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Earthing Design Within Buildings

Introduction

There are presently more publications on this aspect of earthing than on any other and it is the purpose of this chapter to provide just an overview of the more important aspects of earthing within buildings. Those requiring a more detailed coverage are referred to the standards and books listed in chapter 16. Additional material can also be found in books which deal with Building Services. The Institution of Electrical Engineers run a series of courses covering design, installation, maintenance and testing of fixed and portable appliances.

BS 7671, Requirements for Electrical Installations, 1992 is the prime reference document. The main objectives of this and other regulations is to protect persons, property and livestock against hazards arising from an electrical installation. Earthing is fundamental to most of the practices for achieving safety. The earthing system must provide a direct route to the soil for fault current whilst minimising touch and step potentials. The secondary function, is to help mitigate disturbances and serve as a common voltage reference for sensitive electronic equipment. However, with greater use of sensitive electronic equipment, particularly computers, there is a growing awareness of the importance of the secondary function of the earthing system. This is leading to a consensus of opinion that the earthing system must be designed as an overall system such that it fulfills the safety and performance requirements.

Typical TN-S Arrangements

The most common protective measure is earthed equipotential bonding and automatic disconnection of supply. The standards set maximum disconnection times for different types of equipment. In deciding which times are appropriate, the earthing arrangement outside the property, i.e. the supply network, also has to be considered. This is because any earth fault current normally has to return to the source transformer. The earth loop impedance is made up of the impedance of the earthing system at the source transformer, the earth conductors between the transformer and the property and the impedance from the point of fault back to the supply point in the property.

Figure 3-3 illustrated a typical TN-C-S supply and this is the most common arrangement for new and recent supplies to domestic premises in the UK. In this arrangement the neutral and earth conductors are combined in the supply network. However they must be separated within premises.

Figure 8-1 shows a typical arrangement.

Figure 8-1 TN-C-S earthing arrangement in a domestic property

The main earth terminal is installed at the supply position. This is connected to the supply neutral and the earth bar in the consumer unit or distribution board. In addition, the gas, water and other services entering the property are bonded to the main earth terminal. A circuit protective (earth) conductor is run with each electrical circuit which leaves the consumer unit. In a normal wiring arrangement, this would be the uninsulated copper earth wire which is enclosed with the insulated phase and neutral conductors in a PVC sheathed cable. All items of exposed conductive metalwork are bonded together to ensure that there are no potential differences between them during fault conditions.

Now consider a more complex installation, for example part of that within an office or small factory. An arrangement, with emphasis on the earthing arrangement, is illustrated in Figure 8-2. The different types of earth conductor were described in chapter 2 and there is now an opportunity to explain them in more detail.

The supply is TN-C, whilst the installation is TN-S. There is one main earthing terminal which is connected to the supply neutral. The protective conductors and main equipotential bonds are routed back to the main earth terminal. The main earthing terminal acts as the single reference point and can comprise of a bar, a plate or even a copper internal "ring" conductor. This would often be connected directly to an effective earth electrode and this connection must be of copper, as the regulations will not allow use of aluminium or copper clad aluminium because of the corrosion risk involved. The earth electrode should be positioned as close as possible to the main earthing terminal.

Figure 8-2 Typical TN-S installation within a commercial or light industrial property

A circuit protective conductor accompanies all current carrying conductors. If this conductor has a cross sectional area of 10 mm² or less, then it must be of copper. The main equipotential bonds are used to connect incoming services (such as metal gas or water pipes within the property). Supplementary bonding conductors give a visible indication that exposed metal equipment is interconnected and are mainly used when the required disconnection times cannot be met. The circuit protective conductors should already have ensured this, but the supplementary bond is normally shorter and thus more direct. It is not intended to carry fault current, but the minimum sizes are such that it is likely to carry some. Supplementary bonding conductors can (if necessary) also be used to connect external metalwork such as ladders, handrails etc. This would only be necessary if the external metalwork could introduce a potential (normally earth potential) and were within reach of conductive parts of equipment.

The designer must ensure that the protective conductor impedance is co-ordinated with the protective equipment characteristics such that during an earth fault, any voltages on exposed equipment which can be simultaneously touched are of magnitude and duration that they do not introduce danger. The voltage rise within an area during a fault has to be limited to a value stated in the regulations and this value is established by setting a minimum value of earth loop impedance. The maximum values are given in tables 41A1 and 41A2 of BS 7671. It is essential that copper protective conductors used have a sufficiently large cross sectional area and guidance on selecting the appropriate size is included in BS 7671 (tables 54B and 54C) Section 543.

Note that earth connections to metal enclosures should be grouped at one point, to avoid current having to flow through the metal of the enclosure itself. This could create interference. Where cables run between buildings, they should enter/leave at one point and if possible be routed through metal ducts which are electrically continuous. The duct/armouring should be connected back to the main earthing terminal. Surge protection may also be required at this point.

IT type equipment, such as computer power supplies, are now found to be causing particular problems with traditional types of earthing arrangements. This type of equipment has a permanent connection to earth and is a source of earth leakage current which has a high content of harmonics. Single phase rectified loads produce odd harmonics, some of which are additive in the neutral and earth conductors. If we assume that such equipment is situated at locations A, B and C in Figure 8-3, then the route along the protective conductor from C to the main earth terminal can be long, will have an impedance and there will be a voltage difference between the "earth" at C and that elsewhere. The inductance of the protective conductor will be especially important as the voltage difference will be greater for the harmonic currents than those at power frequency. This voltage difference is likely to create noise (or interference) and ultimately a shock risk. Heating and radiated electro-magnetic fields will be produced which could also cause interference. One way of reducing the voltage at C is to route an additional, separate protective conductor directly back to the main earth termination or as close as practical to it. This conductor should preferably be insulated and not run in parallel with cables or steelwork. A route which is as direct as possible will minimise its impedance.

In addition to the voltage reduction gained by this reduced impedance, there would be a further reduction because the leakage current associated with equipment at A and B would no longer follow the same route. This is called a "clean" earth and is shown in Figure 8-3. The "clean" earth could only be taken to a separate earth electrode if this in turn is bonded back to the main earthing terminal. If this bond did not exist, the arrangement would not conform to the regulations and could be dangerous. Other methods of producing a clean earth include use of isolation transformers and power line conditioners (typically an isolation transformer together with voltage regulation and some filtering of harmonics).

Figure 8-3 Earthing problems arising when equipment is interconnected

As mentioned previously, it is essential to select the appropriate cross sectional area and to reduce unwanted interference there is a growing tendency to increase the size of the protective conductors to help reduce interference in such installations. The cost to customers of data loss and equipment failure is often far greater than the initial capital cost of improving the earthing system.

Integrated Earthing Systems

It is not generally possible to have a system consisting of a number of different earthing systems, since these will inevitably interact and it is generally accepted that one integrated design with a low earth impedance is better than a number with medium impedance values. Figure 8-3 helps to illustrate why it is necessary to have one integrated design. It is first assumed that items of equipment at A and B each have their own earth electrode and that the metal enclosures of each are bonded to these. If an earth fault develops at A, possibly due to lightning, then fault current will flow to the ground via Rb and the potential on the exposed metalwork will rise. If there is no connection between A and B, equipment at B will be unaffected.

However, if there is a need to take a communication cable (X-Y) between the two sites, and assuming initially this has its sheath earthed only at A, there will be a potential difference between the sheath and the enclosure at B which may cause a flashover. If the cores of the cable are connected to a signal reference ground (the electronic equipment ground plane) at either end, then significant damage could result due to the potential difference and current flow. If the cable sheath is connected at each end, then some current will flow along it to earth via Rb. The potential difference between A and B will depend on the magnitude of the current, the impedance of the cable sheath and the individual impedances of Ra and Rb. Note that even when using fibre optic cables, care needs to be exercised as these often incorporate metal screens or draw wires.

The accepted way to reduce the potential difference is to bond the two enclosures together as closely as possible, by using a number of parallel connections. This would include copper earth wire, the cable sheaths and conduits etc. If A and B were separate buildings, the preferred way to bond the earthing systems would be a horizontal loop electrode approximately 1m outside each building, with several large electrodes interconnecting these.

Consider now that A and B are within the same building and that B has been provided with a so called "clean" earth. During normal operation the equipment at B will not be affected by interference on the earthing system at A (assuming it is possible to separate them totally - which is unlikely). However, during fault conditions there will be a potential difference between the enclosures (and possibly the reference earths) in exactly the same way as described above. For this reason it is normal to bond the two earthing systems together, although sometimes this is arranged to happen only during fault conditions.

Arrangements to reduce interference

The basic method is to ensure that the supply and return paths for fault current are as close as possible, since this reduces the electromagnetic field produced. This is accomplished with armoured cables and a protective conductor routed with the phases. If single core cables are used with single point bonding, these requirements are normally achieved by running an earth wire with the cables. One source of interference arises when the earthing system forms loops through which leakage and fault currents can circulate. One arrangement which limits the number of such loops and also provides a progressively more protected environment within a building is termed a nested shield arrangement. Whilst this arrangement is mainly concerned with surge protection, it is relevant here as it also involves earthing zones. Figure 8-4 shows three zones. Equipment within zone 2 is connected to the outer earth conductor and shield. This has a single connection to the main earth electrode. Equipment within zone 3 is connected to the shield/earth conductor surrounding it and then via a single connection to the shield of zone 2.

Figure 8-4 Nested shield type arrangement

This progressive shielding arrangement enables different amounts of protection to be afforded and, for example, zone 3 would normally be expected to have the least interference and would be the location for particularly sensitive or critical equipment. Cables passing between zones would require special connections such that the design is not compromised. Surge protection units would also be required at each position where a cable passes through a shield. Any fault current or induced interference current is transferred to the outer shield and eventually into the earth electrode. Faults which do arise should be diverted to ground at the outermost boundary to minimise the effect on equipment within.

Another arrangement designed to minimise interference, whilst ensuring that the earthing system is designed in a controlled manner without loops, is the hybrid design illustrated in Figure 8-5. It is intended to minimise earth loop areas, such as those that can arise when cabling between floors or adjacent areas. This arrangement is particularly applicable to buildings made from non-conducting materials.

Figure 8-5 Hybrid Earthing arrangement to reduce interference (courtesy W J Furse, based on work by Eric Montandon)

Note. The new system blocks (1, 2 & 3) are hybrid-bonded and may be connected to the existing system 4.

Key

Zone 1 = Not directly exposed to lightning

Zone 2 = No partial lightning currents

Zone 3 = Made up of equipment shielding

EB = Equipotential bonding

XXXX = Steel reinforcements in concrete

SERP = System earth reference point. This is the only metallic interface between the system and common earth. It must be directly connected to the structures steel reinforcement where cables leading to the system enter. All conductors that are bonded to the system earth within the system zone must be earthed at the SERP.