Thursday, December 17, 2020

What is the Advantage of IDMT in Protective Relaying?


In protective relaying there are two philosophies available to effectively achieve selectivity and coordination by time grading two philosophies are available, namely: 

  1. Definite time lag (DTL), or 
  2. Inverse definite minimum time (IDMT). 

Traditionally, design engineers have regarded medium- and low-voltage networks to be of lower importance from a protection view, requiring only the so-called simpler type of IDMT overcurrent and earth fault relays on every circuit. In many instances, current transformer ratios were chosen mainly based on load requirements, whilst relay settings were invariably left to the commissioning engineer to determine. Most of the times, the relay settings had been chosen considering the downstream load being protected without an effort to coordinate with the upstream relays. 

Read: Basic Guide in Power System Protection

However, experience has shown that there has been a total lack of appreciation of the fundamentals applicable to these devices. Numerous incidents have been reported where breakers have tripped in an uncoordinated manner leading to extensive network disruption causing longer down times or failed to trip causing excessive damage, extended restoration time and in some cases loss of life. 

In Definite Time Lag or DTL, the relays are graded using a definite time interval of approximately 0.5 s. The relay R3 at the extremity of the network is set to operate in the fastest possible time, whilst its upstream relay R2 is set 0.5 s higher. Relay operating times increase sequentially at 0.5 s intervals on each section moving back towards the source as shown below,

Definite Time Lag or DTL

The problem with this philosophy is, the closer the fault to the source the higher the fault current, the slower the clearing time – exactly the opposite to what we should be trying to achieve. 

Read: IEC 61850 Logical Nodes and Data Classes in Power System Automation Data Modelling

On the other hand, inverse curves as shown in the figure below which describes operating faster at higher fault currents and slower at the lower fault currents, thereby offering us the features that we desire. This explains why the IDMT philosophy has become standard practice throughout many countries over the years

Although not appreciated by many engineers, the widespread use of inverse definite minimum time overcurrent and earth fault (IDMT OCEF) relays as the virtual sole protection on medium- and low-voltage networks requires as much detailed study and applications knowledge as does the more sophisticated protection systems used on higher voltage networks.  

Applying IDMT in the System

When deciding to apply IDMT relays to a network, a number of important points have to be considered. Firstly, it must be appreciated that IDMT relays cannot be considered in isolation. They have to be set to coordinate with both upstream and downstream relays. 

Their very purpose and being is to form part of an integrated whole system. Therefore, whoever specifies this type of relay should also provide the settings and coordination curves as part of the design package to show that he knew what he was doing when selecting their use. This very important task should not be left to others and once set, the settings must not be tampered (even by the operating staff ) as otherwise coordination is lost.

Minimum Grading Intervals

To engineers planning the protection for a medium- to low-voltage network and wishing to adopt the widespread use of the IDMT OCEF relay, the above can be summarized as follows: 

  • Design networks with a minimum number of grading levels possible. 
  • Choose CT ratios based on fault current – not load current. 
  • Consider using 1 A secondary. 
  • Check CT magnetization curves for knee-point voltage and internal resistance. 
  • Connect ammeters, etc. onto own metering cores. 
  • Provide relay settings and coordination curves as part of the design package. 
  • Be careful when choosing relay plug tap setting on electromechanical relays. The lower the tap, the higher the burden. 
  • Relays should not pick-up for healthy conditions such as permissible transient overloads, starting surges and reconnection of loads, which have remained connected after a prolonged outage. 
  • Care should also be taken that the redistribution of load current after tripping does not cause relays on healthy circuits to pick-up and trip. 
  • HV IDMT relays on transformers should trip both HV and LV breakers. 
  • Normal inverse curves should not be selected for overload protection. Rather use the inverse characteristic for this duty. 
  • Take advantage of the additional features offered by the modern electronic relays, e.g. fixed very low burden, integral high-set, breaker fail and busbar blocking protections, event memory, etc. However, remember, one has to do the same calculation exercises for settings and draw coordination curves whether the relays are of the electronic or electromechanical design. 
  • Finally, if the switchgear suppliers also manufacture relays, do not expect them to do the protection application settings free of charge as part of the service. If this is required, specify this as a separate cost item in the specification. 

Many problems down the line can be avoided and the performance, efficiency and safety of the plant improved if a protection engineer is included in the design team, if not full time, but at least to do an audit on the proposals. Finally, remember – while  IDMT relays are the most well known and the cheapest, they are in fact the most difficult relays to set. 


Practical Power System Protection | Download

  • Les Hewitson
  • Mark Brown PrEng, DipEE, BSc (ElecEng), Senior Staff Engineer, IDC Technologies, Perth, Australia  
  • Ben Ramesh Ramesh and Associates, Perth, Australia
Series editor: 
  • Steve Mackay FIE(Aust), CPEng, BSc (ElecEng), BSc (Hons), MBA, Gov. Cert. Comp., Technical Director – IDC Technologies.

Basic Guide for Power System Protection


A power system is not only capable to meet the present load but also has the flexibility to meet future demands. A power system is designed to generate electric power in sufficient quantity, to meet the present and estimated future demands of the users in a particular area, to transmit it to the areas where it will be used and then distribute it within that area, on a continuous basis. 

"To ensure the maximum return on the large investment in the equipment, which goes to make up the power system and to keep the users satisfied with reliable service, the whole system must be kept in operation continuously without major breakdowns". 

Read: What is the Importance of X/R Ratio?

Basic Requirements of Power System Protection

A protection apparatus has three main functions/duties: 

  1. Safeguard the entire system to maintain continuity of supply 
  2. Minimize damage and repair costs where it senses fault 
  3. Ensure safety of personnel. 

These requirements are necessary, firstly for early detection and localization of faults, and secondly for the prompt removal of faulty equipment from service. In order to carry out the above duties, protection must have the following qualities: 

  • Selectivity: To detect and isolate the faulty item only. 
  • Stability: To leave all healthy circuits intact to ensure continuity or supply. 
  • Sensitivity: To detect even the smallest fault, current or system abnormalities and operate correctly at its setting before the fault causes irreparable damage. 
  • Speed: To operate speedily when it is called upon to do so, thereby minimizing damage to the surroundings and ensuring safety to personnel. 

To meet all of the above requirements, protection must be reliable which means it must be: 

  • Dependable: It must trip when called upon to do so. 
  • Secure: It must not trip when it is not supposed to. 

Important Points to Consider in Protection System

  • Protection of any distribution system is a function of many elements and this manual gives a brief outline of various components that go into protecting a system. Following are the main components of protection. 
  • Fuse is the self-destructing one, which carries the currents in a power circuit continuously and sacrifices itself by blowing under abnormal conditions. These are normally independent or stand-alone protective components in an electrical system unlike a circuit breaker, which necessarily requires the support of external components.
  •  Accurate protection cannot be achieved without properly measuring the normal and abnormal conditions of a system. In electrical systems, voltage and current measurements give feedback on whether a system is healthy or not. Voltage transformers and current transformers measure these basic parameters and are capable of providing accurate measurement during fault conditions without failure.
  • The measured values are converted into analog and/or digital signals and are made to operate the relays, which in turn isolate the circuits by opening the faulty circuits. In most of the cases, the relays provide two functions viz., alarm and trip, once the abnormality is noticed. The relays in the olden days had very limited functions and were quite bulky. However, with the advancement in digital technology and the use of microprocessors, relays monitor various parameters, which give a complete history of a system during both pre-fault and post-fault conditions.  
  • The opening of faulty circuits requires some time, which may be in milliseconds, which for a common day life could be insignificant. However, the circuit breakers, which are used to isolate the faulty circuits, are capable of carrying these fault currents until the fault currents are totally cleared. The circuit breakers are the main isolating devices in a distribution system, which can be said to directly protect the system. 

The operation of relays and breakers require power sources, which shall not be affected by faults in the main distribution. Hence, the other component, which is vital in the protective system, is batteries that are used to ensure uninterrupted power to relays and breaker coils.   

Basic Components of Power System Protection

Read: What are the Conditions in Selecting Current Transformer in Protective Relaying?

  1. Voltage transformers and current transformers: To monitor and give accurate feedback about the healthiness of a system. 
  2. Relays: To convert the signals from the monitoring devices, and give instructions to open a circuit under faulty conditions or to give alarms when the equipment being protected, is approaching towards possible destruction. 
  3. Fuses: Self-destructing to save the downstream equipment being protected. 
  4. Circuit breakers: These are used to make circuits carrying enormous currents, and also to break the circuit carrying the fault currents for a few cycles based on feedback from the relays. 
  5. DC batteries: These give uninterrupted power source to the relays and breakers that are independent of the main power source is protected. 


Practical Power System Protection | Chapter 1: Need for Protection | Download

  • Les Hewitson
  • Mark Brown PrEng, DipEE, BSc (ElecEng), Senior Staff Engineer, IDC Technologies, Perth, Australia  
  • Ben Ramesh Ramesh and Associates, Perth, Australia
Series editor: 
  • Steve Mackay FIE(Aust), CPEng, BSc (ElecEng), BSc (Hons), MBA, Gov. Cert. Comp., Technical Director – IDC Technologies.

Saturday, December 05, 2020

What are the Basic Characteristics of an Ideal Transformer?

Substation Transformer


A transformer is an electrical piece of equipment that transforms the AC electrical power from one circuit to another. It makes use of the magnetically coupled coils to transfer energy. 

Basically, it consists of a primary winding and secondary winding. The primary winding and its circuit are called the Primary Side of the transformer. The secondary winding and its circuit are called the Secondary Side of the transformer. Both the primary and the secondary winding of a transformer are electrically isolated from each other, but they are linked through the magnetic field. 


Therefore, the primary and the secondary windings are magnetically coupled to each other. If the primary is connected to an alternating voltage source, an alternating flux is produced. The mutual flux will link the other winding (the secondary) to the primary and will induce a voltage in it. If the secondary winding is open (not connected to a load), the current in the primary winding is determined by its inductive.

The Ideal Transformer

An ideal transformer is a perfect transformer in which there is no power loss. In an ideal transformer: 

  • The windings are purely inductive with no resistance. Therefore, there are no copper losses in the windings.
  • The iron core would not heat up during operation. Therefore, there are no losses in the iron core.
  • The magnetizing current is zero, the current flow in the primary winding is zero when the secondary winding is open circuit.

Ideal Transformer

Transformer Turns Ratio in an Ideal Transformer 

  • The magnetic flux is the same through the primary and the secondary windings.
  • Therefore, the induced voltage per turn is the same in both the primary and the secondary. 
  • That means, Ep and Es are proportional to NP and NS respectively. 

The Basic formula of an Ideal Transformer 

From the given formula we can say the following: 

  • If VP > VS the voltage is stepped down from a higher voltage to a lower voltage the transformer is then called a step-down transformer. 
  • If VP < VS, the voltage is stepped up from a lower voltage to a higher voltage, and the transformer is then called a step-up transformer. 
  • By interchanging the connections of the primary and the secondary windings, a step-down transformer can be made a step-up transformer. 


In the figure shown below, determine the following: 

a. the secondary voltage

b. the secondary current 

c. the primary current

d. the power in the load


a. From the given formula, we can say Vp/Vs = 3/1. Thus Vs = 20. 

b. By ohms law, I = Vs/ R = 20/200; Is = 100 mA

c. From the formula, Vp/Vs = Is/Ip; Ip = Vp/Vs x Is = 1/3 x 100 mA; Is = 33.3 mA

d. Power flow in the load = Vs x Is = 20 x 100 mA; Pload = 2 Watts. 

Sunday, November 29, 2020

Voltage Drop in Consumer Installations According to BS 7671


Voltage drop in a consumer’s installation can be a contentious issue. However, it is an important aspect of installation design since if it is too high some certain equipment will not function correctly or will not function at all.

Rules Applied: 

  • 525.1 In the absence of any other consideration, under normal service conditions the voltage at the terminals of any fixed current-using equipment shall be greater than the lower limit corresponding in the product standard relevant to the equipment.
  • 525.100 Where fixed current-using equipment is not the subject of a product standard the voltage at the terminals shall be such as not to impair the safe functioning of that equipment.
  • 525.101 The above requirements are deemed to be satisfied if the voltage drop between the origin of the installation (usually the supply terminals) and a socket-outlet or the terminals of fixed current-using equipment, does not exceed that stated in Appendix 4 Section 6.4

Calculating Voltage Drop 

When calculating voltage drop due consideration should be given to the following: 
  • motor starting currents; in-rush currents
  • control voltages (particularly those associated with computerized systems).

  1. Motor control contactors and relays can ‘drop out’ if the coil voltages fall towards 80% of the operating voltage.
  2. The effects of harmonic currents may also need to be considered and included in the calculation.
  3. Voltage transients and voltage variations due to an abnormal operation can be ignored.

The maximum voltage drop values can be taken from the table shown below:

For 230 Volts Network

Allowable Voltage Drop Based on BS 7671

*The voltage drop within each final circuit on Private Networks, should not exceed the values given in (i) above for Public Networks

For 400 Volts Network

Allowable Voltage Drop Based on BS 7671

*The voltage drop within each final circuit on Private Networks, should not exceed the values given in (i) above for Public Networks

When calculating the voltage drop in a circuit, the design current can be taken as being either the equipment rated current or, where there are a number of loads, the total connected load multiplied by a diversity factor. 

Note: If the total circuit length exceeds 100 meters, the limits given in Table 4Ab may be increased by 0.005% per meter up to a maximum of 0.5%.

The voltage drop can be apportioned throughout the system circuits as the designer wishes, but the final circuit voltage drop is limited to the values given for Public Networks, regardless of whether it is a Public Network or a Private Network.

In case that the supply voltage at the origin is lower than the nominal 230/400V, the designer needs to consider the effect of the minimum permissible supply voltage. This is a maximum of 6% below the nominal supply voltage, which equates to 216.2V (for 230 V network ) and 376V (for 400 V network) respectively.


Electrical Contractors Association Fact Sheet | Download

Saturday, November 28, 2020

How to Interpret the Different Region of Circuit Breaker Curves


Circuit Breaker
by: EATON 

Circuit breaker time-current characteristic curves are read similar to fuse curves. The horizontal axis represents the current, and the vertical axis represents the time at which the breaker interrupts the circuit. When using molded case circuit breakers of this type, there are four basic curve considerations that must be understood. 

These are:

  1. Overload Region
  2. Instantaneous Region

Overload Region

The opening of a molded case circuit breaker in the overload region is generally accomplished by a thermal element, while a magnetic coil is generally used on power breakers. Electronic sensing breakers will utilize CTs. As can be seen, the overload region has a wide tolerance band, which means the breaker should open within that area for a particular overload current.

Instantaneous Region

The instantaneous trip (I.T.) setting indicates the multiple of the full load rating at which the circuit breaker will open as quickly as possible. The instantaneous region is represented in the following curve and is shown to be adjustable from 5x to 10x the breaker rating. When the breaker coil senses an overcurrent in the instantaneous region, it releases the latch which holds the contacts closed.

Interrupting Rating

The interrupting rating of a circuit breaker is a critical factor concerning protection and safety. The interrupting rating of a circuit breaker is the maximum fault current the breaker has been tested to interrupt in accordance with testing laboratory standards. Fault currents in excess of the interrupting rating can result in the destruction of the breaker and equipment and possible injury to personnel. In other words, when the fault level exceeds the circuit breaker interrupting rating, the circuit breaker is no longer a protective device.

Illustration: Medium to High-Level Fault Currents–Circuit Breakers

The following curve illustrates a 400A circuit breaker ahead of a 90A breaker. Any fault above 1500A on the load side of the 90A breaker will open both breakers. 

The 90A breaker will generally unlatch before the 400A breaker. However, before the 90A breaker can separate its contacts and clear the fault current, the 400A breaker has unlatched and also will open. Assume a 4000A short circuit exists on the load side of the 90A circuit breaker. 

Read: What is the advantage of using Thermal Magnetic Circuit Breaker in motor control?

The sequence of events would be as follows: 

  1. The 90A breaker will unlatch (Point A) and free the breaker mechanism to start the actual opening process.
  2. The 400A breaker will unlatch (Point B) and it, too, would begin the opening process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible. 
  3. At Point C, the 90A breaker will have completely interrupted the fault current. 
  4. At Point D, the 400A breaker also will have completely opened the circuit. Consequently, this is a non-selective system, causing a complete blackout to the other loads protected by the 400A breaker.


Cooper Bussman |  Download

How to Convert IEC 60044-1 Standard Protection Classification to IEEE Standard Voltage Rating?


MiCom P63x Protection Relay

There are a series of protection relays such as MiCom protection relays that are compatible with ANSI/IEEE CTs as specified in the IEEE C57.13 standard. The applicable class for protection is class "C", which specifies a non air-gapped core. The CT design is identical to IEC class P but the rating is specified differently. 

The IEEE C class standard voltage rating required will be lower than an IEC knee-point voltage. This is because the IEEE voltage rating is defined in terms of useful output voltage at the terminals of the CT, whereas the IEC knee-point voltage includes the voltage drop across the internal resistance of the CT secondary winding added to the useful output. The IEC knee-point is also typically 5% higher than the IEEE knee-point. 

Read: What are the Conditions in Selecting Current Transformer in Protective Relaying

Where IEEE standards are used to specify CTs, the C class voltage rating can be checked to determine the equivalent knee-point voltage (Vk) according to IEC. 

The equivalence formula is: 

Vk = (C x 1.05) + (Ksc x In x Rct)

Vk = (C x 1.05) + (100 x Rct)

Note: IEEE CTs are always 5A secondary rated, i.e. In =5A, and are defined with an accuracy limit factor of 20, i.e. Kssc =20.

Read: Types and Classes of Current Transformers According to IEC 60441 

The following table allows C57.13 ratings to be converted to a typical IEC knee-point voltage:

  • * Assuming 0.002/turn typical secondary winding resistance for 5A CTs


Friday, November 27, 2020

Types and Classes of Current Transformers According to IEC 60441

Substation Current Transformer

The behavior of inductive CTs in accordance with IEC 60044-1 and IEEE C57.13 is specified for steady-state symmetrical AC currents. The more recent standard IEC 60044-6 is the only standard that specifies the performance of inductive CTs (classes TPX, TPY and TPZ) for currents containing exponentially decaying DC components of the defined time constant. This section summarises the various classes of CTs.

IEC 60044-1

Class P Class P current transformers are typically used for general applications, such as overcurrent protection, where a secondary accuracy limit greatly in excess of the value to cause relay operation serves no useful purpose. Therefore a rated accuracy limit of 5 will usually be adequate. When relays, such as instantaneous ‘high set’ overcurrent relays, are set to operate at high values of overcurrent, say 5 to 15 times the rated current of the transformer. 

Read: Protection Relays in Power System

The accuracy limit factor must be at least as high as the value of the setting current used in order to ensure fast relay operation. 

Rated output burdens higher than 15VA and rated accuracy limit factors higher than 10 are not recommended for general purposes. It is possible, however, to combine a higher rated accuracy limit factor with a lower-rated output and vice versa. 

When the product of these two exceeds 150, the resulting current transformer may be uneconomical and/or of unduly large dimensions. 

Class P current transformers are defined so that, at rated frequency and with rated burden connected, the current error, phase displacement and composite error shall not exceed the values given in the table below. 

Class PR 

A current transformer with less than 10% remanence factor due to small air gaps for which, in some cases, a value of the secondary loop time constant and/or a limiting value of the winding resistance may also be specified.  

Class PX 

A current transformer of low leakage reactance for which knowledge of the transformer secondary excitation characteristic, secondary winding resistance, secondary burden resistance and turns ratio is sufficient to assess its performance in relation to the protective relay system with which it is to be used. 

Class PX is the definition in IEC 60044-1 for the quasi-transient current transformers formerly covered by class X of BS 3938, commonly used with unit protection schemes. 

Class PX type CTs are used for high impedance circulating current protection and are also suitable for most other protection schemes. 

IEC 60044-6

Class TPS 

Protection current transformers specified in terms of complying with class TPS are generally applied to unit systems where the balancing of outputs from each end of the protected plant is vital. This balance, or stability through fault conditions, is essential of a transient nature and thus the extent of the unsaturated (or linear) zones is of paramount importance. 

It is normal to derive, from heavy current test results, a formula stating the lowest permissible value of Vk if the stable operation is to be guaranteed.

The performance of class TPS current transformers of the low (secondary) reactance type is defined by IEC 60044-6 for transient performance. In short, they shall be specified in terms of each of the following characteristics: 

  • Rated primary current
  • Turns ratio (the error in turns ratio shall not exceed ±0.25%)
  • Secondary limiting voltage
  • The resistance of secondary winding Class TPS CTs are typically applied for high impedance circulating current protection.

Class TPX 

The basic characteristics for class TPX current transformers are generally similar to those of class TPS current transformers except for the different error limits prescribed and possible influencing effects which may necessitate a physically larger construction. 

Class TPX CTs have no air gaps in the core and therefore a high remanence factor (70-80% remanent flux). The accuracy limit is defined by the peak instantaneous error during the specified transient duty cycle. Class TPX CTs are typically used for line protection.

Class TPY

Class TPY CTs have a specified limit for the remanent flux. The magnetic core is provided with small air gaps to reduce the remanent flux to a level that does not exceed 10% of the saturation flux. 

They have a higher error in current measurement than TPX during unsaturated operation and the accuracy limit is defined by peak instantaneous error during the specified transient duty cycle. Class TPY CTs are typically used for line protection with auto-reclose.

Class TPZ 

For class TPZ CTs the remanent flux is practically negligible due to large air gaps in the core. These air gaps also minimize the influence of the DC component from the primary fault current but reduce the measuring accuracy in the unsaturated (linear) region of operation. 

The accuracy limit is defined by peak instantaneous alternating current component error during single energization with maximum DC offset at specified secondary loop time constant. Class TPZ CTs are typically used for special applications such as differential protection of large generators. 


Thursday, November 26, 2020

Star Delta Motor Starting Explained


Star Delta Power Circuit

Star Delta starting is when the motor is connected (normally externally from the motor) in STAR during the starting sequence. When the motor has accelerated to close to the normal running speed, the motor is connected in DELTA. 

The change of the external connection of the motor from Star to Delta is normally achieved by what is commonly referred to as Star-Delta starter. This starter is simply a number of contactors (switches) that connect the different leads together to form the required transition from Star to Delta. 

When the motor is started in the star connection, the phase voltage of the motor is reduced by a factor of √3. The reductions in starting current, starting power, and starting torques for a reduced Voltage can each be calculated by using equation 1 (This ignores other factors like saturation, etc.):  

These starters are normally set to a specific starting sequence, mostly using a time setting to switch between Star and Delta. There can be extensive protection on these starters, monitoring the starting time, current, Voltage, motor speed etc. 

For example, if the supply voltage is 380 Volts. During starting in which the motor is connected into Star, the impressed voltage across each coil is 380/ 1.73 which is 220 Volts. As a result of the reduction of the impressed voltage, the starting torque will also reduce to 67%. 

Control Circuit

From the control circuit above, when switch S1 is pressed, there will be a complete path of electric current that will flow from L1 to L2 causing the following coils to be activated: 

Read: Electric Motor Control in Industrial Plants

  • K1 -  line or main contactor
  • K2 - star contactor 
  • K4 - timer (set at 3 to 5 seconds)

After the predetermined time, there will be a transition of timer contact. As such the time delay close contact (K3)which controls the star contactor will now become open while the time delay close contact (K2) will do the opposite. In this way, the transition from star to delta is executed. 

The auxiliary contact of contactor K1 is connected in parallel with the start button S1 (latched) so that the circuit will remain activated even when S1 goes back to the open position. Note that S1 is characterized by a pushbutton that will return to its original state after being pressed. 

The normally closed contacts K3 and K2 are also interlocked to prevent activating STAR and DELTA connection at the same time that can cause serious damage to the motor. 

What are the advantages of using Star Delta starting? 

The most significant advantage of this starting method is the reduction inrush current during starting. The reduction of the starting current can also reduce the mechanical stress of motor due to high starting torque. Note that when reduce voltage starting is not applied, the starting current could reach as high as 600%. 

What are the General Guidelines in Electrical Design?

Electrical Design

To get the best results, an electrical engineer needs to be aware of specific guidelines so that they can identify the required information that needs to be included. 

Determine the load power demands 

Electrical design starts with the study of the nature of the proposed project in relation to the understanding of all codes, legislations, and standards. The total power demand can be calculated from the data relative to the location and power of each load, together with the knowledge of the operating modes such as the following: 

  • steady state demand
  • starting conditions
  • non-simultaneous operation

Read: How To Prepare Schedule of Loads

From the obtained data, the requirements of power from the utility supply and the number of sources necessary for an adequate supply to the installation are readily obtained, as necessity arise. The local information pertaining to the tariff structures must also be sought so that we can the best option of connection arrangements will be attained. 

Service connection 

Depending on the necessity and the size of the electrical installation, it is better to select properly the service locations. 

  • Medium Voltage Level Service Connections - A consumer-type substation will then have to be studied, built and equipped. This substation may be an outdoor or indoor installation conforming to relevant standards and regulations (the low-voltage section may be studied separately if necessary). Metering at medium-voltage or low-voltage is possible in this case. 
  • Low Voltage level Service Connections- The installation will be connected to the local power network and will (necessarily) be metered according to LV tariffs.

Architecture of the Electrical Distribution

The whole installation distribution network needs to be considered as a complete system. With respect to the necessity of the project, a selection guide is proposed for determination of the most suitable architecture. MV/LV main distribution and LV power distribution levels are covered. Neutral earthing arrangements are chosen according to local regulations, constraints related to the power-supply, and to the type of loads. 

The distribution equipment such as switchgears, panels, MSS and other equipment must be determined from building plans and from the location and grouping of loads. The type of premises and allocation can influence their immunity to external disturbances.

Protection against electric shocks

The earthing system (TT, IT or TN) having been previously determined, then the appropriate protective devices must be implemented in order to achieve protection against hazards of direct or indirect contact.

Read: Basic Safety Provision for Electrical Installation According to BS 7671

Sizing and Protection

Every part of the electric circuit must undergo a detailed study. It starts from the calculation of the rated currents of the loads, the magnitude of short-circuit currents, and the type of protective devices, the sizes of circuit conductors, and the specification of the cable ducts and conduits.  Also, anything that can affect the rating of the conductors must also be identified.  

  • The voltage drop complies with the relevant standard 
  • Motor starting is satisfactory 
  • Protection against electric shock is assured 
  • The possible temperature where the electrical system is to be installed
  • Earth Fault Loop


The short-circuit current must is then determined in order to identify the rating of interrupting capacity of the protective devices. These calculations may indicate that it is necessary to use a conductor size larger than the size originally chosen. 

The performance required by the switchgear will determine its type and characteristics. The use of cascading techniques and the discriminative operation of fuses and tripping of circuit breakers are examined.

Protection against overvoltages

Direct or indirect lightning strokes can damage electrical equipment at a distance of several kilometers. Operating voltage surges, transient and industrial frequency over-voltage can also produce the same consequences. The effects are examined and solutions are proposed.

Energy efficiency in electrical distribution 

Implementation of measuring devices with an adequate communication system within the electrical installation can produce high benefits for the user or owner: reduced power consumption, reduced cost of energy, better use of electrical equipment

Reactive energy 

The power factor correction within electrical installations is carried out locally, globally or as a combination of both methods.


Harmonics in the network affect the quality of energy and are at the origin of many disturbances as overloads, vibrations, ageing of equipment, trouble of sensitive equipment, of local area networks, telephone networks. This chapter deals with the origins and the effects of harmonics and explain how to measure them and present the solutions.

Particular supply sources and loads


Particular items or equipment are studied: b Specific sources such as alternators or inverters b Specific loads with special characteristics, such as induction motors, lighting circuits or LV/LV transformers b Specific systems, such as direct-current networks.


  • Schneider Electric

Wednesday, November 25, 2020

Why is Voltage Rating Important in Selecting Circuit Protection Devices?


This is an extremely important rating for overcurrent protective devices (OCPDs). The proper application of an overcurrent protective device according to its voltage rating requires that the voltage rating of the device be equal to or greater than the system voltage. When an overcurrent protective device is applied beyond its voltage rating, there may not be any initial indicators. Adverse consequences typically result when an improperly voltage rated device attempts to interrupt an overcurrent, at which point it may self-destruct in an unsafe manner. 

There are two types of OCPD voltage ratings: 
  1. straight voltage rated and;
  2.  slash voltage rated. 

The proper application is straightforward for overcurrent protective devices with a straight voltage rating (i.e.: 600V, 480V, 240V) which have been evaluated for proper performance with full phase-to-phase voltage used during the testing, listing and marking. For instance, all fuses are straight voltage rated and there is no need to be concerned about slash ratings. 

However, some mechanical overcurrent protective devices are slash voltage rated (i.e.: 480/277, 240/120, 600/347). Slash voltage rated devices are limited in their applications and extra evaluation is required when they are being considered for use. The next section covers fuse voltage ratings followed by a section on slash voltage ratings for other type devices.


An overcurrent is either an overload current or a short-circuit current. The overload current is an excessive current relative to normal operating current, but one which is confined to the normal conductive paths provided by the conductors and other components and loads of the distribution system. As the name implies, a short-circuit current is one which flows outside the normal conducting paths.


Overloads are most often between one and six times the normal current level. Usually, they are caused by harmless temporary surge currents that occur when motors start up or transformers are energized. Such overload currents, or transients, are normal occurrences. Since they are of brief duration, any temperature rise is trivial and has no harmful effect on the circuit components. (It is important that protective devices do not react to them.) Continuous overloads can result from defective motors (such as worn motor bearings), overloaded equipment, or too many loads on one circuit. 

Such sustained overloads are destructive and must be cut off by protective devices before they damage the distribution system or system loads. However, since they are of relatively low magnitude compared to short-circuit currents, removal of the overload current within a few seconds to many minutes will generally prevent equipment damage. A sustained overload current results in overheating of conductors and other components and will cause deterioration of insulation, which may eventually result in severe damage and short circuits if not interrupted.

Short Circuits 

Whereas overload currents occur at rather modest levels, the short-circuit or fault current can be many hundred times larger than the normal operating current. A high level fault may be 50,000A (or larger). If not cut off within a matter of a few thousandths of a second, damage and destruction can become rampant–there can be severe insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing, and fires. Simultaneously, high level short-circuit currents can develop huge magnetic field stresses. The magnetic forces between bus bars and other conductors can be many hundreds of pounds per linear foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond repair.

Fuse Protection

The fuse is a reliable overcurrent protective device. A “fusible” link or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. Electrical resistance of the link is so low that it simply acts as a conductor. However, when destructive currents occur, the link very quickly melts and opens the circuit to protect conductors and other circuit components and loads. 

Modern fuses have stable characteristics. Fuses do not require periodic maintenance or testing. Fuses have three unique performance characteristics: 

  1. Modern fuses have an extremely “high interrupting” rating–can open very high fault currents without rupturing.
  2. Properly applied, fuses prevent “blackouts.” Only the fuse nearest a fault opens without upstream fuses (feeders or mains) being affected–fuses thus provide “selective coordination.” (These terms are precisely defined in subsequent pages.)
  3. Fuses provide optimum component protection by keeping fault currents to a low value…They are said to be “current- limiting.”

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  • Cooper Bussman

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