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I have a physics book which says that the earth and the neutral wire in a socket are joined at the city substation so that they are at the same potential, indicating that one can easily put a plug in the socket between the live and the earth wire instead of the neutral wire.

One of my teachers says that the earth and the neutral wire are not the same. According to him, if a very low power bulb (about 1W or 2W, or even less) is connected between the earth and the neutral wires, then the bulb glows, suggesting that the neutral wire has some potential.

I haven't tried this experiment.

Can anyone please explain the reality?

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    $\begingroup$ Earth and neutral should only be connected at the panel for your house. It is possible to have a voltage between them elsewhere in the house for various reasons. That can indicate a wiring issue or just older wiring that can't keep up with modern usages. $\endgroup$ – Jon Custer Mar 26 '17 at 16:28
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    $\begingroup$ You might be better asking this on the Electrical Engineering Stack Exchange $\endgroup$ – John Rennie Mar 26 '17 at 18:07
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This is really an Electrical Engineering but seeing as you and I are here I'll have a go with an answer.

It is pretty well universally true that any electrical socket outlet in a domestic property intended to be used to plug in a variety of consumer appliances will have 3 connections; Line(live), Neutral & Earth. It's becoming standard practice in electrical wiring installations to call the "hot" wire the "Line" now instead of the "Live" so I will stick to that so we can get used to it if you want to read further, e.g. Wiring Regulations Handbooks, etc.

The power for the appliance is supplied via the Line and Neutral connections. The current will, under no fault conditions, be the same. If a 10 Amp supply current is flowing through the Line conductor then 10 Amps is also flowing through the Neutral conductor (for that one socket/appliance).

The purpose of the third connection, the Earth conductor, is ONLY for safety. Specifically to reduce or eliminate the possibility of a harmful electric shock AND the possibility of an electrically ignited fire. Many domestic electrical appliances have a case made of metal, this includes items like a convector heater, a fridge, a microwave oven and so on. The case is metal for structural/design reasons (not electrical reasons). For example a plastic cased convector heater might be a very silly and dangerous idea (think about it !). IF we have a metal case it is possible, in the event of an internal fault, for the case to become electrically live and thus impart an electric shock, possibly fatal, to anyone touching it. We therefore connect the metal case to the earth wire of the socket using one separate core of the 3 core flex. The appliance is then "earthed".

Consider now what happens with a faulty appliance. Some electrical current due to the internal fault can flow through the earth wire of the electrical installation back to the source (e.g. if earth and neutral are connected at substation as you say). We can detect this current in the earth wire as a "fault condition" and trip the circuit thus removing the supply and alerting the user to the possibility of a fault. In modern type installations which include an Earth Leakage Circuit Breaker (aka RCD or RCCD) the detection of earth fault current flowing in the earth wire is very fast (< 40mS).

You should under no circumstances attempt to connect a load of any kind between a line conductor and an earth conductor. Neither should you attempt to connect a bulb or anything else across the neutral and earth conductor. In fact it can be dangerous to even use a cheap multimeter to measure voltages on your domestic electrical installation wires (they are not rated for the impulse voltages that may appear on the mains sometimes). By all means read about domestic electrical wiring systems but do not experiment on them ! I once saw someone attempt to measure the internal resistance of the mains using an AVO on the low ohms range while I was in school in the physics lab. The results were not pretty, luckily it was only the AVO that was an irrepaiable heap of black gunge not the human involved. He had a lucky escape.

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  • $\begingroup$ "Neither should you attempt to connect a bulb or anything else across the neutral and earth conductor." - why so (any specific reason)? $\endgroup$ – Wrichik Basu Mar 26 '17 at 19:59
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    $\begingroup$ @WRICHIKBASU I would say it's akin to my standard disclaimer on high voltage. "If you have to ask StackExchange about doing something with high voltage, you shouldn't do it. Only rely on your own knowledge and that of people you trust. You only get one heart, and it's silly to put it at risk trusting someone on the internet." If everything went "normal" you could connect that bulb just fine, but as BetterBuildings points out in his answer, you shouldn't trust your only heart to "normal." Learn what sorts of "abnormal" behaviors you can see first, because one mistake can be your last. $\endgroup$ – Cort Ammon Mar 26 '17 at 20:07
  • $\begingroup$ There's a reason you have to actually undergo training to become an electrician. $\endgroup$ – Cort Ammon Mar 26 '17 at 20:07
  • $\begingroup$ @WRICHIKBASU A huge amount of experience and expert knowledge has gone into designing how domestic electrical installations are configured and constructed. There are wiring regulations of hundreds of pages in length in most countries in the world. NONE take into account the situation of persons with inadequate knowledge partially dismantling systems and conducting home made experiments with bulbs or whatever BECAUSE that is not what they, and their safety systems, were designed for. In other word you could/would defeat the safety systems and hence run the risk of electrocution. $\endgroup$ – BetterBuildings Mar 26 '17 at 20:36
  • $\begingroup$ @WRICHIKBASU all the "because you don't know why, you shouldn't ask why" answers are unhelpful at best. One reason is that wiring standards have changed over time, for example during WWII in the US some appliances used their neutral as ground in order to save materials, and electricians didn't bother to forbid this from their standards until 1996(!!!). So, if you have one of those appliances and there is a fault in them, then one of those lines would actually be live. So, a lot of that "years of training" is learning the dumb stuff that was allowed in past years, and how to test for it. $\endgroup$ – user3685427 Jun 19 at 22:18
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The earth (ground) wire and neutral are both ideally at the same zero volts. They are connected together somewhere nearby (depending on your local safety rules)

The idea of the the earth wire is that if somehow the live wire touches the case of the gadget the electrcity flows to earth through the earth wire (and with enough current to blow a fuse) rather than through you.

Most of the time there is no current flowing in the ground wire and so it is very close to zero volts. But there is current flowing from the appliance through the neutral wire and so because it has some resistance it will be at some small voltage (remember V=IR). You can measure this voltage or use it to light a bub.

ps don't try this at home, it is dangerous if you manage to disconnect the neutral leaving the appliance connected only to live

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  • $\begingroup$ But that current in neutral wire must be very less, very very less, because it neither gives a shock nor lights a line tester. $\endgroup$ – Wrichik Basu Mar 26 '17 at 18:49
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    $\begingroup$ @WRICHIKBASU - it has the same current as the live but is at a very low voltage, that is why there is no shock and it doesn't light the neon tester $\endgroup$ – Martin Beckett Mar 26 '17 at 19:28

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