LEON KALIMA: 2008

PREFACE

On one Sunday morning the habitants of Broadway road in Ndola Zambia woke up expecting a usual Sunday morning. Some left their homes for Sunday church prayers, others left for other places and the majority stayed home. Their television sets, decoders, DVDs, computers, music systems, refrigerators, stoves, microwave cookers etc were in use and on standby. At around 10:00hrs a number of houses in Broadway, Chimwemwe road and Kopa road started experiencing high voltages. Electrical bulbs were abnormally bright, stove plates were extra red hot, television sets, decoders and computers seized working. Something was very wrong with their domestic electrical circuits and Zesco being a power supply company for the area was notified. Faults men were sent to the area and considering the number of complaints received the faults men rushed to Broadway substation supplying this area and immediately switched off the local transformer. Upon inspection of the transformer a neutral terminal at the transformer had snapped. Neutral re-termination was done and everything was normalised again.

A number of complaints of damaged equipment were registered at Zesco Ndola region. Electrical installation inspections were done at the clients who had complained of damaged equipment. Technical reports were written and sent to the legal departments to handle compensation claims for the damaged equipment.

As an individual working as an engineer in Zesco Ndola region the scenario was of great concern. Naturally, malfunction of the flow of power to the supply units is disheartening for a power supply company. Damage to customers’ electrical equipment economically disadvantages the power supply company as the damaged electrical equipment do
not add revenue to the power company. The good image of the company, especially of the technical professionals is highly dented when the system they have built and operate becomes a hazard to the clientele it is suppose to serve.

A lot of questions are raised but the most important of all is who is to blame? The electrical supply circuit in this case is divided in two parts namely the circuit belonging to the power supply company and the circuit belonging to the user. The legal system ably determines the answer to the above question according to the law of the land. As professionals our main concern is what omissions in the protection systems of either the user circuit or the source supply circuit caused this ugly incidence. What technical mechanisms should be put in place in both the power supply circuit and the user circuit to avoid a re-occurrence of the incidence? Why are such types of incidences common to specific areas in case of Ndola region? What are the pros and cons of various Earthing protection systems.

Lightening is a hazard to any electrical circuit regardless of it being a source supply circuit or a user circuit. Lightening during a storm is the cause of many outages, electricity supply faults, domestic electrical shocks and fatalities from being in contact with lightening electrical charge. A securely earthed circuit is protected from the harzard of lightening.
The technical professionals in the Electricity supply industries, Building construction industries, all professionals doing any sort of electrical wiring and anybody living in an installation with electricity is suppose to thoroughly understand and apply the protective earthing guidelines religiously.

Analysis of the Earthing phenomena for protecting the source supply circuit and the user circuits is what this paper will briefly consider. The topics on the following contents page will be exonerated from principle level to application.

Contents Page

1. The earthing principles

2. Earthing systems

3. The principle of earth fault loop impedance

4. Earth electrodes

5. Protective multiple earthing

6. Other protection methods

7. Residual Current Devices

1. The earthing principles

1.1 Earthing
The whole of the world may be considered as a vast conductor which is at reference (zero) potential. In the Zambia we refer to this as 'earth' whilst in the USA it is called 'ground'. People are usually more or less in contact with earth, so if other parts which are open to touch become charged at a different voltage from earth a shock hazard exist. The process of earthing is to connect all these parts which could become charged to the general mass of earth, to provide a path for fault currents and to hold the parts as close as possible to earth potential. In simple theory this will prevent a potential difference between earth and earthed parts, as well as permitting the flow of fault current which will cause the operation of the protective systems.
The standard method of tying the electrical supply system to earth is to make a direct connection between the two. This is usually carried out at the 33/0.4kV or 11/0.4kV step down supply transformer, where the neutral conductor (often the star point of a three-phase supply) is connected to earth using an earth electrode or the metal sheath and armouring of a buried cable.
1.2 Advantages of Earthing
There are two major reasons for earthing:
1. - The whole electrical system is tied to the potential of the general mass of earth and cannot 'float' at another potential. For example, we can be fairly certain that the neutral of our supply is at, or near, zero volts (earth potential) and that the phase conductors of our standard supply differ from earth by 240 volts.
2. - By connecting earth to metalwork not intended to carry current if an extraneous conductive part or a an exposed conductive part get in contact with the metal part a path is provided for fault current to flow to the general mass earth.
The disadvantage is that the installation of a complete system of protective conductors, earth electrodes, etc. is very expensive.

2.1 -System classification
The electrical installation does not exist on its own; the supply is part of the overall system. In some countries the supply company is under legal obligation to provide an earth terminal but in other countries the Electricity supply company has no legal obligation to provide the earth terminal .
As far as earthing types are concerned, letter classifications are used and are listed below with brief explanations of what they stand for.
The first letter indicates the type of supply earthing.
T -indicates that one or more points of the Supply are directly earthed (for example, the earthed neutral at the transformer).

I -indicates either that the supply system is not earthed at all, or that the earthing includes a deliberately inserted impedance, the purpose of which is to limit fault current. This method is not used for public supplies in most countries.
The second letter indicates the earthing arrangement in the installation.
T - all exposed conductive metalwork is connected directly to earth.
N - all exposed conductive metalwork is connected directly to an earthed supply conductor provided by the Electricity Supply Company.
The third and fourth letters indicate the arrangement of the earthed supply conductor system.
S - neutral and earth conductor systems are quite separate.
C - neutral and earth are combined into a single conductor.

2.2 - TT systems
This arrangement covers installations not provided with an earth terminal by the Electricity Supply Company. Neutral and earth (protective) conductors must be kept quite separate throughout the installation, with the final earth terminal connected to an earth electrode by means of an earthing conductor.
Effective earth connection is sometimes difficult. Because of this, socket outlet circuits must be protected by a residual current device (RCD) with an operating current of 30 mA.
Figure 2.1 shows the arrangement of a TT earthing system.

Fig 2.1 - TT earthing system

2.3 - TN-S system
Before 1970s this was the most usual earthing system in Zambia, where the electricity supply company was providing an earth terminal at the incoming mains position. This earth terminal is connected by the supply protective conductor (PE) back to the star point (neutral) of the secondary winding of the supply transformer, which is also connected at that point to an earth electrode. The earth conductor usually takes the form of the armour and sheath (if applicable) of the underground supply cable. The system is shown diagrammatically in Figure 2.2.

Fig 2.2 TN-S earthing system

2.4 - TN-C-S system
In this system, the installation is TN-S, with separate neutral and protective conductors. The supply, however, uses a common conductor for both the neutral and the earth. This combined earth and neutral system is sometimes called the 'protective and neutral conductor' (PEN) or the 'combined neutral and earth' conductor (CNE). The system, which is shown diagrammatically in Figure 2.3., is most usually called the protective multiple earth (PME) system, which will be considered in greater detail in the topics to be considered.

Figure 2.3 - TN-C-S earthing system - protective multiple earthing

2.5 -TN-C system
This installation is unusual, because combined neutral and earth wiring is used in both the supply and within the installation itself. Where used, the installation will usually be the earthed concentric system, which can only be installed under the special conditions to be considered in later topics.
2.6 - IT system
The installation arrangements in the IT system are the same for those of the TT system However, the supply earthing is totally different. The IT system can have an unearthed supply, or one which is not solidly earthed but is connected to earth through a current limiting impedance.
The total lack of earth in some cases, or the introduction of current limiting into the earth path, means that the usual methods of protection will not be effective. For this reason, IT systems are not allowed in the public supply system in most countries. An exception is in medical situations such as hospitals. Here it is recommended that an IT system is used for circuits supplying medical equipment that is intended to be used for life-support of patients. The method is also sometimes used where a supply for special purposes is taken from a private generator.

Earthing conductors
The earthing conductor is commonly called the earthing lead. It joins the installation earthing terminal to the earth electrode or to the earth terminal provided by the Electricity Supply Company. It is a vital link in the protective system, so care must be taken to see that its integrity will be preserved at all times. Aluminium conductors and cables may now be used for earthing and bonding, but great care must be taken when doing so to ensure that there will be no problems with corrosion or with electrolytic action where they come into contact with other metals.
Where the final connection to the earth electrode or earthing terminal is made there must be a clear and permanent label Safety Electrical Connection - Do not remove.Where a buried earthing conductor is not protected against mechanical damage but is protected against corrosion by a sheath, its minimum size must be 16 mm² whether made of copper or coated steel. If it has no corrosion protection, minimum sizes for mechanically unprotected earthing conductors are 25 mm² for copper and 50 mm² for coated steel.
If not protected against corrosion the latter sizes again apply, whether protected from mechanical damage or not.
Earthing conductors, as well as protective and bonding conductors, must be protected against corrosion. Probably the most common type of corrosion is electrolytic, which is an electro-chemical effect between two different metals when a current passes between them whilst they are in contact with each other and with a weak acid. The acid is likely to be any moisture which has become contaminated with chemicals carried in the air or in the ground. The effect is small on ac supplies because any metal removed whilst current flows in one direction is replaced as it reverses in the next half cycle. For dc Systems, however, it will be necessary to ensure that the system remains perfectly dry (a very difficult task) or to use the 'sacrificial anode' principle.
A main earth terminal or bar must be provided for each installation to collect and connect together all protective and bonding conductors. It must be possible to disconnect the earthing conductor from this terminal for test purposes, but only by the use of a tool. This requirement is intended to prevent unauthorised or unknowing removal of protection.
3.1 - Protective conductor types
The circuit protective conductor (increasingly called the 'c.p.c.') is a system of conductors joining together all exposed conductive parts and connecting them to the main earthing terminal. Strictly speaking, the term includes the earthing conductor as well as the equipotential bonding conductors.
The circuit protective conductor can take many forms, such as:
1. - a separate conductor which must be green/yellow insulated if equal to or less than 10 mm2 cross-sectional area.
2. - a conductor included in a sheathed cable with other conductors
3. - the metal sheath and/or armouring of a cable
4. - conducting cable enclosures such as conduit or trunking
5. - exposed conductive parts, such as the conducting cases of equipment
This list is by no means exhaustive and there may be many other items forming parts of the circuit protective conductor as indicated in figure 6. Note that gas or oil pipes must not be used for the purpose, because of the possible future change to plastic (non-conducting) pipes.

Figure 3.1. Some types of circuit protective conductor
It is, of course, very important that the protective conductor remains effective throughout the life of the installation. Thus, great care is needed to ensure that steel conduit used for the purpose is tightly jointed and unlikely to corrode. The difficulty of ensuring this point is leading to the increasing use of a c.p.c. run inside the conduit with the phase conductors. Such a c.p.c. will, of course, always be necessary where plastic conduits are used. Where an accessory is connected to a system (for example, by means of a socket outlet) which uses conduit as its c.p.c., the appliance (or socket outlet) earthing terminal must be connected by a separate conductor to the earth terminal of the conduit box (see {Figure 3.1}). This connection will ensure that the accessory remains properly earthed even if the screws holding it into the box become loose, damaged or corroded.

Figure 3.2. Protective connection for socket outlet in conduit system

Figure 3.3 Separate additional protective conductor with flexible conduit
A separate protective conductor will be needed where flexible conduit is used, since this type of conduit cannot be relied upon to maintain a low resistance conducting path (see {Figure 3.3}).

3.2 - Bonding conductors
The purpose of the protective conductors is to provide a path for earth fault current so that the protective device will operate to remove dangerous potential differences, which are unavoidable under fault conditions, before a dangerous shock can be delivered. Equipotential bonding serves the purpose of ensuring that the earthed metalwork (exposed conductive parts) of the installation is connected to other metalwork (extraneous conductive parts) to ensure that no dangerous potential differences can occur. The resistance of such a bonding conductor must be low enough to ensure that its volt drop when carrying the operating current of the protective device never exceeds 50 V.

Fig 3.4 Main bonding connections
Thus R < 50
Ia
where R is the resistance of the bonding conductor
Ia is the operating current of the protective device.
Two types of equipotential bonding conductor are specified.
1. - Main equipotentiol bonding conductors
These conductors connect together the installation earthing system and the metalwork of other services such as gas and water. This bonding of service pipes must be effected as close as possible to their point of entry to the building, as shown in {Fig 3.4}. Metallic sheaths of telecommunication cables must be bonded with consent of the owner of the cable. The minimum size of bonding conductors is related to the size of the main supply conductors (the tails) and is given in {Table 3.1}.
2. - Supplementary bonding conductors
These conductors connect together extraneous conductive parts - that is, metalwork which is not associated with the electrical installation but which may provide a conducting path giving rise to shock. The object is to ensure that potential differences in excess of 50 V between accessible metalwork cannot occur; this means that the resistance of the bonding conductors must be low (see {Table 3.1}) {Figure 3.5} shows some of the extraneous metalwork in a bathroom which must be bonded.
Table 3.1 - Supplementary bonding conductor sizes
Circuit protective
conductor size Supplementary bonding conductor size
- Not protected Mechanically protected
1.0 mm² 4.0 mm² 2.5 mm²
1.5 mm² 4.0 mm² 2.5 mm²
2.5 mm² 4.0 mm² 2.5 mm²
4.0 mm² 4.0 mm² 2.5 mm²
6.0 mm² 4.0 mm² 4.0 mm²
10.0 mm² 6.0 mm² 6.0 mm²

Fig 3.5 Supplementary bonding in a bathroom
The cross-sectional areas required for supplementary bonding conductors are shown in {Table 3.5}. Where connections are between extraneous parts only, the conductors may be 2.5 mm² if mechanically protected or 4 mm²if not protected. If the circuit protective conductor is larger than 10 mm², the supplementary bonding conductor must have at least half this cross-sectional area. Supplementary bonding conductors of less than 16 mm² cross sectional area must not be aluminium.

There will sometimes be doubt if a particular piece of metalwork should be bonded. The answer must always be that bonding will be necessary if there is a danger of severe shock when contact is made between a live system and the metal work in question. Thus if the resistance between the metalwork and the general mass of earth is low enough to permit the passage of a dangerous shock current, then the metalwork must be bonded.
The question can be resolved by measuring the resistance (Rx) from the metalwork concerned to the main earthing terminal. Using this value in the formula:
Ib = Uo
Rp + Rx
will allow calculation of the maximum current likely to pass through the human body where :
Ib -is the shock current through the body (A)
Uo - Is the voltage of the supply (V)
RP -is the resistance of the human body (Ohms) and
Rx - is the measured resistance from the metalwork concerned
to the main earthing terminal (Ohms)
The resistance of the human body, RP can in most cases be taken as 1000 Ohms although 200 Ohms would be a safer value if the metalwork in question can be touched by a person in a bath. Although no hard and fast rules are possible for the value of a safe shock current, Ib, it is probable that 10 mA is seldom likely to prove fatal. Using this value with 240 V for the supply voltage, Uo, and 1000 Ohms as the human body resistance, RP, the minimum safe value of RP calculates to 23 kOhms. If the safer values of 5 mA for Ib and 200 Ohms for RP are used, the value of Rx would be 47.8 kOhms for a 240 V supply.
{Fig 3.6} shows the application of a supplementary bonding conductor to prevent the severe shock which could otherwise occur between the live case of a faulty electric kettle and an adjacent water tap.

Fig 3.6 Supplementary bonding conductor in a kitchen
To sum up when in doubt about the need to bond metalwork, measure its resistance to the main earthing terminal. If this value is 50 kOhms or greater, no bonding is necessary. In a situation where a person is not wet, bonding could be ignored where the resistance to the main earthing terminal is as low as 25 kOhms. To reduce the possibility of bonding conductors being disconnected by those who do not appreciate their importance, every bonding connection should be provided with a label.
3.3 - Protective conductor cross-section assessment
A fault current will flow when an earth fault occurs. If this current is large enough to operate the protective device quickly, there is little danger of the protective conductor and the exposed conductive parts it connects to earth being at a high potential to earth for long enough for a dangerous shock to occur. The factors determining the fault current are the supply voltage and the earth-fault loop impedance.
The earth fault results in the protective conductors being connected in series across the supply voltage {Fig 5.16}. The voltage above earth of the earthed metalwork (the voltage of the junction between the protective and phase conductors) at this time may become dangerously high, even in an installation complying with the Regulations. The people using the installation will be protected by the ability of the fuse or circuit breaker in a properly designed installation to cut off the supply before dangerous shock damage results.

Fig 3.7 -The effect of protective conductor resistance on shock voltage
a) effective resistance of a ring circuit protective conductor
b) potential differences across healthy protective conductor in the event of an earth fault
Table 3.2 - Main earthing and main equipotential bonding conductor
----------------- sizes for TN-S and TN-C-S supplies
Phase conductor (or neutral for PME supplies ) Earthing conductor (not buried or protected against mechanical damage) Main equipotential bonding conductor for PME supplies Main equipotential bonding conductor
csa mm² csa mm² csa mm² csa mm²
4 4 6 10
6 6 6 10
10 10 6 10
16 16 10 10
25 16 10 10
35 16 10 10
50 25 16 16
70 35 16 25
Remember that lower fault levels result in a longer time before operation of the protective device. Since the cross-sectional area of the protective conductor will usually be less than that of live conductors, its temperature, and hence its resistance, will become higher during the fault, so that the shock voltage will be a higher proportion of the supply potential (see {Fig 5.16}).
{Fig 5.16} shows the circuit arrangements, with some typical phase- and protective-conductor resistances. In this case, a shock voltage of 140 V will be applied to a person in contact with earthed metal and with the general mass of earth. Thus, the supply must he removed very quickly. The actual voltage of the shock depends directly on the relationship between the phase conductor resistance and the protective conductor resistance. If the two are equal, exactly half the supply voltage will appear as the shock voltage.
For socket outlet circuits, where the shock danger is highest, the maximum protective conductor resistance values of {Table 5.3} will ensure that the shock voltage never exceeds the safe value of 50 V. If the circuit concerned is in the form of a ring, one quarter of the resistance of the complete protective conductor round the ring must not be greater than the {Table 5.3} figure. The reason for this is shown in {Fig 5.16(a)}. This assumes that the fault will occur exactly at the mid point of the ring. If it happens at any other point, effective protective conductor resistance is lower, and safer, than one quarter of the total ring resistance.
{Table 5.7} allows selection (rather than calculation) of sizes for earthing and bonding conductors. The rules applying to selection are:
For phase conductors up to 16 mm², the protective conductor has the same size as the phase conductor.
For phase conductors from 16 mm² to 35 mm², the protective conductor must be 16 mm²
For phase conductors over 35 mm², the protective conductor must have at least half the c.s.a. of the phase conductor. The minimum cross-sectional area of a separate CPC is 2.5 mm² if mechanically protected and 4mm² if not.
Note that Regional Electricity Companies may require a minimum size of earthing conductor of 16 mm² at the origin of the installation. Always consult them before designing an installation.
3.4. - Protective conductor cross-section calculation
The c.s.a. of the circuit protective conductor (c.p.c.) is of great importance since
the level of possible shock in the event of a fault depends on it (as seen in {5.4.4}).
Safety could always be assured if we assessed the size using {Table 5.7} as a basis.
However, this would result in a more expensive installation than necessary because we would often use protective conductors which are larger than those found to be acceptable by calculation. For example. twin with cpc insulated and sheathed cables larger than 1 mm² would be ruled out because in all other sizes the CPC is smaller than required by {Table 5.7}.
In very many cases, calculation of the CPC size will show that a smaller size than that detailed in {5.4.4} is perfectly adequate. The formula to be used is:
S = (Ia²t)
k
where S is the minimum protective conductor cross-sectional area (mm2)
Ia is the fault current (A)
t is the opening time of the protective device (s)
k is a factor depending on the conductor material and insulation, and the initial and maximum insulation temperatures.
This is the same formula as in {3.7.3}, the adiabatic equation, but with a change in the subject. To use it, we need to have three pieces of information, Ia, t and k.
1) To find Ia
Since Ia = Uo we need values for Ia = uo and Zs
Zs

Uo is simply the supply voltage, which in most cases will be 240V.
Zs is the earth-fault loop impedance assuming that the fault has zero impedance.
Since we must assume that we are at the design stage, we cannot measure the loop impedance and must calculate it by adding the loop impedance external to the installation (Ze) to the resistance of the conductors to the furthest point in the circuit concerned. This technique was used in {5.3.6}.
Thus, Zs = Ze + R1 + R2 where R1 and R2 are the resistances of the phase and protective conductors respectively from {Table 5.5}.
2) To find t
We can find t from the time/current characteristics of {Figs 3.13 to 3.19} using the value of Ia already calculated above. For example, if the protective device is a 20 A miniature circuit breaker type I and the fault current is 1000 A, we shall need to consult {Fig 3.16}, when we can read off that operation will be in 0.01 s (10 ms). (It is of interest here to notice that if the fault current had been 80 A the opening time could have been anything from 0.04 s to 20 s,so the circuit would not have complied with the required opening times).
3) To find k
k is a constant, which we cannot calculate but must obtain from a suitable table of values. Some values of k for typical protective conductors are given in {Table 5.8}.
It is worth pointing out here that correctly installed steel conduit and trunking will always meet the requirements of the Regulations in terms of protective conductor impedance.
Although appearing a little complicated, calculation of acceptable protective conductor size is worth the trouble because it often allows smaller sizes than those shown in {Table 5.7}.
Table 3.3 - Values of k for protective conductors
Nature of protective conductor Initial temp.
(°C) Final temp
(°C) Conductor
material K
p.v.c. insulated, not in cable
or bunched 30 160 Copper 143
- 30 160 Aluminium 95
- 30 160 Steel 52
-
p.v.c. insulated, in cable
or bunched 70 160 Copper 115
- 70 160 Aluminium 76
-
Steel conduit or trunking 50 160 Steel 47
-
Bare conductor 30 200 Copper 159
- 30 200 Aluminium 105
- 30 200 Steel 58
Example 3.1
A load takes 30 A from a 240 V single phase supply and is protected by a 32 A HBC fuse to BS 88. The wiring consists of 4 mm² single core p.v.c. insulated cables run in trunking, the length of run being 18 m. The earth-fault loop impedance external to the installation is assessed as 0.7 Ohms. Calculate the cross-sectional area of a suitable p.v.c. sheathed protective conductor.
This is one of those cases where we need to make an assumption of the answer to the problem before we can solve it. Assume that a 2.5 mm² protective conductor will be acceptable and calculate the combined resistance of the phase and protective conductors from the origin of the installation to the end of the circuit. From {Table 5.5}, 2.5 mm² cable has a resistance of 7.4 mohms/m and 4 mm² a resistance of 4.6 mOhms/m. Both values must be multiplied by 1.2 to allow for increased resistance as temperature rises due to fault current.
Thus, R1 + R2 = (7.4 + 4.6) x 1.2 x 18 Ohms = 12.0 x 1.2 x 18 = 0.26 Ohms
1000 1000
This conductor resistance must be added to external loop impedance to give the total earth-fault loop impedance.
Zs = Ze + Rl + R2 = 0.7+ 0.26 Ohms = 0.96 Ohms
We can now calculate the fault current:
la = Uo = 240 = 250A
Zs 0.96
Next we need to find the operating time for a 32 A BS 88 fuse carrying 250 A. Examination of {Fig 3.15} shows that operation will take place after 0.2 s.
Finally, we need a value for k. From {Table 5.8} we can read this off as 115, because the protective conductor will be bunched with others in the trunking.
We now have values for Ia, t and k so we can calculate conductor size.
S = (Ia²t) = (250² x 0.02) mm² = 0.97 mm²
k 115
This result suggests that a 1.0 mm² protective conductor will suffice. However, it may he dangerous to make this assumption because the whole calculation has been based on the resistance of a 2.5 mm² conductor. Let us start again assuming a 1.5 mm² protective conductor and work the whole thing through again.
The new size protective conductor has a resistance of 18.1 mOhms/m, see {Table 5.5}, and with the 4 mm² phase conductor gives a total conductor resistance, allowing for increased temperature, of 0.491 Ohms. When added to external loop impedance this gives a total earth-fault loop impedance of 1.191 Ohms and a fault current at 240 V of 202 A. From {Fig 3.15} operating time will be 0.6 s. The value of k will be unchanged at 115.
S = (Ia²t) = (202² x 0.6) mm² = 1.36 mm²
k 115
Thus, a 1.5 mm² protective conductor can be used in this case. Note that if the size had been assessed rather than calculated, the required size would be 4 mm², two sizes larger. A point to notice here is that the disconnection time with a 1.5mm² protective conductor is 0.6 s, which is too long for socket outlet circuits (0.4 s max.).
Example 3.2
A 240 V, 30 A ring circuit for socket outlets is 45 m long and is to be wired in 2.5 mm² flat twin p.v.c. insulated and sheathed cable incorporating a 1.5 mm² cpc. The circuit is to be protected by a semi-enclosed (rewirable) fuse to BS 3036, and the earth-fault loop impedance external to the installation has been ascertained to be 0.3 Ohms. Verify that the 1.5 mm² cpc enclosed in the sheath is adequate.
First use {Table 5.5} to find the resistance of the phase and cpc conductors. These are 7.4 mOhms/m and 12.1 mOhms/m respectively, so for a 45 m length and allowing for the resistance increase with temperature factor of 1.2.
R1 + R2 = (7.4 + 12.1) x 1.2 x 45 Ohms = 19.5 x 1.2 x 45 Ohms = 1.05 Ohms
1000 1000

Zs = Ze + R1+ R2 = 0.3 + 1.05 Ohms = 0.3 + 0.263 Ohms = 0.563 Ohms
4 4
The division by 4 is to allow for the ring nature of the circuit.
Ia = Uo = 240 A = 426A
Zs 0.563
We must then use the time/current characteristic of {Fig 3.13} to ascertain an operating time of 0.10 s.
From {Table 5.8} the value of k is 115.
Then S = (Ia²t) = (426² x 0.10) mm² = 1.17 mm²
k 115
Since this value is smaller than the intended value of 1.5 mm², this latter value will be satisfactory.
Example 3.3
A 240 V single-phase circuit is to be wired in p.v.c. insulated single core cables enclosed in plastic conduit. The circuit length is 45 m and the live conductors are 16 mm² in cross-sectional area. The circuit will supply fixed equipment, and is to be protected by a 63 A HBC fuse to BS 88. The earth-fault loop impedance external to the installation has been ascertained to be 0.58 Ohms. Calculate a suitable size for the circuit protective conductor.
With the information given this time the approach is somewhat different. We know that the maximum disconnection time for fixed equipment is 5 s, so from the time/current characteristic for the 63 A fuse {Fig 3.15} we can see that the fault current for disconnection will have a minimum value of 280 A.
Thus, Zs = Uo = 240 Ohms = 0.857 Ohms
Ia 280
If we deduct the external loop impedance, we come to the resistance of phase and protective conductors.
R1 +R2 = Zs-Ze = 0.857-0.58Ohms = 0.277 Ohms
Converting this resistance to the combined value of R1 and R2 per metre,
(R1 + R2) per metre = 0.277 x 1000 mOhms/m = 5.13 mOhms/m
45 x 1.2
Consulting {Table 5.5} we find that the resistance of 16 mm² copper conductor is 1.15 mOhms/m, whilst 10 mm² and 6 mm² are 1.83 and 3.08 mOhms/m respectively. Since 1.15 and 3.08 add to 4.23, which is less than 5.13, it would seem that a 6 mm² protective conductor will be large enough. However, to be sure we must check with the adiabatic equation.
R1 + R2 = (1.15+3.08) x 1.2 x 45 Ohms = 0.228 Ohms
1000

Zs = Ze+(R1 +R2) = 0.58+0.228 Ohms = 0.808 Ohms

Ia = Uo = 240 A = 297 A
Zs 0.808
From {Fig 3.15} the disconnection time for a 63 A fuse carrying 297 A is found to he 3.8s.
From {Table 5.8} the value of k is 115.
Then S = (Ia²t) = (297² x 3.8) mm² = 5.03 mm²
k 115
Since 5.03 is less than 6 then a 6 mm² protective conductor will be large enough to satisfy the requirements

3.4 - Unearthed metalwork
If exposed conductive parts are isolated, or shrouded in non-conducting material, or are small so that the area of contact with a human body is limited, it is permissible not to earth them. Examples are overhead line metalwork which is out of reach, steel reinforcing rods within concrete lighting columns, cable clips, nameplates, fixing screws and so on. Where areas are accessible only to skilled or instructed persons, and where unauthorised persons are unlikely to enter due to the presence of warning notices, locks and so on, earthing may be replaced by the provision of obstacles which make direct contact unlikely, provided that the installation complies with the Electricity at Work Regulations, 1989.

Earth electrodes
4.1-Why must we have earth electrodes?
The principle of earthing is to consider the general mass of earth as a reference (zero) potential. Thus, everything connected directly to it will be at this zero potential, or above it by the amount of the volt drop in the connection system (for example, the volt drop in a protective conductor carrying fault current). The purpose of the earth electrode is to connect to the general mass of earth.
With the increasing use of underground supplies and of protective multiple earthing (PME) it is becoming more common for the consumer to be provided with an earth terminal rather than having to make contact with earth using an earth electrode.

4.2 - Earth electrode types
Acceptable electrodes are rods, pipes, mats, tapes, wires, plates and structural steelwork buried or driven into the ground. The pipes of other services such as gas and water must not be used as earth electrodes although they must be bonded to earth as described in {5.4.3}. The sheath and armour of a buried cable may be used with the approval of its owner and provided that arrangements can be made for the person responsible for the installation to be told if the cable is changed, for example, for a type without a metal sheath.
The effectiveness of an earth electrode in making good contact with the general mass of earth depends on factors such as soil type, moisture content, and so on. A permanently-wet situation may provide good contact with earth, but may also limit the life of the electrode since corrosion is likely to be greater. If the ground in which the electrode is placed freezes, there is likely to be an increase in earth resistance. In most parts of the UK an earth electrode resistance in the range 1 Ohm to 5 Ohms is considered to be acceptable.
The method of measuring the resistance of the earth electrode will be considered in {8.6.1}; the resistance to earth should be no greater than 220 Ohms. The earthing conductor and its connection to the earth electrode must be protected from mechanical damage and from corrosion. Accidental disconnection must be avoided by fixing a permanent label as shown in {Fig 5.17} which reads:

Fig 4.1 Connection of earthing conductor to earth electrode
4.3 - What is earthed concentric wiring?
This is the TN-C system {5.2.5} where a combined neutral and earth (PEN) conductor is used throughout the installation as well as for the supply. The PEN conductor is the sheath of a cable and therefore is concentric with (totally surrounds) the phase conductor(s). The system is unusual, but where employed almost invariably uses mineral insulated cable, the metallic copper sheath being the combined neutral and earth conductor.

4.4 - Requirements for earthed concentric wiring
Earthed concentric wiring may only be used under very special conditions, which usually involve the use of a private transformer supply or a private generating plant. Since there is no separate path for earth currents, it follows that residual current devices (RCDs) will not be effective and therefore must not be used. The cross-sectional area of the sheath (neutral and earth conductor) of a cable used in such a system must never be less than 4 mm² copper, or 16 mm² aluminium or less than the inner core for a single core cable. All multicore copper mineral insulated cables comply with this requirement, even a I mm² two core cable having the necessary sheath cross-sectional area. However, only single core cables of 6 mm² and below may be used. The combined protective and neutral conductors (sheaths) of such cables must not serve more than one final circuit.
Wherever a joint becomes necessary in the PEN conductor, the contact through the normal sealing pot and gland is insufficient; an extra earth tail must be used as shown in {Fig 5.19}. If it becomes necessary to separate the neutral and protective conductors at any point in an installation, they must not be connected together again beyond that point.

Fig 4.2 Earth tail seal for use in earthed concentric wiring

4.5 - Class II equipment
Class H equipment has reinforced or double insulation. As well as the basic insulation for live parts, there is a second layer of insulation, either to prevent contact with exposed conductive parts or to make sure that there can never be any contact between such parts and live parts. The outer case of the equipment need not be made of insulating material; if protected by double insulation, a metal case will not present any danger. It must never be connected with earth, so connecting leads are two-core, having no protective conductor. The symbol for a double-insulated appliance is shown in {Fig 5.20}.

Fig 4.3 British Standard symbol for double insulation
To make sure that the double insulation is not impaired, it must not be pierced by conducting parts such as metal screws. Nor must insulating screws be used, because there is the possibility that they will be lost and will be replaced by metal screws. Any holes in the enclosure of a double Insulated appliance, such as those to allow ventilation, must be so small that fingers cannot reach live parts (IP2X protection). Class II equipment must be installed and fixed so that the double insulation will never be impaired, and so that metalwork of the equipment does not come into contact with the protective system of the main installation. Where the whole of an installation is comprised of Class II equipment, so that there is no protective system installed, the situation must be under proper supervision to make sure that no changes are made which will introduce earthed parts.
4.6 - Non-conducting location
The non-conducting location is a special arrangement where there is no earthing
or protective system because:
1. - there is nothing which needs to be earthed
2. - exposed conductive parts are arranged so that it is impossible to touch two of them, or an exposed conducting part and an extraneous conductive part, at the same time. The distance between the parts must be at least 2 m, or 1.25 m if they are out of arm's reach. An alternative is to erect suitable obstacles, or to insulate the extraneous conductive parts.
Examples of extraneous conductive parts are water and gas pipes, structural steelwork, and even floors and walls which are not covered with insulating material. Insulation tests on floors and walls are considered in {8.5.2}. There must be no socket outlets with earthing contacts in a non-conducting location. This type of installation could cause danger if earthed metal were introduced in the form of a portable appliance fed by a lead from outside the location.
The potential reached by exposed metalwork within the situation is of no importance because it is never possible to touch two pieces of metalwork with differing voltage levels at the same time. Care must be taken, however, to make sure that a possible high potential cannot be transmitted outside the situation by the subsequent installation of a conductor such as a water or gas pipe. A notice must be erected to state that a non-conducting location exists, and giving details of the person in charge who alone will authorise any work to be undertaken in, or will authorise any equipment to be taken into, the location. If two faults to exposed conductive parts occur from conductors at different potentials (such as a phase and a neutral) and there is a defective bonding system, dangerous potential differences could occur between exposed conductive parts. To prevent this possibility, double pole fuses or circuit breakers must provide overload protection in non-conducting locations.
Non-conducting locations are unusual, and their use must be limited to situations where there is continuous and proper supervision to ensure that the requirements are fully met and are properly maintained. This type of installation should only be considered after consulting a fully qualified electrical engineer.

4.7 - Earth-free bonding
The Regulations permit the provision of an area in which all exposed metal parts are connected together, but not to earth. Inside the area, there can be no danger, even if the voltage to earth is very high, because all metalwork which can be touched will be at the same potential. Care is necessary, however, to prevent danger to people entering or leaving the area, because then they may be in contact with parts which are inside and others which are outside the area, and hence at differing potentials. A notice must be erected to warn that the bonding conductors in the system must not he connected to earth, and that earthed equipment must not be brought into the situation.
As in the case of non-conducting locations, this type of installation is unusual, and must only be undertaken when designed and specified by a fully qualified electrical engineer. The area inside the protected system is often referred to as a 'Faraday cage'.

4.8 - Electrical separation
Safety from shock can sometimes be ensured by separating a system completely from others so that there is no complete circuit through which shock current could flow. It follows that the circuit must be small to ensure that earth impedance's are very high and do not offer a path for shock current (see {Fig 5.3(b)}). The source of supply for such a circuit could be a battery or a generating set, but is far more likely to be an isolating transformer with a secondary winding providing no more than 500 V. Such a transformer must comply with BS EN 60742, having a screen between its windings and a secondary winding which has no connection to earth.
There must be no connection to earth and precautions must he taken to ensure, as far as possible, that earth faults will not occur. Such precautions would include the use of flexible cords without metallic sheaths, using double insulation, making sure that flexible cords are visible throughout their length of run, and so on. Perhaps the most common example of a separated circuit is the bathroom transformer unit feeding an electric shaver. By breaking the link to the earthed supply system using the double wound transformer, there is no path to earth for shock current (see {Fig 5.21}).

Fig 4.4 Bathroom shaver socket to BS EN 60742

Residual current devices (RCDs)
5.1 - Why do we need residual current devices?
It has been stressed that the standard method of protection is to make sure that an earth fault results in a fault current high enough to operate the protective device quickly so that fatal shock is prevented. However, there are cases where the impedance of the earth-fault loop, or the impedance of the fault itself, are too high to enable enough fault current to flow. In such a case, either:
1. - current will continue to flow to earth, perhaps generating enough heat to start a fire, or
2. - metalwork which is open to touch may be at a high potential relative to earth, resulting in severe shock danger.
Either or both of these possibilities can be removed by the installation of a residual current device (RCD).
In recent years there has been an enormous increase in the use of initials for residual current devices of all kinds. The following list, which is not exhaustive, may be helpful to readers:
RCD residual current device
RCCD residual current operated circuit breaker
SRCD socket outlet incorporating an RCD
PRCD portable RCD, usually an RCD incorporated into a plug
RCBO an RCCD which includes overcurrent protection
SRCBO a socket outlet incorporating an RCBO
5.2 - The principle of the residual current device
The RCD is a circuit breaker which continuously compares the current in the phase with that in the neutral. The difference between the two (the residual current) will he flowing to earth, because it has left the supply through the phase and has not returned in the neutral (see {Fig 5.22}). There will always be some residual current in the insulation resistance and capacitance to earth, but in a healthy circuit such current will he low, seldom exceeding 2 mA.

Fig 5.1 The meaning of the term residual current
The purpose of the residual current device is to monitor the residual current and to switch off the circuit quickly if it rises to a preset level. The arrangement of an RCD is shown in simplified form in {Fig 5.23}. The main contacts are closed against the pressure of a spring, which provides the energy to open them when the device trips. Phase and neutral currents pass through identical coils wound in opposing directions on a magnetic circuit, so that each coil will provide equal but opposing numbers of ampere turns when there is no residual current. The opposing ampere turns will cancel, and no magnetic flux will be set up in the magnetic circuit.
Residual earth current passes to the circuit through the phase coil but returns through the earth path, thus avoiding the neutral coil, which will therefore carry less current. This means that phase ampere turns exceed neutral ampere turns and an alternating magnetic flux results in the core. This flux links with the search coil, which is also wound on the magnetic circuit, inducing an e.m.f. into it. The value of this e.m.f. depends on the residual current, so it will drive a current to the tripping system which depends on the difference between phase and neutral currents. When the amount of residual current, and hence of tripping current, reaches a pre-determined level, the circuit breaker trips, opening the main contacts and interrupting the circuit.
For circuit breakers operating at low residual current values, an amplifier may be used in the trip circuit. Since the sum of the currents in the phases and neutral of a three-phase supply is always balanced, the system can be used just as effectively with three-phase supplies. In high current circuits, it is more usual for the

Fig 5.2 Residual current circuit breaker
phase and neutral conductors to simply pass through the magnetic core instead of round coils wound on it.
Operation depends on a mechanical system, which could possibly become stiff when old or dirty. Thus, regular testing is needed, and the RCD is provided with a test button which provides the rated level of residual current to ensure that the circuit breaker will operate. All RCDs are required to display a notice which draws attention to the need for frequent testing which can be carried out by the user, who presses a test button, usually marked T. {Table 5.10} shows the required notice.
This installation, or part of it, is protected by a device which automatically switches off the supply if an earth fault develops.
Test quarterly by pressing the button marked 'T' or 'Test'. The device should switch off the supply, and should then be switched on to restore the supply. If the device does not switch off the supply when the button is pressed, seek expert advice
Table 5.1 - Periodic test notice for residual current device
The test circuit is shown in {Fig 5.23}, and provides extra current in the phase coil when the test button is pressed. This extra current is determined by the value of the resistor R.
There are currently four basic types of RCD. Class AC devices are used where the residual current is sinusoidal - this is the normal type which is in the most wide use. Class A types are used where the residual current is sinusoidal and/or includes pulsating direct currents - this type is applied in special situations where electronic equipment is used. Class B is for specialist operation on pure direct current or on impulse direct or alternating current. Class S RCDs have a built-in time delay to provide discrimination (see below).
It must be understood that the residual current is the difference between phase and neutral currents, and that the current breaking ability of the main contacts is not related to the residual operating current value, There is a widely held misunderstanding of this point, many people thinking that the residual current setting is the current breaking capability of the device. It is very likely that a device with a breaking capacity of 100 A may have a residual operating current of only 30 mA.
There are cases where more than one residual current device is used in an installation; for example, a complete installation may be protected by an RCD rated at 100 mA whilst a socket intended for equipment outdoors may be protected by a 30 mA device. Discrimination of the two devices then becomes important. For example, if an earth fault giving an earth current of 250 mA develops on the equipment fed by the outdoor socket, both RCDs will carry this fault current, and both will become unbalanced. Since the fault is higher than the operating current of both devices, both will have their trip systems activated. It does not follow that the device with the smaller operating current will open first, so it is quite likely that the 100 mA device will operate, cutting off the supply to the complete installation even though the fault was on a small part of it. This is a lack of discrimination between the residual current devices. To ensure proper discrimination, the device with the larger operating current has a deliberate delay built into its operation. It is called a time delayed RCD.

5.3 - Regulations for residual current devices
The primary purpose of the residual current device is to limit the severity of shock due to indirect contact. In other words, it will detect and clear earth faults which otherwise would could lead to dangerous potential differences between pieces of metalwork which are open to touch. If the sensitivity of the device (its operating residual current) is low enough, it may also be used to limit the shock received from direct contact in the case of the failure of other measures. A problem which may occur here is nuisance tripping, because the operating current may be so low that normal leakage current will cause operation. For example an RCD with a sensitivity of 2 mA will switch off the supply as soon as a shock current of 2 mA flows, virtually preventing a fatal shock. The difficulty is that normal insulation resistance leakage and stray capacitance currents can easily reach this value in a perfectly healthy system, and it may thus be impossible to keep the circuit breaker closed. The sum of the leakage currents in circuits protected by an RCD should never be more than 25% of the operating current of the device. Normal earth leakage current from equipment and appliances will, of course vary with the condition of the device. Maximum permitted leakage currents are listed in Appendix L of the 2nd Edition of Guidance Note 1, and vary from 0.25 mA for Glass II appliances to 3.5mA for information technology equipment (see {7.8.2})
Some RCDs (usually electronic types) will not switch off unless the mains supply is available to provide power for their operation. In such a case, mains failure may prevent tripping whilst danger is still present, (due to, for example, charged capacitors). Such RCDs may only be used where there is another means of protection from indirect contact, or where the only people using the installation are skilled or instructed so that they are aware of the risk.
In some cases RCD are designed so that their operating parameters, such as the rated residual current or the time delay, can be adjusted. If such an RCD can be operated by an ordinary person (rather than by a skilled or instructed person then such adjustments must only be possible by a deliberate act using a key or a tool which results in a visible indication of the setting.
If a residual current circuit breaker is set at a very low sensitivity, it can prevent death from electric shock entirely. However, the problem is that a safe current cannot be determined, because it will vary from person to person, and also with the time for which it is applied. The Regulations require a sensitivity of 30 mA for RCDs intended to provide additional protection from direct contact.
An RCD must not be used in an installation with neutral and earth combined (TNG system using a PEN conductor) because there will be no residual current in the event of a fault to cause the device to operate, since there is no separate path for earth fault currents.
RCD protection is required for socket outlets where:
1. - they are part of a TT system (no earth terminal provided by the Electricity Supply Company),
2. - they are installed in a bedroom which contains a shower cubicle, or
3. - the socket outlet(s) are likely to feed portable equipment used outdoors.
4. -they are installed in zones B or C of a swimming pool and comply with BS EN 60309-2.
Protection by an RCD with a rating of 30 mA is required for fixed electrical equipment installed in a bathroom or in zone C of a swimming pool.
Although residual current devices are current-operated, there are circumstances where the combination of operating current and high earth-fault loop impedance could result in the earthed metalwork rising to a dangerously high potential. The Regulations draw attention to the fact that if the product of operating current (A) and earth-fault loop impedance exceeds 50, the potential of the earthed metalwork will be more than 50 V above earth potential and hence dangerous. This situation must not be allowed to arise

Fig 5.3 - Danger with an RCD when earth-fault loop impedance is high. In this case, p.d. from earth to exposed conductive parts will be 1000 Ohms x 0.09 A = 90 V
RCDs must he tested to ensure correct operation within the required operating times. Such tests will be considered in {8.6.3}.
Special requirements apply to RCDs used to protect equipment having normally high earth leakage currents, such as data processing and other computer-based devices. These installations are considered in {7.8.2}.

5.4 - Fault voltage operated circuit breakers
These circuit breakers, commonly known in the trade as 'voltage ELCBs', were deleted from the 15th Edition of the Wiring Regulations in 1985, and should not be installed. It is advisable that installations where they are still in use should be carefully tested prior to a change to residual current device protection

Monday, February 25, 2008

Protective Earth

Tuesday, February 19, 2008

Leon Kalima

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