Education please... or why Neutral?

[ericmark]

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. ?

A friend of mine had a shed at the bottom of his garden which he used as a radio shack. The earth for the radio was four rods one in each corner of garden and buried copper tape between i.e. very good. The electrician who wired the shed had not used the steel wire armour but run a 4mm wire under it to take earth from house (PME) to shed. And had bonded items including the earth bar for the radios.
Workers digging the road hit the cable. The insulation on the earth wire started to melt. I was called and all power switched off. On arriving the 4mm cable was a row of copper bits on the floor.
Removed lid from consumer unit and measured incoming voltage which varied from 10 to 390 volts.
Phoned supplier who told me it is impossible to lose a neutral/earth on a PME supply but workers arrived soon after and compensation was paid to people in road who's equipment had been damaged.
So on a PME system if you are going to install an earth rod make sure the cable connecting it is quite hefty and can take the current from whole system.
Of course with PME it is not an earth rod it is a "extraneous-conductive-part" in the same way as a cast iron waste pipe in coming water main or gas supply. Where homes have metal water and gas supply they do really have extra earths and since all houses will be the same the current down these routes will be shared between all the houses. The problem arises where one house is group is odd one out.
And there are not that many radio hams in the street so he was the odd one out.
As to High voltage and Low voltage getting joined together I know of no time when this has happened. I hope that never changes. However I still think steps should be taken in case. This is now covered in 17th Edition see 44.1

Untitled

• ericmark
• 27 Jun 2009
• 1

 

[Spark123]

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.

The power systems maybe do reference earth potential, however when a system doesn't reference earth potential then why does the voltage to earth, even though it isn't referenced to it, matter?
Using a petrol car as an example, they are not tied to earth and have a few kv kicking about in them. They are fairly well insultated from the ground but the bodywork is a nice conductor should anyone touch it whilst in contact with mother earth. Why don't they get a shock?

 

[ericmark]

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.

The power systems maybe do reference earth potential, however when a system doesn't reference earth potential then why does the voltage to earth, even though it isn't referenced to it, matter?
Using a petrol car as an example, they are not tied to earth and have a few kv kicking about in them. They are fairly well insultated from the ground but the bodywork is a nice conductor should anyone touch it whilst in contact with mother earth. Why don't they get a shock?

You will find there is some graphite in the mix of tyres and wellingtons so they are not perfect insulators this is to stop the build up of static charge.

Also most cars will become Faraday cages if you look at Nikola Tesla's work you will see how he could produce arcs about 7 meters (23 ft) long but the guy in picture is OK.

 

[Spark123]

They will be faraday cages whilst you are inside, I am talking about someone outside the car touching it, unless you have great big holes in the floor of your car!

 

[NotHimAgain]

Spark123 said:

The power systems maybe do reference earth potential, however when a system doesn't reference earth potential then why does the voltage to earth, even though it isn't referenced to it, matter?
Using a petrol car as an example, they are not tied to earth and have a few kv kicking about in them. They are fairly well insultated from the ground but the bodywork is a nice conductor should anyone touch it whilst in contact with mother earth. Why don't they get a shock?

I referred to large power systems - small ones are not usually a problem.

As for the car - I am old enough to remember earth straps and chains being used to try and prevent static.

Go to your local petrol storage depot - is Buncefield anywhere near - you will see the tankers are earthed before they are charged or discharged with fuel 9otherwise big bang ).

 

[Spark123]

Yes, that is to avoid static build up. You see it on aeroplanes too.

It is a bit different to having a neutral not connected to earth, as is similar to a system in a car. My arguament is, in a car you have a few kv kicking about, why don't you get a shock? I understand it is because it doesn't have a reference to earth, hence current can't flow to earth from it. It doesn't matter what voltage is created within that system, it is a separate system.

 

[NotHimAgain]

The car has a small electrical system and it would only reference earth if a fault to earth occurred.

Note that in BS 7671 - if electrical separation is used for more than one appliance it must be supervised - this is because a first fault would earth reference it.

If a system is not using earth for fault interruption i.e. an IT system - it is still referenced through an impedance to allow for fault alarms (Norway has some LV power networks that are IT)

 

[mattylad]

Never mind all those technical explanations, the reality of it is that what the power company sent down the line does not always get fully used up so the neutral wire is so they can take it back then resell it to you.

 

[ban-all-sheds]

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".

More details.

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.

I have 2 shaver sockets in my bathroom.

 

[ban-all-sheds]

http://www.youtube.com/watch?v=9tzga6qAaBA

 

[ericmark]

From my school days electrons flow from negative to positive and flow better if the negative is hot. So cars where changed from negative earth to positive earth.
As time progressed it was realised that this caused corrosion and it was possible to have the LT with negative earth and HT with positive earth.
This was just before the alternator replaced the dynamo and from that point all cars have been negative earth on the LT system.
Petrol tankers and the like have an earth return system where the chassis is not connected to the battery.
Normally the graphite mixed into the type mix is enough to ensure the vehicle remains at earth potential but with some cars i.e. LPG powered then a earth is connected before re-fuelling.
At one time it was common to see cars with chains or heavy impregnated rubber bits which trailed on the ground and also tyre wall paint was available to enhance the connection of the car to ground.
In real terms the raising of the cars potential above earth is more likely due to something lightly touching the fan belt or similar and is static rather than coming from ignition coils and the like.
However cars are earthed and where this fails it is likely to be felt by first person to touch the car.
So cars are earthed but the amount of earthing and reasons for it are completely different to why we have earths on Domestic premises and to try to compare is only going to cause confusion.

 

[ericmark]

Thank you BAS very good you tube link and extra info on hot wire system. I was trying to get away from the idea that the star point is always neutral and connected to earth and those links I am sure helped.

As to bathroom shaver sockets you could have a 100 if you like as long as each one is powered from it's own isolated supply. Does not have to be separate transformer two or more isolated secondary can be induced by one primary on the same transformer.

However on small generators there is often a problem where two sockets are connected to the same windings and it really does need an earth. However on some generators the earth connection is not connected to either of the line outputs i.e. there is no neutral. This is OK with British 110 volt systems as both lines are both switched and fused but where USA equipment is used with UK system there can be problems.
Also where generators are designed to work duel voltage often even in the 230 volt mode no line is connected directly to earth and often the series/parallel switch does not change the tapping to earth so one line is 57 volts to earth and other 173 volts to earth. OK with class 2 equipment but not with class 1.

As a result of these anomalies I am very wary with small generators.

As yet I had not seen any comments from DCC have we helped or hindered him!

 

[ebee]

"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. "

Pardon?

 

[ericmark]

"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. "

Pardon?

Where cable return is used independent of neutral then line - earth 0.8
Where cable and earth rod is used i.e. PME then line - earth 0.35
It will not quite be half of 0.8 for earth line as earth cable is often smaller than line cable but quite obvious the earth is a very good conductor with near the whole of 0.35 figure being taken up with line cable.

Someone was trying to say earth is a bad conductor this is not correct it is a very good conductor.

 

[Spark123]

Mother earth isn't part of the Ze path in a TN system tho.