Nuisance tripping of circuit breakers is a common problem in many commercial and industrial installations. This application note explains the need to use true RMS measurement instruments when troubleshooting and analyzing the performance of a power system.
Nuisance tripping of circuit breakers is often caused by the load current being distorted by the presence of harmonic currents drawn by non-linear loads. Harmonic currents distort the current waveform and increase the load current required to deliver energy to the load. Many measurement instruments, even quite modern ones, use an averaging measurement technique that does not measure harmonic currents correctly. The readings may be as much as 40% too low. Circuit breakers and cable sizes may be underrated as a result.
True RMS meters, which take the complete distorted waveform into account, should be used instead.
This directional over current relay employs the principle of actuation of the relay....It has a metallic disc free to rotate between the poles of two...
Townsend ’s theory
Introduction
Ionization by collision
Townsend’s current growth equation
Current Growth in the Presence of Secondary Processes
Townsend’s secondary ionization coefficient
Townsend’s Criterion for Breakdown
Breakdown in Electronegative Gases
Today, nearly every piece of electrical equipment generates harmonic currents and voltages. This Application Note gives a comprehensive and up-to-date overview of the subject. It explains why harmonic problems have been increasing over recent years, how they are generated, and by which type of equipment. It presents an overview of the various problems harmonic currents can create. Most of them are either related to different kinds of overloading – leading to problems of efficiency loss and overheating – or to disturbances of control and safety devices.
This Application Note also presents a brief overview of the available solutions. The four main mitigation solutions are passive shunt filters, passive series filters, isolation transformers, and active harmonic conditioners. The Application Note concludes that good design practice, the right electrical equipment, and good maintenance are the keys to preventing future problems.
Includes Introduction, Derivation of power flow through transmission line, Single line diagram of three phase transmission, methods of finding the performance of transmission line. 1.Analytical Method 2.Graphical method (circle diagram)., circle diagram of receiving end side and sending end side.
This directional over current relay employs the principle of actuation of the relay....It has a metallic disc free to rotate between the poles of two...
Townsend ’s theory
Introduction
Ionization by collision
Townsend’s current growth equation
Current Growth in the Presence of Secondary Processes
Townsend’s secondary ionization coefficient
Townsend’s Criterion for Breakdown
Breakdown in Electronegative Gases
Today, nearly every piece of electrical equipment generates harmonic currents and voltages. This Application Note gives a comprehensive and up-to-date overview of the subject. It explains why harmonic problems have been increasing over recent years, how they are generated, and by which type of equipment. It presents an overview of the various problems harmonic currents can create. Most of them are either related to different kinds of overloading – leading to problems of efficiency loss and overheating – or to disturbances of control and safety devices.
This Application Note also presents a brief overview of the available solutions. The four main mitigation solutions are passive shunt filters, passive series filters, isolation transformers, and active harmonic conditioners. The Application Note concludes that good design practice, the right electrical equipment, and good maintenance are the keys to preventing future problems.
Includes Introduction, Derivation of power flow through transmission line, Single line diagram of three phase transmission, methods of finding the performance of transmission line. 1.Analytical Method 2.Graphical method (circle diagram)., circle diagram of receiving end side and sending end side.
Power Factor Correction Methods
Fixed Capcitors
Synchronous Condensors
Phase Advancers
Switch Capacitors
Static Var Compensator(SVC)
Static Synchronous Compensator(STATCOM)
Modulated power filter capacitor compensator
Economics of power factor improvement
Economical comparison of increasing the power supply
Application of Capacitors to Distribution System and Voltage RegulationAmeen San
Application of Capacitors to
Distribution System and Voltage
Regulation
POWER FACTOR IMPROVEMENT,
System Harmonics
Voltage Regulation
Methods of Voltage Control
INTRODUCTION BASIC TECHNIQUES TYPE OF BUSES
Y BUS MATRIX POWER SYSTEM COMPONENTS BUS ADMITTANCE MATRIX
Power (Load) flow study is the analysis of a power system in normal steady-state operation
This study will determine:
Power system analysis ETAP. The power system analysis is the field of electrical engineering that focuses on multiple aspects of system studies. It encompasses studies such as load flow, arc flash, short circuit, relay coordination, motor starting study, transient analysis etc.
Nuisance tripping of circuit breakers is a common problem in many commercial and industrial installations. This Application Note explains the need to use ‘True RMS’ measurement instruments when troubleshooting and analyzing the performance of a power system.
Nuisance tripping is often caused by the load current being distorted by the presence of harmonic currents drawn by non-linear loads. Harmonic currents distort the current waveform and increase the load current required to deliver energy to the load. Many measurement instruments, even quite modern ones, use an averaging measurement technique that does not measure harmonic currents correctly.
‘True RMS’ meters take the complete distorted waveform into account. If averaging meters are used to measure distorted current, the readings may be as much as 40% too low. Circuit breakers and cable sizes may be underrated as a result.
Bus ele tech_lib_short_circuit_current_calculations (1)ingcortez
LIBRO DE CALCULOS DE DATOS DE CORTO CIRCUITO ELÉCTRICO PARA CONDUCTORES DE COBRE Y ALUMINIO DEL TIPO MONOPOLARES O TRIFASICOS DENTRO DE CANALIZACIONES ELECTRICAS PLASTICAS O METALICAS EN VOLTAJES DE MEDIA Y BAJA TENSION CON FACTORES O CONSTANTES DE LOS CONDUCTORES ELECTRICOS EN METROS
Power Factor Correction Methods
Fixed Capcitors
Synchronous Condensors
Phase Advancers
Switch Capacitors
Static Var Compensator(SVC)
Static Synchronous Compensator(STATCOM)
Modulated power filter capacitor compensator
Economics of power factor improvement
Economical comparison of increasing the power supply
Application of Capacitors to Distribution System and Voltage RegulationAmeen San
Application of Capacitors to
Distribution System and Voltage
Regulation
POWER FACTOR IMPROVEMENT,
System Harmonics
Voltage Regulation
Methods of Voltage Control
INTRODUCTION BASIC TECHNIQUES TYPE OF BUSES
Y BUS MATRIX POWER SYSTEM COMPONENTS BUS ADMITTANCE MATRIX
Power (Load) flow study is the analysis of a power system in normal steady-state operation
This study will determine:
Power system analysis ETAP. The power system analysis is the field of electrical engineering that focuses on multiple aspects of system studies. It encompasses studies such as load flow, arc flash, short circuit, relay coordination, motor starting study, transient analysis etc.
Nuisance tripping of circuit breakers is a common problem in many commercial and industrial installations. This Application Note explains the need to use ‘True RMS’ measurement instruments when troubleshooting and analyzing the performance of a power system.
Nuisance tripping is often caused by the load current being distorted by the presence of harmonic currents drawn by non-linear loads. Harmonic currents distort the current waveform and increase the load current required to deliver energy to the load. Many measurement instruments, even quite modern ones, use an averaging measurement technique that does not measure harmonic currents correctly.
‘True RMS’ meters take the complete distorted waveform into account. If averaging meters are used to measure distorted current, the readings may be as much as 40% too low. Circuit breakers and cable sizes may be underrated as a result.
Bus ele tech_lib_short_circuit_current_calculations (1)ingcortez
LIBRO DE CALCULOS DE DATOS DE CORTO CIRCUITO ELÉCTRICO PARA CONDUCTORES DE COBRE Y ALUMINIO DEL TIPO MONOPOLARES O TRIFASICOS DENTRO DE CANALIZACIONES ELECTRICAS PLASTICAS O METALICAS EN VOLTAJES DE MEDIA Y BAJA TENSION CON FACTORES O CONSTANTES DE LOS CONDUCTORES ELECTRICOS EN METROS
Voltage Flicker Analysis and its Mitigation by STATCOM for Power Quality Impr...IJMTST Journal
Voltage flicker is considered as one of the most severe power quality problems (especially in loads like electrical arc furnaces) and much attention has been paid to it lately. The reason for this disturbance is mainly due to the large nonlinear loads such as electric arc furnaces. Due to the latest achievements in the semiconductors industry and consequently the emergence of the compensators based on voltage source converters, FACTS devices have been gradually noticed to be used for voltage flicker compensation. This paper covers the contrasting approaches; dealing with the voltage flicker mitigation in three stages and assessing the related results in details. Initially, the voltage flicker mitigation, using FCTCR (Fixed Capacitor Thyristor Controlled Reactor), was simulated. Secondly, the compensation for the Static Synchronous Compensator (STATCOM) has been performed. The voltage flicker compensation by 8– pulse as well as 12 – pulse static synchronous compensator (STATCOM) has been performed. This paper deals with the voltage flicker mitigation and reduction in total harmonic distortion (THD) and compared the results in detail. The obtained results show that STATCOM is very efficient and effective for the compensation and mitigation of voltage flicker and harmonics all the simulation results have been performed on the MATLAB Software.
Research Inventy : International Journal of Engineering and Scienceresearchinventy
Research Inventy : International Journal of Engineering and Science is published by the group of young academic and industrial researchers with 12 Issues per year. It is an online as well as print version open access journal that provides rapid publication (monthly) of articles in all areas of the subject such as: civil, mechanical, chemical, electronic and computer engineering as well as production and information technology. The Journal welcomes the submission of manuscripts that meet the general criteria of significance and scientific excellence. Papers will be published by rapid process within 20 days after acceptance and peer review process takes only 7 days. All articles published in Research Inventy will be peer-reviewed.
A DAPTIVE S UPPLY V OLTAGE M ANAGEMENT F OR L OW P OWER L OGIC C IRCU...VLSICS Design
With the rise in demand of portable hand held devic
es and with the rise in application of wireless sen
sor
networks and RFID reduction of total power consumpt
ion has become a necessity. To save power we
operate the logic circuitry of our devices at sub-t
hreshold. In sub-threshold the drain current is
exponentially dependent on the threshold voltage he
nce the threshold variation causes profound variati
on
of I
ON
and I
OFF
the ratio of which affect the speed of a circuit d
rastically. So to mitigate this problem we
present a adaptive power management circuit which w
ill determine the minimum required supply voltage
to meet the timing requirement. Also to reduce the
power overhead and avoid bulky coil and EMI noise
we used the switch capacitor power regulator to reg
ulate and manage power instead of linear dropout
(LDO) and Inductor base switch mode power converter
A High-Performance SPWM Controller for Three-Phase UPS Systems Operating Unde...ijiert bestjournal
The increased use of rectifiers in critical loads employed by the information technologies,and medical and military equipment mandate the design of uninterruptible power supplies (UPS) with high- quality outputs. The highly nonlinear currents drawn especially by high-power single-phase rectifier loads greatly distort the uninterruptible power supplies (UPS) outputs. The distorted uninterruptible power supplies (UPS) voltages cause generation of low dc voltage at the output of the rectifier loads,which causes high current flow,increased power losses,and possibly the malfunction of the critical load or the uninterruptible power supplies (UPS). As a result,different harmonics mitigating techniques have gained a lot of attention. The main objective of this project is to develop simulation of a high-performance Pulse Width Modulation (PWM) technique based AC-DC converter system operating under highly nonlinear loads using MATLAB/SIMULINK. Here,controlled Insulated-gate bipolar transistor (IGBT) based AC-DC converter is used to supply the load instead of Diode or Thyristor Bridge. The pulse width modulation method is quite effective in controlling the root mean square (RMS) magnitude of the AC-DC converter output voltages and shape of input current. This enables automatic harmonic compensation by Rectifier itself.
A new generation of instruments and tools to monitor buildings performanceLeonardo ENERGY
What is the added value of monitoring the flexibility, comfort, and well-being of a building? How can occupants be better informed about the performance of their building? And how to optimize a building's maintenance?
The slides were presented during a webinar and roundtable with a focus on a new generation of instruments and tools to monitor buildings' performance, and their link with the Smart Readiness Indicator (SRI) for buildings as introduced in the EU's Energy Performance of Buildings Directive (EPBD).
Link to the recordings: https://youtu.be/ZCFhmldvRA0
Addressing the Energy Efficiency First Principle in a National Energy and Cli...Leonardo ENERGY
When designing energy and climate policies, EU Member States have to apply the Energy Efficiency First Principle: priority should be given to measures reducing energy consumption before other decarbonization interventions are adopted. This webinar summarizes elements of the energy and climate policy of Cyprus illustrating how national authorities have addressed this principle so far, and outline challenges towards its much more rigorous implementation that is required in the coming years.
Auctions for energy efficiency and the experience of renewablesLeonardo ENERGY
Auctions are an emerging market-based policy instrument to promote energy efficiency that has started to gain traction in the EU and worldwide. This presentation provides an overview and comparison of several energy efficiency auctions and derives conclusions on the effects of design elements based on auction theory and on experiences of renewable energy auctions. We include examples from energy efficiency auctions in Brazil, Canada, Germany, Portugal, Switzerland, Taiwan, UK, and US.
A recording of this presentation can be viewed at:
https://youtu.be/aC0h4cXI9Ug
Energy efficiency first – retrofitting the building stock finalLeonardo ENERGY
Retrofitting the building stock is a challenging undertaking in many respects - including costs. Can it nevertheless qualify as a measure under the Energy Efficiency First principle? Which methods can be applied for the assessment and what are the results in terms of the cost-effectiveness of retrofitting the entire residential building stock? How do the results differ for minimization of energy use, CO2 emissions and costs? And which policy conclusions can be drawn?
This presentation was used during the 18th webinar in the Odyssee-Mure on Energy Efficiency Academy on February 3, 2022.
A link to the recording: https://youtu.be/4pw_9hpA_64
How auction design affects the financing of renewable energy projects Leonardo ENERGY
Recording available at https://youtu.be/lPT1o735kOk
Renewable energy auctions might affect the financing of renewable energy (RE) projects. This webinar presents the results of the AURES II project exploring this topic. It discusses how auction designs ranging from bid bonds to penalties and remuneration schemes impact financing and discusses creating a low-risk auction support framework.
This presentation discusses the contribution of Energy Efficiency Funds to the financing of energy efficiency in Europe. The analysis is based on the MURE database on energy efficiency policies. As an example, the German Energy Efficiency Fund is described in more detail.
This is the 17th webinar in the Odyssee-Mure on Energy Efficiency Academy.
Recordings are available on: https://youtu.be/KIewOQCgQWQ
(see updated version of this presentation:
https://www.slideshare.net/sustenergy/energy-efficiency-funds-in-europe-updated)
The Energy Efficiency First Principle is a key pillar of the European Green Deal. A prerequisite for its widespread application is to secure financing for energy efficiency investments.
This presentation discusses the contribution of Energy Efficiency Funds to the financing of energy efficiency in Europe. The analysis is based on the MURE database on energy efficiency policies. As an example, the German Energy Efficiency Fund is described in more detail.
This is the 17th webinar in the Odyssee-Mure on Energy Efficiency Academy.
Recordings are available on: https://youtu.be/KIewOQCgQWQ
Five actions fit for 55: streamlining energy savings calculationsLeonardo ENERGY
During the first year of the H2020 project streamSAVE, multiple activities were organized to support countries in developing savings estimations under Art.3 and Art.7 of the Energy Efficiency Directive (EED).
A fascinating output of the project so far is the “Guidance on Standardized saving methodologies (energy, CO2 and costs)” for a first round of five so-called Priority Actions. This Guidance will assist EU member states in more accurately calculating savings for a set of new energy efficiency actions.
This webinar presents this Guidance and other project findings to the broader community, including industry and markets.
AGENDA
14:00 Introduction to streamSAVE
(Nele Renders, Project Coordinator)
14:10 Views from the EU Commission and the link with Fit-for-55 (Anne-Katherina Weidenbach, DG ENER)
14:20 The streamSAVE guidance and its platform illustrated (Elisabeth Böck, AEA)
14:55 A view from industry: What is the added value of streamSAVE (standardized) methods in frame of the EED (Conor Molloy, AEMS ECOfleet)
14:55 Country experiences: the added value of standardized methods (Elena Allegrini, ENEA, Italy)
The recordings of the webinar can be found on https://youtu.be/eUht10cUK1o
This webinar analyses energy efficiency trends in the EU for the period 2014-2019 and the impact of COVID-19 in 2020 (based on estimates from Enerdata).
The speakers present the overall trend in total energy supply and in final energy consumption, as well as details by sector, alongside macro-economic data. They will explain the main drivers of the variation in energy consumption since 2014 and determine the impact of energy savings.
Speakers:
Laura Sudries, Senior Energy Efficiency Analyst, Enerdata
Bruno Lapillonne, Scientific Director, Enerdata
The recordings of the presentation (webinar) can be viewed at:
https://youtu.be/8RuK5MroTxk
Energy and mobility poverty: Will the Social Climate Fund be enough to delive...Leonardo ENERGY
Prior to the current soaring energy prices across Europe, the European Commission proposed, as part of the FitFor55 climate and energy package, the EU Social Climate Fund to mitigate the expected social impact of extending the EU ETS to transport and heating.
The report presented in this webinar provides an update of the European Energy Poverty Index, published for the first time in 2019, which shows the combined effect of energy and mobility poverty across Member States. Beyond the regular update of the index, the report provides analysis of the existing EU policy framework related to energy and transport poverty. France is used as a case study given the “yellow vest” movement, which was triggered by the proposed carbon tax on fuels.
Watch the recordings of the webinar:
https://youtu.be/i1Jdd3H05t0
Does the EU Emission Trading Scheme ETS Promote Energy Efficiency?Leonardo ENERGY
This policy brief analyzes the main interacting mechanisms between the Energy Efficiency Directive (EED) and the EU Emission Trading Scheme (ETS). It presents a detailed top-down approach, based on the ODYSSEE energy indicators, to identify energy savings from the EU ETS.
The main task consists in isolating those factors that contribute to the change in energy consumption of industrial branches covered by the EU ETS, and the energy transformation sector (mainly the electricity sector).
Speaker:
Wolfgang Eichhammer (Head of the Competence Center Energy Policy and Energy Markets @Fraunhofer Institute for Systems and Innovation Research ISI)
The recordings of this webinar can be watched via:
https://youtu.be/TS6PxIvtaKY
Energy efficiency, structural change and energy savings in the manufacturing ...Leonardo ENERGY
The first part of the presentations presents the energy efficiency improvements in the manufacturing sector since 2000, and the role of structural change between the different branches and energy savings. It will compare the improvements in Denmark and other countries with EU average. This part is based on ODYSSEE data.
The second part of the presentation presents the development in Denmark in more detail, and it will compare the energy efficiency improvement, corrected for structural change, with the reported savings from the Energy Efficiency Obligation Scheme.
Recordings of the live webinar are on https://youtu.be/VVAdw_CS51A
Energy Sufficiency Indicators and Policies (Lea Gynther, Motiva)Leonardo ENERGY
This policy brief looks at questions ‘how to measure energy sufficiency’, ‘which policies and measures can be used to address energy sufficiency’ and ‘how they are used in Europe today’.
Energy sufficiency refers to a situation where everyone has access to the energy services they need, whilst the impacts of the energy system do not exceed environmental limits. The level of ambition needed to address energy sufficiency is higher than in the case of energy efficiency.
This is the 13th edition of the Odyssee-Mure on Energy Efficiency Academy, and number 519 in the Leonardo ENERGY series. The recording of the live presentation can be found on https://www.youtube.com/watch?v=jEAdYbI0wDI&list=PLUFRNkTrB5O_V155aGXfZ4b3R0fvT7sKz
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We have compiled the most important slides from each speaker's presentation. This year’s compilation, available for free, captures the key insights and contributions shared during the DfMAy 2024 conference.
Harnessing WebAssembly for Real-time Stateless Streaming PipelinesChristina Lin
Traditionally, dealing with real-time data pipelines has involved significant overhead, even for straightforward tasks like data transformation or masking. However, in this talk, we’ll venture into the dynamic realm of WebAssembly (WASM) and discover how it can revolutionize the creation of stateless streaming pipelines within a Kafka (Redpanda) broker. These pipelines are adept at managing low-latency, high-data-volume scenarios.
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Using recycled concrete aggregates (RCA) for pavements is crucial to achieving sustainability. Implementing RCA for new pavement can minimize carbon footprint, conserve natural resources, reduce harmful emissions, and lower life cycle costs. Compared to natural aggregate (NA), RCA pavement has fewer comprehensive studies and sustainability assessments.
Immunizing Image Classifiers Against Localized Adversary Attacksgerogepatton
This paper addresses the vulnerability of deep learning models, particularly convolutional neural networks
(CNN)s, to adversarial attacks and presents a proactive training technique designed to counter them. We
introduce a novel volumization algorithm, which transforms 2D images into 3D volumetric representations.
When combined with 3D convolution and deep curriculum learning optimization (CLO), itsignificantly improves
the immunity of models against localized universal attacks by up to 40%. We evaluate our proposed approach
using contemporary CNN architectures and the modified Canadian Institute for Advanced Research (CIFAR-10
and CIFAR-100) and ImageNet Large Scale Visual Recognition Challenge (ILSVRC12) datasets, showcasing
accuracy improvements over previous techniques. The results indicate that the combination of the volumetric
input and curriculum learning holds significant promise for mitigating adversarial attacks without necessitating
adversary training.
Student information management system project report ii.pdfKamal Acharya
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Final project report on grocery store management system..pdfKamal Acharya
In today’s fast-changing business environment, it’s extremely important to be able to respond to client needs in the most effective and timely manner. If your customers wish to see your business online and have instant access to your products or services.
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In order to develop an e-commerce website, a number of Technologies must be studied and understood. These include multi-tiered architecture, server and client-side scripting techniques, implementation technologies, programming language (such as PHP, HTML, CSS, JavaScript) and MySQL relational databases. This is a project with the objective to develop a basic website where a consumer is provided with a shopping cart website and also to know about the technologies used to develop such a website.
This document will discuss each of the underlying technologies to create and implement an e- commerce website.
Welcome to WIPAC Monthly the magazine brought to you by the LinkedIn Group Water Industry Process Automation & Control.
In this month's edition, along with this month's industry news to celebrate the 13 years since the group was created we have articles including
A case study of the used of Advanced Process Control at the Wastewater Treatment works at Lleida in Spain
A look back on an article on smart wastewater networks in order to see how the industry has measured up in the interim around the adoption of Digital Transformation in the Water Industry.
Water Industry Process Automation and Control Monthly - May 2024.pdf
Nuisance tripping of circuit breakers
1. APPLICATION NOTE
NUISANCE TRIPPING OF CIRCUIT BREAKERS
(TRUE RMS MEASUREMENTS)
David Chapman
March 2014
ECI Publication No Cu0110
Available from www.leonardo-energy.org
3. Publication No Cu0110
Issue Date: March 2014
Page ii
CONTENTS
Summary ........................................................................................................................................................ 1
Nuisance tripping............................................................................................................................................ 2
Inrush Currents.......................................................................................................................................................2
Case study ................................................................................................................................................2
True RMS – The Only True Measurement ..............................................................................................................5
What is RMS? ...........................................................................................................................................6
The consequences of under measurement..............................................................................................8
Conclusion ...................................................................................................................................................... 9
4. Publication No Cu0110
Issue Date: March 2014
Page 1
SUMMARY
Nuisance tripping of circuit breakers is a common problem in many commercial and industrial installations.
This Application Note explains the need to use true RMS measurement instruments when troubleshooting and
analyzing the performance of a power system.
Nuisance tripping of circuit breakers is often caused by the load current being distorted by the presence of
harmonic currents drawn by non-linear loads. Harmonic currents distort the current waveform and increase
the load current required to deliver energy to the load. Many measurement instruments, even quite modern
ones, use an averaging measurement technique that does not measure harmonic currents correctly. The
readings may be as much as 40% too low, and circuit breakers and cable sizes may be underrated as a result.
True RMS meters, which take the complete distorted waveform into account, should be used instead.
5. Publication No Cu0110
Issue Date: March 2014
Page 2
NUISANCE TRIPPING
Many commercial and industrial installations suffer from persistent so-called ‘nuisance tripping’ of circuit
breakers. The term refers to the apparently random and inexplicable nature of these events which, although
there is no apparent fault, can cause significant disruption and financial loss. Of course, there is always a
reason for the nuisance tripping of a breaker and there are two common causes. The first possible cause is the
inrush currents that occur when some loads, particularly personal computers and other electronic devices, are
switched on. The second possible cause is that the true RMS current flowing in the circuit has been under-
measured – in other words, the true current really is too high and the trips are valid.
INRUSH CURRENTS
Modern electronic equipment, such as personal computers, monitors, television sets and office equipment,
uses a type of power supply that converts mains electricity to the low voltage direct current without a low
frequency transformer. This type of supply is known as a switched mode power supply (SMPS) and works by
rectifying mains current directly and storing the direct voltage on a large capacitor which charges to the peak
of the supply voltage. Conversion circuits draw current from the capacitor and generate the required low
voltage, usually via a high frequency transformer to provide galvanic isolation. SMPS is very cheap, but it
causes problems in installations because it produces large harmonic currents and draws very large inrush
currents to initially charge the storage capacitor.
Many PCs are never actually ‘turned off’ – in the sense of being electrically isolated from the supply – but
remain powered in standby mode and can be ‘woken up’ by user input, modem activity, or by a network
message. On wake up, they draw a starting current very similar to that drawn when starting from cold.
CASE STUDY
In a small computer room at the University of Sheffield (UK) circuit breakers were being tripped apparently at
random but particularly during the night. Groups of 16 computers were connected to standard final circuits,
each protected by a 32 A miniature circuit breaker. A preliminary investigation revealed nothing – the
installation was apparently correctly installed and functioning correctly. As the problem persisted, further tests
revealed that the trips occurred at the start of the nightly maintenance procedure as the computers turned
back on and it was realized that the inrush current was responsible. Further inspection revealed that the MCBs
were type B devices, so they were replaced with type D devices of the same rating – this will be discussed in
detail later. Although this change resolved the problem, a measurement exercise was undertaken to verify the
conclusion.
A logic controlled switch was interposed at the load side of one of the breakers together with a transient
recorder to measure the applied voltage and current. The switch was capable of applying the supply voltage at
a defined point on the voltage waveform.
A number of startup cycles were conducted and the inrush currents recorded. Figure 1 shows the inrush
current for a typical personal computer and (CRT) monitor set with the supply voltage applied close to the
positive voltage peak. At 155 A, this was the worst case – i.e. the maximum – inrush current recorded during
the testing for this configuration. The voltage waveform is shown only for clarity but it is interesting to note
the resulting distortion of the supply waveform, especially on the first half cycle. The steady state current
consumption was 0.75 A. The current and time resolutions of the measurements are 0.28 A and 0.8 ms
respectively.
6. Publication No Cu0110
Issue Date: March 2014
Page 3
Figure 1 – Inrush current for one computer/monitor pair fed by a normal final circuit.
Figure 2 shows the result with 16 computer/monitor pairs connected, the worst case peak current being 583 A.
The current and time resolutions of the measurements in this case are 1.12 A and 0.8 ms respectively.
Figure 2 – Inrush current for 16 computer/monitor pairs fed by a normal final circuit.
7. Publication No Cu0110
Issue Date: March 2014
Page 4
MCB CHARACTERISTICS
Although the case study relates to personal computers, the principle applies to most modern electronic
equipment so it is prudent to design installations to survive large inrush currents.
Figure 3 shows the envelope of the characteristic curves for type B, C and D MCBs. The so-called ‘inverse time’
part of the characteristic is designed to protect against over-current. It allows for substantial short-term
overload without tripping, taking advantage of the inherent short time over-current tolerance of the cable. As
the over-current level increases, the time to respond reduces rapidly to restrict the rise in temperature and
reduce the risk of damage. The instantaneous characteristic is intended to respond very rapidly to fault current
to reduce the risk of damage to load circuits.
Types B, C and D MCBs are differentiated by their instantaneous tripping current, shown as Bmin, Bmax, etc. in
Figure 3.
Figure 3 – Characteristics of type B, C and D MCBs.
Taking the example of a type B MCB, Figure 3 shows that the breaker will not trip ‘instantaneously’ at any
current below 3 times nominal rating, but must trip at or above 5 times nominal. In the case of a nominal 32 A
device, instantaneous tripping will occur between 96 A and 160 A with a type B device, and between 320 A and
640 A for a type D device. The minimum duration required to trip is not well defined.
8. Publication No Cu0110
Issue Date: March 2014
Page 5
From the case study results, it is clear that a type B MCB could trip due to the inrush current from one
computer/monitor pair while selecting a type D device would provide a degree of protection against nuisance
tripping. Changing the MCB type means that the potential fault current is increased so the loop impedance of
the final circuit should be carefully checked to ensure compliance with the applicable regulations. In cases
where the inrush is too high for any available breaker, problem loads should be distributed among more final
circuits.
TRUE RMS – THE ONLY TRUE MEASUREMENT
Why do under-measurements occur frequently in modern installations even though digital test instruments
are so accurate and reliable? The answer is that many instruments are not suitable for measuring distorted
currents - and most currents these days are distorted. Since currents are being under-measured, the real
current is much closer to the nominal breaker current than believed, leading to genuine trips that are
misinterpreted as nuisance trips.
This distortion is due to harmonic currents drawn by non-linear loads, especially electronic equipment such as
personal computers, electronically ballasted fluorescent lamps and variable speed drives. Figure 6 shows the
typical current waveform drawn by a personal computer. Obviously, this is not a sine wave and all the usual
sine wave measurement tools and calculation techniques no longer work. This means that, when
troubleshooting or analyzing the performance of a power system, it is essential to use the correct tools for the
job – tools that can deal with non-sinusoidal currents and voltages.
Figure 4 shows two clamp-meters on the same branch circuit. Both the instruments are functioning correctly
and both are calibrated to the manufacturer’s specification. The key difference is in the way the instruments
measure.
Figure 4 - One current, two readings. Which do you trust? The circuit feeds a non-linear load with distorted
current. The True RMS clamp (left) reads correctly but the average responding clamp (right) reads low by 32%.
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The left-hand meter is a true RMS instrument and the right-hand one is an averaging reading RMS calibrated
instrument. Appreciating the difference requires an understanding of what RMS really means.
WHAT IS RMS?
The ‘Root Mean Square’ magnitude of an alternating current is the value of equivalent direct current that
would produce the same amount of heat in a fixed resistive load. The amount of heat produced in a resistor by
an alternating current is proportional to the square of the current averaged over a full cycle of the waveform.
In other words, the heat produced is proportional to the mean of the square of the current, so the equivalent
current value is proportional to the root of the mean of the square or RMS. The polarity is irrelevant since the
square is always positive.
For a perfect sine wave, such as that seen in Figure 5, the RMS value is 0.707 times the peak value. Or the peak
value is 2 (= 1.414) times the RMS value. In other words, the peak value of 1 A RMS pure sine wave current
will be 1.414 A. If the magnitude of the waveform is simply averaged (inverting the negative half cycle), the
mean value is 0.636 times the peak, or 0.9 times the RMS value. There are two important ratios shown in
Figure 5:
And
Figure 5 - A pure sine wave.
When measuring a pure sine wave – but only for a pure sine wave – it is quite correct to make a simple
measurement of the mean value (0.636 x peak) and multiply the result by the form factor 1.111 (making 0.707
times peak) and call it the RMS value. This is the approach taken in all analogue meters (where the averaging is
performed by the inertia and damping of the coil movement) and in all older and many currently available
digital multimeters. This technique is described as ‘mean reading, RMS calibrated’ measurement.
The problem is that the technique only works for pure sine waves and pure sine waves do not exist in the real
world of an electrical installation. The waveform in Figure 6 is typical of the current waveform drawn by a
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personal computer. The true RMS value is still 1 A, but the peak value is much higher, at 2.6 A, and the average
value is much lower, at 0.55 A.
Figure 6 - Typical waveform of current drawn by a personal computer.
If this waveform is measured with a mean reading, RMS calibrated meter it would read 0.61 A, rather than the
true value of 1 A, nearly 40% too low. Figure 7 below gives some examples of the way the two different types
of meters respond to different wave shapes.
Figure 7 – Response of mean reading and true RMS meters to various waveshapes.
A true RMS meter works by taking the square of the instantaneous value of the input current, averaging over
time and then displaying the square root of this average. Perfectly implemented, this is absolutely accurate
whatever the waveform. Implementation is, of course, never perfect and there are two limiting factors to be
taken into account: frequency response and crest factor.
For power systems work it is usually sufficient to measure up to the 50
th
harmonic, i.e. up to a frequency of
about 2500 Hz. The crest factor, the ratio between the peak value and the RMS value, is important; a higher
crest factor requires a meter with a greater dynamic range and therefore higher precision in the conversion
circuitry. A crest factor capability of at least three is required for accurate measurement in power installations.
It is worth noting that, despite giving different readings when used to measure distorted waveforms, meters of
both types would agree if used to measure a perfect sine wave. This is the condition under which they are
calibrated, so each meter could be certified as calibrated – but only for use on sine waves.
True RMS meters have been available for at least the past 30 years, but they used to be specialized and
expensive instruments. Advances in electronics have now resulted in true RMS measurement capability being
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built into many handheld multimeters. Unfortunately, this feature is generally found only towards the top end
of most manufacturers’ ranges, but they are still cheap enough to buy as ordinary instruments for everyone
and every day.
THE CONSEQUENCES OF UNDER MEASUREMENT
The limiting rating for most electrical circuit elements is determined by the amount of heat that can be
dissipated so that the element or component does not overheat.
Cable ratings, for example, are given for particular installation conditions (which determine how fast heat can
escape) and a maximum working temperature. Since harmonic polluted currents have a higher RMS value than
that measured by an averaging meter, cables may have been under-rated and will run hotter than expected;
the result is degradation of the insulation, premature failure and the risk of fire.
Busbars are sized by calculating the rate of heat loss from the bars by convection and radiation and the rate of
heat gain due to resistive losses. The temperature at which these rates are equal is the working temperature
of the busbar, and it is designed so that the working temperature is low enough so that premature ageing of
insulation and support materials does not result. As with cables, errors measuring the true RMS value will lead
to higher running temperatures. Since busbars are usually physically large, skin effect is more apparent than
for smaller conductors, leading to a further increase in temperature.
Other electrical power system components such as fuses and the thermal elements of circuit breakers are
rated in RMS current because their characteristics are related to heat dissipation. This is the root cause of
nuisance tripping – the current is higher than expected so the circuit breaker is operating in an area where
prolonged use will lead to tripping. The response of a breaker in this region is temperature sensitive and may
appear to be unpredictable. As with any supply interruption, the cost of failure due to nuisance tripping can be
high, causing loss of data in computer systems, disruption of process control systems, etc.
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CONCLUSION
This paper has described two common causes of nuisance tripping. In each case there are some simple
preventive steps. Avoiding tripping due to inrush currents simply requires selection of the correct type of
breaker and sensible distribution of loads among circuits.
Under-measurement is easily avoided by ensuring that true RMS meters are used routinely. Knowledge of the
real current in a circuit allows corrective action to be taken. For example, redistributing loads across final
circuits.