0

I am aware that the neurtal wire provides a return path for the current to the power source. But that neutral wire too, is connected to the ground at somewhere between the load and the source. My question is: Why use three wires Live, Neutral and Ground? Instead we can use two wires Live and Ground which does the same work, as Neutral and Ground are joined.

Another question: Does two grounds of different localities have same potential? If they have different potentials, is this the reason why we use neutral as the common reference for both the load and the supply?

Shashank
  • 51
  • 1
  • 5
  • 4
    Does this answer your question? [AC: Why differentiate between Ground and Neutral?](https://electronics.stackexchange.com/questions/189867/ac-why-differentiate-between-ground-and-neutral) – JRE Jun 19 '20 at 17:34
  • Note that many types of plugs are reversible - you can swap live and neutral by plugging them in upside down. But ground is NEVER reversible – user253751 Jun 19 '20 at 23:03

3 Answers3

1

In typical north American house wiring, ground and neutral are tied together back at the panel, and also to an earthing rod. Nevertheless, the neutral and ground wires have different missions:

  • Neutral carries return current. It is always needed for a single-leg feed (like a 2-wire plug.) It is optional for a 2-leg feed to carry any imbalance current. More about that below.

  • Ground carries current only in case of a fault. It provides a path for a line-to-chassis short back to the panel, preventing or reducing the possibility of electric shock.

Examples of how neutral and ground are used in normal 120V/240V wiring:

  • 2-prong 120V receptacle: L1 and neutral; all return is on neutral

  • 3-prong 120V receptacle: L1, neutral, ground; all return is on neutral

  • 3-prong 240V dryer outlet: L1, L2, neutral; neutral carries 120V unbalance return (e.g., dryer motor)

  • 4-prong 240V outlet: L1, L2, neutral, ground; neutral carries 120V unbalance return (e.g., dryer motor)

  • 3-wire 240V oven feed: L1, L2, ground: load is balanced so no neutral is needed.

hacktastical
  • 49,832
  • 2
  • 47
  • 138
0

Why use three wires Live, Neutral and Ground?

With only two wires, you would have to use the neutral for "earthing" the metallic part of the equipment. I'm thinking of cookers, refrigerators, Hi-Fi, iron etc..

You do this because should the internal live wire break and unexpectedly touch the appliance metal case, you want a significant "earth" current to flow and blow the fuse thus revealing a dangerous situation.

However, with only two wires, if live and neutral got swapped the problem would not be self-revealing and would be highly dangerous to anyone coming into contact with the case of the equipment. The equipment would appear to work but be highly dangerous because the exposed metal parts of the case are at live potential.

With a third wire (specifically an earth wire), if the wiring to the plug got messed up and live and neutral got swapped, the third earth wire is still connected to the exposed metal parts of the case and there wouldn't be a safety issue. In all likelihood the equipment would still operate correctly but is still safe.

If neutral and earth got inadvertently swapped, again there should be no real safety issue because the neutral wire that is incorrectly connected to the exposed metal parts would still be able to "earth" those exposed metal parts and, safety is preserved.

If live and earth got messed up then this is a self revealing fault because the equipment could not operate with neutral and earth connected to where neutral and live would normally go. It's not great but it is self-revealing and relatively safe. Of course with modern day gizmos like residual current devices (RCD), those exposed metal parts that are improperly connected to live would likely trip the RCD.

First and foremost it's about safety.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • The part where mains live and neutral can get inadvertently swapped only applies to fixed installation and to countries with polarized mains plugs that can be plugged in only one way. In countries with non-polarized mains plugs, you are free to plug it in any orientation so a device could get mains and live either way, and that's perfectly normal. – Justme Jun 19 '20 at 20:38
  • @Justme I don't think I've contradicted that have I? I tried to cover the options. Anyone with a microscopic brain using a screwdriver can swap live and neutral in a UK plug!! – Andy aka Jun 19 '20 at 20:40
  • No, it does not contradict anything. My point was that sometimes the live/neutral being swapped at a device is an installation error, and sometimes it is perfectly normal usage. So a device made in 2020 should be made to operate safely no matter which way live/neutral is applied. Devices made in 1980 had different power supply sections regarding mains input, depending on the geographic location it was sold, for example if it had a polarized plug, only the live wire had a single pole mains switch, and devices with unpolarized plugs had dual pole mains switch. – Justme Jun 19 '20 at 21:04
0

It is for safety. The dedicated ground wire is not used for anything else except fault currents. It is always available and never switched. Sort of like an emergency lane on the freeway. All exposed metal in appliances is bonded to this ground wire.

It is like they said hey, we should run a separate neutral wire all the way from the fuse box (breaker box) to every place that electricity is used. We will connect it to all the exposed metal on every device... refrigerators, fans, heaters, etc to make sure nobody ever gets a shock. That neutral will never be switched and won't carry any current (except for short circuit or fault current). Then they said we should give that wire a special name, and also use a different color insulation on it so it is easy to recognize. We can call it protective earth, or ground or something like that.

user57037
  • 28,915
  • 1
  • 28
  • 81