Electrical safety systems

The provision of protective devices in an electrical installation is fundamental to the whole concept of the safe use of electricity in buildings. The electrical installation as a whole must be protected against overload or short circuit and the people using the building must be protected against the risk of shock, fire or other risks arising from their own misuse of the installation or from a fault. The installation and maintenance of adequate and appropriate protective measures is a vital part of the safe use of electrical energy. I want to look at protection against an electric shock by both direct and indirect contact, at protection by equipotential bonding and automatic disconnection of the supply, and protection against excess current.

Let us first define some of the words we will be using. how the NEC describes the earthing (grounding) arrangements for an electrical installation:

Earth/Ground - the conductive mass of the earth (Ground).

Bonding Conductor - a protective conductor pro viding equipotential bonding.

Circuit Protective Conductor (CPC) - a protective conductor connecting exposed conductive parts of equipment to the main earthing terminal. This is the green and yellow insulated conductor in twin and earth cable.

Exposed Conductive Parts - this is the metalwork of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live, but which may become live under fault conditions.

Extraneous Conductive Parts - this is the structural steelwork of a building and other service pipes such as gas, water, radiators and sinks. They don't form a part of the electrical installation but may introduce a potential, generally earth potential, to the electrical installation.

Ill. 17 Touch voltage curve.

Direct contact protection

The human body's movements are controlled by the nervous system. Very tiny electrical signals travel between the central nervous system and the muscles, stimulating operation of the muscles, which enable us to walk, talk and run and remember that the heart is also a muscle.

If the body becomes part of a more powerful external circuit, such as the electrical mains, and current flows through it, the body's normal electrical operations are disrupted. The shock current causes unnatural operation of the muscles and the result may be that the person is unable to release the live conductor causing the shock, or the person may be thrown across the room. The current which flows through the body is determined by the resistance of the human body and the surface resistance of the skin on the hands and feet.

This leads to the consideration of exceptional pre cautions where people with wet skin or wet surfaces are involved, and the need for special consideration in bathroom installations.

Two types of contact will result in a person receiving an electric shock. Direct contact with live parts which involves touching a terminal or phase conductor that's actually live. Indirect contact results from contact with an exposed conductive part such as the metal structure of a piece of equipment that has become live as a result of a fault.

The touch voltage curve in Ill. 17 shows that a person in contact with 120/230V must be released from this danger in 40ms if harmful effects are to be avoided.

Similarly, a person in contact with 400V must be released in 15ms to avoid being harmed.

In installations operating at normal mains voltage, the primary method of protections against direct con tact is by insulation. All live parts are enclosed in insulating material such as rubber or plastic, which prevents contact with those parts. The insulating material must, of course, be suitable for the circumstances in which they will be used and the stresses to which they will be subjected.

Other methods of direct contact protection include the provision of barriers or enclosures which can only be opened by the use of a tool, or when the supply is first disconnected. Protection can also be provided by fixed obstacles such as a guard rail around an open switchboard or by placing live parts out of reach as with overhead lines.

Earth (Ground) fault protection

In the NEC we are told that where the metalwork of electrical equipment may become charged with electricity in such a manner as to cause danger, that metalwork will be connected with earth so as to discharge the electrical energy without danger.

There are five methods of protection against contact with metalwork which has become unintentionally live, that's , indirect contact with exposed conductive parts recognized by the NEC. These are:

1 Earthed (Grounded) equipotential bonding coupled with automatic disconnection of the supply.

2 The use of Class II (double insulated) equipment.

3 The provision of a non-conducting location.

4 The use of earth free equipotential bonding.

5 Electrical separation.

-- limited to special situations under the effective supervision of trained personnel.

-- electrical separation, is little used but does find an application in the domestic electric shaver supply unit which incorporates an isolating transformer.

-- use of Class II insulated equipment is limited to single pieces of equipment such as tools used on construction sites, because it relies upon effective supervision to ensure that no metallic equipment or extraneous earthed metalwork enters the area of the installation.

The method which is most universally used in grounded/ earthed equipotential bonding coupled with automatic disconnection of the supply.

This method relies upon all exposed metalwork being electrically connected together to an effective earth connection. Not only must all the metalwork associated with the electrical installation be so connected, that's conduits, trunking, metal switches and the metalwork of electrical appliances, but NEC tells us to connect the extraneous metal work of water service pipes, gas and other service pipes and ducting, central heating and air conditioning systems, exposed metallic structural parts of the building and lightning protective systems to the main earthing terminal. In this way the possibility of a volt age appearing between two exposed metal parts is removed. Main equipotential bonding is shown.

The second element of this protection method is the provision of a means of automatic disconnection of the supply in the event of a fault occurring that causes the exposed metalwork to become live.

The NEC recognize that the risk of an injurious shock is greater when the equipment concerned is portable and likely to be hand held, such as an electric drill, than when the equipment is fixed.

The NEC, therefore, specify that the disconnection must be effected within 0.4 seconds for circuits, which include socket outlets, but within 5.0 seconds for circuits connected to fixed equipment.

The achievement of these disconnection times is dependent upon the type of protective device used, fuse or circuit breaker, the circuit conductors to the fault and the provision of adequate equipotential bonding. The resistance, or we call it the impedance; of the earth fault loop must be less than the values. (Later in this Section shows the maximum value of the earth fault loop impedance for circuits protected by a semi-enclosed fuse). We will look at this again later in this Section under the heading

'Earth Fault Loop Impedance Z_S'. The NEC gives details of the earthing arrangements to be incorporated in the supply system to meet these NEC and these were described under the heading 'Low Voltage Supply Systems' and are shown in Ill. 106 to 08.

Residual current protection

The NEC recognize the particular problems created when electrical equipment such as lawnmowers, hedge-trimmers, drills and lights are used outside buildings. In these circumstances the availability of an adequate earth return path is a matter of chance. The NEC, therefore, require that any socket intended to be used to supply equipment outside a building shall have the additional protection of a residual current device (RCD), which has a rated operating current of not more than 30 milliamperes (mA).

An RCD is a type of circuit breaker that continuously compares the current in the phase and neutral conductors of the circuit. The currents in a healthy circuit will be equal, but in a circuit that develops a fault, some current will flow to earth and the phase and neutral currents will no longer balance. The RCD detects the imbalance and disconnects the circuit.

Ill. 18 shows an RCD.

Ill. 18 Construction of a residual current device (RCD).

Isolation and switching

The NEC deals with the application of protective measures for safety with the regulations for switching devices or switchgear required for protection, isolation and switching of a consumer's installation.

The consumer's main switchgear must be readily accessible to the consumer and be able to:

** isolate the complete installation from the supply,

** protect against overcurrent,

** cut off the current in the event of a serious fault occurring.

The NEC identify four separate types of switching: switching for isolation; switching for mechanical maintenance; emergency switching; and functional switching.

Isolation is defined as cutting off the electrical sup ply to a circuit or item of equipment in order to ensure the safety of those working on the equipment by making dead those parts which are live in normal service.

An isolator is a mechanical device which is operated manually and used to open or close a circuit off load.

An isolator switch must be provided close to the sup ply point so that all equipment can be made safe for maintenance. Isolators for motor circuits must isolate the motor and the control equipment, and isolators for high-voltage discharge lighting luminaires must be an integral part of the luminaire so that it's isolated when the cover is removed. Devices which are suitable for isolation are isolation switches, fuse links, circuit breakers, plugs and socket outlets.

Isolation at the consumer's service position can be achieved by a double pole switch which opens or closes all conductors simultaneously. On three-phase supplies the switch need only break the live conduct ors with a solid link in the neutral, provided that the neutral link can't be removed before opening the switch.

The switching for mechanical maintenance requirements is similar to those for isolation except that the control switch must be capable of switching the full load current of the circuit or piece of equipment.

Switches for mechanical maintenance must not have exposed live parts when the appliance is opened, must be connected in the main electrical circuit and have a reliable on/off indication or visible contact gap (NEC). Devices which are suitable for switching off for mechanical maintenance are switches, circuit breakers, plug and socket outlets.

Emergency switching involves the rapid disconnection of the electrical supply by a single action to remove or prevent danger. The device used for emergency switching must be immediately accessible and identifiable, and be capable of cutting off the full load current.

A fireman's switch provides emergency switching for high-voltage signs.

Electrical machines must be provided with a means of emergency switching, and a person operating an electrically driven machine must have access to an emergency switch so that the machine can be stopped in an emergency. The remote stop/start arrangement shown could meet this requirement for an electrically driven machine (NEC). Devices which are suitable for emergency switching are switches, circuit breakers and contactors. Where contactors are operated by remote control they should open when the coil is de-energized, that's , fail safe. Push-buttons used for emergency switching must be colored red and latch in the stop or off position. They should be installed where danger may arise and be clearly identified as emergency switches. Plugs and socket outlets can't be considered appropriate for emergency disconnection of supplies.

Functional switching involves the switching on or off or varying the supply of electrically operated equipment in normal service. The device must be capable of interrupting the total steady current of the circuit or appliance. When the device controls a discharge lighting circuit it must have a current rating capable of switching an inductive load. Plug and socket outlets may be used as switching devices and recent years have seen an increase in the number of electronic dimmer switches being used for the control and functional switching of lighting circuits.

Where more than one of these functions is per formed by a common device, it must meet the individual requirements for each function.

Overcurrent protection

The consumer's mains equipment must provide protection against overcurrent; that's , a current exceeding the rated value. Fuses pro vide overcurrent protection when situated in the live conductors; they must not be connected in the neutral conductor. Circuit breakers may be used in place of fuses, in which case the circuit breaker may also provide the means of isolation, although a further means of isolation is usually provided so that maintenance can be carried out on the circuit breakers themselves.

Overcurrent can be subdivided into overload current, and short-circuit current. An overload current can be defined as a current which exceeds the rated value in an otherwise healthy circuit. Overload currents usually occur because the circuit is abused or because it has been badly designed or modified. A short circuit is an overcurrent resulting from a fault of negligible impedance connected between conductors. Short circuits usually occur as a result of an accident which could not have been predicted before the event.

An overload may result in currents of two or three times the rated current flowing in the circuit. Short circuit currents may be hundreds of times greater than the rated current. In both cases the basic requirements for protection are that the fault currents should be interrupted quickly and the circuit isolated safely before the fault current causes a temperature rise which might damage the insulation and terminations of the circuit conductors.

The selected protective device should have a cur rent rating which is not less than the full load current of the circuit but which does not exceed the cable cur rent rating. The cable is then fully protected against both overload and short-circuit faults. Devices which provide overcurrent protection are:

** HBC fuses. These are for industrial applications having a maximum fault capacity of 80 kA.

** Cartridge fuses. These are used for a.c.

circuits on industrial and domestic installations having a fault capacity of about 30 kA.

** Cartridge fuses. These are used in 13A plug tops and have a maximum fault capacity of about 6 kA.

** Semi-enclosed fuses. These were previously called rewirable fuses and are used mainly on domestic installations having a maximum fault capacity of about 4 kA.

** MCBs. These are miniature circuit breakers which may be used as an alternative to fuses for some installations. The British Standard includes ratings up to 100A and maximum fault capacities of 9 kA. They are graded according to their instantaneous tripping currents - that's , the current at which they will trip within 100ms. This is less than the time taken to blink an eye.

MCB Type 1 will trip instantly at between 2.7 and four times its rated current and is therefore more suitable on loads with minimal or no switching surges such as domestic or commercial installations.

MCB Type B will trip instantly at between three and five times its rated current and is also suitable for domestic and commercial installations.

MCB Type 2 will trip instantly at between four and seven times its rated current. It offers fast protection on small overloads combined with a slower operation on heavier faults, which reduces the possibility of nuisance tripping. Its characteristics are very similar to those of an HBC fuse, and this MCB is possibly best suited for general commercial and industrial use.

MCB Type C will trip instantly at between five and ten times its rated current. It is more suitable for highly inductive commercial and industrial loads.

MCB Type 3 will trip instantly at between seven and ten times its rated current. It is more suitable for protecting highly inductive circuits and is used on circuits supplying transformers, chokes and lighting banks.

MCB Type D will trip instantly at between 10 and 25 times its rated current. It is suitable for welding and X-ray machines where large inrush currents may occur.

MCB Type 4 will trip instantly between 10 and 50 times the rated current and is more suitable for special industrial applications such as welding equipment and X-ray machines.

We will now look at the construction, advantages and disadvantages of the various protective devices.

Semi-enclosed fuses. The semi-enclosed fuse consists of a fuse wire, called the fuse element, secured between two screw terminals in a fuse carrier. The fuse element is connected in series with the load and the thickness of the element is sufficient to carry the normal rated circuit current.

When a fault occurs an overcurrent flows and the fuse element becomes hot and melts or 'blows'.

The designs of the fuse carrier and base are also important. They must not allow the heat generated from an overcurrent to dissipate too quickly from the element, otherwise a larger current would be required to 'blow' the fuse. Also if over-enclosed, heat will not escape and the fuse will 'blow' at a lower current. This type of fuse is illustrated in Ill. 19. The fuse element should consist of a single strand of plain or tinned cop per wire having a diameter appropriate to the current rating as given in Tbl. 1.

Ill. 19 A semi-enclosed fuse.

Tbl. 1 Size of fuse element: Current rating (A) | Wire diameter (mm) 5 0.20 10 0.35 15 0.50 20 0.60 30 0.85

ADVANTAGES OF SEMI-ENCLOSED FUSES

** They are very cheap compared with other protective devices both to install and to replace.

** There are no mechanical moving parts.

** It is easy to identify a 'blown fuse'.

DISADVANTAGES OF SEMI-ENCLOSED FUSES

** The fuse element may be replaced with wire of the wrong size either deliberately or by accident.

** The fuse element weakens with age due to oxidization, which may result in a failure under normal operating conditions.

** The circuit can't be restored quickly since the fuse element requires screw fixing.

** They have low breaking capacity since, in the event of a severe fault, the fault current may vaporize the fuse element and continue to flow in the form of an arc across the fuse terminals.

** There is a danger from scattering hot metal if the fuse carrier is inserted into the base when the circuit is faulty.

Cartridge fuses: The cartridge fuse breaks a faulty circuit in the same way as a semi-enclosed fuse, but its construction eliminates some of the disadvantages experienced with an open-fuse element.

The fuse element is encased in a glass or ceramic tube and secured to end-caps which are firmly attached to the body of the fuse so that they don't blow off when the fuse operates. Cartridge fuse construction is illustrated in Ill. 20. With larger-size cartridge fuses, lugs or tags are sometimes brazed on to the end-caps to fix the fuse cartridge mechanically to the carrier. They may also be filled with quartz sand to absorb and extinguish the energy of the arc when the cartridge is brought into operation.

ADVANTAGES OF CARTRIDGE FUSES

** They have no mechanical moving parts.

** The declared rating is accurate.

** The element does not weaken with age.

** They have small physical size and no external arcing which permits their use in plug tops and small fuse carriers.

** Their operation is more rapid than semi-enclosed fuses. Operating time is inversely proportional to the fault current.

Ill. 20 A cartridge fuse.

DISADVANTAGES OF CARTRIDGE FUSES

** They are more expensive to replace than rewirable fuse elements.

** They can be replaced with an incorrect cartridge.

** The cartridge may be shorted out by wire or silver foil in extreme cases of bad practice.

** They are not suitable where extremely high fault currents may develop.

HIGH BREAKING CAPACITY FUSES

As the name might imply, these cartridge fuses are for protecting circuits where extremely high fault cur rents may develop such as on industrial installations or distribution systems.

The fuse element consists of several parallel strips of pure silver encased in a substantial ceramic cylinder, the ends of which are sealed with tinned brass end-caps incorporating fixing lugs. The cartridge is filled with silica sand to ensure quick arc extraction.

Incorporated on the body is an indicating device to show when the fuse has blown. HBC fuse construction is shown in Ill. 21.

ADVANTAGES OF HBC FUSES

** They have no mechanical moving parts.

** The declared rating is accurate.

** The element does not weaken with age.

** Their operation is very rapid under fault conditions.

** They are capable of breaking very heavy fault currents safely.

Ill. 21 HBC fuse.

** They are capable of discriminating between a persistent fault and a transient fault such as the large starting current taken by motors.

** It is difficult to confuse cartridges since different ratings are made to different physical sizes.

DISADVANTAGES OF HBC FUSES

** They are very expensive compared to semi enclosed fuses.

Miniature circuit breakers

The disadvantage of all fuses is that when they have operated they must be replaced. An MCB overcomes this problem since it's an automatic switch which opens in the event of an excessive current flowing in the circuit and can be closed when the circuit returns to normal.

An MCB of the type shown in Ill. 22 incorporates a thermal and magnetic tripping device. The load current flows through the thermal and the electromagnetic mechanisms. In normal operation the current is insufficient to operate either device, but when an over load occurs, the bimetal strip heats up, bends and trips the mechanism. The time taken for this action to occur provides an MCB with the ability to discriminate between an overload which persists for a very short time, for example the starting current of a motor, and an overload due to a fault. The device only trips when a fault current occurs. This slow operating time is ideal for overloads but when a short circuit occurs it's important to break the faulty circuit very quickly.

This is achieved by the coil electromagnetic device.

When a large fault current (above about eight times the rated current) flows through the coil a strong magnetic flux is set up which trips the mechanisms almost instantly. The circuit can be restored when the fault is removed by pressing the ON toggle. This latches the various mechanisms within the MCB and 'makes' the switch contact. The toggle switch can also be used to disconnect the circuit for maintenance or isolation or to test the MCB for satisfactory operation.

The simplified diagram in Ill. 23 shows the various parts within an MCB.

Ill. 22 (a) Interior view of Wylex 'plug-in' MCB; (b) 'plug-in' MCB fits any standard Wylex consumer's unit.

ADVANTAGES OF MCBS

** Tripping characteristics and therefore circuit protection are set by installer.

** The circuit protection is difficult to interfere with.

** The circuit is provided with discrimination.

** A faulty circuit may be easily and quickly restored.

** The supply may be safely restored by an unskilled operator.

DISADVANTAGES OF MCBS

** They are very expensive compared to re-wirable fuses.

** They contain mechanical moving parts and there fore require regular testing to ensure satisfactory operation under fault conditions.

Fusing factor

The speed with which a protective device will operate under fault conditions gives an indication of the level of protection being offered by that device. This level of protection or fusing performance is given by the fusing factor of the device:

Fusing factor = Minimum fusing current / Current rating

The minimum fusing current of a device is the cur rent which will cause the fuse or MCB to blow or trip in a given time (gives this operating time as 4 hours). The current rating of a device is the current which it will carry continuously without deteriorating.

Thus, a 10A fuse which operates when 15A flows will have a fusing factor of 15 ÷ 10 = 1.5.

Since the protective device must carry the rated current it follows that the fusing factor must always be greater than one. The closer the fusing factor is to one, the better is the protection offered by that device.

The fusing factors of the protective devices previously considered are:

** semi-enclosed fuses: between 1.5 and 2

** cartridge fuses: between 1.25 and 1.75

** HBC fuses: less than 1.25

** MCBs: less than 1.5.

In order to give protection to the conductors of an installation:

** the current rating of the protective device must be equal to or less than the current carrying capacity of the conductor;

** the current causing the protective device to operate must not be greater than 1.45 times the current carrying capacity of the conductor to be protected.

The current carrying capacities of cables given in the tables of the NEC assume that the circuit will comply with these requirements and that the circuit protective device will have a fusing factor of 1.45 or less. Cartridge fuses, HBC fuses and MCBs do have a fusing factor less than 1.45 and therefore when this type of protection is afforded the current carrying capacities of cables may be read directly from the tables.

However, semi-enclosed fuses can have a fusing factor of 2. The wiring regulations require that the rated current of a rewirable fuse must not exceed 0.725 times the current carrying capacity of the conductor it's to protect. This factor is derived as follows:

The maximum fusing factor of a rewirable fuse is 2.

Now, if In _ current rating of the protective device Iz _ current carrying capacity of conductor I_2 = current causing the protective device to operate.

Then I_2 = 2 In _1.45 Iz therefore I_n = 1.45 Iz 2 or I_n = 0.725 Iz When rewirable fuses are used, the current carrying capacity of the cables given in the tables is reduced by a factor of 0.725, as detailed in Appendix 4 item 5 of the NEC.

Position of protective devices

Isolation, switching and protective devices can be found at the consumers mains equipment position such as that shown. The general principle to be followed is that a protective device must be placed at a point where a reduction occurs in the current carrying capacity of the circuit conductors. A reduction may occur because of a change in the size or type of conductor or because of a change in the method of installation or a change in the environmental conditions. The only exceptions to this rule are where an overload protective device opening a circuit might cause a greater danger than the overload itself - for example, a circuit feeding an overhead electromagnet in a scrapyard.

Disconnection time calculations

The overcurrent protection device protecting socket outlet circuits and any fixed equipment in bathrooms must operate within 0.4 seconds. Those protecting fixed equipment circuits in rooms other than bath rooms must operate within 5 seconds.

The reason for the more rapid disconnection of the socket outlet circuits is that portable equipment plugged into the socket outlet is considered a higher risk than fixed equipment since it's more likely to be firmly held by a person. The more rapid disconnection times for fixed equipment in bathrooms take account of a possibly reduced body resistance in the bathroom environment.

The NEC permit us to assume that where an overload protective device is also intended to provide short-circuit protection, and has a rated breaking capacity greater than the prospective short circuit current at the point of its installation, the conductors on the load side of the protective device are considered to be adequately protected against short circuit currents without further proof. This is because the cable rating and the overload rating of the device are compatible. However, if this condition is not met or if there is some doubt, it must be verified that fault currents will be interrupted quickly before they can cause a dangerously high temperature rise in the circuit conductors. NEC provides an equation for calculating the maximum operating time of the protective device to prevent the permitted conductor temperature rise being exceeded as follows:

where t = duration time in seconds; S = cross-sectional area of conductor in square millimeters; I = short-circuit rms current in amperes; k = a constant dependent upon the conductor metal and type of insulation.

EXAMPLE

A 10mm PVC insulated copper cable is short-circuited when connected to a 400 V supply. The impedance of the short-circuit path is 0.1_.

Calculate the maximum permissible disconnection time and show that a 50 A type 2 MCB will meet this requirement:

For copper conductor and PVC insulation, Table 43A gives a value for k of 115. So, t = k^2 S^2 / l^2 (x)

The maximum time that a 4000 A fault current can be applied to this 10mm^2 cable without dangerously raising the conductor temperature is 82.66 ms. Therefore, the protective device must disconnect the supply to the cable in less than 82.66 ms under short-circuit conditions. Manufacturers' information and the NEC give the operating times of protective devices at various short-circuit currents in the form of graphs, similar to those shown.

Ill. 24 Time/current characteristic of an overcurrent protective device.

Ill. 25 Time/current characteristics of (a) a type 2 MCB; (b) semi-enclosed fuse.

Time/current characteristics of protective devices

Disconnection times for various overcurrent devices are given in the form of a logarithmic graph. This means that each successive graduation of the axis represents a ten times change over the previous graduation.

These logarithmic scales are shown in the graphs of Ill. 24 and 25. From Ill. 24 it can be seen that the particular protective device represented by this characteristic will take 8 seconds to disconnect a fault current of 50A and 0.08 seconds to clear a fault current of 1000A.

Ill. 25(a) shows the time/current characteristics for a type 2 MCB. This graph shows that a fault current of 4000A will trip the protective device in 20 ms. Since this is quicker than 82.66 ms, the 50A type 2 MCB will clear the fault current before the temperature of the cable is raised to a dangerous level.

The NEC gives the time/ current characteristics and specific values of prospective short-circuit current for a number of protective devices.

These indicate the value of fault current which will cause the protective device to operate in the times indicated by NEC, that's 0.4 and 5 seconds in the case of domestic socket outlet circuits and distribution circuits feeding fixed appliances.

The NEC deal with fuses and with MCBs.

It can be seen that the prospective fault current required to trip an MCB in the required time is a multiple of the current rating of the device. The multiple depends upon the characteristics of the particular devices. Thus:

** type 1 MCB has a multiple of 4

** type 2 MCB has a multiple of 7

** type 3 MCB has a multiple of 10

** type B MCB to has a multiple of 5

** type C MCB to has a multiple of 10

** type D MCB to has a multiple of 20.

EXAMPLE

A 6 A type 1 MCB used to protect a domestic lighting circuit will trip within 5 seconds when 6 A times a multiple of 4, that's 24 A, flows under fault conditions.

Therefore if the earth fault loop impedance is low enough to allow at least 24 A to flow in the circuit under fault conditions, the protective device will operate within the time required by Regulation.

The characteristics shown in the NEC give the specific values of prospective short-circuit current for all standard sizes of protective device.

Ill. 26 Effective discrimination achieved by graded protection.

Ill. 27 Earth fault loop path for a TN-S system.

Discrimination

In the event of a fault occurring on an electrical installation only the protective device nearest to the fault should operate, leaving other healthy circuits unaffected. A circuit designed in this way would be considered to have effective discrimination. Effective discrimination can be achieved by graded protection since the speed of operation of the protective device increases as the rating decreases. This can be seen in Ill. 25(b). A fault current of 200A will cause a 15A semi-enclosed fuse to operate in about 0.1 seconds, a 30A semi-enclosed fuse in about 0.45 seconds and a 60A semi-enclosed fuse in about 5.4 seconds. If a circuit is arranged as shown in Ill. 26 and a fault occurs on the appliance, effective discrimination will be achieved because the 15A fuse will operate more quickly than the other protective devices if they were all semi-enclosed types fuses with the characteristics shown.

Security of supply, and therefore effective discrimination, is an important consideration for an electrical engineer and is also a requirement of NEC.

Earth fault loop impedance, Z_s. In order that an overcurrent protective device can operate successfully, meeting the required disconnection times, of less than 0.4 seconds for socket outlets and 5.0 seconds for fixed equipment, the earth fault loop impedance value measured in ohms must be less than those values given for socket outlet circuits for circuits supplying fixed equipment. The value of the earth fault loop impedance may be verified by means of an earth fault loop impedance test as described later in this Section. The formula is:

Z_S = ZE = (R1 x R2) (_)

Here ZE is the impedance of the supply side of the earth fault loop. The actual value will depend upon many factors: the type of supply, the ground conditions, the distance from the transformer, etc. The value can be obtained from the area electricity companies, but typical values are 0.35_ for TN-C-S (PME) supplies and 0.8_ for TN-S (cable sheath earth) supplies. Also in the above formula, R1 is the resistance of the phase conductor and R2 is the resistance of the earth conductor. The complete earth fault loop path is shown in Ill. 27.

Values of R1 _ R2 have been calculated for copper and aluminum conductors and are given in Table 9A of the On Site Guide as shown in Tbl. 2.

EXAMPLE

A 20A radial socket outlet circuit is wired in 2.5mm^2 PVC cable incorporating a 1.5mm^2 CPC. The cable length is 30m installed in an ambient temperature of 20°C and the consumer's protection is by semi-enclosed fuse. The earth fault loop impedance of the supply is 0.5_.

Calculate the total earth fault loop impedance Z_S, and establish that the value is less than the maximum value permissible for this type of circuit.

We have ZS _ZE _(R1 _R2) (_) ZE _0.5_ (value given in the question) From the value given a 2.5mm phase conductor with a 1.5mm protective conductor has an (R1 _R2) value of 19.51_10_3 _/m (R1 _R2)_19.51_10_3_/m_30m_0.585_ However, under fault conditions, the temperature and therefore the cable resistance will increase. To take account of this, we must multiply the value of cable resistance by the factor given in Table 9C of the On Site Guide. In this case the factor is 1.20 and therefore the cable resistance under fault conditions will be:

0.585 __1.20_0.702 _ The total earth fault loop impedance is therefore ZS _0.5 __0.702 __1.202 _ The maximum permitted value given for a 20 A fuse protecting a socket outlet is 1.48_ as shown by Tbl. 3. The circuit earth fault loop impedance is less than this value and therefore the protective device will operate within the required disconnection time of 0.4 seconds.

Size of protective conductor

The circuit protective conductor forms an integral part of the total earth fault loop impedance, so it's necessary to check that the cross-section of this conductor is adequate. If the cross-section of the circuit protective conductor complies with NEC, there is no need to carry out further checks. Where phase and protective conductors are made from the same material:

** for phase conductors equal to or less than 16mm^2, the protective conductor should equal the phase conductor;

** for phase conductors greater than 16mm2 but less than 35mm^2, the protective conductor should have a cross-sectional area of 16mm^2;

** for phase conductors greater than 35mm^2 , the protective conductor should be half the size of the phase conductor.

However, where the conductor cross-section does not comply with this table, then the formula given in Regulation must be used:

S = SQR_RT (I^2 t) /k (mm^2) where

S = cross-sectional area in mm^2

I = value of maximum fault current in amperes

t = operating time of the protective device

k = a factor for the particular protective conductor

EXAMPLE 1

A 230 V ring main circuit of socket outlets is wired in 2.5mm single PVC copper cables in a plastic conduit with a separate 1.5mm CPC. An earth fault loop impedance test identifies Z_S as 1.15_. Verify that the 1.5mm CPC meets the requirements of Regulation when the protective device is a 30A semi-enclosed fuse.

A 1.5mm^2 CPC is acceptable since this is the nearest standard-size conductor above the minimum cross-sectional area of 1.10mm^2 found by calculation.

EXAMPLE 2

A domestic immersion heater is wired in 2.5mm^2 PVC insulated copper cable and incorporates a 1.5mm^2 CPC. The circuit is correctly protected with a 15 A semi-enclosed fuse. Establish by calculation that the CPC is of an adequate size to meet the requirements of Regulation. The characteristics of the protective device are given.

For circuits feeding fixed appliances the maximum operating time of the protective device is 5 seconds. It can be seen that a current of about 44A will trip the 15A fuse in 5 seconds. Alternatively he NEC gives a value of 43A. Let us assume a value of 43A:

I = 43A

t = 5 seconds for fixed appliances

k = 115

The circuit protective conductor of the cable is greater than 0.836mm^2 and is therefore suitable. If the protective conductor is a separate conductor, that's , it does not form part of a cable as in this example and is not enclosed in a wiring system as in Example 1, the cross-section of the protective conductor must be not less than 2.5mm^2 where mechanical protection is provided or 4.0mm^2 where mechanical protection is not provided n order to comply with NEC.

Cable selection/calculation

The size of a cable to be used for an installation depends upon:

++ the current rating of the cable under defined installation conditions and

++ the maximum permitted drop in voltage.

The factors which influence the current rating are:

1 the design current - the cable must carry the full load current;

2 the type of cable - PVC, MICC, copper conduct ors or aluminium conductors;

3 the installed conditions - clipped to a surface or installed with other cables in a trunking;

4 the surrounding temperature - cable resistance increases as temperature increases and insulation may melt if the temperature is too high;

5 the type of protection - for how long will the cable have to carry a fault current? Regulation 525-01 states that the drop in voltage from the supply terminals to the fixed current-using equipment must not exceed 4% of the mains voltage.

That is, a maximum of 9.2V on a 230V installation.

The volt drop for a particular cable may be found from VD = Factor _ Design current _ Length of run

The cable rating, denoted It, may be determined as follows:

It = Current rating of protective device / Any applicable correction factors

The cable rating must be chosen to comply with NEC. The correction factors which may need applying are given below as:

Ca the ambient or surrounding temperature correction factor.

Cg the grouping correction factor given.

Cr the 0.725 correction factor to be applied when semi-enclosed fuses protect the circuit as described in the NEC Ci the correction factor to be used when cables are enclosed in thermal insulation. Regulation 523-04 gives us three possible correction values:

++ Where one side of the cable is in contact with thermal insulation we must read the current.

++ Where the cable is totally surrounded over a length greater than 0.5m we must apply a factor of 0.5.

++ Where the cable is totally surrounded over a short length, the appropriate factor given.

Having calculated the cable rating, the smallest cable should be chosen from the appropriate table which will carry that current. This cable must also meet the voltage drop and this should be calculated as described earlier. When the calculated value is less than 4% of the mains voltage the cable may be considered suitable. If the calculated value is greater than the 4% value, the next larger cable size must be tested until a cable is found which meets both the current rating and voltage drop criteria.

EXAMPLE

A house extension has a total load of 6 kW installed some 18m away from the mains consumer unit. A PVC insulated and sheathed twin and earth cable will provide a submain to this load and be clipped to the side of the ceiling joists over much of its length in a roof space which is anticipated to reach 35°C in the summer and where insulation is installed up to the top of the joists. Calculate the minimum cable size if the circuit is to be protected (a) by a semi-enclosed fuse and (b) by a type 2 MCB. Assume a TN-S supply, that's , a supply having a separate neutral and protective conductor throughout.

Let us solve this question using only the tables. The tables will give the same values, but this will simplify the problem.

Nominal current setting of the protection for this load I_n = 30A.

For (a) the correction factors to be included in this calculation are:

Ca ambient temperature; the correction factor for 35°C is 0.97.

Cg the grouping correction factor is not applied since the cable is to be clipped direct to a surface and not in contact with other cables.

Cr the protection is by a semi-enclosed fuse and , therefore, a factor of 0.725 must be applied.

Ci thermal insulation is in contact with one side of the cable and we must therefore assume installed method 4.

The cable rating, I_t = is given by:

A 10mm cable having a rating of 43 A is required to carry this current.

Now test for volt drop: The maximum permissible volt drop is 4%_230 V_9.2 V. From Table 6E2 shown in Table 2.6 the volt drop per ampere per meter for a 10mm cable is 4.4mV.

Therefore, the volt drop for this cable length and load is equal to 4.4_10_3 V/(A m)_26.09 A_18m_2.07 V Since this is less than the maximum permissible value of 9.2 V, a 10mm cable satisfies the current and drop in voltage requirements and is therefore the chosen cable when semi-enclosed fuse protection is used.

For (b) the correction factors to be included in this calculation are:

Ca ambient temperature; from the correction factor for 35°C is 0.94.

Cg grouping factors need not be applied.

Cr since protection is by MCB no factor need be applied.

Ci thermal insulation once more demands that we assume installed method 4.

The design current is still 26.09A and we will therefore choose a 30 A MCB for the nominal current setting of the protective device, I_n.

A 6mm cable, having a rating of 32 A, is required to carry this current.

Now test for volt drop: from -- the volt drop per ampere per meter for a 6mm cable is 7.3mV. So the volt drop for this cable length and load is equal to 7.3_10_3 V/(A m)_26.09 A_18m_3.43 V

Since this is less than the maximum permissible value of 9.2 V, a 6mm cable satisfies the current and drop in voltage requirements when the circuit is protected by an MCB. From the above calculations it's clear that better protection can reduce the cable size. Even though an MCB is more expensive than a semi-enclosed fuse, the installation of a 6mm cable with an MCB may be less expensive than 10mm cable protected by a semi-enclosed fuse. These are some of the decisions which the electrical contractor must make when designing an installation which meets the requirements of the customer.

If you are unsure of the standard fuse and MCB rating of protective devices, Google for more info.