Protect Against Overload and Short Circuit Currents

8/22/2019 Protect Against Overload and Short Circuit Currents

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MCAST BTEC National Diploma in Building Services Engineering

Electrical Installations

Protection Against Overload and Short-circuit Currents

Joseph Gatt


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Assignment C March 2010

Joseph Gatt Page 2 of 18

ContentsContentsContentsContents

Task 1 P.40.2 Cables ................................................................................ 3Task 2 P.40.6 Over-current Devices............................................................ 8Task 3 M.40.1 The Use of IEE Wiring Regulations ........................................ 11Task 4 M.40.5 Protection for Installations ................................................... 13


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Task 1Task 1Task 1Task 1 P.40.2P.40.2P.40.2P.40.2 CablesCablesCablesCables

A. State which are the external influences that affect the choice of acorrect cable for a particular installation

Ambient temperature (Ca)Ambient temperature refers to the surrounding temperature of the

cable installed, and is based on 30C. It should be noted that the

temperature and the current flow are inversely proportional. The

higher the surrounding temperature, the less is the current flow. Heat

is generated and released when a cable is carrying current. Hence,

we must make sure that the surrounding temperature is not greater

than the cable temperature in order for the cable to release heat. An

overheated cable would be in the risk of fire. For safety reasons,

correction factor must be applied if the ambient temperature exceeds30C. Such factors are found on the IEE Wiring Regulations.

Grouping factor (Cs)As previously mentioned, cables generate heat as they carry current.

Grouping factor refers to the factor which must be applied when

cables are grouped together in order to reduce the overall heat. Such

factors also are found on the IEE Wiring Regulations.

Thermal insulation (Ci)This refers to cables which would be embedded in thermal insulation.

Heat being generated when a cable is carrying current cannot bereleased due to the insulation. Hence, a correction factor must be

applied for cable lengths from 50-500mm, stated in regulation 523-

04-01. 0.5 is the correction factor which must be applied for lengths

that exceed 500mm.

B. List the methods of installation considered by the IEE wiringregulations

The following is quoted from table 4A1 of IEE Wiring Regulations 16th

Edition (please refer to appendix 1)

Method 1o Sheathed cables clipped direct to or lying on a non-metallic

surface

o Sheathed cables embedded directly in masonry, brickwork,concrete, plaster or like (other than thermally insulating

materials)


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Method 3o Single core non-sheathed cables in metallic or non-metallic

conduit on a wall or ceiling. This method is commonly used

locally for installations

o Multi-core cables having non-metallic sheath, in metallic ornon-metallic conduit on a wall or ceiling

o Cables in conduit embedded in masonry, brickwork, concrete,plaster, or like (other than thermally insulating materials).

This method is also commonly used

o Cables in trunking on a wall or suspended in the airo Cables in flush floor trunkingo Single core cables in skirting trunking

Method 4o Single core non-sheathed cables in metallic or non-metallic

conduit in a thermally insulating wall or above a thermally

insulating ceiling, the conduit being in contact with a thermally

conductive surface on one side

o Sheathed cables in conduit in a thermally insulating wall etc.(otherwise as reference method 4)

Method 11Sheathed cables on a perforated cable tray bunched and unenclosed.

A perforated cable tray is a ventilated tray in which the holes occupy

30% or more of the surface area


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C. Explain what is meant by voltage drop and list themillivolt/ampere/metre for both single core and multi core cables

used in domestic installations. Also state the maximum current that

can be carried out by each cable

Voltage drop refers to the reduced voltage affected by the length and cross-

sectional area of a cable. The more length, the more is the voltage drop. The

thinner the cable, the more is the voltage drop. The voltage drop allowance

for lighting and power is 3% and 5% respectively.


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Task 2Task 2Task 2Task 2 P.40.6P.40.6P.40.6P.40.6 OverOverOverOver----current Devicescurrent Devicescurrent Devicescurrent Devices

A. Explain by means of diagrams the following protective devicesI. Semi-enclosed fuse

A wire of circular section which is connected to two screw terminalson the fuse carrier is the construction of the fuse element. The

combination of the element consists of an alloy containing 63% tin

and 37% led. The low cost and simplicity are considered

advantageous to the fuse, but it has many disadvantages, including;

Aging due to oxidisation Time delay in operation Fire risk because the element becomes white hot when

operates

II. Cartridge fuseIt consists of a porcelain tube with

metal end caps to which the element is

attached. The tube is filled with silica

gel. These fuses are normally found in

plugs used within 13A socket outlets.They are very accurate in breaking and

unlike rewireable fuses, they do not

deteriorate. In turn, these are quite

expensive to replace.


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III. MCB (thermal and magnetic type)An MCB is a mechanical device which

has many advantages over other fuses.

The major advantage is to reset after

tripping and do not need to replace it.

They also offer a better protection than

other fuses. Disadvantages include

regular tests since they may jam if

unused for long time, and expensive to

buy. There are three types of MCBs

which are type B domestic, C Industrial,

and D Motor Rating.

I. Thermal trippingThe load current is passed through a small heater, the

temperature of which depends on the current it carries.

The heater is arranged to warm a bimetallic strip. This

strip is made of two different metals, which are securely

riveted or welded together along their length. The rate of

expansion of the metals is different, so that, as the strip is

warmed, it will bend and will trip the latch. The bimetallic

strip and heater are so arranged that normal currents will

not heat the strip to tripping point. If the current increases

beyond the rated value, extra power is dissipated by the

heater, and the bimetallic strip is raised in temperature to

trip the latch.

II. Magnetic typeThe principal used here is the force of attraction which can

be set up by the magnetic field of a coil carrying the load

current. At normal currents, the magnetic field is not

strong enough to attract the latch, but overload currents

operate the latch and trip the main contacts.

B. Explain fusing factor and state typical factors of commonly usedfuses

The maximum current which the fuse can support without tripping is defined

as the current rating. The minimum current causing the fuse to blow is


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defined s the fusing current. The level of protection or fusing performance is

given by the fusing factor of the device.

The fusing factors of protective devices are;

Semi enclosed 1.8 and 2 Cartridge 1.25 and 1.75 HBC less than 1.25 MCBs up to 1.5

Example: current rating of a fuse is 16A and the fusing factor is 2. Calculate

the fusing current.

2 16 32

C. With reference to table 4D2A from the IEE wiring regulations, findthe CSA of a copper cable to be used for the following miniature

circuit breakers;

Assuming reference method B, 1 two core cable single-phase a.c. or d.c.

(Please refer to the table below)

I. 16A x 0.67 = 10.7A = 1mm2II. 40A x 0.67 = 26.8A = 4mm2


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Task 3Task 3Task 3Task 3 M.40.1M.40.1M.40.1M.40.1 The Use of IEE Wiring RegulationsThe Use of IEE Wiring RegulationsThe Use of IEE Wiring RegulationsThe Use of IEE Wiring Regulations

A. Explain in detail the purpose of the IEE wiring regulations andexplain how this is used

The scope of the IEE Wiring Regulations is to rely on it regarding electricalinstallations. The regulations apply to the design, erection and verification of

electrical installations such as those of;

Residential premises Commercial premises Public premises Industrial premises Caravans, caravan parks and similar sites Construction sites

Marinas Street furniture And many others

IEE wiring regulations is the 17 editions, and has been published since 1982

till 2008. The book contains the rules for the design and erection of electrical

installations so as to provide for safety and proper functional for the

intended use. The requirements of the book are intended to provide for

safety of persons, livestock and property against dangers and damage which

may arise in the reasonable use of electrical installations. It is divided into

chapters, and these chapters are sub-divided into points. At the end of the

book, one can find the appendix list of various tables and standard

requirements.

B. Highlight special installations for which particular requirements arelisted in the IEE regulations

Locations containing a bath or shower Swimming pools and other basins Rooms and cabins containing sauna heaters Construction and demolition site installations Agricultural and horticultural premises Conducting locations with restricted movement Electrical installations in caravan/camping parks and similar locations Marinas and similar locations Medical locations, reversed for future use


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Exhibitions, shows and stands Solar photovoltaic (pv) power supply systems Mobile or transportable units Electrical installations in caravans and motor caravans Temporary electrical installations for structures, amusement devices

and booths at fairgrounds, amusement parks and circuses

Floor and ceiling heating systemsC. Name the items excluded from the scope of the IEE regulations

The regulations do not apply to the following installations

Systems for distribution of electricity to the public Railway traction equipment, rolling stock and signalling equipment Equipment of motor vehicles, except those to which the requirements

of the regulations concerning caravans or mobile units are applicable

Equipment on board ships covered by BS 8450 Equipment of mobile and fixed offshore installations Equipment of aircraft Those aspects of mines and quarries specifically covered by statutory

regulations

Radio interference suppression equipment, except far as it affectssafety of the electrical installation

Lighting protection systems for buildings and structures covered byBS EN 62305

Those aspects of lift installations covered by relevant parts of BS5655 and BS EN 81-1

Electrical equipment of machines covered by BS EN 60204


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Task 4Task 4Task 4Task 4 M.40.5M.40.5M.40.5M.40.5 Protection for InstallationsProtection for InstallationsProtection for InstallationsProtection for Installations

A. Compare different types of overload/short circuit devices and statethe advantages and disadvantages of both types

MCBThe MCB is an automatic, electrically operated

switching device that was designed to

automatically protect an electric circuit from

overload currents and short circuit currents. It

is a complicated construction made up of

almost 100 individual parts. It has the ability

to respond within milliseconds when a fault

has been detected. MCBs find wide application

in residential, commercial and industrialoperations. These applications include;

Power Supplies Lighting circuits Solenoids Relay/Contactor coils Appliances Control circuits Motor circuits

The advantages of the MCB can be summarized as follows:

Closed overload protection compared to HRC fuses Stable tripping characteristics Common tripping of all the phases of a motor Instant re-closing of the circuit after a fault has been cleared Safety disconnect features for circuit isolation Terminal insulation for operator safety Ampere ratings that can be fixed and modified compared to

the possibility of introducing overrated fuses

It is reusable, hence very little maintenance and replacementcosts

Lower power losses Simplicity of mounting and wiring Lower space requirements


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Provision of accessories e.g. auxiliary switch Stable arc interruption Discrimination can be achieved either based on current or

based on time

The disadvantages of the MCB can be summarized as follows:

More expensive than the fuse Difficult to identify where the fault occurred Fault can be cleared in any time up to 10 cycles of the current

waveform

Large amount of energy let through (10 times that releasedby the fuse)

FuseThe word fuse is a short form of fusible

link and it is also protection device

capable of protecting a circuit from

overload currents and short circuit

currents. Fuses are rated in terms of

many aspects. These include voltage,

current and the type of application. A

high rupturing capacity (HRC) fuse hasa high breaking capacity. A fuse should

be selected with a rating just above the normal operating current of

the device to be protected. A general approach is that it should

operate at 1.2 times the rated current. A typical fuse is made of

silver-coated copper strips and granular quartz. Fuses find application

in systems where the load does not vary much above the normal

value (overload protection). They also find application in systems

where the loads vary considerably (short-circuit protection). These

applications include:

Transformer circuit Capacitor bank Motor circuit Fluorescent lighting circuit


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Control circuitsThe advantages of the fuse over can be summarized as follows:

Cheaper when compared to MCBs It is easy to identify where the fault is due to the open air gap It can cut-off fault current long before it reaches its first peak Perfect discrimination easily achievable due to the low cut-off

value

The disadvantages of the fuse can be summarized as follows:

The abrupt introduction of high resistance in the circuit by abadly designed and assembled fuse can create unwanted

effects while clearing the fault

Although this is very rare, fuses are likely to produce highpeak voltage which is much higher than the system voltage

and can puncture the insulation of the rest of the circuit

A lot of maintenance and replacement costs. Maintenance inthe form of continuously monitoring the state of the fuse; and

replacement after each and every fault

The cut-off current increases with the fuse rating Fuse of incorrect ratings can easily be installed in the fuse

holders

In a three phase power circuit, if one fuse blows, all the fusesmust be replaced at the same time

B. Explain what the term discrimination isA number of fuses connected in series are found in a normal installation. For

instance, the main fuse for a single-phase domestic installation is 40A which

protects several sub-circuits rated less than the main fuse. Logically, the

fuse of the final circuit must trip in case of overload. It will show a lack of

discrimination on parts of the fuses if the main fuse trips instead the final

circuit fuse. For example; the discrimination for a local single phase

domestic installation is as follows;

40A main fuse 32A MCB for lighting circuit 32A MCB for power circuit


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13A cartridge fuse in plugs used within 13A socket outletsC. Explain the terms prospective short circuit current and earth loop

impedance

Prospective short circuit currentBefore such current cause danger due to thermal or mechanical

effects produced in those conductors or the associated connections,

the protective devices shall be provided in a circuit to break any fault

current flowing in conductors of that same circuit. This is stated in

regulation 434-01 in IEE wiring regulations. When selecting the type

of over current device to be installed, this prospective short circuit

current must be taken care of. The effects of short circuit current

are:

The thermal effect, which can cause melting ofconductors/insulation, fire, alteration of the properties of

materials etc.

The mechanical effects of large magnetic fields that can buildup when short circuit currents are flowing, resulting in

conductor distortion, breaking of supports/insulators etc.

To prevent this damage, rapid disconnection of the supply is

essential. Possible causes of occurrence of a short circuit fault are;

Contact between two poles of the supply due to incorrectconnection

Equipment failure Ingress of moisture Accidental damage.

A fault may be derived from many causes and it varies, such as a

broken underground cable with construction plant. A connection

between two conductors including the neutral conductor refers to a

short circuit.

Earth fault loop impedanceThe over current protective device will operate, causing the

automatic disconnection of the supply on a full earth fault. The over

current protective device will not detect the fault on a low level earth

leakage fault. Metalwork may become live under fault conditions with


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a faulty or disconnected earth-

fault loop path. Zs is the sum

of Ze (the supply earth loop

impedance) plus R1 (the

resistance of the circuits phase

conductor) and R2 (the

resistance of the circuit

protective conductor)

1 2

Regulations state that the protective device protecting a circuit must

operate within a time of:

I. 0.4 seconds for portable equipment supplied from socketoutlet circuit

II. 5 seconds for fixed equipmentIII. 0.2 seconds for construction sites and agricultural or

horticultural premises.

D. With reference to the correct tables in the IEE wiring regulationsfind the time taken to operate a BS88 40A fuse on a 230V system.

Explain how you achieved the answer and attach a copy of the tables

used

According to the table below, the time taken to operate a BS88 fuse varies

according to the fault current. For instance, if a fault current would be 100A

on, the tripping time would be 100 seconds. The same principle was used in

the table below. First, I took the reading from the x-axis (100A). I followed

the vertical line till the point met on the curve of the 40A fuse. Then, I

followed the horizontal line to the left and determined the tripping t ime.


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Protect Against Overload and Short Circuit Currents

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