Education please... or why Neutral?

[DCC]

Although I understand some of the practical side of electrics on an amateur basis I notice my head shies away from ideas around 'earth' and 'neutral' and AC power distribution, give me a battery and all is clear -point me at what comes out of the ground in my understairs cupboard and the theory of it sort of makes my brain jitter a bit and then reboot. So, with the attitude that learning is good...

Correct me when I go wrong but I've been trying to understand how things work: If I understand it tight both earth and neutral are essentially physically tied to the ground beneath my feet, the difference being earth is relatively local to my house but neutral may be connected to dirt anywhere between my TV and Drax depending on what the supplier does. Neutral may not be at the same potential as my earth because its physically connected to earth at a different location.

*If* the above is more or less right it begs the question - why do we have neutral at all? Why wouldnt every property have a big earth spike, connect all CPCs to the earth and also connect all current neutral connections to it as well.

I guess I am touching on the same question as "why do we have different supply types" which would probably explain much of what I've been pondering.

 

[ColJack]

the "general mas of earth" is a relatively poor conductor of electricity..

the idea is to get the electrons back to the other end of the coil that supplies your live..

the neutral would be quite happy if it wasn't tied down to earth, but then you'd have no reference for the live to earth.. it might float up as high as several thousand voilts relative to earth....

not a problem if there was no earth anywhere at all on the supplies..

 

[ericmark]

First I will try to explain earth and why.
To transmit power over long distances we raise the voltage with transformers. Then local to point of use we transform the voltage to a usable value. To ensure if any fault should occur a fuse or other safety device will blow we use earth mats or rods to tie the voltage to a maximum value in respect to earth.
There are a number of ways this can be done but the most popular is to use a star wound secondary and to connect the star point to earth.
As well as the safety from transformer failure there are other reasons to use an earth.
If as with the isolation transformer used for shavers in a bath room we did not connect a point of the transformer to earth for a single item this would make it safer but as we use multi items unless we tie one point to earth two faulty pieces of equipment can make different pieces of metal have different voltages and one could get a shock by touching for example the kettle and toaster at the same time.
What ever point is selected as neutral is therefore connected to earth.

But as we leave the point where neutral and earth are bonded together different amounts of current flow in the earth to the neutral point and both the wires and the earth have some resistance so the two voltages will get further and further apart as you travel away from the point where they are bonded together. It is considered that under fault conditions this differential should not exceed 50 volts.

In ac systems the resistance is called impedance and we have to measure the resistance of both the earth and the line wire to ensure should the two become connected together enough current will flow to open circuit the safety device before the voltage exceeds 50 volt.

This is one of the reasons why it is not considered safe to allow people to DIY on electrical systems because an electrician will always measure the "Earth loop Impedance" to ensure the system will fail safe in the event of a fault. However the meters required to measure this are expensive and while doing the tests dangerous voltages can be generated.

Three phase to single

• ericmark
• 3 Mar 2009
• 2

the picture shows how three phases are connected to different houses and the centre point neutral is also connected to earth it is the only common wire to all three houses so it has to be that wire.

Untitled

• ericmark
• 3 Mar 2009
• 4

the second picture shows how the 230 volt we get in each house can also supply 400 volt between phases so if you had a fault on your lawn mower and your next door neighbour also had a fault on their lawn mower as you shake hands you could get a 400 volt shock. In real terms because of the safety built in to these items it is not likely to happen.

Be aware I am showing British system in the States a different system is sometimes used. So an internet search may turn up conflicting information.

There are some internet sites with animated diagrams hopefully someone can point these out to you.

 

[Spark123]

The main concept of tying a system down to earth isn't to clear a fault however it does become part of the fault path for faults in some systems.
The main reason for earthing the star point of a 3 phase system is to limit the potential above mother earth of all of the phases so only a 230v nominal shock is possible between any phase and earth.
If a 400v 3 phase system wasn't earthed and one phase became in contact with mother earth through a fault, (this first fault being undetected could sit there for years), then a second fault occurs with someone touching another phase they would get the full 400v potential across them to mother earth.
We therefore apply this link to mother earth at the star point to reduce this risk.
The neutral is tied to earth so should sit around about 0v with respect to mother earth, the neutral and earth both share the same conductor which goes back to the transformer from the service head in a TN-CS system. This type is often tied down at multiple locations along its length too.

 

[ban-all-sheds]

the "general mas of earth" is a relatively poor conductor of electricity..

Not unknown for it to be used as the return path for EHV DC transmission lines....

 

[ban-all-sheds]

the neutral would be quite happy if it wasn't tied down to earth, but then you'd have no reference for the live to earth.. it might float up as high as several thousand voilts relative to earth....

not a problem if there was no earth anywhere at all on the supplies..

I can see it now in the 18th

All supplies for low-voltage installations not intended to be under the supervision of a skilled or instructed person shall be SLV.

And in Wiring Matters Mark Coles will be saying that if you change a light switch you must install the whole-house isolation transformer.

 

[Adam_151]

All supplies for low-voltage installations not intended to be under the supervision of a skilled or instructed person shall be SLV.

Skipping over the joke... as it says on the schedule of inspections, separation for multiple pieces of current using equipment is recognised as being a special protective measure (like placing out of reach, or earth free bonding is) and is not recognised for places which are *not* under the supervision of a competant person...

Makes sense really, an undetected fault does not cause loss of fuctionality, but does collaspse the system into a an earthed system...

 

[ericmark]

In Hong Kong I worked on a Robin tunnel boring machine. This was supplied from an 11kv to 10kv transformer on the surface. Then on the machine it was transformed to 660v for main motors and then further to 220v for control and lighting. This 220v was supplied from a delta secondary and used the IT system. It was very hard to fault find on the 220v system. If one phase went down to earth the other two would show 220v to earth but with all phases floating there was no voltage to earth and one had to always measure against another phase. Although the voltage was a lot lower than the 400v used in the UK between phases it was more dangerous.

The neutral point is not always the star point. Delta output transformers are used in other countries and one winding is centre tapped to earth. This is what coined the term "Hot wire" so to earth with 220v between phases you get 110v on two phases and 190v on third to earth, this third is known as the "Hot Wire".

There are two methods used in UK. In the main the three phase system is used giving 230v phase to star point and 400v between phases. In out lying areas a single phase system is used with centre tap giving 230v phase to centre tap and 460v between line wires. The star point or centre tap is connected to earth.

Without this earth connection if a transformer was to develop a short the 3.3kv or 11kv supply could end up on the output of the transformer without causing excessive current flow. Because with an earth there would be excessive current flow it will blow a fuse or open some other protective devise making the system fail safe.

Of course some outlying villages even in 2000 still had their own generators so the need to protect against transformer faults did not exist. However they still need to connect an earth. In this case it is to make sure you don't have two faulty bits of equipment which can be touched at same time and have 230 or 400v between the exposed metal parts. So when using a kettle and a toaster you don't get a shock by touching both at the same time.

If you use a generator to supply one item only i.e. an electric drill then there is not really a need for an earth. This is used with shavers which in bathrooms are supplied with an isolation transformer. But as soon as two or more items are used then you need the earth.

There may be exceptions and there is provision for working in an earth free environment but this will not apply to domestic supplies.

As to the resistance of earth it as a mass is very low. Up until well after the conflict in the Falklands they used a telephone system which only had one wire. The earth was used as second wire. However the problem was in high winds static was produced between the ground and the wire. One cure to this was to use digital signals instead of voice and the railway networks used this quite a lot using a series of dots and dashes. The fact that more current was being used got around the problem of static.

However the baud rate was very low on this system. It was considered enough to be allowed to operate when one could send and receive 12 words per minute. At best unlikely 50 words per minute could be bettered.

Where the earth is used as a fault return path as well as a wire the resistance of both feed and return to a domestic is lower than 0.35 ohms however when only the wire is used for return the resistance is only lower than 0.8 ohms. Remember this is both feed and return so the resistance of ground will be very low.

However to connect to the ground is a problem. To get a figure of 8 ohms at Point of Ayr near Rhyl I had to use between 3 and 6 rods each 1.2 meters long. This is only time I have used the special meter to measure the resistance of the rod to earth. Would be interesting to know what is measured else where. I know in Algeria we had to dig earth pits to get the connection needed to work the Cathodic protection used on the pipe line. But fit two earth rods at 8 ohms each and measure resistance between them and it does not seem to matter how far apart they are you still get 16 ohms between them.

 

[Spark123]

I haven't come across a transformer which has failed and put 11kv through the 400v network as of yet, don't they have HV earths to them as well for the HV side?
I was under the impression they are condstructed to minimise the risk of this type of fault happening.

Where the earth is used as a fault return path as well as a wire the resistance of both feed and return to a domestic is lower than 0.35 ohms however when only the wire is used for return the resistance is only lower than 0.8 ohms. Remember this is both feed and return so the resistance of ground will be very low.

If you have metal connecting you directly to the star point (TN) then you are going to have a low impedance.
This has nothing to do with tie to mother earth - iirc that resistance can be up to 21 ohms.

 

[Spark123]

the "general mas of earth" is a relatively poor conductor of electricity..

No it isn't, if you think about it as a lot of conductances in parallel then it can be a very good conductor of electricity. The ties to earth and conductivity of the soil etc will have an effect on the overall resistance of the path which can give an overall poor path.

the neutral would be quite happy if it wasn't tied down to earth,
but then you'd have no reference for the live to earth.. it might float up as high as several thousand voilts relative to earth....

What would it matter? It would still be a separate system which isn't referenced to earth.

 

[OilLecky]

Well DCC...did you get that?...'

(A very thorough synopsis.....(debate?)...... gentlemen. Well done!

 

[bernardgreen]

the neutral would be quite happy if it wasn't tied down to earth,
but then you'd have no reference for the live to earth.. it might float up as high as several thousand voilts relative to earth....

What would it matter? It would still be a separate system which isn't referenced to earth.

In the perfect system having neutral and live NOT referenced to earth would be safe as there would be no pressure ( voltage ) on the electrons to get back to the source transformer via a persons body and the earth.

It however would fail and become un-safe as soon as there was one fault.

A second fault could then prove fatal. If Mr Smith standing on wet ground held the Live ( first fault ) he would be OK. But if then at the same time Mr Bown held the Neutral ( second fault ) while both were standing on the ground they would both receive electric shocks. From Live down through Mr Smith across the earth and up through Mr Brown to the neutral. One or two could be dead.

 

[bernardgreen]

While on the topic of earthing,

PME systems. Protective Multiple Earths, On overhead systems there are on some poles a wire down the pole from the neutral to an earth rod. ( some of these do appear to need maintainance where dog pee and other things have severed the connection at ground level ).

As the network distributed neutral is also the source of the "earth" in the domestic property it would seem logical that an earth rod at a house and connected to the MEB would be doing exactly the same job as the (possibly defunct) earthing of the neutral at the distribution pole.

So why is it considered not a good idea to have an earth rod at a property that has a PME supply to it. ?

 

[NotHimAgain]

Spark123 said:

What would it matter? It would still be a separate system which isn't referenced to earth.

It matters because all large power networks reference earth potential, even if just by capacitive coupling. If you don't know the voltage to earth at some point in the system how do you decide how much insulation to use on your conductors.

This is one of the main reasons why supply networks are referenced to earth deliberately. This is not always by a low impedance (IT system).

BAS said:

Not unknown for it to be used as the return path for EHV DC transmission lines....

Yes but it can cause severe corrosion in underground services etc and the worms don't like it either

 

[OilLecky]

Earth's a "funny thing" you know. It ain't always what you think....I think.
Here's an analogy that is true. Once upon a time I worked in an electrical workshop and despite equipment passing an insulation resistance test, people were receiving electric shocks from that equipment. How?
When they were doing the insulation resistance test, using an "electrostatic meter", the meter showed low resistance for a short while and then infinity, despite there being a short circuit in the equipment. The equipment under test was bolted to a large metal test rig and the rig was absorbing the electrons from the meter until it couldn't supply any more. (i.e. like a capacitor) No more current flow so it read infinity!! This also happened when the two probes from the meter were put about 1 mtr apart on a large piece of sheet metal but showed a short when the probes were put close together. I had to do this several times to persuade the idiot in charge that the meter was unsuitable for the job.
A similar thing happened when some "experts" carried out an insulation test on a train (as in railway) power supply using a normal electronic insulation tester. They found good high insulation resistance but the train kept tripping the RCD at the "shore supply". This baffled the "experts"!!

An air-conditioning unit about 3/4 of the way down the train had a direct short circuit but the electrons never got that far. They were dissipated in the large cable (length & diameter) between the front of the train and the defective carriage.
In the same way a discharge (fault or otherwise) to a local "earth" may not actually return to a centre tapping of a transformer but can dissipate into the surrounding earth. It may be that an "electrostatic" effect then discharges electrons from the local area around the transformer centre tapping earth into the transformer to balance things up but I believe that it may not always. So I think that earth in fact is a large mass that absorbs excess electrons from faults but does not necessarily return them to where they came from. ............Thoughts on that gents?