Electrical Earthing

The first question any budding electrician asks "If a bird can land on a pylon and not get a shock because there is not earth why don't we use an earth free system." so first I need to explain why we need earths.

Having worked on a tunnel boring machine where there was no earth I soon found out any large system will find it's own earth. Only by using an earth monitor can you possible keep any large piece of plant earth free. And even then it can be a problem I worked on a batching plant where we had an earth monitor and it took 16 hours to find the fault which was a horn that was on plans as being 24v so was last to test. One of the problems is any earth free system need all pole switching but on an earthed system we can use single pole switching as any connections between neutral and earth will not cause danger. And for many years it did not cause any other problems either until the earth leakage trips was born.

So what is the earth for?
The earth ensures in the event of a fault the supply is automatically disconnected. There are two main ways of doing that the first is to open an over current device i.e. blow a fuse and the second is by measuring power in and power out and if they are not equal it is assumed the difference is going to earth and it operates what is called a residual current device RCD in both cases the supply is automatically disconnected.

Blowing a fuse with an earth fault.
When there is an earth fault we not only want the fuse to blow but we also want it to blow quickly and it has been calculated to get a 13 amp fuse to blow in 0.4 seconds it will take 95 amp. In order for 95 amp to flow using ohm's law (230/95) we know we know the resistance needs to be 2.42Ω this is referred to as the earth loop impedance. Continuing I would start to make this look like a maths lesson but fortunately for us the IET and BS have produced a book called the 17th Edition Requirements for electrical installations or BS7671:2008 which has a load of tables that we can use to reduce the amount of maths we have to do.

The TN system.
With the TN system a really good earth is supplied by the electricity supplier either completely separate called TN-S or at some point combined with neutral TN-C-S also know as PME. As the user we may not know which system is used but results are the same and we can take supplies to fixed appliances like cookers relying on this earth to make it safe. (This assumes the cable does not need extra protection) With a system like this very large currents can run for a short time so we need to have quite large cables.
Note:- For TN-S system we expect the Earth loop Impedance to be 0.8Ω or better outside the consumers installation.
For TN-C-S system we expect the Earth loop Impedance to be 0.35Ω or better outside the consumers installation.

The TT system.
With this system we use earth rods and because earth rods do not give anywhere need as good of an earth as the TN system we have to employ an extra method of automatic disconnection the RCD but on the plus side the cables don't need to be quite as thick.
Note:- We expect the Earth loop Impedance to be 21Ω or better outside the consumers installation. This means even with a perfect earth rod on the consumers premises we still may have an earth loop impedance of 21Ω. Since a value over 200Ω may be unstable this is considered as highest acceptable value.
The Electrical Safety Council do a PDF to explain this more fully.

Supplementary equipotential bonding.
Although normally supplementary bonding will be connected to the main earth system via electrical equipment in the area it does not directly connect back to the main earth. The idea is that if all metalwork is at the same polarity what ever that is then you can't get a shock. To get a shock there must be a potential difference between two items to which you are simultaneously touching. Back to bird on high voltage wire. If I take the bathroom as an example the water pipes may be connected to the earth rod near the front of the house and due to a fault this rod is at 45 volts above the normal earth voltage and the soil pipe metal goes into the ground at the back you could then in the bathroom get a shock of 45 volts by touching soil pipe an water pipe which while dry would only be uncomfortable but wet from the bath it would be a different story. 

Definitions
I have already covered some of this but there are one or two other bits one needs to know.
Earth fault current. An overcurrent resulting from a fault of negligible impedance between a line conductor and an exposed-conductive-part or a protective conductor. This is a direct short to earth and the maximum that can flow.
Earth leakage current also called Prospective conductor current. Electric current appearing in a protective conductor, such as leakage current or
electric current resulting from an insulation fault. In this case not necessary a short circuit maybe only a few milliamp.
Circuit protective conductor (cpc). A protective conductor connecting exposed-conductive-parts of equipment to the main earthing terminal.
Main earthing terminal. The terminal or bar provided for the connection of protective e conductors, including protective bonding conductors, and conductors for functional earthing, if any. to the means of earthing.
Exposed-conductive-part. Conductive part of equipment which can be touched and which is not normally live, but which can become live when basic insulation fails. For example the metal of an electric towel rail.
Extraneous-conductive-part. A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation i.e. the soil pipe I talked about.

The main earth may be a block as shown far right or part of the consumer unit enlargement shown above. The earth rod is normally housed in a pit of some kind an example is shown, so that no one can normally touch the rod it's self or get too close to it. Where earths are connected to water pipes and the like earth clamps are used which have warning sign warning that it should not be removed. The regulations changed in 2008 and there are houses wired to both old and new regulations and it is important that they are not mix and match as because the use of RCD's is now required for nearly every circuit in a house the requirements for supplementary protective bonding have been relaxed but they have not been removed where the RCD requirements have not been met.
As well as what must be earthed there are a host of regulations telling us what size cable and how long a cable all tied up with how it is protected and these do become complex.
With the restrictions of Part P regulations it has become costly for the DIYer to work in bathrooms, Kitchens, and outside on electrical installation and in the main registered electricians who pay a lot less to register their work with building control will do work in these locations. So as a result these have been skipped over.

Cable sizes
Since the main incoming earth is often either from under ground or a TN system 16mm² is considered as the safe size where reference to regulations does not need to be too precise shown as 3 on diagram. Where the earth wire is protected against mechanical damage (542.3.1) it may be smaller but where there is no corrosion protection it raises to 25mm² but (544.1.1) never less than 6mm². (543.1.3) gives a formula to work out cable sizes but (table 54.8) gives 10mm² as min where neutral supply cable is under 35mm² no earth wire not mechanical protected will be under 4mm² (544.2.1).
So an a TT system you could be OK with 6mm² earth cables for 2 and 16mm² for 3 but on a TN system both 2 and 3 would need to be 10mm² in both systems 4 would be 4mm² and wire 1 if part of supply cables then 1mm² (assuming that the supply is 13 amp) if not the 4mm². 
Had I not shown minimum sizes I am sure someone would try to tell off some electrician for getting it wrong. One problem is where the supply authority up grade the supply from TT to TN and I would always use 10mm² cable for extraneous conductive parts. When fitting an earth rod one needs to visit supplier to get rod, box and clamps so also getting some 16mm² cable at the same time is not problem so most electricians would only carry 4 and 10mm² earth cable with them. The fact that electricians could use the 543.1.3 formula means that one can't automatically say it is wrong when under size.

Earths outside
Exporting power from the house to an out building can introduce even more problems in keeping earths within the regulations. If 6mm² 3 core SWA cable is used normally all earthing requirements will be met and if not the addition of an earth rod will be all that is required. The extraneous conductive parts again is the problem in that they require 10mm² earth or equivalent which is why I would use 6mm² SWA cable as the steel in 6mm² 3 core is above the 10mm² equivalent. With buried cables if not protected i.e. part of the supply cable and in same sheaf needing to be 16mm² it can work out more expensive using smaller cables. Where there are no extraneous conductive parts in the out building i.e. wooden garden shed then smaller earths can be used. There is also the possibility of using a TT supply but this can sometimes cause more danger and since any work in the garden comes under Part P I see no point of expending further.

The RCD
Most new houses will have an RCD to protect them from electric shock but older properties had v-ELCBs (Voltage operated Earth leakage circuit breaker) to assist with earthing arrangements. Years ago earth leakage trips worked with voltage and not current and to the left are two examples. The enlarged picture shows how earth wires are connected to this unit and it worked by passing all the house earths through the unit but where extraneous conductive parts are earthed the effectively shorts out all the protective part of this unit and it was so hard to ensure it was not shorted out when the current sensing versions became available these where phased out and I think are now banned. For many years they were identified by the yellow button but now there are some current RCD's with yellow buttons. I have never seen this type of v-ELCB being built into a consumer unit all the versions I have seen to date are stand alone units.
These and the latter current type were primary designed for where earth rods were used and would allow more than the 30ma considered as safe for personnel to flow before they would trip the 100ma version was popular often with a built in delay called S type. Many older houses had two RCD's a 100ma S type would supply the whole house then a further 30ma would supply sockets that may be used out side. Houses built today have all sockets protected and also everything in the bathroom which has resulted in extraneous conductive parts not being required to be bonded. One of the big failings of the 30ma trip has been unwanted tripping and regulation 314.1 has tried to redress this problem and to satisfy this regulation multi RCD's are required in most houses often being provided by using RCBO's which is a RCD and MCB (Miniature circuit breaker) combined.

Cable length
Before the use of RCD's as already explained it was required that enough current could flow Line to Earth to operate the MCB or fuse. With the RCD this restriction is no longer valid and many now consider they can use more cable in ring mains etc. But Line to Earth is not only consideration and Line to Neutral impedance and volt drop are still valid restrictions so no more than 80 meters of 2.5mm² twin and earth in a ring main is still valid. Because the meters used to measure PFC and ELI are so expensive I see no point in listing the requirements. The same applies for insulation and low ohm meters together with RCD testers. Although you may be able to hire an earth rod testing meter in most cases it still can't be used as you need to have access to at least 30 meters around the earth rod.

Conclusion
This is only designed to give the DIY person an inside to earthing for the sake of interest and is not designed to instruct electricians or allow anyone to undertake DIY earthing. The skill and tools required are unlikely to be available but it may help to identify faults which will in turn get the home owner to get an electrician to correct. In theory on change of occupancy or every 10 years all homes will be electrical tested and inspected but in reality only rented properties seem to get this done so out of date and faulty items remain undetected for years. I hope this helps to redress some of those problems.

Note:- The earth system outside a building becomes complex. The supply transformer is earthed very well and if we consider that as Zero then as we move away from the transformer there can be a gradient even with no fault, with a fault the gradient can become steep.