The K-Zone: A guide to selection of electrical cable

©2001 Kevin Boone, all rights reserved. No part of this publication may be reproduced in material form (including photocopying or printing), or transmitted in electronic form, without the express permission of the copyright holder, except for the purposes of private study, review, and criticism, or in accordance with the provisions of the Copyright, Designs and Patents Act (1988). So there.

Overview

Cable selection is guided by two main principles. First, the cable should be able to carry the current load imposed on it without overheating. It should be able to do this in the most extreme conditions of temperature it will encounter during its working life. Second, it should offer sufficiently sound earthing to (i) limit the voltage to which people are exposed to a safe level and (ii) allow the fault current to trip the fuse or MCB in a short time.

To meet these requirements requires consideration of the circuit load current, the ambient temperature, installation technique, cable thickness and length, and the over-current protection device. In some cases you may need to consider factors that are outside your control, like the external earth loop impedance. Typical `worst-case' values for these factors are given in the article.

Scope of this article

In most domestic wiring scenarios, the principles and techniques described in this article are simply not relevant. The materials and equipment currently available are designed to simplify installation, and common sense and the ability to read the manufacturer's instructions are all that is required. Ordinary domestic power and lighting circuits do not require any special skills or knowledge to install, beyond what you would find in a DIY handbook. This article covers the issues that DIY books steer clear of, like running long cables to outbuildings, installing supplies for electrical showers, and electrical wiring in bathrooms. It assumes that the reader has sufficient time and enthusiasm to get to grips with the theory, which can be rather technical in places.

This article is intended for readers in the UK, and in places where UK practices and regulations are followed.

How this article is organized

This article has three chapters and an appendix. Chapter 1 describes the theory of over-current protection, and discusses the properties of cables, fuses, MCBs and related devices. Chapter 2 describes principles of electrical shock protection and the effect of cable length and thickness on shock voltage and disconnection time. Chapter 3 describes a practical calculation based on the principles from the first two chapters. Finally, the appendix provides design tables for cable selection, based on the IEE Wiring Regulations and various manufacturer's product data sheets. Please don't use the design tables without reading the text; it will be easy to misinterpret the information if you do.

Warning and disclaimer

I would hate to think of anyone coming to harm as a result of reading this article. It describes procedures which, if not carried out competently, could lead to death or serious injury, or substantial damage to property. Please be careful. Always ensure that before starting to work on an electrical system, the relevant circuit has been isolated from the supply, and you have taken steps to ensure that it remains that way until you have finished work. Ensure that you understand the consequences and implications of any work you intend to carry out. While I have taken every effort to ensure that the information in this article is accurate, and will lead to a safe and reliable installation, I do not accept any responsibility for any adverse consequences arising from its use. Please note that the article is about modern domestic installations; the procedures, design tables, and calculations described may well be unsuitable for commercial or industrial premises and equipment. In particular, the article assumes the use of a single-phase supply, and predominantly resistive loads. If you don't know what these terms mean, I respectfully suggest you ought not to be reading this yet. In addition, this article does not describe any procedures for dealing circuits protected by semi-enclosed (re-wirable) fuses. Although they are allowed with the terms of the IEE Wiring Regulations, they are obsolete and ought to be replaced.

Note on the text

The symbol `[IEE]' in the text indicates a guideline that should be followed to ensure conformance with the IEE Wiring Regulations, 16th edition. Where this symbol is followed by numbers, e.g., [IEE 528-01], this refers to a specific regulation in that document. Note that the IEE Wiring Regulationsis equivalent in practice to British Standard 7671 ``Requirements for Electrical Installations''. To the best of my knowledge, this article complies strictly to the IEE Wiring Regulations.

1. Over-current protection

1.1 Overloads and short-circuits

Over-current is one of the two major safety hazards that must be controlled in a wiring system. The other, of course, is electric shock. Protecting against over-current provides a measure of protection against electric shock as well, as we shall see. Over-currents are dangerous because they lead to a risk of fire. In the UK every year about 50,000 fires are attributed to electrical faults. So it's worth paying a bit of attention to this issue.

This chapter provides a fair amount of technical detail, which you won't always need to know. For many applications, provided that you choose a fuse or MCB (see below) that has the same current rating as the cable to which it is connected, this will work nicely. For example, if the current rating of the circuit is 35 amps, say, then a 32 amp MCB (that's the nearest size available below 35 amps) should do the trick for most applications.

There will be situations, however, where this simple rule won't work. This chapter explains what they are, and what to do about them. It also explains why fuses sometimes blow when there's nothing wrong.

1.2 Types of over-current

An over-current is any increase in the current in the electrical system, above the level for which it is designed.

Electrical cables are intended to become warm in operation; heat is generated whenever a current flows in anything, and this is perfectly normal. However, the level of heat generated by electrical cables is only safe when it is kept within reasonable limits. Standard PVC-insulated cables are designed to run at temperatures up to 70 degrees Celsius; beyond this there is a risk of damage.

The ability to handle short-circuits is not just important to protect cables, it is part of the protection against electric shock. If a live conductor in, say, an electric kettle becomes loose and touches the metal case, we hope that a large fault current will flow. This current will flow from the live, through the case, and back to earth via the earth wire. The fault current will blow the fuse or trip the MCB, thus rendering the circuit dead. If this does not happen, then we have a potentially very dangerous situation: a metal casing with a live voltage on it.

In practice, in domestic installations overload protection and short-circuit protection are both provided by the same device: either a fuse of an MCB. Additional shock protection may be provided by an RCD. Whether a fuse or an MCB is used, when the current exceeds a certain limit for a certain time, the fuse will `blow' (break) or the MCB will `trip'. In both cases this will open the circuit and prevent the flow of further current. For simplicity, I will use the term `trip' for both these events.

1.3 Over-current protection devices

1.3.1 Fuses

A fuse is a simple device that will limit the current flowing in an electrical circuit. In practice a fuse normally consists of a piece of wire of exactly the right length and thickness to overheat and break when the current gets to a particular level.

There are two sorts of fuse normally used in houses. Cartridge fuses have the wire enclosed in a sealed cylinder, with a contact at each end. You should be familiar with this kind of fuse: it's the kind that goes in a plug. Larger versions are available for distribution boards as well.

Semi-enclosed or re-wirable fuses are the kind that can be rewired with fuse wire. Although they are still widely used, they are discouraged by most authorities for two reasons: a common-sense reason and a technical reason. The technical reason will be discussed later. The common-sense reason is simply that it is very easy to rewire it with the wrong size of fuse wire, so that we end up with, for example, a 30-amp fuse `protecting' a 5-amp circuit. This is exceptionally dangerous.

Fuses are an effective basic method of over-current protection, but they have a practical disadvantage: if a fuse blows and you haven't got one of the right rating, what do you do? Of course you won't use a fuse of the wrong rating, or wrap a bit of tinfoil around a blown one, but someone will.

Because of this failing, and for technical reasons that will be discussed, permanently-installed equipment (particularly mains distribution panels) often have electrical over-current protectors rather than fuses.

1.3.2 MCBs

The most popular of these electrical protection devices is the `miniature circuit breaker' (MCB). An MCB can usually act as an ordinary switch as well as an over-current circuit breaker, and so has a lever on the front for manual operation. This is very convenient, and MCBs are universally used in new domestic distribution boards (and most industrial ones as well).

MCBs are available in various types^1.1: `1', `2', `3', `B' and `C'. Each has different characteristics, and is appropriate for a particular application. In a domestic system, we will normally use a type `1' or a type `B' device, as these are general-purpose units.

If you have a distribution board with re-wirable fuses, and don't want to replace it (yet), you can get adapters that will let you plug in an MCB in place of the fuse. If you are replacing wiring with a system that is rated on the basis that you will eventually be using MCBs and not fuses, this is a very sensible thing to do.

1.4 Fuse and MCB characteristics

Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current. We normally assume that if the current in the circuit is lower than the nominal current, the device will not trip, however long the current is maintained. This isn't quite true, but it's a reasonable design assumption.

Many people think that if the current exceeds the nominal current, the device will trip, instantly. So if the rating is 15 amps, for example, a current of 15.00001 amps will trip it, right? This is not true. There isn't any reason why it should be true: the MCB or fuse is designed to protect the circuit cabling, and a current of 15.00001 amps won't damage a 15-amp cable. So when will it trip?

This is where things start to get interesting. It turns out that there's a rather complex relationship between the tripping current and the time for which an over-current is maintained. As an example, the relationship between time and level of over-current that will trip either a 32-amp type-1 MCB or a 30-amp cartridge fuse are shown in figure 1.1.

**Figure 1.1:** time for which a 32-amp MCB or 30-amp fuse will stand an over-current before tripping
$\begin{figure} \epsffile {fusecharacteristic.eps}\par\end{figure}$

The horizontal axis of this graph shows the current flowing in the fuse/MCB and the circuit it is protecting. The vertical axis shows the duration for which the device can stand this current before it trips. There are a few things to note about this graph.

If the fuse is marked `30 amps', but it will actually stand 40 amps for over an hour, how can we justify calling it a `30 amp' fuse? The answer is that the overload characteristics of fuses are designed to match the properties of modern cables. For example, a modern PVC-insulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse should as well.

In fact it would be very impractical to use a fuse or MCB that tripped at a current very close to the nominal value. This is because many electrical devices take higher currents for the first fraction of a second after they are switched on, compared to normal running. Take an ordinary lightbulb, for example. The resistance of all metals increases as they heat up. When the lightbulb is first switched on, its filament is cold, and it has a very low resistance. As it heats up, the resistance increases, so the current decreases. For the first tenth of a second or so, the current flowing in a lightbulb may be 5-10 times higher than its normal running current^1.2.

So we have to allow some margin for start-up currents, or the fuse or MCB will tend to trip by accident, which is inconvenient.

Because the MCB trips very quickly once a particular threshold is reached, the concept of an `instantaneous trip current' is appropriate for MCBs. This is the current that will trip the device in 0.1 seconds. For type 1 MCBs the instantaneous trip current is guaranteed to be between 2.7 and 4 time the nominal current; for type B it is 3 to 5 times the rating.

1.5 Fuse/MCB selection

When selecting the correct MCB or fuse to use, we have to consider its role in both over-current protection, and short-circuit protection. The basic principles are as follows.

The `disconnection time' rule is the most awkward to ensure compliance with in a domestic installation; it will be discussed later. In practice it doesn't affect what rating of fuse or MCB to use, but it often affects whether to use a fuse or an MCB, and may impose the use of additional protective devices.

1.6 Example

Assume we are installing a lighting circuit, which will nominally have 8 light fittings of 100 watts each. The current is 800 / 230 or about 3.5 amps. 1 mm

cable appears to be appropriate, as it has a rating of at least 11 amps (see table A.1), even when concealed in a plastered wall^1.4.

So the MCB must have a nominal current (that is, the current marked on its body) of at least 3.5 amps, and less than 11 amps. Furthermore, its tripping current must be less than 1.45 times 11 amps, in order to protect the cable. Looking in the manufacturer's catalogue, I find a 6 amp MCB, that has a trip current of 8.7 amps. This appears to be just right. These currents are shown in figure 1.2.

**Figure 1.2:** Relationship between the fuse nominal and tripping currents, and the current carrying capacity of the cable, for the example given in the text
$\begin{figure} \epsffile {fusecharacteristics.eps}\end{figure}$

Note that in this example, the MCB trip current is not only below the short-term current capacity of the cable (which it must be), but it is even below the nominal current rating of the cable. This means that the MCB will prevent the cable reaching even it normal working current. Of course that's fine in this case, because we know exactly what the load will be: eight 100 watt bulbs. The tripping current does not have to be below the nominal current of the cable, but it does have to be below 1.45 times the nominal current of the cable.

Note that we need also to check the disconnection time in the event of a live-earth fault, but doing so requires more information than has been supplied; see below.

1.7 Common examples of bad design

Some wiring configurations can lead to overload currents that cannot be detected.

**Figure 1.3:** Extending a spur with a second spur: not a good idea
$\begin{figure} \epsffile {overloadspur.eps}\end{figure}$

Consider, for example, the system shown in figure 1.3. Suppose we have a standard ring circuit wired using 2.5 mm

cable and protected by a 30 amp MCB. This is a perfectly standard, reasonable configuration. Now suppose we extend this circuit with a spur (`spur 1' on the diagram). What is the maximum current that can flow in the spur cable? Assume that we have used standard single socket outlets. We can plug an appliance rated up to 13 amps into the outlet, so in the worst case the current load could be 13 amps in the spur cable. This is well within the current carrying capacity of the spur cable, so no problem.

Now suppose we extend the system further by connecting a second spur (`spur 2') to the first spur. Now the maximum current that could be carried by spur 2 is 13 amps, and the maximum current that could be carried by spur 1 is twice 13 amps: 13 amps from each appliance. That's 26 amps. The cable used might carry 26 amps, in ideal circumstances, but we shouldn't rely on it. The IEE Wiring Regulationsthe capacity of this cable as between 18.5 amps - if it is concealed in a wall - to 30 amps if it is exposed to air all around^1.5.

If we plug several heavy duty appliances into the outlets in the main ring, we may cause an overload that will trip the MCB. But this isn't the problem: the MCB will always trip at a lower current than will damage the cable. In the spur, the current carried could be too high for the cable, but too low to trip the MCB. This is a potentially dangerous situation.

It's worth noting that we could, in principle, avert this dangerous situation by using a heavier cable for spur 1. 4 mm

would probably do the job. However, spur 1 was probably put in before the need for another spur was recognized, and it would probably have been constructed from the same size cable as the ring.

The simple, robust solution to this problem is to connect spur 1 to the ring using a fuse unit, as shown in figure 1.4.

**Figure 1.4:** extending a ring with a fused spur
$\begin{figure} \epsffile {fusedspur.eps}\end{figure}$

The spur is connected to the ring using a fuse or MCB. The rating of the fuse or MCB should be no higher than 13 amps, which means that the total current taken by the spur can never be higher than the current that would be taken by a single outlet attached to the ring by an unfused spur. Since we have already established that this current is insufficient to damage the cable, the fused spur allows the ring to be extended in a safe way. Note also that we can wire the fused spur in lighter cable (1.5 mm

should do) because the fuse limits the nominal current to 13 amps.

The circuit shown in figure 1.3 only becomes at risk of danger from overload if two high-current appliances are plugged into the spur. You could argue that this won't ever happen: this spur will only ever be used to supply a pair of desk lamps, for example. The problem with this logic is that you may know this, but not everyone else does. What's to stop someone (a visitor to your house, for example) from plugging an electric fire into each one? The example described above crops up in two common situations.

The first is that of powering a large, complex hi-fi/TV system. Suppose we have ten electronic appliances of this sort, all stacked one on top of the other in a small area. It might be convenient to fit a row of, say, six double socket outlets just behind the system. We might do this by running a spur from the main power ring. One could reasonably argue that these appliances actually have a small power consumption. A 100-watt amplifier is staggeringly loud in a domestic lounge, but it takes less current on average than a lightbulb. The complete hi-fi/TV system probably takes less than 5 amps. In the circumstances one might be tempted to use an unfused spur. The problem, as before, is that although you only plug hi-fi equipment in here, who's to say what the next owners of your house will do?

A related problem is this one: I am commonly asked whether it is safe to fit an extra mains outlet in a room, by connecting it to an existing one on the other side of the wall. This is a very handy way to add an extra socket, if there's an existing one in position. In a modern house, wired to comply with the IEE Wiring Regulations, then this almost certainly is safe. Such houses are wired to a standard scheme, where all the power outlets on each floor (perhaps excluding the kitchen) are wired into a simple ring system. If you tap one of the socket outlets then you will be effectively installing an unfused spur. As there will only be one new socket on the spur, this is fine.

However, if there's any possibility that the socket you want to tap is itself a spur, then you should probably not do it. If the existing socket has only one cable entering it, this is probably what it is. Even if the socket has two cables, you can't be certain it's part of a ring. It might be part of radial system, or even a dodgy unfused spur fitted by someone else. The older your house is, and the more haphazard the wiring is, the more likely this is to be the case. There is no straightforward solution to the problem: you need to determine how the various sockets are connected to one another before you can decide whether the extra socket is safe, or not.

1.8 Disconnection times

When an overload occurs, the protective device (fuse or MCB) must cut off the supply within a time short enough to prevent damage to the installation. This time may, in some circumstances, be as much as an hour. However, when a short-circuit fault occurs, it may be because a live part has come into contact with a metal casing. In this case lives are at risks unless the supply is cut off very quickly.

Short-circuits may be between live and earth, or live and neutral. In the case of live-neutral shorts, the current flow could be enormous (thousands of amps). It is limited only by the resistance of the cable between your house and the supply system, which is usually less than an ohm. In this kind of fault, the MCB or fuse will trip in its shortest possible time: usually about 0.1 seconds for a fuse and 0.01 seconds for an MCB. In domestic installations we usually don't have to worry about this, because if we can meet the requirements for disconnection time in the case of a live-earth fault, we will also meet the requirements for a live-neutral fault.

The notion of disconnection time in the event of a live-earth fault will be discussed in much more detail on page

, when we consider the effect of disconnection time on electric shock protection.

2. Shock protection

This chapter describes the measures that are taken to protect against electric shock. For most simple jobs, ensuring that the earth conductors are properly connected is all that is necessary to ensure adequate shock protection. The more detailed information in this chapter is intended for use in more substantial work, like supplying power to a garden or outbuilding, or adding a new power circuit.

2.1 The nature and risk of shock

Most electric shocks that are received in the home are dangerous because of the effects they have on the heart or respiratory system. Relatively small currents may be sufficient to kill or injure. Larger currents can also cause burns and tissue damage. Shocks occur when electric currents flow through the body between points at different voltage. For example, if you touch a live electrical terminal in a lampholder (230 volts) while standing on the ground (assumed 0 volts), a voltage of 230 volts is developed between your hands and feet. This voltage causes a current to flow through the body, including the heart and lungs. The current causes stimulation of the muscle mass of the heart, and of the nervous system controlling the lungs. Clearly this is a bad thing.

Increasing either of these risk factors increases the risk of injury or death. Later in this chapter we will discuss how knowledge of the shock risk factors is used in specifying the performance of the shock protection system.

2.2 Types of shock

2.2.1 Direct contact

`Direct contact' occurs when a body part touches a live part directly. This type of shock is particularly dangerous, as the full voltage of the supply can be developed across the body. In a well-designed electrical installation there should be little or no risk of direct contact; in most cases it arises out of carelessness (e.g. changing a lightbulb with the outlet switched on). However, it can sometimes arise from wear and tear, such as the breakdown of insulation on a flexible cable that is badly stressed. RCDs (see below) provide some measure of protection against direct contact.

2.2.2 Indirect contact

Indirect contact occurs when a live part touches a piece of metal, and the body comes into contact with the live metal. Indirect contact can occur as a result of faults in electrical appliances, particular with metal casings.

Your main protection against indirect contact is earthing, combined with an overcurrent cut-out device. This works because the large current that will flow to earth in the event of a fault should activate the overcurrent device.

A fault in an appliance where a live part comes into contact with an earthed enclosure is called a live-earth or live-to-earth fault.

2.3 Earthing and bonding

These terms are often confused, and it is important to understand the distinction.

`Earth' is that big lump of rock and mud we all live on. Most electrical power systems currently in use are connected, somewhere, to earth. This helps to keep the voltage at a well-defined level. For convenience we treat the voltage of the earth as `zero', and everything else is measured with respect to it. When we talk about a 230-volt supply, we mean 230 volts higher than earth. `Earthing' is the process of connecting parts of the electrical system to earth.

`Bonding', on the other hand is the connecting of metal equipment together. It does not necessarily mean connecting to earth, although in practice there usually is an earth connection somewhere.

The main purpose of bonding is to keep all metalwork that can come into contact with an electrical potential at the same voltage. If two pieces of metal are at the same voltage, then it is impossible to get a shock by touching them simultaneously. No current will flow. Earthing ensures that if a fault does occur, the current that flows to earth is sufficient to activate an overcurrent device and shut off the supply.

So earthing and bonding have complementary functions, and in domestic practice are always used together^2.1.

2.4 Types of primary earth connection

Your house will (or at least should) contain, in or near the main distribution board, a primary earth terminal. This is the main point to which all circuit earths will be run back. Of course, there may be other paths to earth for current elsewhere in the premises. It stands to reason that the main earth terminal should provide a very low resistance path to true earth.

There are three main ways that this earth terminal may be connected to a true earth. These are identified by the abbreviations^2.2 shown in table 2.1.

Table 2.1: typical methods of provision of the main earthing terminal

Supply type code	Meaning

TN-S	Supplier provides a separate earth connection, usually direct from the distribution station and via the metal sheath of the supply cable
TN-C-S	Supplier provides a combined earth/neutral connection; your main earth terminal is connected to their neutral
TT	Supplier provides no earth; you have an earth spike near your premises

Most houses have `TN-S' supplies at present, but the `TN-C-S' method is increasing in popularity because of its lower cost and superior earth contact resistance. TN-C-S is also called `protective multiple earthing', or PME, because of the additional earthing required inside the house. If your power supply is from an overhead cable, then you may well have a TT arrangement. The earth spike will usually be located in a small pit, with a cable run into the house. TT earthing presents significant challenges for the electrical installer, as its earth resistance is uncomfortably high (see below), and may vary with the weather. When it is very wet, the contact with earth will be better than when it is dry.

2.5 The earth loop

To understand how earthing requirements are to be met, it is important to understand where current flows in the event of a live-earth fault (figure 2.1).

**Figure 2.1:** the earth loop: the path for current when a live-earth fault occurs in an appliance. Note that in the worse case the neutral part of the system carries no current; it all flows in the appliance casing and earth. The rectangles represent the resistances of the various conductors in the circuit.
$\begin{figure} \epsffile {earthloop.eps}\end{figure}$

If a live part in the appliance comes into contact with the casing, a current will flow from the live of the supply company's apparatus to the premises, along the live conductor to the appliance, through the fault to the casing, from the casing to the earth terminal via the earth conductor, and from the earth terminal back to the supply company's apparatus via its earth connection.

This circuit is called the `earth loop'. Note that part of the earth loop is outside your premises, and in the supply company's cables and apparatus. You have no control over that part of the loop. The part of the loop inside your house has a resistance which can be calculated, because we know the resistances per metre of the various cables that are likely to be used.

If an earth fault occurs (that is, a short-circuit between live and earth), the path for current includes that supply company's live conductor into your premises, the live part of your cabling, the earth part of your cabling, and the earth part of the supplier's system. You can calculate the resistance of your part this system, or measure it, but you may need to approach the supply company for the resistance of their part. Suppliers are legally obliged to tell you this; it is, after all, very important for ensuring safety.

When a short-circuit from live to earth occurs, the earth loop is the path that the current will flow in. This current could be very large; it should certainly be large enough to blow the fuse or trip the MCB before serious injury occurs. This suggests that the earth loop resistance should be as low as possible.

For initial and approximate design calculations, you can use the `worst case' values of earth loop resistance given in table A.11. If your installation appears safe with these worst case figures, it will almost certainly prove to be safe with the true figures. However, the earth loop resistance figures depend on your knowing the supply type of your premises. If you don't know this, you will need to ask the supply company anyway.

2.6 Main and supplementary bonding

Normally all electrical appliances with metal cases will be earthed. The case will be connected to the main earth terminal via the appliance cable and the power circuit itself. If a fault occurs in the appliance, and a live part touches the case, the earthing prevents injury to the user.

However, what happens if a fault occurs in, say, a mains cable, and a live conductor comes into contact with a central heating pipe? In theory, this could result in the whole system becoming live, as the metal pipework will carry the live potential through the house. We avoid this problem by ensuring that the pipework is sufficiently well earthed to prevent a potential being developed. The same considerations apply to water mains, gas mains, structural metalwork, and metal ventilation ducts. These items should all be earthed.

Normally gas, water and ventilation systems will be earthed at at least one point by a connection direct to the main earthing terminal. This is called main bonding. In a house, you will normally see a heavy earth cable running from the earth terminal to clamps on the incoming service pipes. If you have a lightning conductor, this must be earthed as well.

You could argue that the service pipes are earthed automatically, by virtue of being buried in the ground. The problem with this argument is that the earthing is uncertain. In some district, plastic water pipes are used for the mains, and these provide no significant earth contact at all. In others, there may be a contact of uncertain resistance. Main bonding eliminates this uncertainty.

We don't have to earth every piece of metal in the house, but everything that may be in contact with earth should be well-bonded. We need to avoid the situation where metalwork is able to carry live voltage between rooms, but is not sufficiently well earthed to protect the occupiers. The basic principle is this: Any earth should be a good earth.

We don't need to earth doorhandles, or filing cabinets, or window frames, as these don't present a significant risk of carrying a potential. However, we do have to earth pipework and structural metal. By `structural' is meant central support beams and joists, not metal window frames. In practice, most houses do not have a steel frame, and it won't be necessary to earth the structure.

In some places, main bonding is not considered to be sufficient, and we need to employ supplementary bonding as well. This is the connection of metalwork together in a small area, to prevent voltages being developed between different parts of a room. In a house, this is only likely to be necessary in a bath or shower room.

Where supplementary bonding is used, it should be used thoroughly. In particular, you need to take the trouble to bond all metal parts that have a connection outside the room. In a bathroom, this includes pipework, tubs and sinks, taps, radiators, etc. In older houses it may also include waste pipes, which may be of cast iron. The bonding connections need to be made with a stout earth cable, typically^2.34 mm

, connected to clamps with integral warning plates.

Note that the supplementary bonding does not have to be run back to the main earth terminal. A connection to, say, the earth terminal of a lighting outlet would be fine. It is the bonding that is important, rather than the earthing.

The benefits of supplementary bonding have always been contested; many authorities believe that in some circumstances it reduces electrical safety rather than improving it. The whole discussion has recently been re-opened, with the widespread use of plastic plumbing. Clearly, plastic pipes don't conduct electricity, and a radiator connected by plastic pipes is not able to carry a current out of the room. This issue is discussed in more detail later in this chapter.

2.7 Shock voltage: the limitation of earthing

It might be thought that if a metal appliance is soundly earthed, then it is impossible for a person touching it to receive a shock, regardless of the nature of the fault. This is an untrue and dangerous misapprehension. Consider the following example.

A live-earth fault occurs in an electrical appliance. The appliance is connected by its own dedicated cable to the main distribution board. The cable is 2.5 mm

two-core-and-earth, 10 metres long. This type of cable has a 1.5 mm

earth conductor, which has a resistance of 0.015 ohms per metre at 70 degrees Celsius^2.4. The live conductor has a resistance of 0.009 ohms per metre. So the total resistance of the earth conductor is 10 x 0.015 ohms, or 0.15 ohms. The total resistance of the live conductor is 10 x 0.009, or 0.09 ohms. Assume that the resistance of the supply company's part of this loop has a resistance of 0.3 ohms. (see table A.11 for typical figures for various types of earthing).

So the total resistance in the current path (the earth loop) is 0.3 + 0.09 + 0.15 ohms, or 0.54 ohms. With a supply voltage of 230 volts, the fault current will be (230 / 0.54) amps, or about 426 amps. This current flowing in the earth conductor (1.5 ohms) will develop a voltage of (426 x 0.15) volts, or about 64 volts.

This 64 volts is called the `prospective shock voltage' or `prospective touch voltage', because this is the voltage at the casing of the appliance when the fault occurs, that is, the voltage that a person will experience if he or she touches the appliance in the fault condition. The term `prospective' is used because, in practice, the use of an overcurrent device (e.g., MCB) or an earth fault device (e.g., RCD) may stop this shock voltage being reached. For example, if an MCB will always interrupt the supply when the current reaches 20 amps, the true shock voltage will be resistance of the earth conductor multiplied by 20 amps (as discussed later).

In summary, a fault in this appliance could cause the casing to develop a voltage of 64 volts, which is potentially dangerous. Note that the only practical ways to reduce the shock voltage are (i) to shorten the cable, (ii) to reduce the earth resistance, or (iii) to shut off the supply before the fault current reaches the calculated figure.

There are two ways to ensure that the supply is disconnected automatically. The `traditional' approach is to ensure that the current that flows to earth is enough to trip the overcurrent device for the circuit. The modern approach is to use an RCD.

Without automatic supply disconnection, the shock voltage will be given by multiplying the fault current by the earth conductor resistance, as calculated above. In that example, the shock voltage would have been 64 volts. However, suppose the cable had been protected by a 20 amp type-1 MCB. The `instantaneous trip voltage' of the MCB will be approximately four times the rating, or 80 amps. This means that as soon as the current rises to 80 amps, the supply will be cut off. With 80 amps flowing, the shock voltage is 80 multiplied by the earth conductor resistance (0.15 ohms), which is 12 volts. So the overall shock voltage is set by the trip current of the overcurrent device, and the resistance of the cable.

Of course, this approach will only work if the overcurrent device actually cuts off the current. In the case described, the fault current of 426 amps will trip the MCB very quickly indeed, probably in less than a hundredth of a second. However, if the fault current is low, it may not trip the device at all, or may not do so quickly enough to prevent danger. So we need to consider the notion of `disconnection time' in an earth fault.

2.8 Disconnection time and shock voltage

We have already seen that the use of earthing alone will not necessarily protect against electric shock, because in a fault the current flow may be so large that it still develops a dangerous voltage between earth and the fault. To prevent injury, a protective system must be available to shut off the supply in the event of a fault. Obviously, a quick disconnection is to be preferred.

It is difficult to specify the performance of an electric shock protection system in terms of tissue current, since this is not readily measurable. It is relatively straightforward to specify in terms of `shock voltage', which is the voltage at the point where the live part touches the body, as described above. Because the shock voltage and the tissue current are not directly related, large safety margins are specified.

The disconnection time given by regulations [IEE 413-02-09] will depend on the level of risk to which a user of the circuit is exposed; for this discussion I will divide circuits into `high', `medium' and `low' risks categories. Note that `low risk' here means comparatively low risk; no mains electrical system can really be classified as low-risk.

2.9 Checking disconnection time

This section describes how to calculate disconnection times, and the circumstances in which you need to do this.

2.10 When do we need to be concerned about disconnection times?

Even in substantial projects you may not need to be concerned about disconnection times. The following points should be considered.

Bear in mind that in some cases you will need to know the external earth loop resistance to do these checks.

2.10.1 Disconnection time in high risk circuits

In these circuits, disconnection must occur in 0.4 seconds, whatever the shock voltage. Therefore we don't need to calculate the shock voltage. The check procedure is a follows.

Alternatively, use the figures in table A.12, columns 4 or 5; this gives maximum lengths that will allow a disconnection in 0.4 seconds for a range of popular cable/MCB/fuse combinations.

2.10.2 Disconnection time in medium risk circuits

In these circuits, disconnection must occur in 0.4 seconds, unless the shock voltage is less than 50 volts. In the latter case, we are allowed a disconnection time of 5 seconds. So we should first calculate the shock voltage, and then check the disconnection time according to whether the shock voltage is less than 50 volts or not.

Alternatively, use the figures in table A.12, column 3 to determine what length of cable will give an acceptable shock voltage.

2.10.3 Disconnection time in low risk circuits

In these circuits, a disconnection time of 5 seconds is allowed. The check procedure is as follows.

Alternatively, use the figures in table A.12, columns 6 or 8; this gives maximum lengths that will allow a disconnection in 5 seconds for a range of popular cable/MCB/fuse combinations.

The process of checking disconnection times is complicated by the fact that the external earth loop resistance cannot easily be measured or calculated: it must be obtained from the supply company.

2.10.4 General guidelines on disconnection time

2.11 RCDs

We have seen that we can't always rely on the use of earthing to disconnect the mains supply in a fault. Even if it does disconnect, it may not do so quickly enough, or keep the voltage at a safe level. Increasing awareness of these problems has led to the widespread uptake of other devices that can detect and isolate earth faults. The most popular at present is the residual current device.

The residual current device measures the difference in current between the live and neutral conductors of a system. In all normal circumstances these should be equal. If the live current and the neutral current are different, this indicates that some current is flowing somewhere other than the live and neutral. There are only a few places that the extraneous current can flow, all of them bad. In many cases the difference will indicate a current flow to earth, via a fault.

RCDs are available with different sensitivities. That is, they trip at different levels of current leakage. In the UK, the most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps).

An RCD does not necessarily require an earth connection itself (it monitors only the live and neutral). In addition it detects current flows to earth even in equipment without an earth of its own. This means that an RCD will continue to give shock protection in equipment that has a faulty earth. It is these properties that have made the RCD more popular than its rivals. For example, earth-leakage circuit breakers (ELCBs) were widely used about ten years ago. These devices measured the voltage on the earth conductor; if this voltage was not zero this indicated a current leakage to earth. The problem is that ELCBs need a sound earth connection, as does the equipment it protects. As a result, the use of ELCBs is no longer recommended.

RCDs are now available in all sorts of guises. For example, an RCD plug replaces a standard three-pin plug, but has a built in RCD. For permanent installations, you can get RCDs built into socket outlets (figure 2.2). Of course, RCDs are available to fit standard distribution boards alongside MCBs.

**Figure 2.2:** a socket with integral RCD protection.
$\begin{figure} \epsffile {rcd-socket.eps}\end{figure}$

2.11.1 Where to use RCDs

Houses built in the last few years probably have an overall RCD for the main supply. This means that no further RCD protection is necessary, or helpful, from a safety perspective. Additional RCDs may, perhaps, have a practical benefit, as will be discussed below.

There are a few places around the home where RCD protection is stipulated by the IEE Wiring Regulations: fixed equipment in bathrooms, socket outlets in rooms that contain a shower cubicle (see below), gardens, and outbuildings.

Remember that you can provide RCD protection in various places: a distribution board, a spur unit, or a single socket outlet.

2.12 Where not to use RCDs

Remember that RCDs are only available for mains-voltage supplies. 12-volt and 24-volt systems do not require RCD protection, and none is available.

You may want to consider avoiding RCD protection on lighting circuits, with the possible exception of kitchen and bathroom lighting. You need to weigh up the advantages and disadvantages quite carefully. The risk of electrocution from a lighting system is actually quite small, and the victim is unlikely to be grasping the faulty equipment strongly (contrast this to the use of, say, an electric drill or a hairdryer). If someone does receive a shock from a light fitting, it may not be to the victim's benefit if the house is plunged into darkness. You decide.

Another place to avoid RCDs is in the supplies for equipment that really must run without interruption. In the home this applies particularly to freezers, but you may want to consider aquarium pumps and some kinds of computer equipment. It would not be in your best interests if an electrical fault in the garden caused your freezer to shut down. In addition, RCDs do sometimes trip by accident, as will be discussed.

Remember that RCDs detect an imbalance in the live and neutral currents. A current overload, however large, cannot be detected. It is a frequent cause of problems with novices to replace an MCB in a fuse box with an RCD. This may be done in an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or an overload), the RCD won't trip, and may be damaged. In practice, the main MCB for the premises will probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it may be inconvenient.

It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below). Replacing an MCB with an RCBO of the same rating is generally safe.

2.12.1 RCD discrimination

Common sense, and the IEE Wiring Regulations, suggests that if a system has multiple RCDs, then when a fault does occur the RCD nearest the fault should be the one to trip. This is called discrimination. If we have, for example, an RCD socket in a garage (with protect on the individual socket), and this is fed by a distribution board with overall RCD protection, then the two RCDs need to have different sensitivities. Specifically the one in the garage needs to be more sensitive than the one in the distribution board. This ensures that a fault in the garage will shut off the garage socket, not the distribution board. If the garage socket has 30 mA sensitivity, then using a 100 mA device, with a small trip delay, in the board will ensure the correct discrimination.

However, there is no safety benefit in this approach: two RCDs aren't safer than one. The only reason for installing a system with multiple RCDs in the same circuit is to localize the isolation in the event of a fault. For example, suppose you have a mains power ring (with RCD protection) feeding a single socket in the garden. If you provide the garden socket with its own RCD, this will prevent faults in the garden equipment tripping the RCD in the power ring. Of course, for this to work, you would need to use two different sensitivities.

Bear in mind that if you have a 30mA RCD as the overall protection for your house, then there is absolutely no advantage in using any other RCD anywhere in the premises. If you want to fit additional RCDs, you need first to change the main RCD for a 100 mA device.

2.12.2 Nuisance tripping

In my experience, modern RCDs on modern wiring systems with modern appliances are not prone to trip by accident. Any tripping of an RCD should be taken seriously. However, there are well-known reasons for nuisance tripping.

Sudden changes in electrical load can cause a small, brief current flow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly; they may well trip when the motor of an old freezer switches off. Some equipment is notoriously `leaky', that is, generate a small, constant current flow to earth. Some types of computer equipment, and large television sets, are widely reported to cause problems.

Persistent nuisance tripping, or an RCD that won't switch on at all, are causes for concern. Either can indicate an intermittent or permanent live-earth fault somewhere in the circuit (e.g., a faulty appliance flex).

2.12.3 RCD hazards

RCDs are an extremely effective form of shock protection. When properly selected and installed in a system that is generally sound, they render it almost impossible to receive a dangerous shock. This degree of protection is not entirely without problems, as it is very easy to overestimate the protection that RCDs provide. Increasingly RCDs are seen as a cure for any kind of electrical problem. Many people evidently think that by having and RCD somewhere in the house this gives license to use shoddy and poorly-planned wiring and equipment. This can be very dangerous.

RCDs don't offer protection against current overloads. The widespread, dangerous practise of taking long spurs from a power ring will be just as dangerous. Neither does an RCD protect against long-term overloads that are just below the tripping current of the protective device. Both these problems are consequences of poor design. The latter, for example, often arises when the same ring circuit is used for a kitchen and the rest of the socket outlets on one floor of a building.

An RCD will not protect against a socket outlet being wired with its live and neutral terminals the wrong way round.

An RCD will not protect against the overheating that results when conductors are not properly screwed into their terminals.

An RCD will not protect against live-neutral shocks, because the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a nasty shock.

In summary, an RCD is intended to shorten the disconnection time in the event of a live-earth fault, and to give a measure of protection against earth failure in an appliance. If you rely on it for anything else, you're living on borrowed time.

2.13 RCBOs

As discussed above, an RCD does not provide overcurrent protection. If you rely on an RCD to trip in an overload it will probably be damaged and stop working completely. To make it possible to replace an MCB with an RCD, manufacturers have introduced a new range of equipment called RCBOs. An RCBO combines an RCD with an RCB, in an enclosure that will fit into the same distribution board as the RCD. When choosing an MCBO, remember that its overload and its earth-fault protection capabilities are completely separate. You should apply exactly the same design and selection procedures as you would for two separate units.

2.14 Special considerations for bathrooms

As far as possible, mains-operated equipment should be kept away from a tub. In general, there should not be electrical socket outlets in a bathroom. Fixed equipment should be protected by an RCD, unless specifically designed for bathroom use. Any electrical equipment in a bathroom (including light fittings) should be of a type that is specifically designed to be safe for a bathroom. Shaver sockets can be fitted, provided they're the type with an isolating transformer that are specifically designed for bathrooms. [IEE 601-08] allows socket outlets that are placed at least 3 metres from a shower, in a room containing a shower cubicle, but this does not appear to apply to bathrooms that contain a bath. Under-floor heating grids should be protected by an earthed metal screen.

2.14.1 What constitutes a bathroom?

The IEE Wiring Regulationsnot mention bathrooms, it deals with `locations containing a bath or shower' [IEE 601, 3rd amendment]. Specifically, the Regulations recognize that rooms other than a `bathroom' may contain a bath or shower. For example, if you are fitting a shower cubicle in a bedroom, then you will want to take some care with electrical fittings in the vicinity of the shower. For example, you need to be careful about locating it with respect to mains outlets and lightswitches. Any electrical outlet in a room that contains a bath or shower should be protected by an RCD [IEE 601-08-01], even if outside the 3-metre zone.

2.15 Special considerations for gardens and outbuildings

In general, extra-low voltage (12V) equipment is to be preferred for garden use. It is now possible to get 12V lighting, pumps, lawn sprayers, among others. Where you must use mains power, you will need to use RCD protection for socket outlets.

There is a widespread misconception that the use of RCD protection outside the house is because the environment may be wet, and increase the electrical contact between person and ground. Clearly this is likely to be a problem in a garden. However, it is less obvious why we should need to provide RCD protection in outbuildings.

The IEE Wiring Regulationsto something called the `main equipotential zone'. This is, essentially, the part of the premises that has a very low resistance contact to the main earthing terminal. Any part of the site that is outside this zone should be considered a candidate for RCD protection, as it will be difficult to meet disconnection time regulations otherwise. The `equipotential zone' probably does not include a separated garage, and certainly does not include the garden. The reason for using an RCD in a garage is therefore not that it may get wet, but because it may be difficult to control shock voltage and disconnection time otherwise.

2.16 Special considerations for kitchens

You may well feel that special consideration needs to be given to electrical equipment in kitchens; there is, in fact, no legal or regulatory requirement for this. However, it is probably sensible to use RCD protection on kitchen power outlets, and to use supplementary bonding for metal sinks and taps where the pipework is metallic.

2.17 Effect of plastic water pipes on earthing

There has been a lot of controversy lately about the benefits and hazards of plastic plumbing in kitchens and bathrooms. Remember that traditional metal pipework is a very good conductor of electricity, and can be used to supplement earthing. Plastic piping is a very good insulator, but the water it contains may conduct, to a degree. As has already been suggested, if the basic principle `any earth should be a good earth' is applied, this implies that metal equipment (e.g., radiators and sinks) supplied by plastic piping should be earthed by bonding cables.

The electrical resistance of a 15 mm pipe containing tap water is about 100 kilohms per metre; with corrosion inhibitor (in radiator pipes) it may be as low as 20 kilohms per meter (source: Electrical Research Association). While water purity and content varies from place to place, it is unlikely that there will be much variation from the above figure for any tap water. Even in the worst case (radiator pipes), the amount of current that can be carried by the water in the pipes will be about 11 milliamps (230/20,000) divided by the length of the pipe to the nearest good earth. So even if a radiator is connected by a plastic pipe to a metal pipe at as little as 1 metre away, it cannot carry enough current to present a significant hazard. This means that earthing the radiator will increase the risk of electric shock, rather than reduce it.

It is increasingly common, therefore, to treat plastic pipe - even containing water - as an insulator. It makes no more sense to earth a radiator with plastic piping than it does to earth a filing cabinet or window frame, even in the bathroom. There is no particular requirement to earth bath or sink taps that are connected by plastic pipe, any more than you would earth a metal shelf or towel rail.

It is increasingly common to find that radiators are plumbed using plastic pipe, except for the part that is visible above floor level. There is no need to use supplementary bonding on the visible part.

2.18 Special considerations for PME supplies

Remember that if you have a PME supply (and you have no control over that, except by moving house), that your main earth is the supply company's neutral. There is no earth connection brought into the house. This is fine so long as the supply neutral remains intact. If the neutral connection breaks, then you could be left with no effective earth. A fault in these conditions could be extremely dangerous.

While it is good practice with all forms of supply to enhance the earth connection by connecting the main earth terminal to the incoming service pipes (e.g., gas and water), with a PME supply this is absolutely essential. Moreover, the use of supplementary bonding within the premises, is even more important than with other supplies. In fact, you - the householder, not the supply company - are legally obligated to ensure the integrity of supplementary bonding in this case. The main bonding conductors (from the earth terminal to incoming services) should normally be 10 mm

with a PME supply, but check with the supply company because larger sizes are required in some cases.

If you have a PME supply, there may be a label near the incoming cables indicating that fact, but it is probably safer to check with the supply company or the local authority.

3. Example of selecting cable size

This appendix describes how to select the appropriate cable size for a given application, to be in accordance with the IEE Wiring Regulations. For your convenience I have extracted the information from the tables given in the Regulations that are likely to be applicable to typical domestic installations and summarized it in appendix A. However, this appendix makes certain assumptions about your installation, and if these assumptions are not true, you should not use this information. Instead, you should refer to the approriate sections of the IEE Wiring RegulationsIEE appendix 4] or, better still, calculate the appropriate figures yourself.

Note that in domestic wiring you will hardly ever need to use this procedure. It is nearly always appropriate to use 2.5 mm

cable for power circuits and 1.5 mm

cable for lighting. You will need to apply this procedure if you are planning long runs of cable (e.g., to an outbuilding with a power circuit) or high current appliances (e.g., electric showers or cookers).

The procedure to be described can be followed by anyone who has a pocket calculator and can follow instructions no more complicated than ``multiply this number by the one you calculated in step 2''. However, it will make more sense if you have a basic understanding of electrical theory and Ohms's law.

3.1 Outline of the procedure

The basic procedure is to select a cable whose nominal current rating appears appropriate for the load it will have to carry - or the overcurrent device fitted, then check it meets the `current carrying capacity' requirements, the `voltage drop' requirements, and the `disconnectiont time' requirements, after applying various correction factors. If any requirement is not met, you should try again with the next largest cable. If you can't meet the disconnection time requirements with the largest cable, then you will need to use an RCD to protect against earth faults.

3.1.1 Step 1: calculate the nominal current

3.1.2 Step 2: pick a cable size that appears appropriate for the nominal current

To do this, refer to table A.1. In this table, `enclosed in a wall' refers to a cable that is concealed in plasterwork, either directly or in a protective conduit. `Enclosed in conduit' means fully enclosed in conduit or trunking that is fastened to the surface of a wall or ceiling. `Clipped to a surface' means fastened at intervals to a flat surface such as brickwork or joists. `Free' means completely surrounded by air. A cable that is passed across a line of joists, running through ample holes, could probably be classed as `free' (but see discussion of thermal insulation below). `Free' also applies to cables that are laid on metal cable trays, but this is unusual in domestic installations.

There will be some cases where your installation does not conform to any of the installation methods shown in table&nbs