Low Voltage
Power Supply System
POWER TRANSFORMER
Is used to transfer electrical energy in any part of the
electrical circuit between the generator and the distribution
primary circuits.
used in transmission and distribution systems to
interface step up and stepdown voltages.
DISTRIBUTION POWER LINE
TRANSFORMER (OVERHEAD) FEEDER
Performs the last voltage Is a structure used to Voltage power line
transformation in a transmit electrical energy transferring power from a
distribution grid across large distances. distribution substation to
It converts the voltage the distribution
used in the transmission transformers
lines to one suitable for wire/line that carries
household and power from a
commercial use, typically transformer or switch
down to 240 volts. gear to a distribution
panel
As per Bahraini standards a As per Bahraini standards
level of 400 V (three phase) stepped down voltage is
or 230 V (single phase) 66kV/ 33 kV/11 kV
Initial testing of an installation
• Electrical continuity and conductivity tests of protective, equipotential and earth-
bonding conductors.
• Insulation resistance tests between live conductors and the protective conductors
connected to the earthing arrangement.
• Test of compliance of SELV (Safety Extra Low Voltage) and PELV (Protection by Extra
Low Voltage) circuits or for electrical separation.
• Insulation resistance/impedance of floors and walls.
• Protection by automatic disconnection of the supply.
Periodic check-testing of an installation
• Verification of RCD (Residual Current Device) effectiveness and adjustments
• Appropriate measurements for providing safety of persons against effects of
electric shock and protection against damage to property against fire and heat.
• Confirmation that the installation is not damaged.
• Identification of installation defects.
BAHRAINI REGULATIONS
LOW VOLTAGE SUPPLY VOLTAGE SUPPLY AT 11 KV
All electrical switchgear and accessories shall For electricity supply at 11,000 Volts, the
be so designed and manufactured to operate characteristics of the electricity supply system will
continuously in the electricity supply system of be as follows
Bahrain having the following characteristics.
• Nominal voltage:400/230 volt ± 6%,three-phase,four-wire. • Nominal voltage:11,000 volt ± 6%,three-
• Frequency: 50 Hz. ± 2%. phase,three-wire.
• Neutral arrangement: Solidly earthed. • Frequency: 50 Hz ± 2%.
• Installation earthing: “TT” system as per IEC 60364-3. • Neutral arrangement: The system neutral is
• Maximum prospective fault level:50 kA (31 MVA) at 400 V connected to earth through a low resistance at the
and short time duration of 0.5 s (Maximum). sending end station.
• Maximum prospective fault level: 20 kA (350 MVA)
at 11 kV for a short time duration of 1 s (Maximum)
General rules of
electrical
installation design
General rules of electrical installation design
Induction motors
➢ Rated Current demand
▪ The rated current 𝐼𝑎 supplied to the motor is given by the following
formulae:
𝑃𝑛 ×1000
▪ 3-phase motor: 𝐼𝑎 =
3×𝑈 × 𝜂 × cos 𝜑
𝑃𝑛 ×1000
▪ 1-phase motor: 𝐼𝑎 = )
𝑈 × 𝜂 × cos 𝜑
Where:
▪ 𝑰𝒂 : rated current (in amps)
▪ 𝑷𝒏: nominal power (in kW)
▪ 𝑼: voltage between phases for 3-phase motors and voltage
between the terminals for single-phase motors (in volts).
▪ 𝜼:per-unit efficiency, i.e. output kW / input kW
▪ 𝒄𝒐𝒔 𝝋:power factor, i.e. kW input / kVA input
Motor starting current
Starting currents of high-efficiency motors are approximately 7.5 × 𝑰𝒏 , but using a star-delta starter
reduces them to around 4 × 𝑰𝒏 , offering a practical way to minimize electrical network stress and
safeguard the motor.
𝐼𝑛 is the current when the motor run normally
Power-factor improvement of power-factor-correction.
Using capacitors can give advantages for technical and financial; it will reduce the
current supplied to induction motors without affecting the power output of the motors
𝑘𝑊
cos 𝜑 = ;
𝑘𝑉𝐴
cos 𝜑
𝐼 = 𝐼𝑎 ×
cos 𝜑 ’
Where:
cos φ = power factor before compensation
cos φ’ = power factor after compensation. 𝐼𝑎 is the original current.
Resistive-type heating appliances and incandescent lamps (conventional or halogen)
The current demand of a heating appliance or an incandescent lamp is easily obtained
from the nominal power 𝑷𝒏 quoted by the manufacturer (i.e. cos φ =1)
Fluorescent lamps and related equipment
The power 𝑃𝑛 (watts) indicated on the tube of a fluorescent lamp doesn’t include the power dissipated
in the ballast.
𝑃 +𝑃
The current is given by 𝐼𝑎 = 𝑏𝑎𝑙𝑙𝑎𝑠𝑡 𝑁
𝑈 cos 𝜑
Where:
U= voltage applied to the lamp.
If no power-loss value is indicated for the ballast, a figure of 25% of 𝑃𝑛 may be used.
cos φ = 0.6 with no-power factor correction capacitor
cos φ = 0.86 with power factor correction capacitor (single or twin tubes)
cos φ = 0.96 for electronic ballast.
Discharge lamps
These lamps depends on the luminous electrical discharge through a gas or vapor of a metallic
compound.
These lamps have long start-up time, during which the current 𝑰𝒂 is larger than the nominal current 𝑰𝒏.
LED lamps &fixtures
A lamp or luminaire with LED technology is powered by a driver.
This technology has a very short start-up time.
On the other hand, the inrush current at the powering is generally much higher than for fluorescent
lamp with electronic ballast.
Installed power (kW)
Most electrical appliances and equipment are marked to indicate their nominal power rating 𝑷𝒏.
Installed apparent power (kVA)
η = the per-unit efficiency = Output in kW/Input in kW
cos φ = the power factor = kW/kVA.
𝑃𝑛
𝑃𝑎 =
𝜂 𝑐𝑜𝑠 𝜑
1000 × 𝑃𝑎
𝐼𝑎 = 𝑓𝑜𝑟 𝒔𝒊𝒏𝒈𝒍𝒆 − 𝒑𝒉𝒂𝒔𝒆 − 𝑡𝑜 − 𝑛𝑒𝑢𝑡𝑟𝑎𝑙 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑙𝑜𝑎𝑑
𝑉
1000 × 𝑃𝑎
𝐼𝑎 = (𝑓𝑜𝑟 𝟑 − 𝒑𝒉𝒂𝒔𝒆 𝑏𝑎𝑙𝑎𝑛𝑐𝑒𝑑 𝑙𝑜𝑎𝑑)
3 ×𝑈
The total kVA of apparent power is not the arithmetic sum of the calculated kVA
ratings of individual loads (unless all loads are at the same power factor).
Factors ku and ks allow the determination of the maximum power and apparent
power demands actually required to dimension the installation.
Factor of maximum utilization (ku)
In normal operating conditions the power consumption of a load is sometimes less than that
indicated as its normal power rating
This factor must be applied to each individual load, with particular attention to electric motors,
which are very rarely operated at full-load.
Diversity Factor (ks)
The ratio, expressed as a numerical value or as a percentage of the simultaneous maximum demand of a
group of electrical appliances or consumers within a specified period, to the sum of their individual maximum
demands within the same period. As per this definition, the value is always ≤ 1 and can expressed as
percentage.
The factor ks is applied to each group of loads.
The diversity factors are given for domestic consumers without electrical Heating and supplied at
230/400V(3-phase 4-wires).
𝑃𝑎 × 103
Example of a 5-storey apartment block 𝐼𝑎 =
𝑈 3
5 - storeys apartment building with 25 consumers, each having 𝑃𝑎 = kVA rating of the transformer
6k VA of installed load. The total installed load for the building 𝑈 = Line voltage at no-load in volts (400V)
is:
36 + 24 + 30 + 36 + 24 = 150𝑘𝑉𝐴
150 × 𝑘𝑠 = 150 × 0.46 = 69𝑘 𝑉𝐴
The rising main at ground level is:
150 × 0.46 × 103
= 100𝐴
400 × 3
The current entering the third floor is:
Take from 3rd and above it
3rd + 4th = 4+6= 10 consumers
(36 + 24) × 0. 63 × 103
= 55𝐴
400 × 3
Rated Diversity Factor for distribution switchboards
Diversity factor according to circuit function
• ks factor may be used for the circuits supplying
commonly
• occurring loads as given in the table.
LOW VOLTAGE
DISTRIBUTION
EARTHING CONNECTIONS
Earth electrode (1): A conductor or group of conductors in intimate contact
with, and providing an electrical connection with Earth
Earth: The conductive mass of the Earth, whose electric potential at any point is
conventionally taken as zero
Electrically independent earth electrodes: Earth electrodes located at such a
distance from one another that the maximum current likely to flow through one of
them does not significantly affect the potential of the other(s)
Earth electrode resistance: The contact resistance of an earth electrode with
the Earth.
Earthing conductor (2): A protective conductor connecting the main earthing
terminal (6) of an installation to an earth electrode (1) or to other means of
earthing (e.g. TN systems);
Exposed-conductive-part: A conductive part of equipment which can be
touched, and which is not a live part, but which may become live under fault
conditions.
Protective conductor (3): A conductor used for some measures of protection against
electric shock and intended for connecting together any of the following parts:
Exposed-conductive-parts
Extraneous-conductive-parts
The main earthing terminal
Earth electrode(s)
The earthed point of the source or an artificial neutral.
Extraneous-conductive-part: A conductive part liable to introduce a potential,
generally earth potential, and not forming part of the electrical installation (4)
Bonding conductor (5): A protective conductor providing equipotential bonding
Main earthing terminal (6): The terminal or bar provided for the connection of
protective conductors, including equipotential bonding conductors, and conductors
for functional earthing, if any, to the means of earthing.
Low-voltage power distribution systems
1. TT
2. TN
• S
• C
• CS
3. IT
1. TT system (earthed neutral)
TN systems
(exposed conductive parts connected to the neutral)
• TN-C system
PEN (Protective Earth and Neutral)
This system is not permitted for
conductors of less than 𝟏𝟎 𝒎𝒎𝟐 or
for portable equipment.
PEN conductor is both the neutral conductor
and carries phase unbalance currents as
well as 3rd order harmonic currents.
The PEN conductor must therefore be
connected to a number of earth electrodes
in the installation.
• TN-S system
(5 wires)
is obligatory for circuits with cross-sectional areas less than
𝟏𝟎 𝒎𝒎𝟐 for portable equipment.
TN-C-S system
Conclusion:
• A TN-C system should not be used after a TN-S system.
• PEN should not be connected directly to the N terminal.
• The PEN wire must be ≥ 10 mm² when used in a TN-C
system.
• Always ensure that the N and PE terminals are properly
separated when transitioning from TN-C to TN-S.
3. IT system
(isolated or impedance-earthed neutral)
(isolated neutral)
No intentional connection is made between
the neutral point of the supply source and
earth. Exposed- and extraneous-conductive-
parts of the installation are connected to an
earth electrode.
THE MEANING OF THE LETTERS I, T, N, C, S
First Letter: Describes the power system's relationship to ground:
T: Directly grounded neutral point (from French "Terre" – earth).
I: Isolated from ground or connected through high impedance (from French "Isolé" – isolated).
Second Letter: Indicates the ground connection of exposed conductive parts (like equipment
enclosures):
T: Exposed parts are directly grounded (independent of the power system’s grounding).
N: Exposed parts are connected to the system's neutral point (zero potential), providing protection.
Third Letter: Specifies the combination of neutral and protective conductors :
C: Neutral (N) and Protective Earth (PE) functions are combined in a single conductor (common in
TN-C systems).
S: Neutral and PE are separate , with a dedicated protective conductor (as in TN-S systems).
This classification helps standardize electrical safety and design across different systems globally.
The quality of an earth electrode:
• Installation method
• Type of soil.
Installation methods:
• Buried ring
• Earthing rods
• Vertical plates
BURIED RING
buried around the outside wall of the premises to a depth of at least 1 meter.
General rule: all vertical connections (at least 4) from an electrode to above-ground
level should be insulated for the nominal LV voltage (600-1000 V).
The conductors may be:
• Copper: Bare cable (>= 25 mm2)or multiple-strip (>= 25 mm2
and >= 2 mm thick)
• Aluminium with lead jacket: Cable (>= 35 mm2)
• Galvanized-steel cable: Bare cable (>= 95 mm2) or multiple-
strip (>= 100 mm2 and >= 3 mm thick).
The approximate resistance R of the electrode in ohms:
𝟐𝝆
𝑹 =
𝑳
L =length of the buried conductor in meters
ρ =soil resistivity in ohm-meters
EARTHING RODS
Copper or (more commonly) copper-clad steel:
• The latter = 1 or 2m
• provided with screwed ends
• sockets in order to reach considerable depths
Galvanized steel pipe
• >= 25 mm diameter
• rod >= 15 mm diameter
• >= 2 meters long in each case.
𝟏𝝆
𝑹 = If the distance separating the rods> 4 L
𝒏𝑳
n = the number of rods
VERTICAL PLATES
Rectangular plates each side of which must be >=0.5 meter
buried at least 1m below the surface of the soil.
The plates may be :
• Copper of 2 mm thickness
• Galvanized steel of 3 mm thickness
The resistance R in ohms is given (approximately):
𝟎. 𝟖𝝆
𝑹=
𝑳
The resistance of the electrode/earth interface rarely remains
constant Among the principal factors affecting this resistance are
the following:
• Humidity of the soil
• Frozen earth
• Ageing
• Oxidation
Measure the resistance of earth electrode:
• Ammeter Method
• Use of a direct-reading earthing-resistance ohmmeter
Ammeter Method
Direct-reading earthing-resistance ohmmeter
Test Setup Overview:
• Equipment : Two temporary test electrodes/spikes (T1 and T2) connected to an earth tester.
• Connections :
C1 : Earth electrode under test (Ra).
C2 : Connected to T1 via a long lead (~30–50 m from Ra).
P2 : Connected to T2 , placed midway between Ra and T1.
Key Guidelines:
• Distance between Ra and T1 should ideally be 10× the length of the earth electrode .
• T2 is moved during testing to verify consistency:
• For a C1–T1 distance of 30 m , move T2 by ±10% (i.e., ±3 m) for second and third readings.
Example Readings (Good Soil/Clay):
• T2 central (at 15 m) : 72 Ω
• T2 moved 3 m closer to Ra : 70.5 Ω
• T2 moved 3 m closer to T1 : 73.5 Ω
The three readings obtained should fall within a tolerance of ±5% of the average
Direct-reading earthing-resistance ohmmeter
INSTALLATION SYSTEM
• Distribution Switchboards
• Cables and busways
Distribution switchboard: Types of distribution switchboards are:
is the point at which an incoming-power supply divides • The main LV switchboard- MLVS
into separate circuits, each of which is controlled and • Motor control centers- MCC
protected by the fuses or switchgear of the switchboard. • Sub-distribution switchboards
• Final distribution switchboards
Enclosure Protection Functions :
• Equipment Protection:
• Shields internal components (switchgear, instruments, relays, fuses, etc.) from:
• Mechanical damage
• Vibrations
• Dust, moisture
• Electromagnetic interference (EMI)
• Vermin or other environmental hazards
• Human Safety:
• Prevents electric shock (both direct and indirect contact) by enclosing live parts and ensuring
safe access conditions.
FUNCTIONAL DISTRIBUTION BOARDS
Fixed functional units:
These units cannot be isolated from the supply so that any intervention for
maintenance, modifications and so on, requires the shutdown of the entire
distribution switchboard. Plug-in or withdrawable devices can however be used to
minimise shutdown times and improve the availability of the rest of the installation.
Disconnectable functional units:
Each functional unit is mounted on a removable mounting plate and provided with a
means of isolation on the upstream side (busbars) and disconnecting facilities on
the downstream (outgoing circuit) side. The complete unit can therefore be removed
for servicing, without requiring a general shutdown.
Drawer-type withdrawable functional units:
The switchgear and associated accessories for a complete function are mounted on a
drawer-type horizontally withdrawable chassis. The function is generally complex
and often concerns motor control.
SWITCHBOARD - FORMS OF INTERNAL SEPARATION
A-Incoming Device
• Receives power from the supply source
• Controls and protects the entire distribution board.
B- Main Busbar
• Distributes electricity from the incoming device to the other parts of the panel.
• Made of copper or aluminum for efficient conductivity.
C-Distribution Busbar
• Sub-busbars that take power from the main busbar and distribute it to outgoing
circuits.
D-Outgoing Device
• Circuit breakers, switches, or fuses connected to the distribution busbar.
• Protect individual circuits from overloads and faults.
E-Terminals for External Conductors
• Connection points for external wiring that supply power to different loads such as
lighting, appliances, or machinery.
F- Enclosure (IP2X Minimum)
• The protective casing that houses all the components.
• IP2X rating ensures safety by preventing accidental contact with live parts while
allowing ventilation.
CABLES AND BUSWAYS
Conductor
A conductor comprises a single metallic core with or without an insulating envelope.
Cable
A cable is made up of several conductors, electrically separated, but joined
mechanically, generally enclosed in a protective flexible sheath.
Cableway
The term cableway refers to conductors and/or cables together with the means of
support and protection, etc. for example :cable trays, ladders, ducts, trenches, and so
on… are all “cableways”.
Conductor marking
Rule 1
The double colour green and yellow is strictly reserved for the PE and PEN protection conductors.
Rule 2
• When a circuit comprises a neutral conductor, it must be light blue or marked “1” for cables
with more than five conductors.
• When a circuit does not have a neutral conductor, the light blue conductor may be used as a
phase conductor if it is part of a cable with more than one conductor
Rule 3
Phase conductors may be any colour except:
• Green and yellow
• Green
• Yellow
• Light blue (see rule 2).
Conductors in a cable are identified either by their colour or by numbers.
PROTECTION
AGAINST ELECTRIC
SHOCK AND
ELECTRICAL FIRES
Dangerous contact
Protective measures:
1- Basic protection
• Protection by the insulation of live parts.
• Protection by means of barriers or enclosures.
• Protection by means of obstacles, or by placing out of arm’s reach.
• Protection by use of Extra-Low Voltage (ELV) or by limitation of the energy of
discharge.
2- FAULT PROTECTION
• The earthing of all exposed-conductive-parts of electrical equipment in the
installation and the constitution of an equipotential bonding network.
• Automatic disconnection of the supply of the section of the installation
concerned, in such a way that the touch voltage/time safety requirements
are respected for any level of touch voltage Uc.
The higher the value of Uc, the higher the rapidity of supply disconnection is
required to provide protection. The highest value of Uc that can be tolerated
indefinitely without danger to human beings is 50 V AC. In DC the highest value of Uc
that can be tolerated indefinitely without danger is 120 V.
HIGH SENSITIVITY RCDS
which operates at 30 mA or less,
All of the above preventive measures cannot be infallible because:
• Lack of proper maintenance
• Imprudence, carelessness
• Normal (or abnormal) wear and tear of insulation; for instance flexure and abrasion of
connecting leads
• Accidental contact
• Immersion in water, etc. A situation in which insulation is no longer effective.
According to IEC 60364-4-41, additional protection by means of high sensitivity RCDs
(IΔn ≤ 30 mA) must be provided for:
• circuits supplying general use socket-outlets with a rated current ≤ 32 A liable to be used by
ordinary persons,
• for circuits supplying mobile equipment with a rated current ≤ 32 A for use outdoors,
• AC final circuits supplying luminaires.
Principle:
• In this system, all exposed-conductive-parts and extraneous-conductive-parts of the installation must
be connected to a common earth electrode.
• The impedance of the earth fault loop:
• Source
• installation electrodes
• Protection by automatic disconnection of the supply used in TT system is by RCD of sensitivity:
50
• 𝐼∆𝑛 ≤
𝑅𝐴
• 𝐼∆𝑛 is the rated residual operating current of the RCD
• 𝑅𝐴 is the resistance of the earth electrode for the installation (maximum value over time)
• For temporary supplies (to work sites, …) and agricultural and horticultural premises, the value of 50
V is replaced by 25 V.
RCCB (Residual Current Circuit Breaker)
Selectivity is the selection of RCD devices so that the device nearest to a fault will
operate rather than any upstream device. The purpose is to ensure that the fault is
isolated and supply is maintained to other parts of the installation without
disruption.
SIZING AND
PROTECTION OF
CONDUCTORS
Conditions for cables to ensure a safe and reliable installation:
• It should carry the permanent full load current, and normal short-time over-currents.
• It should not cause voltage drops because it results in poor performance of certain loads.
𝑰𝒁 is the maximum permissible that the cabling for the circuit can carry indefinitely,
without reducing its normal life expectancy.
The current depends, for a given cross sectional area of conductors, on several parameters:
• Constitution of the cable and cable-way (Cu or Al conductors; PVC or EPR etc. insulation; number
of active conductors)
• Ambient temperature
• Method of installation
• Influence of neighboring circuits.
Overcurrent
Overcurrent occurs when current exceeds the maximum load current (𝑰𝑩 ).
Types of over-currents:
1. Overloads:
• Occur in healthy circuits due to short-duration loads (e.g., motor starting).
• If sustained, protective devices automatically cut off the circuit.
2. Short-Circuit Currents:
• Caused by insulation failure between live conductors or to earth.
• Can occur in various forms:
• Three-phase short-circuit (with/without neutral/earth).
• Two-phase short-circuit (with/without neutral/earth).
• Single-phase short-circuit (to neutral or earth).
I²t:
• I²t is a measure of the thermal energy (heat) that a cable can withstand during a
short-circuit or overcurrent event.
• The I²t rating defines a curve between current and time to blow a fuse or trip a
circuit breaker.
• It helps determine the minimum cable size required to prevent the cable from
overheating and potentially catching fire.
• 𝐼²𝑡 = 0.495
Correction factors:
𝑰𝒁
𝑰′𝒁 =
𝑪𝒂 . 𝑪𝒃 …
𝑪𝒂 : correction factor for ambient temperature
𝑪𝒃 : correction factor for the depth of burial of a buried cable or duct
𝑪𝒄 : overload correction factor for buried cables or cables in buried ducts
𝑪𝒅 : correction factor for type of overcurrent protective device
• 𝐶𝑑 = 1 for HBC fuses and mcbs
• 𝐶𝑑 = 0.725 for semi-enclosed fuses
Note: Cc and Cd are combined in BS 7671:2008 as Cc but they are, in fact, two separate factors
𝑪𝒈 : correction factor for grouping
𝑪𝒊 :correction factor for conductors embedded in thermal insulation
𝑪𝒓 :correction factor for grouping of ring circuits
𝑪𝒔 :correction factor for the thermal resistivity of the soil surrounding a buried cable or duct
DETERMINATION OF
VOLTAGE DROP
Maximum voltage drop limit
In general, satisfactory motor performance requires a voltage within ±5% of its rated
nominal value in steady state operation.
Starting current of a motor can be 5 to 7 times its full load value(or even higher). If an 8% voltage drop occurs
at full-load current, then a drop of 40% or more will occur during start-up.
In such conditions the motor will either:
• Stall (i.e. remain stationary due to insufficient torque to overcome the load torque) with consequent over-
heating and eventual trip-out.
OR
• Accelerate very slowly, so that the heavy current loading (with possibly undesirable low voltage effects on
other equipment) will continue beyond the normal start-up period.
Calculation of voltage drop in steady load conditions
φ: phase angle between voltage and current in
the circuit considered, generally:
• Incandescent lighting: cos φ = 1
• Led lighting: cos φ > 0.9
• Fluorescent with electronic ballast: cos φ > 0.9
Motor power:
• At start-up: cos φ = 0.35
• In normal service: cos φ = 0.8
𝑈𝑛 : phase-to-phase voltage
𝑉𝑛 : phase-to-neutral voltage
𝑽𝒅𝒓𝒐𝒑 𝑽𝒐𝒍𝒕𝒂𝒈𝒆 = 𝑲 × 𝑰𝑩 × 𝑳
SHORT-CIRCUIT
CURRENT
A knowledge of 3-phase symmetrical short-circuit current values (Isc) at strategic
points of an installation is necessary in order to determine switchgear (breaking
capacity), cables (thermal withstand rating), protective devices (selective trip settings) and
so on...
3-phase short-circuit of zero impedance (the so-called bolted short-circuit) fed through
a typical MV/LV distribution transformer will be examined.
𝐈𝐧 × 𝟏𝟎𝟎 𝐒 × 𝟏𝟎3
𝐈𝐒𝐂 = ; 𝐈𝐧 =
𝐔𝐬𝐜 𝑼𝟐𝟎 × 3
Short-circuit current at the secondary terminals
𝐈𝐧 × 𝟏𝟎𝟎 𝐒 × 𝟏𝟎3
𝐈𝐒𝐂 = ; 𝐈𝐧 =
𝐔𝐬𝐜 𝑼𝟐𝟎 × 3
𝐒 =kVA rating of the transformer
𝑼𝟐𝟎 = phase-to-phase secondary no-load voltage.
𝐈𝐧 = rated current of the transformer.
𝐈𝐒𝐂 = short-circuit current in amps.
𝐔𝐬𝐜 = short-circuit impedance voltage of the transformer in %.
For Bahrain: Oli- immersed :: Can work for 1MVA
Other: Cast-resin
Problem-1
The phase-phase no-load voltage of a 400 kVA MV/LV three phase distribution transformer is
420V. Estimate the short circuit current that would flow at its secondary terminals in the event of
a 3-phase solid fault.
The case of several transformers
in parallel feeding a busbar
• You can estimate the fault current on a circuit right after
busbars by adding the calculated short-circuit currents 𝑰𝒔𝒄 of
each connected transformer.
• This assumes all transformers are supplied by the same MV
network.
• The summed fault current will be slightly higher than the
actual value.
• This overestimation doesn't consider the impedance of
busbars and cables.
Problem-2
Estimate the value of fault current on an outgoing circuit immediately downstream of
the busbars as shown in the figure. Neglect the impedance of the busbars and of the
cable between transformers and circuit breakers. Assume that the transformers has
420V phase-phase no-load voltage.
In a 3-phase installation 𝑰𝒔𝒄 at any point is given by:
𝐔𝟐𝟎
𝐈𝐬𝐜 =
3 × 𝐙𝐓
𝐙𝐓 = total impedance per phase of the installation upstream of the fault location (in Ω).
𝐙𝐓 = 𝑅𝑇2 + 𝑋𝑇2
Network upstream of the MV/LV transformer
𝑼𝟐𝟐𝟎
𝒁𝒂 =
𝑷𝒔𝒄
𝒁𝒂 =impedance of the MV voltage network, expressed in milli-ohms
𝑷𝒔𝒄 = MV3-phase short-circuit fault level, expressed in kVA
