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Neutral and Earth wires.

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alphacat

New Member
I once raised this subject in the forum and understood that not in every house, the Neutral and Earth wires are connected to each other inside the house walls.

But on the other hand, it cant be that the Neutral and Live wires are floating, since each of them could reach a very high voltage that would damage the plugged appliances.

So, either inside the walls of the house, or where the (high voltage to low voltage) transformer is located, or at both ends (house and transformer), the Neutral is connected to the Earth wire, right?
 
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indulis

New Member
Both are "ground", but the neutral wire carries current, whereas the ground (earth) wire shouldn't and is considered a "safety ground". They are wired together in the power panel (fuse box... at least in the USA).
 

alphacat

New Member
Thank you.
I think (as was said here once) that not in all buildings, the Earth and Neutral wires are connected to each other in the power pannel.
I think that some are connected where the High-V to Low-V transformer is installed.

When too much current flows through the Earth wire, then the panel box cuts off the power right?
How come current can flow through the Earth wire if R_Earth = infinity?
 

indulis

New Member
I can't speak for anywhere outside the USA. Here the NEC mandates that both are tied together in the panel.

"Earth" is actually considered "0"... the path of least resistance, not infinity.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
In the UK (and in most places I think?), the neutral is earthed at the substation. In the house, earth may be either a correctly installed earth spike, or connected to the incoming neutral (PME earth).
 

indulis

New Member
Yes, I forgot about the "spike" (ground rod). Although, here there is often a "ground strap" from the fuse box to a copper water pipe (point of entry into the house and on the street side of the water meter).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, I forgot about the "spike" (ground rod). Although, here there is often a "ground strap" from the fuse box to a copper water pipe (point of entry into the house and on the street side of the water meter).

That's more to bond the copper piping to ground than a grounding system.
 

indulis

New Member
In the States...



The Neutral Bar and Ground Wire

House Ground Wire Ties Into Neutral Bar


The main house Grounding wire also connects to the Neutral bar thereby grounding the circuits to an earth ground directly via ground rods or by attachment to a metallic cold water pipe.

(from Inside the Electrical Service Panel-The Neutral Bar and Ground Wire)
basically the same verbiage can be found in the NEC see section 250.52.1 (a).
 

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alphacat

New Member
Thank you very much guys!

I was wondering about the sentence: "Earth" is actually considered "0"... the path of least resistance, not infinity.

Current will flow from the Live wire into the Earth wire only if it can return from there to the Earth wire, in order to form a close loop, right?

So, is what i described in the picture correct?
plastic-wedges-suggestion-jpg.35134
 

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Chippie

Member
The current will flow for a very short time until the fuse blows if the domestic appliance develops an internal Live-Earth short.. If the wiring to the socket outlet develops a L-E short then the fuse or ELU should interrupt the supply

Here in the UK, the power distribution system feeds the domestic user using a single core conductor with the Neutral tied to Earth at the Substation. Inside the 'Utility box' the cable is terminated and then fed via a high capacity fuse- somewhere in the region of 60-80 amps, from there it feeds the meter and then to the consumer unit housing either re-wireable fuses or more recently mcbs...More modern installations include an earth leakage unit that will disconnect the supply sooner rather than later.....

The Wiki article referenced above is good for explaining all of this better than I can...
 
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indulis

New Member
Think about what happens when lightning strikes... why people have lightning rods and how they are connected.

Also, current in a circuit wants to return to it's source, which may not be ground.
 
In the UK the Live connection supplies current from the sub station and the Neutral is there to return it.

It would be very nice if you guys could all keep it like that! That is the intended circuit. No, don't do anything else.

At the sub station the Neutral is connected to earth. Now if your neigbour (on the same phase) is using 1000A then the resistance of the wiring between his Neutral pin and the connection to 'Earth' will develop a voltage that you will 'see' on your neutral pin. That's Ohms law. So Neutral isn't at the same potential as Earth - necessarily, it could be, depends on the current you, or your neighbours, are drawing. So, in the UK some other nutter can make your Neutral very high!! (normally though the wiring is so thick that they don't)

What is Earth for then?

Firstly it's there to distribute charge. My neighbours had their metal gas pipe changed for a plastic one and the dopey workmen reconnected the houses 'Earth' to the new PLASTIC gas pipe. As my neighbours activities in the house generated static they would find that they got nasty shocks from metal and water pipes in their house. The 'Earth' wire, properly re-connected then allowed this charge to flow to the 'Earth' - and literally cover the earths surface. The Earth, being a little bit larger than their house, spread out the charge so much that the potential was effectively zero.

Secondly 'Earth' protects you from being electrocuted - well provided that things are connected correctly! OK, so imagine you have a metal box with Mains Wiring in it and the Live wire disconnects and touches the metal case, which is 'Earthed'. Using just Ohms law we can see that the large current that flows (before the fuse blows - hopefully) will attempt to raise the potential of the metalwork with respect to, well, the 'Earth' you are standing on. Should the metal casing and eath wiring have such a large resistance to 'Earth' that the potential developed gets to above 50V (I think) - you will FEEL it!! So, if you keep the resistance between your metal box and the 'Earth' around you to milli ohms - then you will be able to tolerate 100s of amps of fault current before you can 'feel' it (read 'die from electric shock'). Note that it's not the potential that the metal box gets to with respect to the sub station - it's the potential relative to the Earth you are standing on that is important!

To answer your specific question - yes of course mains leakage current will flow back to its source - That's ohms law. Don't think that current flows in wires just because humans have printed words on them!! If current can find a path - it will use it!! (Did I mention Ohms Law?) Naturally the Neutral wiring will have a much smaller resistance than the Earth route so the currents will flow in the same ratio as the resistances.

Considering the two different resistances of the wet/dry Earth route and the Neutrals copper wiring, I would assume that the earth 'spikes' are there for the static reasons mostly (includes lightning) since it doesn't matter how big a spike you drive into the ground - the resistance presented to the sub station will largely depend on how wet the ground is and the distance to the sub-station.

Finally - try not to think of any such thing as perfect 0V, chassis, common, ground, +15V return, etc, etc. Ohms law says that if you have a current and a resistance - then you get a voltage. I bet that every point you would call 0V has some potential relative to anyone elses '0V', chassis, common, ground, etc. If you connect a 'charge' to the Earth then the charge spreads out evenly across the planet - that spreading reduces it to a very low potential since the 'Earth' is massively bigger than your house- but NOT zero, at least not while we still have thunderstorms!!

James
 
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