Electrical System Design Initial

Ministry of Awqaf and Islamic Affairs

Introduction to Design course on Electrical Power System.

Criteria : The design course based on theory and the most practical point of view.

General :

1.0 Basic Design consideration:

1.1. The power supply system shall be designed to provide safe ,economical operation and ensure safetyOf equipments and persons. The power system should be economic, reliable, long service life and panels should be simple in operation and maintenance.

1.2 The following factors shall be taken into consideration while designing the Electrical system:-a) Safety : Personnel safety, efficiency of operation and precaution of equipments are the most

vital factors to be considered while designing the electrical system.

b) Reliability : Reliability of electrical supply is essential for the continuous operation. The electrical system shall be designed so that faults can be isolated with minimum disturbance to the system as well as short time.

c) Easy operation : Simplicity of operation is very important factor in achieving safe and reliable electrical system. The operation should be simple as possible to meet the requirements.

d) Voltage Fluctuation : Voltage shall be maintained within the equipments tolerance limit underall load condition in order to enhance the service life of electrical equipments.

e) Flexibility : Flexibility of electrical system means the adaptability to meet the various requirement. utilization voltage, equipment rating, space for additional equipments, capacity for increased load, spare capacity for future load.

f) Installation :Power installation shall be designed to give enough power ,easy control of electrical devices. materials, components, accessories used for installation shall be identical type.

f) Maintenance : The electrical system shall be design to allow easy maintenance without major interruption and disturbance of users.

g) Equipments: All equipments and materials selected shall be well known manufacture , spare parts are available,manufactures are according to local and international standard.

h) Cost : Cost should be economical on the basis of initial, running and maintenance purpose.

Introduction of Electrical system design course on Electrical Power system.


2. Source of Power Supply :

a) Normal : MEWb) Emergency : Stand by Generatorc) Critical : UPS

3. Supply System : All electrical works, equipment, accessories, fittings shall be designed and manufactured operate continuously in the power supply system having the following characteristics.

a) Voltage : 415+-6% 3 phase, 4 wire 240+-6%, single phase

b) Frequency : 50 hertz+-4%c) Neutral : Solidly earthed T.T system.d) Fault level : 31MVA at 415 voltse) Fault duration : 0.5 second.

4. Climate condition : The environmental condition in Kuwait are severe and shall be consider carefully while designing the electrical power system, selecting

equipments. Electrical panels for outdoor installation shall be suitable for continuous operation at dusty, windy and rainy weather under high

ambient temp. 50 degree centigrade and high humidity 100%.

5. Electrical Load : Load is the is key factor to design electrical power system. Type of

Electrical load are as follows :1) General loads :

a) Dynamic load which consists of electric motors driving rotating equipments. Loads are A/C Equipments, Lifts,Plumbing Equipments(Pumps), Mechanical Equipments like exhaust fan, Fresh air Fans, Smoke Fans, Central water Heater.

b) Static load which consists of non moving type

equipments, like as cooling ,lighting, heating, small power (socket outlet) load.

2) Critical loads : These are loads of prime importance to the safety Of installation and operation like as computer and Control room load ( no break power supply by UPS)

3) Essential load : These are the loads whose power loss would affect the continuity of operation ,resulting loss of important data and damage of equipments. stand by Generator can be provided to take care

of this matter. Loads are Elevators, Pumps( all kind ), Smoke Fans, Fire alarm system ,security



system and Communication system. It is very importto prepare a complete LOAD SCHEDULE for all type of loads before starting design

6. Basic theory of Electrical Power System : Basic theory of Electrical Engineering is very important to design of electrical power system

. So, here some important tropics which are related to design is review

Tropics are as follow :1) Ohm, s Law 2) Voltage3) Current4) Frequency5) Resistance6) Inductance7) Capacitance8) Power 9) Voltage Drop10) Power Factor11) Conduit12) Conductor (cables, wire )13) Protection14) Measuring Instrument15) Method of Installation16) Transformer17) Motor18) Lux calculation19) Earthing system20) Switchgear21) Sub-Station

Ohm, s Law The potential Difference (voltage) across an ideal conductor (copper) is proportional to the current through it. The constant of proportionality is called Resistance.

So, V ∞ I or V=IR, Where V= voltage, I= Current, R= Resistance.

Voltage : Voltage is the electrical force that causes free electron to move one atom to aanother Voltage is also called Potential, Potential difference, Potential drop, Electromagnetic force. Its unit is volt.

Current : Current is the rate of flow of charge or in an electrical circuit, the number of electrons are moving is called the amerage or the current and its unit is ampere (A).

Frequency : In ac circuit the current wave how many times change it direction is known as frequency. It units is hertz (f ).



Power : The power is energy produced by one second or electric power is the rate of doing electrical works . It units is kilowatt and symbol is P.

Resistance : Resistance is the retarding force in a conductor which opposes the flow of current. The voltage is equivalent to water pressure, the current is equivalent to the flow rate, the resistance is like the pipe size.

R= p L /A where R is the resistance of conductor ,L is the length Of Conductor, p is resistivity of conductor.

Or, R ∞ 1/A if L ,p are constant.

So, Resistance is inversely proportional to the cross-section of conductor.Resistance is responsible for resistive loss in a electrical circuit. It loss as aform of heat.

From ohm, s law current I= V/R Or, I ∞ 1/R, Current is inversely proportional to R, if V is constant

If R is low the current I is high, so by increasing the cable size load current can be increased.

Inductance : Inductance is that property of a conductor (coil) which oppose to build up current before voltage ,first voltage build up due to inductance property, then flow in the circuit. Voltage is lead and current is lag. Most of bulk loads are lagging power factor due to inductance. It symbol is L and unit is henry.

Capacitance : Capacitance is that property of a conductor which oppose to build up voltage. But storage energy as a charge. In a capacitive circuit first current flow then voltage. It symbol is C and unit is microfarad

Power : The capacity to do the works is known as power, energy produced a electrical circuit in

one second, electric power is rate of doing electrical works. It unit is P and unit is watt. For a d.c circuit power P= V x I but in a.c circuit power P= V x I x P.F for single phase

For three phase circuit Power P= √3 V X I X P.F (P.F= power factor) Or, P ∞ P .F ,Power P is proportional power factor. So, by increasing the power factor

apparent power can be increased.

Voltage Drop : It is the potential difference between two point in electrical circuit. Voltage drop is the reduction in voltage in the passive element of an electrical circuit. The in the A.C circuit the product of current I and impedance Z, V.P=I X Z .

Power factor : The cosine angle between voltage and current in an a.c circuit is known as power fa. Factor Or it the ratio of real power to apparent power. Power factor is very important in Electrical system. unit is microfarad. b) At constant voltage the load current will

be more , this increase conductors size. c) Voltage drop is high due higher load current d) Poor voltage regulation. Now a day energy is saving by improve the power factor ,like by capacitor banks.



Conduit : An electrical conduit is an electrical piping used for safety, protection and route of electrical wiring. Conduits are classified by the wall thickness ,mechanical stiffness and material used to make the tubing. For ,electrical works generally

PVC, UPVC ,GI , and flexible are used on the basis of application. Different sizes of conduits are 20mm, 25mm, 38mm, 50mm ,UPVC pipes sizes are

3”, 4” and 6”.

Conductors : Conductors (cables & wires ) of electricity are materials that electricity pass through

them with safety. Copper is good conductor, it has so many advantages rather than other conductor, like aluminum, silver and gold. Advantages bearing temperature is high, current carrying capacity is very high, conductivity is very high and mechanical stress is very high. Generally XLPE and PVC insulated cables are used in Kuwait.

XLPE Cable : XLPE (Cross linked polyethelene) cables are mostly in Kuwait. The Cables consists of a) conductor b) Insulation c)PVC sheath d)Armour

Other material used for bedding and serving .

a) Conductors : The function of conductors is to carry the current. Conductors made of pure copper or alumunium. The

Diameter or size of conductor depend upon current Carrying capability, conductors are solid , stranded. Stranded conductors are more flexible. Cross section Of conductors are circular and oval shape.

b ) Insulation : The function of insulation is to separate phase and Earth conductors from each other. Xlpe insulation are white in colour, for phase identification Red, Yellow, Blue, Black and Green.

c ) PVC sheath : In order to protect the cables from moisture, water damaging liquids in the soil and atmosphere PVC sheath is used.

d ) Armour : Armour which consists of one or two layer of galvanized steel wire is provided the cables from mechanical injury during lying and use as earth.

c) Bedding : A layer of bedding which consists of fibrous material is applied over the XLPE insulation to protect cables from armour and provide smooth bed for armouring.

c ) Serving : In order to protect the armouring from atmospheric condition ,a layer of fibrous material is proved over the armour.

Protection : Protection or Trip system is the main feature of a electrical system. Protection criteria should have following characteristics a) Reliability : Perform correctly when required operation and avoid unnecessary

interruption.

b) Speed :Disconnect the fault at shortest time



c) Selectivity : Ability to locate the fault and trip

the minimum number of Breakers to isolate only fault.d) Economics :Maximum protection at optimum

Cost. e) Simplicity :Minimum equipment and circuitry

for the required protection.

The main protection system are Short circuit , Over-current, Earth fault, Under voltage protection. When a fault occurs in a electrical circuit, first in CT or Toroid current increases, relay sense this increased current and energized the relay coil which in turn closed the tripping circuit ( normally tripping circuit is open ) then trip coil energized and pull the breaker down by magnetic induction.

Measuring Instruments : The device which are used to measure parameters of electrical system like

Current ,voltage ,frequency and power are called measuring instrument.

Method of Installation : Installation method of complete electrical works is a important factor whiledesign the electrical works. It define how to execute the whole electrical works to finalized the installation, it require coordination with other services like civil Architecture, Air-condition ,mechanical and plumbing works.

Light fitting fixing method, arrangement depend on the basis false ceiling type whether it is recessed or suspended type or ceiling mounted or Latch to ceiling or wall mounted. Cables may be laid in underground or in cable tray suspended from ceiling

.

Transformer : Transformer is the static device which transfer a.c energy from one level toanother at the same frequency and power factor through electromagnetic induction .Transformers are used in transmission and distribution system

in consumer premises to step up and step down voltage. Power transformers are used to get various voltage in Sub-Station.

It normally consists of a ferromagnetic core and two or more coil or windings. When one of the winding (primary) is connected to a.c supply , alternating current magnetic flux is produced in the core. The flux linkage of other windings on the same core alternates. Thereby e.m.f. of same frequency is induced in other windings called secondary windings. The e.m.f. is induced in the transformer by electromagnetic induction of alternating magnetic flux. Power transformer rated 11/0.415kv is normally is used to provide power supply in project.Kuwait transformer rating are 1000 KVA ,1250 KVA and 1600 KVA.

Electrical Motor : Motor is rotating device which convert electrical energy to mechanicalIntroduction of Electrical system design course on Electrical Power system.


energy. Main part of a motor are stationary stator and rotating rotor. The motor have three phase distributed a.c. winding on stator ,rotor has a closed short-circuited winding. When three-phase a.c. supply is given to stator winding, the resulting stator magnetic flux formed by circulating current goes on varying in magnitude and direction with respect to each coil with time at synchronous speed. As the rotor has closed short-circuited winding , its induced emf in rotor produced short-circuited rotor current.

A current carrying conductor placed in rotating magnetic field experiences torque. So, the rotor conductors carrying induced currents placed in rotating magnetic field experiences torque. Motor are mainly two type induction motor for bulk load and synchronous motor for light load constant speed. Basically , motor used as a drive unit with pump such as Booster pump Water transfer pump, Irrigation pump ,submersible pump, domestic water pump ,fire fighting water pump and jockey pump.

Lux Calculation : Lux calculation is a technique to find out the total number of light fittings are required for a room on the basis of light fitting type ,installation method , working Plane and mounting height. Later chapter detail lux calculation will discussed.

Earthing system : To protect the personnel and equipments earthing is very important. By proper earth human life and equipments damage can be save. so, all metal part involving with the electrical system must be earthed for safe and secure operation of electrical system. From the soil test report ,soil resistivity can

be find out. Earth resistance must be less than one ohm for ring earth. By calculation total number of earth electrode can find out to get resistance value less than one ohm. When fault occurs short-circuit current is very high, so , low resistance path is highly required to clear fault current in short time.

Switchgear : The apparatus used for switching ,controlling ,protecting ,the electrical circuit

and equipment is known as switchgear. The main parts of switchgear are Enclosure, Busbar, Breakers (MCCB), control, protection , measuring devices Low voltage switchgear are three type :



1) Cubical type (MSB ,fixed type)2) Cellular type (MLTB ,fixed and draw out type)3) Metal Clad type (draw out type)

In MSB all MCCBs are fixed to common centrally busbar in one enclosure.

from central busbar different rating MCCB are tape out for different feeder. DBsSMSBs, S/Fs feed from MSB. If the building is multi-stored then each floor have separate MSB. DB (distribution board) are used to control, protection & switching of small power load like Light fitting ,small power socket outlet for Kitchen equipments, home appliances’ and office equipments .

S/Fs is final device, power supply given to electrical equipments like a/c package unit ,Smoke & fresh air fan, various pump motors via through it for safe and secure operation & maintenance.The rating of S/F depend on equipments load and type of load. Most of S/F are outdoor type and locatednearby equipments. The S/F used only for short circuit protection.

Where as in MLTB (Main Low Tension Panel) enclosure have facility of separate section with individual compartment housing incoming and outgoing feeder. Low tension panel provide power supply to all feeder of project. LT panel size depend on total connected MSBs ,EMSBs ,MCC ,S/F.s ,FP etc feed from it. The output single core cables ( LV ) side of power transformer isconnected to incoming ACB of LT panel.

ATS (Automatic Transfer Switch : Automatic transfer switch is an electrical switch that reconnects Electric power source from its primary source to a standby source (Generator) automatically. An ATS is often installed where a backup Generator is located ,so that generator may provide temporary Electric power if the main source is fails. It senses when voltage on the utility falls below predetermined limits then initiates gen set start up. It transfer load to the gen set when the engine has sufficiently warmed up and the generator reached operating voltage and frequency. It also returns the to the utility (main supply) when the utility power is restored.

ATS also provides a timer so the engine runs long enough to warm up fully. ATS command the generator to turn off after specified amount of “cold down “ time generator will completely stop.

There are two type of Automatic transfer switch, Circuit breaker and Contactor. The circuit breaker type has two interlocked circuit Breakers, so only one breaker can be closed any time.

The contactor type is simpler that is electrically operated and Mechanically held. It operates faster than circuit breaker type, which reduces transfer time. Rating of ATS are from 100A to 2500A.



Generator :Generator is a machine which convert mechanical energy to electric energy. Generator components are prime mover, Stator and Rotor. Prime mover is the drive unit which rotate the generator shaft.

Stator is the stationary part of generator. The has three phase distributed winding in Armature slot. The e.m.f induced in armature coil when rotating magnetic field of rotor cuts it.

Rotor is the moving part of the generator, rotor has d.c field winding which is excited by d.c current The rotor either Salient pole type (low speed generator ) or Cylindrical type (high speed generator ).

According to the faraday,s law of electromagnetic induction whenever a conductor cuts magnetic flux the emf induced in the conductor (coil ) is given by the rate of change of flux linkage of the coil. The value of emf generated depend on three thing

a) Numbers of turn in the coilb) Strength of the fieldc) Speed at which coil or magnetic filed rotate.

The induced emf is given by

E.M.F =n Φ Km

Where Km = design constantn = Speed in rpmΦ = Flux per pole

Magnetic flux Φ is proportional to field excitation If.Φ∞If

So, E ∞Φ∞If

A diesel generator is the combination of diesel engine (prime mover )With an electric generator ( Alternator ) to generate electric energy.Diesel generator are best due best due to their longevity and lower operatingCost.

Generator has three rating a) Stand by b) Prime c) Continuous.A stand by gen set is used is a back up to normal utility power. StandbyUnits are used when normal utility power is not available and will not usedFrequently. Another word for Stand by is “Emergency “.

Prime rated generator is required when there is no other source .Any Generator that is every day or a fixed schedule to provide power is consideredA prime power genera tor .Another word for prime Is” continuous “.Generators have several rating from 50KVA upto 2500KVA.



Sub-Station : Sub-Station provide the power supply to project. The electrical panel like as HT & LT, Power transformer, capacitor bank are equipped in Sub-Station. So, Sub-Station is very important on basis of dimension and location at design stage. Electrical engineer has sole responsibility to coordinate with Architecture,Civil and other services engineers to ensure incoming & outgoing HT cables, unloading HT & LT panel ,Transformers in Sub-Station without any interruption of the others services. Transformers room should be in between HT room and LT room to easy HT cables acces to Transformer and LT cables out from Transformer to LT panel incoming ACB. It is responsibility of Elec.

Engineer to prepare Sub-Station equipment layout drawing and handover to Architecture. Other important thing is Electrical rooms dimension and location. If the building is multi-stored then it is advisable each floor has separate Electrical room at same location to easy access feeder cables from LT panel to MSB at the different floor of the building.

Now a days,most of Sub-Stations are in basement that case if the cables riser are inside the electrical room, then all feeder cables can pull from LT panel to all MSB without disturbance of other services. Electrical engineer also check all electrical room dimension to ensure enough space for fixing MSB,s & DB,s.

Generator room dimension should be suitable to accommodate Generator on

basis of its rating and others facility like daily fuel tank, pumps. Finally Sub- Station building drawing should be approved from concern ministries.

Project Drawing Study : Project drawing study of other services is very important to design electrical power system. Study of Architecture, Civil, Air Condition,

Mechanical, Plumbing( Drainage, Sewerage ) ,Interior design drawing to know room identity & user requirements and choice.Location of Air-Condition, Mechanical ,Plumbing and Irrigation equipments nearby the power supply have to provide through S/Fs., Furniture layout study is essential to know location & number of power ,data Telephone socket outlets in each room False ceiling

drawing study help to select light fitting type, installation method. Light may be recessed or ceiling mounted or suspended type

,it depend on décor type. At beginning stage Electrical engineer have to inform Architecture Engineer,25cm space above décor is required to fix the light fixture.



Project load :Total load of the project is key for electrical system design, without this electrical Panels(LT, MSB, EMSB, SMSB and DB ) drawing cannot be prepared. Basically Load s are two category a) Small power load b) Bulk load (Equipment load).

Small power Load : Small power are minor load 10-25% of project load like lighting General socket outlets load, house and office hold equipments Load.

Bulk load : The equipments load of electro-mechanical system are bulk or major load of the project. They are 75-90% load of project. The equipments load are as follow ;

Air condition system load : a) The A/C system load are several Air cooled or Water cooled chiller System. Motor drive unit AHU give Cool air, so ,Electrical engineer must check data sheet of motor to know starting current how many more times of running current to star delta or VFD panel.

b) The A/C package & DX unit ,electrical engineer should check electrical data sheet from manufacture catalogue. Electrical engineer should take full A/C equipments schedule with load from HVAC engineer.

Mechanical equipment load : c) Electrical engineer should coordinate with Mechanical engineer to get

equipments schedule with load for equipment like Fresh air fans, Extract fan, Smoke fans and Lifts.

Plumbing equipment load :d) Electrical engineer also coordinate with Plumbing engineer to get load schedule of plumbing equipment like Booster, Submersible, irrigation,



Lifting, hot water circulating pumps. Central water heater , fire fighting and Jockey pumps.

Others Load : e) The others load are (special project) Kitchen Equipment load, swimming pool equipments

load,

Now summation of small power , bulk or equipments and others load will be the total load of project .To get design load add 20 -25% ofproject load as extra load for future. This will be final load of project

Checklist prior Design : The following item reviewing is very important for electrical system design.

a) Architecture drawing study done for electrical works requirement yesb) A/C drawing study done for electrical works requirement yes c) Plumbing drawing study done for electrical works requirement yesd) Furniture layout drawing study done for electrical works requirement yese) False ceiling layout drawing study done for electrical works requirement yesf) Interior design layout drawing study done for electrical works requirement yesg) General load schedule prepared yesh) Air-Condition load schedule prepared yesi) Mechanical load schedule prepared yesj) Plumbing load schedule prepared yes k) Emergency load schedule prepared yesl) Total load schedule prepared Yesm) Sub-Station equipments layout drawing completed yesn) Electrical room equipments layout drawing completed yeso) Generator Room equipments layout drawing completed yes p) Electrical Shaft or Cables Riser shown in drawing yes q) Feeder cables route from LT to all panels(MSB, EMSB etc.) finalized yes r) Coordination meeting with others services Engineers done yes

Electrical Design : The electrical design system mainly based on :1) Power system design2) Lighting system design.

Power system Design : The power system design based on items are electrical panels, Cables/wires, wiring devices.

Selection for cables : The factors need to consider to design cables size are as follows. a) Load in KW



b) Distance between source and equipmentc) Voltage drop calculationd) Number of parallel cables rune) Group factor ( spacing between two cables )f) Temperature factorg) Installation method( depth of lying cable in ground or duct )h) Drop factor per KM ( gulf cable catalogue ) i) Soil resistivity factor of ground.j)k)

Example for cable size: Let us consider, a MSB has load 150KW which is giving power supply to DB,s and S/FOf A/C unit. Distance between MLTB to MSB is 125meter.Nos. of parallel cables is 1 but spacing between them is 15cm, Cables buried 75 cm below direct in ground , Temperature of soil below 75 cm is 40degree centigrade and soil resistivity at 40 degree is 0.8. What is suitable cable size of that feeder.

We know, Group factor for 15 cm space = 0.8 Depth of lying factor at 0.75m = 1 Temperature variation at 40 degree =0.95 Soil resistivity factor =0.8

Now, Derating factor = group factor x depth of lying factor x Temperature factor x soil resistivity

=0.87 X 1X 0.95 X 0.8 =0.66Load current = 1.732 x 150 = 259.8 amp.Equivalent current = load current / derating factor

= 259.8/ 0.66 = 393.63 amp.

Let us consider propose cable size is 4c x 300mm.sqr cu/xlpe/pvc/swa/pvc .

Now, voltage drop = equivalent current x distance in km x drop factor / no. of parallel cables

= 393.63 x0.125 x 0.19 /1 = 9.35 volt (drop factor=0.19 gulf cable catalogue )% of voltage drop = 9.35 x 100 /415 = 2.25 %

Voltage drop from LT panel to MSB = 2.25%

Let us consider , a A/C package unit has load 75 KW at a distance 70 m feed from MSB with cable size is4c x50 mm xlpe.

Voltage drop for 4c x50mm cable= 1.732x 75x .070x 0.87 7.910 volt. (Drop facto =0.87 gulf cable cat)% of voltage =7.910x100/4.15 = 1.90%Now, total % of voltage drop = 2.25+1.9 =4.15 % which is more than Mew approved voltage drop 2.5% .



In this case cable size from LT panel to MSB and MSB to S/F of A/C unit have to change, Cable size LT panel to MSB is 2Rx 4Cx 240 mm sqr cu/xlpe /pvc/swa/pvc and MSB to A/c unit is 4Cx 95mm xlpe

Voltage drop for main cable =393.63x 0.125x 0.21/2 = 5.166 volt Voltage drop for sub cable = 1.732x 75x 0.070x 0.45 = 4.09 volt Total voltage drop from LT panel to final equipment A/c unit = 5.166+4.09=9.256 volt % of voltage drop = 9.256x 100/415 =2.23% which is acceptable as per MEW regulation.

Finally, Main feeder cable is 2Rx4Cx240mm sqr. And Sub feeder cable is 4Cx95mm sqr. Selection of Breaker (MCCB) Rating : The mould case circuit breakers (MCCB) are used as a

protection device in MSB and MLTB. Normally, it is tri-pole breaker and breaking current capacity from 22KA

to 70KA.It is used for over-current and short circuit fault protection. It has provision of shunt trip coil for Earth Leakage protection. To select MCCB rating factor need to Consider are as follow.

a) Load in KWb) Type of load c) Temperature derating factor (if any)d) Breaker Type (Fixed or Adjustable)e) Protection scheme (Over curent or

short circuit)f) Fault level

Example : Let us consider a package unit has load 60 KW and AHU unit has load 20KW. What are breaker rating of two cases.

Load current = 1.732X 60 = 103.92 Amp Maximum rating of protective device= 1.45X load current=1.45X103.92=150.68 amp. As ,most of A/c package unit fan motor rating low less than 10 HP. So, there is no high starting current.

For, first case MCCB rating is 150 amp.(adjustable type)

Load current for AHU unit = 1.732X 20=34.64 amp.

We know, AHU unit has big size fan and it drive unit is motor whose rating is most more than 15 HP, have high starting current. The starting current differ from manufacture to manufacture. Consider, starting current is 2-3 times more than running current.

Now, load current=34.64X 2.5 =86.6 amp Maximum rating of protective device =1.15X86.6 =99.59 amp =100 amp.

( note: we consider starting current factor so, protective device factor will be low )

For, second case MCCB rating will be 100 amp.(adjustable type)



Design of Lighting schemes : The lighting scheme should be such that it may,1) Provide adequate illumination2) Provide light distribution all over working plane as uniform as

possible3) Provide light of suitable colour4) Avoid glare and hard shadows as far as possible.

Type of Lighting Schemes : The interior light schemes may be classified as 1) direct lighting 2) semi-direct

Lighting 3) indirect lighting 4) general lighting.

1) Direct lighting : It is most commonly used type of lighting scheme. In this lighting scheme more than 90 percent of total light flux is made to fall directly on the working plane with the help of deep reflector. Though it is most efficient but hard shadow and glare. It is mainly used for industrial and general out door lighting.

2) Semi-direct lighting : In this lighting scheme 60 to 90 percent of the total light flux is made to fall down Wards directly with the help of semi-direct reflector, remaining light us used toilluminate the ceiling and walls. Such a lighting system is best suited to room with high ceiling where the high level of uniformly distributed illumination is desirable. Glare in such units is avoided by employing diffusing globes which not only improve the brightness towards the eye level but improve the efficiency of the system with reference to the working plane.

3) Semi-indirect lighting : In this lighting scheme 60 to 90 percent of total light flux is thrown to the ceiling for diffuse reflection and the rest reaches the working plane directly except some absorption by the bowl . This lighting scheme is with soft shadow and glare free. It is mainly used for indoor light decoration purposes.

4) Indirect light : In this lighting scheme more than 90 percent of total light flux is thrown upwards to the ceiling for diffuse reflection by used inverted or bowl reflectors. In such a system the ceiling acts as light source and glare is reduced to minimum. The resulting illumination is softer and more diffuse, the shadow are less prominent and the appearance of the room is much improved over that which results from direct lighting. It is used for decoration purpose in cinemas, theatres and hotels etc and in workshop where large machines and other obstructions would cause trouble some shadow if direct lighting is employed.

5) General lighting : In this lighting scheme lamps made of diffusing glass are used which give nearly equal illumination in all direction.

Lighting System Design: The following factor need to consider while design the lighting scheme.a) Illumination level :It is vital factor as because the light incident on the

working plane depends on its value like 300lux, 500lux, higher the value more light and lower the value less light.

b) Uniformity of illumination : The human eye adjust itself automatically to the brightness within the field of vision .If there is a lack of uniformity pupil or iris of the eye has to adjust more frequently and thus fatigue is caused



to be occurs.c) Colour of light : The appearance of the body colour entirely depend upon

the colour of the incident light. The composition of the light should be such that the colour appears neutral.

d) Glare : The size of opening of the pupil of human eye is controlled by iris If the eye exposed to a very bright source of light the iris automatically contracts in order to reduced the amount of light admitted and prevent damage to retina ; this reduce sensitivity so that other object with in the field of vision can be only imperfectly seen. This effect referred to an glare and any one feels discomfort, eye fatigue with in the field of vision. Glare may be direct or reflected i.e it may be come directly from the light source or it may be reflected brightness such as desktop , nickel machine parts.

e) Shadow : In lighting installation formation of long and hard shadow causefatigue. Hard and long shadows can be avoided by using number of small luminaries mounted at height not less than 2.5m

.f) Mounting height : Mounting height of the luminaires will largely be govern

by type of building and type of lighting scheme used coefficient of utilization factor is depend on mounting of light fitting.

g) Spacing of luminaires : Correct spacing is of great importance to provide uniform illumination over the whole area.

h) Colour of surrounding Area : The illumination in any room depends upon The light reflected from the walls and ceiling. White ceiling reflect more light as comparedto coloured ones.

i) Utilization factor :It is defined as the ratio of total lumen reaching the Working plane to total lumen given out by the lamp. It is value between 0 and 1.It takes into account the room reflectance room shape, polar distribution and light out.

j) Maintenance factor : Due to accumulation of dust, dirt and smoke on lamp they emit less light than that they emit when they are new ones .Similarly the walls and ceiling etc. after being covered with dust, dirt and smoke do not reflect the same output light which is reflected when they are new the ratio of illumination under normal working condition the illumination when the things are

perfectly clean is known as maintenance factor. Its value in between 0.6 to 1.

k) Light fitting type : Light fitting selection is very important to lighting scheme Light fitting are different type, like down light at various watt, linear light fitting at different wattage. Lumen out-

put from the lamps of fitting is the vital to select total



lumen output Factor need to consider to select fitting type are as follows:

a) Room identity and dimensionb) Room users and utilityc) Décor or without décord) Room wall coloure) Room height f) Room floor g) Room furniture layout

Finally, Light fitting and its control (dimming provision) depends on the Room application and users satisfaction, proper light distribution depends on the illumination level. Its value higher or lower has significant effect on lighting.

Method of Lighting design : 1) Manual method a) Watt per square method b) Lumen method.

2) Soft wire method like Dialux etc.

Watt per square method : It consist in making an allowance of watts per square meter of area (15-30) watt/m sqr. to be illuminated according to

the illumination desired on assumption of an average figure of overall efficiency of the system.

Lumen or light flux method : This method is applicable of source of light provide

approximate uniform illumination on working plane.

Formula for design calculation : No. of light fitting n=Ax E/U.Fx D.FxΦ

Where, n= Number of light fittingA= Area to be illuminatedE= Illumination level in lumen/m sqr.U.F= utilization factorD.F= Depreciation factor or light loss factor

Φ = Lumen output for one particular light fitting

Utilization factor depend on depreciation factor, reflectance factor and room index. Its valueIn between 0 and 1.

Typical table for depreciation factorAir condition office 0.8Clean Industry 0.7Dirty industry 0.6



Typical Reflectance values Ceiling Walls Floor

Air condition office 0.7 0.5 0.2Industrial 0.5 0.3 0.2

Room index : The room index is a number that describes the ratio of the room length, width height.

Formula K = Lx W/Hm(L+W)

K = Room index, (this value usually between 0.75 and 5) Where, L = Room length

W =Room width Hm = Mounting height of fitting (from working plane =0.75m FFL )

Utilization Factor Reflectance Room Index K

Ceiling Wall Floor 0.75 1.00 1.25 1.5 2.0 3.0 4.0 5.00.5 0.3 0.2 0.3 0.37 0.42 0.46 0.52 0.59 0.63 0.66

Numbers of fitting required in axis of room

Number in length = √Total no. x L/WNumber in width = √Total no. X W/L

Spacing between each fitting : It is good practice to aim at a horizontal spacing Between two rows approximately equal to the height of the ceiling above of working plane.

Example of lighting calculation : Let us consider a room has width 6 m , length 8m and room height is 4m. Room has décor zip- sum board wall white colour

and floor marble .Propose Light fitting is down light 2x 26w and illumination level is 500 lux.

No. of light fitting n=Ax E/U.Fx D.FxΦ Where,

n= Number of light fittingA= Area to be illuminatedE= Illumination level in lumen/m sqr.U.F= utilization factorD.F= Depreciation factor or light loss factor Φ = Lumen output for one particular light fitting

n =6x 8 x500/0.8x u.f x 1800 x 2 =24000/u.f x 2880Introduction of Electrical system design course on Electrical Power system.


n = 24000/u.f x 2880 = 19.84 Formula K = Lx W/Hm(L+W)

K = Room index, (this value usually between 0.75 and 5) Where, L = Room length

W =Room width Hm = Mounting height of fitting (from working plane =0.75m FFL )

Room index = 8x 6/3(8+6) = 1.15Utilization factor is 0.42

Now, n= 24000/0.42x 2880 =19.84 = 20 Number in length = √Total no. x L/W = √20x 8/6=6 nos.Number in width = √Total no. X W = √20x 6/8 =3 nos.

For uniform distribution of light ,in length fitting will be 5 nos. and in width fitting will be 4 nos.





Schedule points of DB (Distribution Board ) : Basically , DB is used to control, protection of small power load like lights, power socket outlets,

household and office equipments. DB should have detail information and contain the following coloum.

a) Circuit numberb) Phase indicationc) MCB ratingd) Wire sizee) Schedule of pointf) Load description g) Location of pointsh) Controlled by i) Phase (R,Y,B ) load in watt j) Total load

Location of DB is very important factor , maximum distance of DB from respective MSB should not be more than 50 meter as because of pulling main wire inside conduit also keep load less than 30kw to avoid voltage drop and for power & lighting load 25mmsqr or 16mmsqr is used as main wire in DB. There are several way to select the type of DB like as Single Bus-bar, Double Bus-bar and Split Bus-bar on the basis of load ,power supply (Single phase or Three phase). The internal component of DB are ELCB 30ma sensitivity (Power) ,ELCB 300ma sensitivity (Light ), MCBs 10A (Light) ,MCB 15A( power socket outlet for general load) ,MCB 20-32A for W.H, A/C unit, Freezer.For lighting circuit wire size 1.5mm+1mmsqr,maximum load 1000 watt, for power Socketoutlet circuit wire size 2.5mm+1.5mmqr,maximum load 1500 watt and for Equipments(W.H, A/C) circuit wire size 4mm+2.5mmsqr,maximum load 3000W

Schedule of MSB: MSB (main switch Board ) is vital panel to provide, protect, control the power supply for DB ,SMSB and all the S/F,s of Electro-Mechanical Equipments (a/c, plumbingmechanical).To develop the schematic or single diagram of MSB the following points need to be considered:

a) List out the load according to the equipments and application.

b) Short out general (DBs) load, Air condition, Mechanical, Pluming load separately.

c) From each load of equipments finalized the rating of MCCB.

d) From each rating of MSB finalized the feeders cable size.

e) From total load finalized the incoming breaker of MSB.

f) Bus-bar rating will be 25% more from incoming main MCCB rating.



g) Prepare individual MSB for Air-condition, Mechanical Plumbing and General small power load.

h) Load should distributed in three phase for balanced.i) Measuring ,indication devices should shown in

drawing.j) All pumps motors, lifts, Fresh air fans, Smoke fans

and A/c equipments outgoing MCCB should have separate ELR of 300mm sensitivity.

Necessity of Capacitor Bank : Most of bulk load like Motor, Choke, Transformer, Electric Heater, Compressor, Arc-welding transformer are inductive and therefore, taking lagging current. So, in generation stage power factor is

lagging (0.8-0.85). The cosine angle in between voltage and current is known as power factor.The power factor results in a higher energy consumption and cost, less power distribution via network, a power loss in network (transmission line ), a increased voltage drop in

network results poor voltage regulation, large KVA rating of equipments ( Transformer, Alternator, Switchgear ),as because

KVA= KW/cosΦ , if KW is constant then KVA = 1/ cosΦ i.e KVA is inversely proportional to Power factor. And also high voltage variation in transmission lines.

To Transmit or distribute a fixed amount of power at constant voltage , the conductor will have to carry more current at low power factor

, this require large conductor size which is more costly due to higherinsulation cost.

Power factor improvement can be achieved by a compensation of reactive power with Capacitor, a active compensation using semiconductor, a over excited synchronous machine( motor or generator ).

There are so many benefits of power factor correction improvement like as :

1) Power factor correction reduces the reactive power and power cost drop in proportion.

2) Smaller transmission losses and saving the energy.3) Improve voltage quality and reduce conductor size for same amount of power

supply.4) Fewer voltage drop and improve voltage regulation.5) All Electrical equipments KVA rating reduced.



Choice of compensation type : The capacitor bank or power factor corrector has two type of compensation

a) Individual compensation : Power factor correction is wired at each single load

b) Central compensation :There is only one bank of capacitor on the main Power distribution switch or LT panel.

The individual compensation the capacitor and the user equipment follow the sorts during the daily work ,power factor correction is simple with low cost.

The central compensation is best suited for system where the load fluctuates.

Key component of capacitor bank :

a) PFC controller :Modern PFC controllers are microprocessor type. The Microprocessor analyzes the signal from current generator and produced Switching command to control the contactors that add orremove capacitors stages.

b) Detuned Reactor : The detuned reactor are designed to avoid any amplification of the harmonics present on the network and protect the capacitors. Harmonics are dangerous for capacitor connected in PFC circuit ,especially if the capacitors operate at resonant. frequency. The series connection of reactor and capacitor to detune the capacitor resonant frequency help to prevent capacitor to damage.

c) Fuse/MCCB : An HRC fuse or MCCB acts as a safety device for short circuit protection.

d) Capacitor : Power factor correction capacitors produce the necessary leading power to compensate the lagging reactive power. PFC capacitors should be capable of withstanding high inrush current by switching current.

Capacitor bank size calculation : The rating of capacitor bank can be calculated as follows ; Qc = P*{tan[acos(pf1)}]-tan[acos(pf2)}] = P.( tanΦ1-tanΦ2 ) Where, Qc = required capacitor output in kVAr

Pf1 =actual power factor Pf2 = target power factor P = real power in KW



The required capacitor output may be calculated as followsSelect the factor ( matching point of actual and target power factor ) kQc = k*P

Example :Actual power factor = 0.70, target power factor = 0.96, Real power ,P = 500 KWThen , Qc = 0.73*500KW = 365 KVAR

Target Power Factor0.7 0.75 0.8 0.85 0.9 0.92 0.94 0.96 0.98 1

Actual power factor0.4 1.27 1.41 1.54 1.67 1.81 1.87 1.93 2 2.09 2.290.45 0.96 1.1 1.23 1.36 1.5 1.56 1.62 1.69 1.78 1.980.5 0.71 0.85 0.98 1.11 1.25 1.31 1.37 1.44 1.53 1.730.55 0.6 0.64 0.77 0.9 1.03 1.09 1.16 1.22 1.32 1.520.6 0.31 0.45 0.58 0.71 0.85 0.91 0.97 1.04 1.13 1.330.65 0.15 0.29 0.42 0.55 0.68 0.74 0.81 0.88 0.97 1.170.7 0 0.14 0.27 0.4 0.54 0.59 0.66 0.73 0.82 1.020.75 0 0.13 0.26 0.4 0.46 0.52 0.59 0.68 0.880.8 0 0.13 0.27 0.32 0.39 0.46 0.55 0.750.85 0 0.14 0.19 0.26 0.33 0.42 0.620.9 0 0.06 0.12 0.19 0.28 0.48

Design Drawing : Drawing is the language of Engineers .So, it is very important to present design Works in a professional way .The electrical design drawing should be contain the following information.

a) Drawing should have proper title, number and revision.b) Drawing legend should be meaningful.c) Legend symbol should be matching with light fitting , wiring devices

Ex-fand) Drawing should have proper scale.e) Mounting height of respective item should be shown in lighting and

power drawing.f) All the low current ( Fire alarm, Sound , Central Clock , cctv, Access

control Quenching, Telephone, Data,) system must be in separate drawing.Drawing should show the master plan in Top corner and shaded portion is current one.




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Electrical System Design Initial