is safe to touch, even if a hot AC wire comes loose inside and comes in contact with the chassis. The neutral wire still has some voltage to earth, so touching it may still shock you, though to a much lesser degree than the hot wire!

\$\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.