Does current flow in both wires or in one in our homes?

[Mike Bergen]

Your house current is 1 of 3 phases AC that has had its voltage reduced to 240 nominal volts with a connection at both ends. If you put a volt meter on the 2 ends you'll read 240 volts the Utility taps the center of the 240 volt transformer coil in the middle so that the voltage reads 120 from either end to the middle tap. Now we have 3 connection points If you connect any 2 of the 3 points you'll complete the circuit and get current flow. If the current is through a device like a motor or light bun it will work. If any of those wire touch each other without a load then we have a SHORT CIRCUIT where an uncontrolled amount of current flows and can melt the wire or worse could burn the person.THis is why we put a fuse or breaker in the line to keep the whole wiring system from destroying itself
If you touch any 1 of the 3 connections and nothing else ( we haven't grounded anything yet) you won't get a shock unless you complete the circuit. With me so far?

Now for safety reasons we ground our home electrical systems by connecting 1 of those 3 points to a conductor buried in the ground so it will always have a path back to the transformer and therefore trip the fuse/ breaker. We pick the middle 1 because it is common to the 2 ends and has 120 volts from each end. ( in some Industrial 3 phase applications we could connect 1 of the "hots" without a 4 th Neutral Wire) We do this because we want to make sure that the "Load" like the motor has a path so we won't become the path and get burned and hurt

To sum up when things get messed up we can have a short circuit between any of those 3 wires or a Ground fault if 1 of the 2 ends touches any thing else.
The center neutral wire is grounded like you are so current won't flow UNLESS the wire becomes disconnected and you become part of the path that completes the circuitt. SO you can get a shock and get hurt, SO it is always a good idea to make sure the wires are OFF before you work on them It is call Lockout/ Tag Out

Tough to explain in Words, Hope you'll consider becoming an Electrician some day

 

[Svein]

Except it is normally tied to an Earth, not far away.

Maybe in the US, but (as I said) not in Europe. I normally measure about 150VAC between any of the conductors and Earth. This is due to the fact that the distribution does not include "Earth".

To be technical: The classic distribution is three-phase delta to each house. In the main fuse box, the phases are distributed as evenly as possible to the outlets and other loads. Therefore one internal circuit will be R-S (with one of them arbitrarily marked "live"), another S-T and a third T-R.

Due to the increased use of electricity, a new system is being slowly phased in: three-phase star. This system uses four instead of three wires, with the fourth being "neutral". The advantage is that the "neutral" wire now is very close to "protective Earth". The disadvantages are:

• Four wires instead of three, making the distribution cables more expensive
• A higher voltage between the phases. Since the house outlets at 230VAC are now connected between the phases and "neutral", the voltage between phases are now 230VAC*√3 ≈ 400VAC.

 

[DaveC49]

To get back to the OP's question, if his Ac source is isolated , i.e. it has a transformer between it and the mains distribution power then either terminal is equivalent and the discussion about mains power is irrelevant. If one terminal of the isolated AC source is tied to a ground, then the other terminal will not be safe to touch while also touching the ground. If neither terminal is grounded then you would have to touch both terminals simultaneously to get a shock.

 

[sophiecentaur]

To be technical: The classic distribution is three-phase delta to each house.

What part of the world does that refer to? All over Europe, individual homes are supplies with single phase 240V with one conductor at near zero PD (the neutral). In the US, they have a single phase 240V with a centre tap (the neutral), giving the option of 120V appliances, half in phase and the other half in antiphase, or 240V (nominally balanced) appliances.
It is only in large, industrial and commercial premises that Three phase is supplied to the consumer. As far as I know, the arrangement is WYE, with the neutral conductor at the centre of the Y.
Distribution is a totally different matter and, looking at the overhead wires - three conductors and a small Earthy conductor, linking all the towers - I get the impression that it is a delta connection. But in the 'last' substation in a UK chain, the secondary of the transformer is a WYE, with the Neutral, branching out to all consumers and phases 1,2,3,1,2,3,1,2,3 etc fed to houses along a street in turn. So, even when a premises is supplied with three phase, there is a WYE arrangement to the consumer because the same bug cable feeds both single and three phase consumers.
If anyone wants to make assertions about a mains distribution system, they should specify where their particular model is used.

 

[Svein]

If anyone wants to make assertions about a mains distribution system, they should specify where their particular model is used.

As I said above, this system is used in Norway (and I suspect Sweden and Finland).

 

[sophiecentaur]

As I said above, this system is used in Norway (and I suspect Sweden and Finland).

I can't find a reference to that in the post I was replying to.)?) What you describe does not apply to all Europe. I see that the info is in an earlier post. Do keep up, Sophiecentaur - sorry.

 

[Svein]

What you describe does not apply to all Europe.

Well, I know that it applies to a large part of it, since I designed a kWh meter from those specifications about 10 years ago. The specifications were very clear: I had to create a virtual center between the phases in order to calculate some of the parameters the customer wanted (true RMS, phase angle etc.).

 

[sophiecentaur]

So, can you confirm that domestic houses in Norway have a three phase supply with delta connection? It sounds an expensive sort of installation, compared with single phase supply. How are single phase appliances connected? How is it metered? What happens about shock protection? I am not surprised that they are moving to, at least, a star system.
Are there special mains connectors?
There are endless questions about this interesting idea.

 

[sophiecentaur]

Well, I know that it applies to a large part of it,

I think there must be some misunderstanding here. I have been searching on Google for some mention of a non-standard system in Norway and I cannot find a hint of it. I have to ask you to back up your statement with a reference to domestic wiring regulations in Norway. If it really existed, it would surely be mentioned, along with the US and UK systems - after all, it would be revolutionary and potentially dangerous if people didn't know about it.
Your Power meter design could not be the first - every home must have one. Are there any manufacturer's adverts for such meters?

 

[Svein]

So, can you confirm that domestic houses in Norway have a three phase supply with delta connection?

Yes. The voltage between the phases is 230VAC, so it is trivial to take out a standard 230VAC between two phases (see https://en.wikipedia.org/wiki/Three-phase_electric_power).

I use the standard names for the phases - R, S and T. They all go into the main fuse box, as does the protective ground. The various circuits inside the house are connected as shown in the figure (I did not have symbols for the standard European wall outlet, so I used what I found). Thus, there are only standard European outlets and plugs. In some cases (like my heat exchanger) a three-phase connection is necessary and is separately fused and connected.

This is not theory, this is a simplified schematic of the electrical system in my own house (built in 2010). And no, I did not install it myself, it was done by professionals using the current standards.

I know that in some old houses they did not deem it necessary to connect all three phases, but just connected two. The power company then had to ensure that these houses were equally distributed among the phase pairs.

 

[sophiecentaur]

In some cases (like my heat exchanger) a three-phase connection is necessary and is separately fused and connected.

If you have a need for three phase in your home then you are in a non-standard situation (by UK standards) . But you are implying that all standard european mains plugs and sockets, when connected in Norway, have no Neutral (How do people connect them?). That was absolutely gob-smacking but, I now realize that Norway is not an EU member. The gnomes of Brussels cannot interfere with what your electricity supply industry chooses to do. You would need to modify your statement about "In Europe (at least in Norway)" so that it doesn't include EU countries.
I have one comment about the delta system, in a domestic situation. Imbalance in load can alter the volts on the other phases and that could be an embarrassment. A three phase system with a neutral (WYE) reduces that problem.
Are all your major heating appliances fed from three phases? I'm still trying to get my head round the fact that, only a few hundred miles away from the UK and in such a civilised country, people do things so differently. And they get away with it. But you do mention that they are moving to WYE, which is fully compatible.
You learn something every day on PF.

 

[Mark Harder]

I'm surprised to read that neutral and ground are tied together at the service box in a building. I've always wondered about the fact that the neutral and ground wires (or outlet boxes, etc.) usually are at different potentials - as much as tens of volts - when I measure them. Ideally, neutral and ground (the bare metal or green wire) should be at the same potential since they are tied to the same ground. The only explanation I have is that there is too much resistance in either the neutral or ground. Perhaps the electrician used wire that was higher gauge # (thinner) than the codes specify (or else the codes themselves are inadequate). If there's enough current flowing in either wire, that and the finite difference between the resistances in the neutral and ground wires would introduce a voltage drop between the point of measurement and the common ground. Am I correct? I'm not inclined to take my measurements inside the service box in order to verify this hypothesis .

As for the question of currents in the neutral and hot wires, Kirchoff's Current Rule says that the supply and return currents must be the same in a DC circuit. But AC circuits with capacitative and inductive loads are different. The KCR doesn't apply to regions in an AC circuit (See Wikipedia.), so one is not guaranteed that the current flow is the same anywhere in an electrical loop. But, does the KCR apply when one measures RMS current at different points in an AC circuit? Here, Wikipedia is no help. When I have a chance, I'll look at my electronics references to find out. Curious minds want to know.

 

[Svein]

Imbalance in load can alter the volts on the other phases and that could be an embarrassment.

Since all houses on a distribution stretch are connected to all three phases, the loads will average out. And - we have gone from the old-fashioned air stretches to cables below ground and those are thick!

You would need to modify your statement about "In Europe (at least in Norway)" so that it doesn't include EU countries.

Again, see https://en.wikipedia.org/wiki/Three-phase_electric_power. A quote from that article:
For domestic use, some countries such as the UK may supply one phase and neutral at a high current (up to 100A) to one property, while others such as Germany may supply 3 phases and neutral to each customer, but at a lower fuse rating, typically 32 A per phase, and "shuffled" to avoid the effect that more load tends to be put on the first phase.

I'm surprised to read that neutral and ground are tied together at the service box in a building.

Again - not in Norway. Protective ground are wired separately and each circuit is protected by a fuse with ground fault circuitry.

Technical: The reason for protective ground is that the water mains used to be copper pipes dug into the ground (very good "Earth" connection). Thus, if there was a current leakage anywhere, a person could easily get a mild shock if touching a stove and a water spout at the same time. Because of this, protective ground is mandatory on each outlet (it used to be mandatory only in "wet" rooms). Anyhow, the water mains are now supplied using plastic pipes (which do not conduct), so the reason for protective ground has more or less disappeared.

 

[sophiecentaur]

some countries such as the UK may supply one phase and neutral at a high current (up to 100A) to one property, while others such as Germany may supply 3 phases and neutral to each customer,

And my point is that the neutral on each single phase supply is the same and at around ground potential. This cannot be the same for a delta configuration. I don't think you will find that in the EU.
Anyone can apply for a three phase connection, if they have a high enough load but the installation is costly and would involve re- laying a cable from the road to the fuse box. In the UK, they can only supply you with a WYE connection because is how the wires are arranged, coming out of the transformer at the substation. (Without exception, ifaiaa)

 

[David Lewis]

In a simple AC circuit, there is no live or neutral wire. Both wires are the same, just like a DC circuit. The only difference is that the current reverses polarity, that's all.