Electrical Earthing Explanation Needed

[sossage]

Hi,

I know that metal light switches etc. / wallboxes need to be earthed.

I know that mains wiring is earthed.

I also know that the earth connection should basically travel back to the Consumer Unit.

My question is - where does it go from there ? i.e. is "earthing" a piece of equipment just ensuring that an earth connection goes into the ground or is there something more sinister.

If this is the case, wouldn't the fact that a wallbox is in a wall mean that is is already "earthed" - i.e. through the wall into the ground.

I'm not about to remove all my earth cabling, but was just interested as to the definition of this.

Thanks in Advance,
Tarqs.

 

[breezer]

An interesting question with an equally interesting answer.

The earth actually comes from the centre of the "star point" at the transformer where it is generated, it has a very large "earth rod" which indeed is sunk into the soil, hence the name "earth"

The cable that connects your house to the "substation" is actually a wire armoured cable (That means it isprotected from mechanical damage by steel stands that are wrapped around the cable)

The Earth block at your consumers unit is actually connected to the wire armouring of this cable (Although some times it is a lead sheathed cable)

By screwing a metal box into the wall does not make it earthed, because the resistance is too high. (electricity will always flow down the easiest path, no matter what or who that is)

So anything electrical should have a connection to earth so that if something goes wrong the current will flow to earth.

There are a few exceptions to this, "double insulated" or Low voltage as used in pond lights are just a couple

There is also a system called P.M.E. but it is rarely used and it is another story, but if you do see a notice saying this installation is P.M.E DO NOT tamper with any of the connections, you should never remove the earth anyway.

If you are using an appliance outside you should always use an rcd (see http://www.mywatergarden.co.uk/rcd.html for an explanation of how this works)

 

[sossage]

Thanks Breezer,

I bet you're good at Trivial Pursuit !

 

[amtodd]

This is lifted from [url]http://www.diyfaq.org.uk/electrical.html [/url] because this was easier than trying to explain it myself.....

A fault current flowing in the earth wiring will cause the voltage on that wiring to rise relative to true earth potential. This could cause a shock to someone touching, for instance, the case of a faulty washing machine and a water tap at the same time. In order to minimise this risk, an 'equi-potential zone' is created by connecting the services to the main earthing point. Such services are: Water Pipes; Gas Pipes; Oil Pipes; Central Heating; Metallic Ventilation Trunking; Exposed Parts of Building Structure;
Lightning Conductor; Any other Metallic Service.

The equi-potential bonding reduces the voltage difference which could exist between the metalwork of these services if an earth fault occurred to any one of them. It does not necessarily reduce the voltage to true earth. For this reason, metal window frames or patio doors should not be included in the bonding system - it could lead, for instance, to a window cleaner receiving a shock if an earth fault occurred inside the building.

The equi-potential bonding connections for incoming services should be made close to where the service enters the building on the consumer's side of the meter, stop cock etc. It is convenient to use purpose-made bonding clips (obtainable from most d-i-y stores) which include a label "SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE". The connections must, of course, be made to metal pipes - not plastic. The bonding conductors back to the main earthing block should be 6 sq mm minimum with green/yellow insulation.

Bathrooms require special attention: The aim is to create a local equi-potential zone, so all extraneous metalwork should be bonded together. This could include:

Hot & Cold water pipes to sink, bath, shower; Waste pipes (metal); Central heating pipes; radiator; towel rail; Electric towel rail; radiant or convector heater; The bath (if metal - don't rely on the pipes alone).[/url]