Domestic electrical installation earthing and circuit protection – part 1

This is a, rather lengthy (even though it was intended to be brief), bit on the history and background of domestic electricity supply earthing and circuit protection systems, and why they are important, to help understand why we have earthing and circuit protection schemes in the first place, what they are for, how they’ve evolved, and why, together with the expertise of the electricians we employed, I made the choices I did.

Like any history, it’s written from my personal perspective and reflects my view on why certain things were done at certain times.  Much of the reasoning for some of the decision making has been lost, so inevitably some of the historical aspects are a reasonable guess. using information from multiple sources.  The later history is more accurate, as I was indirectly involved with rule making from around the time of harmonisation onwards, specifically with the formulation and application of the LV Directive and the EMC Directive.

This may sound an odd thing to write, but there is NO such thing as a safe electrical installation, and there never can be.  Safety is not a finite entity; no matter how hard we try we cannot remove all of the risk, and some risk ALWAYS remains.  All we can ever do is try to mitigate the remaining risks by a carefully thought-through design, balancing what we personally find acceptable in terms of risk, cost and convenience, with what is within the requirements of the regulations that apply at the time your installation is installed.

This article is not directly related to our build, but it is background that may help explain some aspects of electrical installation design, specifically why we have two different earthing systems on our site, one on the supply side and a different one on one external part of the protected consumer side.  I’ll go into more detail about our external wiring in a later post that addresses that specifically, and gives clear reasons as to why it was done that way, as several people have asked me about it and it seems to have caused some controversy elsewhere amongst the less well-informed.

I’ve decided to write it here, primarily as advice to self-builders, as whether you employ an electrician, or feel competent enough to do your own electrical work (with the appropriate inspection and testing by a competent person), there is information you need, and decisions that you need to make, in order to determine what best fits your own personal balance between safety, risk, cost etc.  There is no “one-size-fits-all” solution, in my opinion.  It is my view that only you will know what risks you are prepared to accept, how much more you are prepared to pay to reduce the risk level further, and what level of inconvenience you will accept.  If you can decide what is acceptable to you, then it goes some way to helping with the overall design of your electrical installation.

You may have some planned future uses for areas that impact on the design of your electrical installation, and rather than just let your electrician decide what to fit based on what you’ve specified now, it’s useful if you know enough about the impact of future plans on the electrical installation so that you, or an electrician, can plan ahead and choose the best solution specifically to meet your anticipated requirements.

Also, although I hate to say it, there are some electricians around who are not as competent, knowledgeable or free-thinking as they should be, so the better informed you are, the more chance you have of getting an installation that is acceptably safe, convenient to use, meets your needs and is within your budget, without the need for later costly modification.

First some definitions, as they may make reading this, and the second part of this article, that will cover circuit protection, a little easier:

Line (sometimes colloquially called “live”) is the power feed wire FROM the supply to the Load.  In simple terms, this is a really dangerous connection in terms of getting an electric shock.

Load is the equipment or device that is using electricity when it is operating.

Neutral is the power return wire FROM the Load to the supply

Earth is the potential of the ground around the Load, but is colloquially used as the term for the wire that protects either the  supply, or the Consumer, by being connected to the ground potential.  It is sometimes referred to as the Protective Earth, however, strictly speaking, these may have either the same, or different, roles, depending on the nature of the system.

Consumer is the generic term for the user of the electricity supply.

Conductor is generally a wire or cable that conducts electricity, but can refer to anything that can conduct electricity, such as a metal case or pipe.

Protective Earth and Neutral (PEN) is the term used when a supply conductor Neutral is also the Protective Earth connection, and may, infrequently in my experience, also be referred as the Combined Neutral and Earth

Fuse or Circuit Breaker or Miniature Circuit Breaker (MCB) is any device that is intended to break or disconnect a circuit whenever the current flowing through that circuit exceed a certain rated value.  In a domestic electrical installation it is there to protect the conductors in the circuit from over-heating, becoming heat damaged or even being a possible cause of fire, nothing more.

Switch is a device for making and breaking one or more ciruits when operated.

Pole refers to any individual and separate electrical supply conductor, and in the context of this article normally refers to either the Line or Neutral

Voltage is the electrical equivalent of pressure in a water system, it is the “force” available to drive a current through a circuit.

Current is the electrical equivalent of the rate of flow of water in a plumbing system.

Potential refers to a voltage difference between two conductors or a conductor and an area, such as the voltage difference between the Line conductor and the local Earth.

Impedance is a measure of the opposition an electrical circuit presents to the initial current flowing through it when a voltage is applied.  It is not the same as resistance.

Resistance is the steady-state opposition an electrical cicruit presents to a steady current flowing though it.  It may well be lower than the impedance in the specific case of measuring domestic electrical installation circuits.

Single Pole (SP) refers to any switching device that only makes and breaks the Line conductor, rather than both the Line and Neutral conductors.

Double Pole (DP) refers to any switching device that makes and breaks both the Line and Neutral conductors at the same time.

Isolator is another term for a specialised type of DP Switch, one that may or may not have additional special  capabilities.

Residual Current Device (RCD) is a device that measures the difference between the current flowing through the Line conductor and that flowing back from the load through the Neutral conductor, and breaking the circuit if there is a difference between these two currents.  A difference indicates that current is “leaking” out of the circuit, so in some countries you will find these devices referred to as Earth Leakage Breakers.  In many ways this is a more accurate description, as the “leaking” current is almost always flowing to Earth.

Residual Circuit Breaker with Overload (RCBO) is a combination device that combines the function of an RCD with an MCB.  In other words, it will break the circuit in the event of either a current overload, or a current imbalance between Line and Neutral.

There are many other terms, but I’ll try and keep this as simple as I can, and define any thing else as I go along.

A Much Simplfied History Of Domestic Wiring Development

Years ago, when we first had electricity supplies in homes, we had no earthing or circuit protection systems at all, power was distributed around a house most commonly by two core cable  and two pin sockets and one of the two pins was usually close to earth potential, because it was connected to the earth (literally)  back at the power station or substation, or even at the house battery pack or generator in some cases.  Inevitably there were deaths and injuries from this system.  Plugs could be inserted either way around,  a broken connection or lead could make an appliance case live and create an electric shock hazard and wiring wasn’t protected from overload, could over heat and start fires.  As most appliances were made of, or contained exposed, metal parts, the electric shock risk was very real.

To make things worse, plugs and sockets weren’t to any particular standard, they varied from one manufacturer to another.  Between the wars, standards were first created, and one of the first things that were standardised were the dimensions of 2 pin plugs.  Standardising plugs didn’t address the safety issues but it did mark the effective beginning of the document that we still rely on as the “bible” of electrical safety, by creating the first electrical system regulations.

As soon as the use of electricity became fairly widespread, it was clear that there was a significant risk of electric shock from the two pin system.  The plug  standard was changed to a 3 pin design for higher power appliances, still with round pins, but with the third, larger and longer, pin being connected to earth.  Lower power appliances continued to use two pin plugs, and two core cables, for many years, despite the greater risk of electric shock.  At the time that three pin plugs were introduced, it wasn’t usual for there to be an earth connection provided on the incoming supply cable to a house, just a line and a neutral, or a positive and negative on DC household supplies, as at that time we had a mix of supply types and voltages.  Some were Alternating Current (AC) and some were Direct Current (DC) and the supply voltage varied, too.  The system you had depended on where you lived, as each supplier tended to have a slightly different system.

Although the neutral in an AC supply (or either one of the two terminals in a DC supply) at this time was usually close to earth potential, it could well have been connected to that earth a long way away, so have a fairly high impedance compared to the local earth.  Impedance is important, as it determines the “instant resistance” of the cable when a fault current starts to flow through it, and may well be higher than the steady resistance of the same cable when the current has been flowing for some time.  This is down to some detail I won’t go into here, but suffice to say that, when a fault occurs you want that initial resistance to earth to be as low as possible, so high fault currents can quickly be conducted to earth, without increasing the potential (voltage above local earth in this case) of the faulty appliance parts that can be touched, to a dangerous level.

When three pin plugs were introduced, they needed an earth connection for safety, but generally no such connection was provided with the electricity supply.  This meant that the earth connection had to be created, either locally, or by a third conductor provided by the electricity supplier.  Interestingly, this connection gets its name from the way it made, the earth conductor, wherever it happens to be located, is physically connected by a conductor to a rod or plate that was buried in the earth outside, so it was, originally, literally a connection to the earth.  The idea was that exposed metals parts, or the metal case, of every electrical appliance would be connected to this earth terminal through the third earth pin on the plug and because this terminal was close to the same potential as the floor the person was standing on the risk of electric shock was very much reduced.

For example, say someone was using an electric iron and was standing on a flagstone floor, when the iron developed a fault, say a frayed cable that was touching the case or handle inside the iron.  With a two pin system there was a near-certain risk that the person using it would get an electric shock, because the current could only pass from the live case of the iron to the persons hand and then through their body to the ground they were standing on, in order to get to the earth beneath.  With the three pin system, by connecting an earth cable to the case of the iron, the fault current would rather flow through the lower impedance earth cable, to the external earth connection, than through the much higher impedance of the person holding the iron, so greatly reducing the risk of electric shock.

One consequence of adopting this system, was that fault currents through the wiring and earth cable under a scenario described above, could be very high, high enough to overheat the wiring and perhaps start a fire.  So, one risk had been reduced, but another made significantly greater.  The risk of fault currents over heating a conductor had previously only been from the line and neutral, or positive and negative, conductors touching, or shorting out, so earthing introduced an additional conductor overload risk.  This risk was mitigated by adding a fuse into each circuit in the house.  This fuse was only there to protect the cable in the house, an important  principle that still applies today in a domestic electrical installation, even though we’ve mainly replaced fuses with different types of over-current protection devices in our fixed wiring today.

For decades this three pin plug system worked well, but there were still hazards with it.  Although the plugs were now a standard size, the same wasn’t true for the design of sockets, and most did not include any means of preventing something being poked into the line connection, giving that person a potentially lethal shock.  Also, there was no specific fuse protection for the flexible lead from the plug to the appliance, as the fuse protecting the house wiring would usually be rated to blow at a much greater current than the thin bit of flexible cable connecting something like a bedside light.  We also still had two pin plugs and two core cables for low power loads, like lights, and so there was still an electric shock risk from those.

Around WWII, a new “universal” standard of plug was introduced, that corrected both these failings, and a standard was also made for the front face of sockets, to add further protection to consumers.  This lead directly to the plugs and sockets we use today, those made to BS1363.  They overcame the need to protect the appliance cable from over load by having the facility to fit a small fuse inside the plug.  They also reduced the risk of a young child poking something conductive into the line connection, by having a shutter that blocks the line and neutral, and which is opened by the longer earth pin being inserted.  The longer Earth pin also meant that the Line or Neutral side of the circuit would be broken first, if the plug was pulled out, leaving the appliance earthed right up the the point where the plug was fully removed from the socket.  This is probably still one of the safest domestic electrical connectors around, especially now it’s been slightly modified to remove the risk of small fingers getting in the gap of a partially inserted plug and touching the line pin, by adding insulating sleeves over that exposed part.  At the time that the BS1363 plug was introduced, two pin connectors ceased to be allowed, except for a few, very specific, applications.

Back to earthing systems.  In the early years, a locally-connected earth, to an earth rod buried in the ground adjacent to the house, was the only way of providing an earth connection.  It has some advantages, in that is ensures that the local earth potential in the house cabling is the same as the earth potential of the floor of the house, but it also has some disadvantages.  One disadvantage is that, being buried in the ground its effectiveness depended on how good a connection there was between the earth and the rod and between the house earth cabling and the rod.  Dry ground often meant a poor earth, with the house owners being unaware of the potential problem.  External cable connections were prone to corrode, or get physically damaged, again without the house owners being aware.  There were (and still are) ways to enhance the local conductivity of an earth rod, by placing more conductive compounds around it and making sure it is in good physical contact with the surrounding soil, but there are some soils that are notoriously difficult to make a reliable, long-term, connection to.  What’s more, the only way to be sure that such an earthing system was really working was to have an electrician come in and regularly test it, and few households bothered to do this.  A poor, or disconnected, earth, created significant and unseen safety hazard, and made the installation no safer than the old two pin plug system we started out with.

With the introduction of three pin plugs, and the need for a reliable earth connection, it became normal for the electricity supply companies (later the Electricity Boards, and currently the Distribution Network Operator (DNO) to provide an additional earth connection conductor to new houses.  This conductor was often in the form of the protective metal sheaths that were around the line and neutral conductors.   Having a reliable and well-protected earth connection supplied to the house removed much of the risk associated with the older earth rod system, and this soon became the standard way of providing an electricity supply.

There are some limitations to doing this, though, and they come back to something I mentioned earlier, the earth connection impedance.  If this is too high, because the supply earth cable is too long or too small, then the earth potential (voltage above the true local earth in this case)  at the house under a fault condition could rise to too high a level, and the level of electric shock protection could be reduced.  Nevertheless, this was not really an issue in towns and cities, where supply cables were relatively short, and had multiple earth connections along their length from the electricity boards equipment and connections.  Before long, with the boom in post-WWII housebuilding,  it became standard to use the earth supplied by the electricity board.

The only disadvantage of this system was really at the suppliers side, as they had to provide three conductors, insulated from each other, a line, neutral and earth.  Given that the neutral was ultimately connected to earth somewhere in the supply network, this was wasteful of cable, as the neutral and earth were always at almost the same potential with respect to each other.

Once the distribution network had a lot of earth connections in their local supply network, due to the need to earth every supply network transformer, it seemed obvious that there was now no real need to run a separate earth lead to a house, the neutral could easily be made to be at earth potential, so it could also be used as the main earth for the house.  In doing this, the neutral had to be renamed, so it was now know as the PEN, Protective Earth and Neutral, or occasionally the CNE, Combined Neutral and Earth.  At the house connection point, on the DNO side (as it is now, Electricity Board side as it used to be) the incoming neutral and the house earth were connected, with the earth made by a connection adjacent to the main neutral connection on the block that houses the incoming cable termination and company fuse.

Before harmonisation, this earthing system was termed PME, or Protective Multiple Earth on the consumer side of the installation, and term that is still in common use, even though, strictly speaking, it may not always apply to it’s post-harmonisation cousin, TN-C-S.

PME was, generally, no less reliable than the former system of supply a separate earth conductor, and was still significantly more reliable than the system where each house had it’s own earth rod.  It also saved the supply companies (by then nationalised Electricity Boards) money, as they only needed to provide a two conductor cable to many houses, rather than three.   There were still some installations where a separate earth conductor was needed,  usually because of the need to reduce the overall earth impedance at the consumer end to an acceptable level.

Also in many rural areas a separate earth connection or PEN connection was either never provided by the Electricity Board, or if it was, the impedance of it was too high to be safe, so the practice of having a local earth rod continued, and still does.  There are also specific applications where having a local earth rod has an advantage over a PME scheme, particularly where the PME provided earth has to be exported via a long cable on the consumer side of the installation.

Bringing things up to date

With harmonisation with the EU, all the standards were revised yet again,  with pressure being applied for the whole of the EU to use the same power supply, the same wiring standards, the same earthing systems and the same type of plugs and sockets.  It fair to say that the UK had to fight to retain its, very much safer, BS1363 plug, as many EU states still use non-fused plugs with no appliance cable current overload protection.  We had probably the safest domestic system in Europe, and weren’t going to give it up just because the EU said we should!  We did, however, have to standardise terminology, particularly relating to earthing methods, and standardise some wiring standards, as well as accept the EU 230V 50Hz system, rather than our older 240V 50Hz system.  In fact, we didn’t really ever accept the EU 230V system at all, all we did was change the allowable voltage tolerance, so that it’s now 230V +10%, -6%%, which, in reality, means we still run closer to our old 240V standard all the time, but can call it 230V to keep the EU happy………………..

Needless to say there were, and still are, other earthing systems around, some which were mainly used with commercial 3 phase power systems and some that were unique to other EU countries.  There is a good description of all the earthing methods allowable in the UK in BS7671, “The Requirements for Electrical Installations, IET Wiring Regulations” that’s been colloquially known as “the wiring regs” for decades.  That document, in principle, dates right back to that old two wire, no earth, system I described at the start, when it was decided that the government should create a standard for two pin plugs.

Nowadays these regs are produced by a joint committee, that is made up of three bodies, one of which I used to be a member of, as it happens.  The regs still retain the IET name, because the IET publish it and and remain the lead body, but it is produced by a joint committee, recognising that the importance of British Standards has been largely subsumed by EU standards, (and members of my former staff sat on the EU EMC Directive and LV Directive working groups too – not a lot of fun for them, I have to say).

You have to buy a copy of BS7671, and it’s not cheap, as there has been a drive to ensure that all this information is only made available to electricians, and they also have to pay to belong to one of bodies that allows them to certify domestic work that needs it if they are to do this type of work.  All told it’s a bit of a closed shop, intended to prevent ordinary members of the public from getting hold of technical information – not a policy I agree with at all.  Luckily there are sources online for most of the important information in the wiring regs, but some have to be viewed with suspicion, as the internet contains more false information that true information, unfortunately.

I have a copy of BS7671, Amendment 2, but sadly it’s now out of date, as Amendment 3 came out in 2015, after our electrical installation was completed.  I will happily let anyone who asks have an electronic version of my copy, but please just use it for background reading, as I haven’t yet had a chance to see the full extent of the changes that Amendment 3 has brought in (other than the need for non-flammable consumer units).

Getting back to earthing, there are currently five approved methods, all subtly, or not so subtly, different that are allowable, although only three are in common domestic use in the UK.  They are detailed on the IET website ( ) if you are a member or have access some other way, perhaps as a student, but most of the useful stuff needs to be paid for.  There is some good stuff on their forum and some free stuff for students too, which is useful, but no substitute for having the real standards and Guidance Notes.

In the harmonised earthing terminology there are five meaningful and descriptive letters:

T – from the French, Terre = Earth – you can tell where this lot’s come from, can’t you! (actually, some will argue it came from the Latin root, Terra, but blaming it on the French makes for a better story).





There is a very specific way of using this terminology, in order to avoid confusion, and believe me, it can be confusing for the uninitiated, because there are seemingly different meanings for Combined and Separate, depending on context.  For the purposes of most domestic installations, and in the context of self-build, there are really only three common systems that need to be considered, as it’s unlikely that you’d have anything other than these on a new build or new installation.  In all probability you would have the choice of just two of these three, making life even simpler.  There are other systems, but because they are fairly uncommon in the context of a domestic self build I’ve left them out of this bit of background reading.

The system that’s still in fairly common usa, is the oldest, the one where the earth connection is provided by a local earth rod or buried plate and there is either no earth provided by the DNO, or the earth provided is too high an impedance (greater than 0.35 ohms) to use safely.  Under the current terminology, this is referred to as TT, literally Earth – Earth, meaning the house wiring earth is connected to the local earth.  For me this is really the only term that is really meaningful; the abbreviation TT clearly indicates that the “Terre” is connected to the “Terre”, which seems logical when you’re looking at the house earth cable connected to an earth rod, or very rarely, a buried earth plate.  This diagram shows how a TT earthing scheme is connected, and also, most importantly, shows (by means of the dashed lines) who owns, and has responsibility for, each part:

In this diagram I’ve gone no further than show the three wires that connect to the consumer installation.  These wires must be 25mm² in cross sectional area for the Line and Neutral, and be no longer than 3m in length from the meter termination to the consumer equipment, be double insulated for the Line and Neutral, with a brown core for Line and a blue core for Neutral and would normally terminate in a nearby consumer unit (what would have been called the “fuse box” in times gone by).  The Earth wire should 16mm² in cross sectional area, must have a green/yellow sheath, and needs to be physically protected by conduit from the earth rod termination box to a protected location, like the meter box.  I’ll go into more detail about the consumer side of things later.

The next system is probably still the most common, largely because it was introduced during a period when large numbers of new houses were being built just after WWII,  and is the one where the electricity supplier (the DNO today) provides three separate conductors to the house, Line, Neutral and Earth.  The system is called TN-S, which, quite frankly, isn’t entirely logical, in my view.  The nomenclature is read from supply side on the left, to consumer side on the right, with the hyphen supposedly signifying a potential conductor separation point or terminal connection (except that it doesn’t, really, in this case!).  Here’s a diagram, similar to that above, that shows the separation of responsibilities as well as the conductor connections for a TN-S earthing scheme:

So, reading from left to right we have T for Earth, N for Neutral and then S for Separate.  There’s a sort of logic there, but not very clear in my opinion.  It translates as the incoming supply contains an Earth and a Neutral, and that they are separated.  This sort of makes sense, except that, in my view, the abbreviation would make more sense without the hyphen.  Just a personal thing, but I rather like abbreviations to be very easily understood.  Note that there is a change of responsibility with this system.  The Earth connection is now the responsibility of the DNO, so as well as offering a physically better protected Earth, this system also removes the responsibility for maintaining it from the consumer to the DNO.  The consumer only retains responsibility for the Earth conductors from the screw connection on the DNO head.

We  now get to the last system, the one where the Neutral and Earth are combined at the termination or within the incoming supply cable and are at the same potential, the one that had been referred to as PME for years.  This system is called TN-C-S .  Translated literally, this means Earth Neutral – Combined – Separate.

Nice and logical, isn’t it?  I mean, you can read that and immediately see what it means………………

Here’s a diagram of a typical TN-C-S earthing scheme:

As above, the dashed lines denote who owns and is responsible for, each part.  Note that, as with TN-S,  responsibility for providing the Earth changes from the consumer to the DNO with TN-C-S .

Looking at the installation it can be hard to spot an obvious difference between the the last two, as some DNOs use the same head for both TN-S and TN-C-S, just with an internal link removed for the former.  As before with TN-S, the first bit means that the Earth and the Neutral are both provided by the incoming supply cable,  but in this case the Combined term means that the supply is providing a Combined Earth and Neutral in the supply side, right at the interface between the DNO and the consumer.  This connection is properly known as the PEN, Protective Earth and Neutral.

The Separate term could be confusing, but in this case it means that the Earth and Neutral are not Combined at the consumer side, but kept separate, again following the “left is DNO, right is Consumer” sequence, and taking heed of those all-important hyphens in this case.  This is a bit more logical than  TN-S, in my view, as you can at least interpret where the actual connection point(s) between Earth and Neutral is/are, thanks to those pesky hyphens.

Most self builders can expect to get a TN-C-S supply provided by their DNO, unless there are some fairly unusual reasons as to why they can’t.  It’s now pretty common, even in rural areas, to provide a TN-C-S connection, as the DNOs have been gradually improving their networks over the years and there are now a lot more intermediate earth’s on the DNO network side than there used to be years ago and they tend to be a bit better maintained in my opinion(at least around here they seem to be, from what I’ve seen when driving around and about).

I’ll stop here for now, and continue with the consumer side, circuit protection methods, why we need them, how they work and what the differences are between different devices, in the next article, which will concentrate on external wiring, and use some of the external electric systems we have in our build as examples as to how I chose to balance risk, convenience, cost etc, whilst still remaining fully compliant with the regulations.


Leave a Reply

Your e-mail address will not be published. Required fields are marked *