As mentioned earlier, all electrical systems depend on there being a complete circuit from the sources of supply to the appliance, such as a lamp, that is to be fed with current.
In an installation supplied in the ordinary way from the electricity board’s mains, the source of supply is the supply terminals on the consumer’s fuseboard.
Every circuit must start from the phase fuse terminal and return to the neutral terminal.
Circuits are interrupted so that the power may be controlled by:
Miniature circuit breakers
Contactors and relays
The simplest circuit consists of a pair of wires from the mains terminals supplying one appliance, say a lamp.
This requirement, which is of course part of the Regulations, is especially important. If an appliance, for example, a vacuum cleaner, is connected to a switched socket-outlet, and the plug is left inserted, the appliance may not be in operation because the switch is off, and therefore any uninstructed person, or perhaps a child, might start to tinker with it, thinking that because it is not running, it is dead. But if the switch were placed so that it interrupted the neutral wire and not the phase wire, the live phase of the supply would be carried through the flexible cable to the appliance, even if it is not running, and a fatal shock might result from anyone interfering with the appliance.
In this circuit, the switches can have two positions, either of which can light the lamp. There is no circuit, so the lamp is out.
Now imagine a person near switch B turns that switch to the upper position. A circuit is established, and the lamp lights. He goes downstairs and reaches switch A, and wishes to turn the lamp out. He brings switch A to the lower position, and the lamp is extinguished. A second person, at switch B, has only to move his switch to the lower position for the lamp to light once more.
Many switches, notably those of the rocker type, are often made only as two-way switches and care must be taken when necessary to see that the two-way design is bought. Any two-way switch can be used as a single-way switch, if needed.
Suppose now that on a long staircase, for example, with several landings, it is desired to arrange for the light to be switched on and off at several points. In this case intermediate switches are used.
If, in the case shown, the wires were to be reversed, a circuit would be established and the lamp would light.
The two positions of the switch contacts can be seen from the two diagrams. Fuses
Fuses are found on domestic installations at three points.
It should be made clear that with normal, standard supplies only the phase wire is fused. The neutral connection simply has a link. It is totally incorrect, and in contravention of the Regulations, to fuse the neutral side.
There are the board’s main fuses, or cut-outs as they are called. They are the third line of defence. These are sealed, and must not be interfered with. If the main fuse blows, the electricity board’s engineers must be called out to replace it.
Secondly, there is the consumer’s unit, with a fuse or MCB for each circuit or group of circuits, and a main switch for that fuseboard.
The fuses used in the consumer’s unit may be of one of two types: rewireable fuses, or cartridge fuses.
Rewireable fuses are perhaps the most commonly used. A fuse bridge, or fuseholder, of non-flammable material, is equipped with screws for holding a suitable length of fuse wire. This wire is usually threaded through a ceramic tube, or otherwise held in some way that will ensure that if the wire gets hot and ultimately melts or fuses, no fire damage can result.
The rewireable fuse is convenient in the sense that if a reel of fuse wire is handy a blown fuse can be rapidly replaced at any time, provided the faulty appliance or section of wiring has been repaired or isolated from the mains.
Perhaps the rewireable fuse may sometimes be considered as too convenient, because it is easy for unwise people to replace a blown fuse wire with a wire of the wrong size.
If a 30 A circuit is properly fused, the fuse will blow if the current exceeds 30 A for any length of time, and if all parts of the circuit wiring are correctly chosen, no harm will result. But if the fuseholder is renewed with thicker fuse wire, which will allow, say a 50 A current to flow continuously, some part of the circuit -perhaps a switch, or a flexible cable, or a socket-outlet – may become overheated and a fire could possibly result.
When replacing a fuse wire, always make sure that the screws are properly tight, but not too tight to damage the wire and reduce its current-carrying capacity. Check that the wire is properly fitted into the safety tube or path through the fusehol-der. Do not strain the wire too tightly between the terminals, as tightening up may stretch it and reduce the copper section.
Fuseholders are usually marked with the maximum size of fuse wire they should carry. Cartridge fuses
The best type of fuse is one in which the actual fusible element is enclosed in a flame-proof cartridge. In some cases the cartridge is filled with a type of sand intended to extinguish any flame that might result from a fuse blowing as a result of a heavy excess of current.
The cartridge fuse is made in various sizes, such as 5, 10, 15, 20, 30, 60 A and so on.
It is slightly more expensive than the rewireable type, and there is always the problem of being caught with no spare cartridge of the correct size. Do not try to repair a blown cartridge fuse.
The best consumer’s intake point installation will consist of cartridge equipment, each circuit being properly labelled as to its destination and the size of fuse needed, with an adequate supply of cartridge fuses in a convenient spot nearby.
This plug, now the recognised standard has three ‘square’ pins and the phase side is connected through a fuse. The fuses commonly available are: 3 A, red; 13 A, brown.
Care should always be taken to ensure that the correct size of fuse is inserted in the plug to suit the appliance to which it is connected.
As an alternative to fuses, small automatic circuit-breakers are now available, which are much the same size as the equivalent fuse.
A circuit-breaker is an automatic switch. It is so arranged that if a current of greater value than that for which it is set should pass through the device, the switch will open automatically and cut off the circuit, so preventing damage in the same way as a fuse.
The automatic operation of a circuit-breaker is known as ‘tripping’ – the switch trips a catch that holds it in. Trip mechanisms take two forms, both of which may be used on the same switch. An electromagnet – a coil of wire wound on an iron frame – has a mechanical pull that corresponds to the current passing through its coil. The current through the switch is taken through such a coil, arranged with an arm, attracted by the magnet, that can flick the switch to the off position if the current is too great.
In the second type of trip mechanism, a bimetal strip is used. All metals expand when heated, some more than others. If two strips of metal, one with a high rate of expansion with heat, and the other with a low rate, are joined only at their ends, when heat is applied the combined strip will bow, or bend, as the different rates of expansion come into play. Inside the switch there is a very small heater element that carries the main current. If the current is excessive, this element gets hot and causes the nearby bimetal strip to bend, and a linkage then trips the switch.
With the electromagnetic type of trip, the switch can be closed again immediately it has tripped. But with the thermal, or bi-metal strip type, the switch cannot be closed again for a minute or two, as the heater element and the bi-metal strip have first to cool down.
A time switch is often used for controlling such circuits as shop-window lighting and central heating. Like all other switches, the actual contacts of the time switch must be wired into the phase side of the circuit, and not into the neutral, and care must be taken that the rating of the time-switch contacts – for example, 10 A, as indicated on the nameplate – is not exceeded by the appliances on its circuit.
Time switches may be of several kinds. In some, known as the spring-rewind type, an electric motor winds a clockwork system, so that if the supply should fail, the clock will continue to run, and will open and close the contacts at the proper time, for a period which may be a few hours or a few days. In other designs, the clock is electrically operated, and will stop if the mains supply fails; it must be set to the correct time when the supply is resumed.
In both cases, there must be a separate, fuse-protected circuit to the clock motor. Some clocks have the motor-circuit fuse incorporated in the case. In other instances, it is necessary to provide a 2 A fuse and a separate connection.
Thermostats are temperature-operated switches, which are arranged to open or close a circuit as the temperature rises or falls. For room heating control, they may work in the range of 10°C to 20°C, while for refrigerator applications they may operate in the range of 0°C to -20°C.
In many cases, bimetal strips, mentioned earlier, are used. As the temperature rises, the bimetal strip heats and bends so that ultimately the contact is broken and the heater circuit switched off. To prevent the contacts ‘dithering’ and consequently arcing, the contact piece is usually equipped with a small magnet, and as the bi-metal strip slowly bends it is suddenly snapped to the open or closed position, as the case may be, thus giving a clean break.
Thermostats should always be wired into the phase side of the circuit, and again the current rating of the contacts should never be exceeded.
Relays and contactors
Devices such as time switches and thermostats have switching contacts that are limited in their capacity, usually to circuits of about 3kW. If the time switch, for example, is to be used for larger circuits, some auxiliary device is necessary. Small devices of this kind are called relays, larger examples are known as contactors.
An electromagnetic coil is supplied with current by the closing of the time-switch contact, and when this coil is energised it attracts an armature that in turn closes much larger contacts than those with which a time switch could be fitted: in fact, a contactor could control a load of 100 kW or more.
A relay is a smaller version of the contactor, and is usually of the type where the originating current is very small, such as the output from photoelectric cells which automatically switch on lighting equipment at dusk.
In connecting up contactors and relays, care must be taken to fuse the control circuit with a light fuse and then to see that the fusing on the main circuit is adequate.
Both the control circuit and the main circuit switches must be wired into the phase side, as for all switches or circuit-breaking devices.
The circuit layout
We can now consider the circuit layout for a typical installation, for example, a four-bedroomed house.
Bearing in mind the first principle that each circuit must have proper protection by being adequately fused, one’s mind must first turn to a system whereby every light, every appliance and every socket-outlet had its own separate cable back to the main consumer’s fuseboard, and was individually connected to a separate fuse of the appropriate size.
This would indeed be the perfect and ideal system, but would be extremely expensive on account of the lengths of cable needed and the number of fuses required. It is also unnecessary.
To consider the extreme alternative, suppose an installation had only one main fuse, to which all the circuits were connected. If this fuse blew, through a fault of any one appliance, such as a desk lamp, the whole house would be plunged in darkness. In any case, proper graded protection for the various circuits could not be provided in this way.
The practical solution obviously lies somewhere between these two extremes.
The number of circuits that may be grouped together on one fuse depends on the total demand on the circuit, evolved according to certain rules. For example, consider a lighting circuit. This might well originate at a 5 A fuse in the consumer’s unit. It is laid down that each fixed lampholder must be assumed to carry a 100 W lamp. Now a 100 W lamp consumes 0.416 A. So 12 lampholders could legitimately be supplied from a single 5 A lighting circuit – providing the correct size of cabling was used throughout the circuit.
But this would not be practicable in the house we are discussing, since the wiring would become somewhat cumbersome and in any case it would be undesirable for every light in the house to go out if a single lampholder developed a fault.
A commonly used system would be to have two 5 A lighting circuits, one for the first floor and one for the ground floor, with perhaps ten or so landholders wired to each. In this way there would be some light left in the house if one lampholder failed, and in addition the circuits would be a little underloaded so that extensions would always be possible.
Turning now to fixed appliances – cookers, water heaters, fixed radiators, and refrigerators – some of these are heavily loaded appliances, often calling for 12 kW or more in the case of cookers, 3kW for water heaters, and so on.
There should be a separate final circuit for every appliance rated at 15 A and above, except in the special circumstances mentioned later when the ring circuit is discussed.
Therefore each of these larger fixed appliances should in fact have a separate circuit back to its own fuse, of appropriate size, in the consumer unit. Even if some appliances, such as a particular fixed radiator, do not initially require more than 2kW it is strongly recommended that the circuit should be wired back to the consumer unit on the basis of a single feed of 15 A capacity: no one can tell if at some time in the future a larger radiator may not be needed at that point. Experience shows that the usage of electricity steadily grows, not only for installing more appliances but in substituting new, more heavily loaded appliances for older ones. For example – to mention a portable appliance – the elements fitted to electric kettles have grown in loading from the initial 600 W, and have now reached 3000 W or even more.
The ring circuit
Anyone who thinks carefully about the circuit arrangements set out above will soon reach the conclusion that there is some inevitable wastage of copper – that is, there is more current-carrying capacity than is needed for most of the time.
On the whole, it is better to have a little extra capacity in the circuits, to allow for extension. But there is a very widely used system that allows much fuller use to be made of the current-carrying capacity of the circuits installed.
This system is called the ring circuit, and it is based on the application of what is called the principle of ‘diversity’.
Take any ordinary house, and consider the time and place at which various appliances are used.
The largest portable electricity-consuming appliance commonly used in domestic premises is the 3kW fire. It is extremely unlikely that more than three such fires will be in use, at full load, at the same time, even in the coldest weather, at any rate on the ground floor.
Therefore, if there are, say, eight socket-outlets on the ground floor, each of 13 A capacity it would be extremely generous in copper to wire each one separately back to the consumer unit. When the three large fires are in use, the remaining five sockets on the ground floor are likely to be used only for very light current appliances like lamps, radio and television sets.
Now suppose all the 13 A standard socket-outlets on the ground floor were connected in a ring. That is, a pair of 2.5 mm2 wires starts at one 30 A fuse at the consumer unit and runs to the first socket, on to the second, the third, and so on, and then back to the same 30 A fuse.
The ring circuit is based on the employment of a standard socket-outlet, of 13 A capacity, and having a fuse in the plug top. This socket-outlet will allow for an appliance up to 3 kW being connected, as such an appliance will need 12.48 A.
In this system, each 13 A socket has two routes back to the mains. The maximum use is made of the minimum amount of copper in the cables: that is, the minimum length of cabling is employed for a given number of socket-outlets.
The use of the ring circuit, as mentioned above, is only possible because of the diversity of usage that naturally evolves. In an ordinary dining room, where there might be perhaps six 13 A standard socket-outlets, it is very unlikely that more than two 3kW fires would be in use at the same time, even under arctic conditions. Fires, as mentioned earlier, are the largest portable current-consuming appliances likely to be used in domestic premises. Any other appliances use so much less current that they do not need to be taken into account here.
The Regulations regarding the use of the ring circuit are: If the floor area concerned does not exceed 100 square metres there can be an unlimited number of 13 A standard socket-outlets on the ring, which must consist of 2.5 mm2 conductors and must terminate in a 30 A fuse.
Note: In practice, in domestic premises, the requirement mentioned above may mean that two separate rings are needed. A convenient division is to have a downstairs ring and an upstairs ring. The more the rooms embraced by the ring, the greater the diversity.
There are several additional features of the ring circuit to be mentioned. In addition to the main 13 A socket-outlets, it is possible to install any number of specially designed connections for very small current appliances such as shaver supplies, providing each connection is made by means of the proper fused connector designed for the purpose, complying with British Standard 3052.
Although the rule still holds good that appliances consuming above 3kW, such as cookers and large immersion heaters, should be wired back separately and individually to appropriate fuses in the consumer unit, nevertheless there is a strong tendency, in domestic wiring practice, for more and more fixed appliances to be wired into the ring, to secure the maximum economy in the use of cable.
It should be noted that special regulations, to be mentioned later, apply to the method of connection of certain fixed appliances to the ring.
There are two types of spur connection that may be made to a ring circuit – unfused spurs and fused spurs. This avoids the need to run two wires to an isolated area, providing the following points are observed:
When spurs supplying outlying socket-outlets are connected to a ring circuit, not more than one single socket, or one twin socket, or one fixed appliance shall be fed from each, and the total number of unfused spurs shall not exceed the total number of socket-outlets and stationary appliances connected directly to the ring. Unfused spurs shall be connected to a ring circuit at socket-outlets, or in suitable joint boxes.
The conductors supplying the unfused spur shall not be smaller than those forming the ring itself.
The fused spur is connected to the ring via a fused connection unit, the rating of the fuse in the unit not exceeding that of the cable forming the spur, and in any event not exceeding 13 A. The connection unit is provided with a cartridge fuse which can be replaced easily, usually from the front of the box. It is useful in cases where an isolated lighting circuit, made up of fixed lampholders, needs to be supplied. The use of this unit, fitted, say, with a 3 A fuse, obviates the need for a special wiring run back to the consumer unit to supply one or two lighting points. In this case, the conductors on the spur need only be of a size suitable for the fuse fitted within the connection unit, except that if the spur supplies a socket the cable must be a minimum of 1.5 mm2.
Radial circuits are circuits which also utilise 13 A flat pin sockets, to British Standard 1363, except that the circuit is not wired in the form of a ring. Radial circuits may supply an unlimited number of socket-outlets, providing the floor area served by the circuit does not exceed: (a) Sercing one room only, which is not a kitchen, and is less than 30 m2 floor area – 4 mm2 conductors protected by 20A fuse or miniature circuit breaker supplying up to six outlets none of which supplies a fixed water heating appliance. (b) Serving rooms other than above a radial final circuit -2.5 mm2 conductors protected by a 20 A fuse or miniature circuit breaker supplying two socket outlets, or fixed appliances.
Immersion heaters supplying vessels in excess of 15 litres capacity, or heating appliances which are permanently connected and form part of a space heating installation, should NOT be connected to ring circuits. Provided that in total they do not exceed the circuit rating they may be connected to radial circuits in accordance with and above.
In this system, the area electricity board provides a time switch to allow the off-peak supply to be taken at the specified hours only. Sometimes there may be provided, in addition, a contactor to switch on the off-peak circuits, since the time-switch contacts may not be of large enough current-handling capacity. The consumer himself has to supply the extra consumer unit, allowing for the required number of fuses, and the requisite off-peak circuits.
Off-peak supplies are generally used in three ways: for thermal storage heaters, under-floor warming, storage water heating.
In addition, battery chargers are sometimes fed on the off-peak system.
Until 1970, no other circuits could be connected to the off-peak system. However, with White Meter and now Economy 7 tariffs, all the house circuits are connected for cheap night rate electricity.
Each off-peak circuit is wired back to the off-peak consumer unit. The termination points may be in the form of fused spur units – that is an outlet fitted with replaceable cartridge fuses and the connections being permanently made.
White Meter and Economy
These are now the only methods of providing domestic cheap off-peak rates. Using a special meter these systems allow the same appliances to be used both on and off peak. At the beginning of the off-peak period a sealed time switch energises a relay in the meter which transfers the meter drive from one set of recording dials to another. At the end of the period the time switch de-energises the relay, switching the meter dials back to their daytime recording positions.
In the Economy 7 tariff the ‘off-peak’ period is seven hours, and in the White Meter it is eight hours, both of which occur during the night. It will often be necessary for the heating appliances to be wired to their own consumer unit or fuseboard. For convenience of control, the consumer may use an additional time switch of his own, this time switch being the responsibility of the consumer, as it is quite permissible for the heating appliances to be used at any time of the day, however a higher rate will be charged during the ‘normal’ period. For the above reason, it is in the consumer’s interest to use electricity during the night and it will be cheaper to use, say an automatic washing machine connected by a time clock, in this period.
To sum up, the circuit layout in a typical house comes under the following headings: Fixed appliances
Cookers and immersion heaters, and any other fixed ap pliances using 3kW or above.
Each appliance usually wired directly and individually back to the consumer unit. (b) Other fixed appliances may be connected to a ring circuit. (c) Lighting circuits for fixed lamps.
Grouped in circuits, each circuit wired back to the consumer unit.
Off-peak circuits, for storage heaters, floor heating, immer sion water heaters.
Each appliance wired singly or in groups back to the consumer unit on separate circuits controlled by a time switch.
The ring circuit is used, with unfused spurs as required.
As mentioned earlier, earthing is a means of ensuring electrical safety. All metal parts of all appliances used in the installation should be connected solidly to earth at all times that the appliance is connected to live electric mains.
For fixed appliances, earthing is ensured by a permanent earth connection, which must be solidly connected to an earth point, a feature to be mentioned later. The connection must be made by means of the steel conduit, if such a system is properly installed, or a protective conductor of suitable size.
For portable appliances, fed by means of socket-outlets, the earth connection is ensured by means of the earth pin in the plug. The earth socket, into which this pin enters, must be connected to the earth point by proper permanent means, as in the case of fixed appliances.
In the majority of cases in domestic premises the earth socket is connected to the earth point by means of the uninsulated protective conductor in the sheathed cable itself.
If the existing cable has no protective conductor, the question arises as to how to provide an earth connection, if a new socket-outlet is being installed.
The earth connection
This raises the question as to what is an ‘earth’ connection. A wire buried in the earth itself forms a kind of earth connection, but an earth electrode of this kind has to be very carefully designed so that it will not corrode or, for example, find the earth around it becoming so dry that the electrode becomes insulating instead of conducting.
Wherever practicable, the earth terminal is nearly always provided by the area electricity board, although this is not obligatory. In the case of an underground service the incoming supply cable has a lead sheath and steel armouring which is connected to the carefully designed earth electrode at the substation. This sheath is usually provided, at the consumer’s metering point, with a connection to an earthing terminal. All the earthing points of the whole installation should be connected back to this earthing terminal, which is the official earth point.
But if such a terminal does not exist, what can be done? In any case, overhead supplies, such as those provided in many rural areas, do not allow for the provision of an ‘official’ earth point.
The cold water system has often been used as the protective conductor system, however, this is insufficient for the sole means of earthing of an installation as modern systems often use plastic pipes, and a separate earth electrode must be provided.
The purpose of bonding the services is to ensure that all metal work within the premises is at the same potential, so reducing the risk of electrical shock.
An earth electrode is a metal rod or rods, or even plates, set in the ground providing an effective connection with the general mass of earth. The size of the earth electrode, and of the earthing conductor which makes the connection from the electrode to the earthing terminal, should be of the correct size, calculated according to IEE Regulation 542-16.
But even with this arrangement, we are not out of all our earthing difficulties where no ‘official’ earthing point is provided. The ground near the installation may be rocky, or exceptionally dry, or there may be some other condition that leads to a high resistance between the earth electrode and the general mass of earth.
This could give rise to dangerous conditions. The whole ground might become alive, in the neighbourhood of the earth electrode, at the time of a fault, and a child, for example, touching perhaps a rainwater down-pipe with a water flow into a gutter, could be killed by electrocution, the current passing from the ground through his body, to the rainwater pipe.
Earth leakage circuit-breakers
The problem of providing proper earthing facilities under these conditions is solved by the use of the earth leakage circuit-breaker.
Suppose an earth spike, or electrode, is provided. Assume also that very dry or rocky ground means that its resistance to the general body of earth is rather high – say up to 100 Q, instead of much less than 1Q, as it should be.
Now imagine a wire connected from the earth terminal of the wiring installation as a whole to this electrode. If then an appliance fails – say an electric iron becomes faulty internally -the phase wire will become connected to the earth electrode, through the green and yellow protective conductor on the iron.
Under ideal conditions this occurrence would immediately cause enough current to flow to blow the fuse, and any danger to the person using the iron would be averted.
But with our high-resistance earth electrode, enough current does not flow to blow the fuse. The body of the iron remains alive.
However, some current does flow, and it is this current that is used to safeguard the installation.
The connection between the earth terminal of the whole installation and the earth electrode is taken through an electromagnetic coil attached to a circuit-breaker, or automatic switch, that is connected in series with the main fuse. Any current flowing through this coil will cause the switch to trip, or open, so disconnecting the mains, and removing all source of danger.
Earth leakage circuit-breakers can be made to operate on several thousandths of an ampere, so that despite a high resistance at the earth electrode, the circuit-breaker will operate. It should be realised, however, that unlike a fuse it will cut off the whole supply and not only the faulty section.
More than one type of earth leakage circuit-breaker is available. Some – as mentioned above – operate through the passage of the leakage circuit through the trip coil. That is the type must commonly employed, since it is the cheapest. It is referred to as a ‘voltage-operated earth leakage circuit-breaker’.
It has one or two disadvantages. First, the current path through which the fault current must flow, to earth, may be paralleled somewhere else on the system, as for example in an immersion heater, where the water piping may well provide a second earth path for the fault current, in addition to the path provided by the proper protective conductor. This may affect the sensitivity of the earth leakage circuit-br.eakers.
Secondly, it has to be realised that any fault anywhere on the installation will cause the earth leakage circuit-breaker to trip and so cut off the whole installation. Unlike normal fusing arrangements, there is no selectivity.
The first of these problems can be solved by the use of the somewhat more expensive current balance earth leakage circuit-breaker.
The current in the phase wire, in any normal circuit, must at all times exactly equal the return current in the neutral wire. If some current is passing from the phase wire to earth, these two currents no longer balance.
The current balance ELCB uses this condition, by employing in effect two coils, one carrying the phase current and the other the neutral, and so arranging the mechanical parts that if the force exerted by these two coils becomes unequal, the circuit-breaker will trip. This type of device is referred to as a ‘current-operated earth leakage circuit-breaker’.
The second problem, the lack of selectivity, can be solved only by splitting the installation up into sections that are entirely separate electrically, each being fitted with a separate ELCB, whatever type may be used. The circuit-breakers all have a common earthing point, but on the installation side the protective conductors must be kept completely separate from each other. If this separation is carried out, the occurrence of a fault will result in only one circuit-breaker tripping, leaving the remainder of the installation in service.
All ELCBs are provided with a test knob, so that the user can make sure that his essential protection against shock is in working order. Even the best earth leakage circuit-breakers, after some years of installation without faults occurring, may tend to stick, if not regularly tested.
This matter of earth leakage protection should be discussed with the electricity board in each individual case. Their engineers will probably have carried out extensive earth resistance tests in the area concerned, and will be able to advise on the most suitable kind of ELCB to install.
Protective multiple earthing
Another solution to the problem of satisfactory and safe earthing where difficulties are encountered is protective multiple earthing, but this is not a system that can be used by the electrician himself.
As we have seen earlier, the normal system of supply has the neutral conductor earthed at the supply transformer only. Normally, this neutral conductor must not be earthed anywhere else, because this could cause stray currents to flow in such other services as telephone cables.
However, sanction from the appropriate ministry is required before one can apply PME in a particular area. In this case, the neutral conductor is connected to the earth connection at each consumer.
When this PME system is applied, the neutral conductor on each installation becomes the earth point, and all the protective conductor connections are joined to the neutral, at the supply intake position.
The electrician should consult the electricity board about earth connection made in an area in which PME is being employed. He can learn about this likelihood by consulting the electricity board before any installation work commences.