Whether you are new to the industry or are well seasoned and want to review some of the basic information that you learned in the past, these courses and lessons will clearly outline what you are looking for. “Power Systems Electric” is On-Line training that bridges the gaps between textbook theories and practical power systems experience. As a retired electrical engineer who has gone through the experience of developing a career in electrical power systems, I know how frustrating it can be to try and find the answers, so I put together these courses that I feel would have been a major help to me in my development. I have also taken suggestions from students of what they would like to see in addition to these courses which I have given live in person.

Basic Electrical Learning

For those new to the industry, there is a beginner’s group of courses that cover the “Fundamentals of Electricity ” including DC and AC Circuit Analysis. These lessons examine such basics as Ohm’s Law, Series, and Parallel Circuits. The first, and perhaps most important, relationship between current, voltage, and impedance, “Ohm’s Law”, and its relevance to Series and Parallel Circuits. Subsequently, this will lead to the development of Kirchhoff’s Laws as they help to further analyze Network Analysis & Metering Circuits.

Conductors and Insulators are investigated along with their connected components, Capacitors, Inductors, and how they are influenced by Electromagnetism.

Alternating Current (AC – an electric current that periodically reverses direction) is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans, and electric lamps into a wall socket.

The abbreviations AC and DC are often used to mean simply alternating and direct, as they are applied to current or voltage.

These AC courses deal specifically with sinusoidal waveforms and will provide the student with the basic understanding of working with circuits involving Alternating Current, which includes sinusoidal waveforms, vectors & phasors, reactance & impedance of R,L,C circuits, as they relate to the basic laws and theorems of electricity. This includes working with AC Power, Power Factor, Resonance, Complex Numbers, Reactance, and Impedance

Advanced Electrical Learning

These Courses involve subjects such as “Short Circuit Analysis for HV Three-Phase Systems ” which introduces the student to the basic concepts of fault studies on a high voltage three-phase system. System modeling is then used in order to aid in this process, with the ability to move between asymmetrical and symmetrical systems. With hey Searle and extensive study of “Per Phase” & “Per Unit” methodologies system faults are analyzed with the use of symmetrical components.

“A Per-Unit System ” is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per unit do not change when they are referred from one side of a transformer to the other.

In these courses, you will learn exactly what Per Unit Analysis is, the main advantages of using it, how manufacturers of equipment use and rate their products, and the technique of converting to and from the Per Unit system.

Several examples of working with Per Unit are demonstrated in this crisp clear presentation. When you finish you will have a though understanding of this subject.

It is important for all power engineers and technicians to be familiar with the concept of Per Unit as it is being used and referred to every day in power flow, short circuit evaluation, and motor starting studies.

The method of “Symmetrical Components ” is used to simplify asymmetrical three-phase voltages and current analysis by converting the unbalanced system into two sets of balanced phasors and a set of single-phase phasors, or symmetrical components. These sets of phasors are called the positive, negative, and zero sequence components.

An understanding of this method is essential for the understanding of fault analysis and modern-day protection schemes. These courses will provide you with the knowledge to comprehend the concept and how it is applied.

Supplemental Electrical Lessons

In all power electrical analyses, the student will encounter special Trigonometric and Mathematical identities and equations. This site contains special supplemental lessons that zero in on those identities and equations. For example, there are lessons that take you from the “Fundamentals of Trigonometry ” to the more sophisticated requirements in electrical engineering. As you work and study in electrical engineering you are going to run into proofs and equations that are based on trigonometry. A good example of this is when studying AC current, voltage, and impedance calculations, phasors or vectors are used and combined mathematically. Further adventures into complex power will also require a knowledge of trig functions and identities. As a student of this course, you will be introduced to these or at least re-introduced to these that may have been long since forgotten.
In mathematics, “The Derivative of a Function ” of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. For example, the derivative of the position of a moving object with respect to time is the object’s velocity: this measures how quickly the position of the object changes when time advances.

Specialized Electrical Lessons

Specialized lessons include the “Protection & Control(P & C)” principles of high voltage stations. HV Bus Differential Protection is studied along with restrictions due to CT saturation & mismatch and its solution, “Restraint Differential Protection”.

High Voltage Circuit Breakers are examined as to the various Types (Oil, Vacuum, Air Blast, SF6), their Controls as well as the introduction of potential transformers & current transformers, and their use in conjunction with the relevant instruments such as ammeters, voltmeters, watt meters, and energy meters.

Other lessons include the study of “Transformers ” including core construction along with losses, cooling, and mitigation techniques. Three-phase transformer configurations are studied along with harmonic distortion, CT saturation, and on-load tap-changer problems and how these problems are dealt with. Over-current and restraint differential transformer protection is developed along with a look at some examples of ” Old School relays” as well as modern IDE (intelligent Electrical Devices) relays.

Transformer Connections: (Y – Y; Delta – Delta; Y – Delta; Delta – Y & Y – Zag Zig) are examined along with Transformer Clock System Vector Nomenclature.

Lastly, there is a special section dedicated to “Single and Three-Phase Metering “, including old analog and new digital kilowatt-hour meters.

Conclusion

Regardless of whether or not you are new to the industrial power system, you’ll find what you’re looking for on this site in the way of training. In order to review or select any of these courses, left-click on any of the blue highlighted hyperlinked words of this posting or on “14 Courses Available” from the top menu of the landing page. All of the courses have a free introduction

As a bonus and in the way of a thank you, for your interest in PSPT’s WEB (and Blog) page, I’m making available my 50-page “Electrical Power” crib sheets. These were prepared for use with my courses that are available on this site. There is one section associated with each course and is extremely valuable while viewing the course, as well as a recall of the pertinent formulas and information after the fact. The contained information is also useful during any technical career as a quick reference from time to time. Simply click here or on the picture to the right to be taken to where you can download this item.

September 17, 2022

Blog #8 - Residential Electrical Power Chapter Three

September 17, 2022/ Graham VanBrunt

Residential Electrical Power Chapter Three

Low Voltage Circuit Protection

American wire gauge (AWG), also known as the Brown & Sharpe wire gauge, is a logarithmic stepped standardized wire gauge system used since 1857, predominantly in North America, for the diameters of round, solid, nonferrous, electrically conducting wire. The cross-sectional area of each gauge is an important factor for determining its current-carrying ampacity.

Low Voltage wiring will involve the requirement of different sizes or gauges of wire depending on the load that it feeds or more precisely, the amperage it will have to carry. Wire sizes are categorized by what is standard in North America as the American Wire Gauge. This is a measure of wire thickness (which also dictates cross-sectional area, and for a given material, ampacity). For example…24 AWG wire has a nominal diameter of 0.0201in or 0.511mm. AWG only applies to wire used to conduct electricity.

These above two charts describe the characteristics and capabilities of the various wire gauges of copper wire.The chart with the white background is the current caring capacity of the various copper conductors…for example for a load that draws 15 amps #14 AWG wire is required. The chart with the green background, along with describing the diameter indicates the resistance of the wire in ohms per foot. This is only significant for long runs of cables.

Due to the current carrying capacity of the various gauges of wire sizes, the figure on the left shows the breaker rating and type, single pole single throw (SPST) and single pole double throw (SPDT), required for the various wire gauges.

Circuit protection is employed to protect wires and electrical equipment from damage in the event of an electrical overload, short circuit, or ground fault. Lightning storms, overloaded power outlets, or a sudden electrical surge may result in a dangerous situation with the potential to cause fire, equipment damage, or personal injury. Circuit protection is designed to eliminate this risk before it occurs by cutting off the power to the circuit.

A circuit breaker is a protection device employed in every electrical circuit to prevent any potential hazard. There are different types of circuit breakers used all over the world due to their various characteristics & applications. It is necessary to have a circuit breaker that offers adequate protection so that one can work safely around it without having fear of any potential hazards. That is why it is best to know about these kinds of circuit breakers & what kinds of protection do they offer before buying one.

A circuit breaker breaks the circuit in case of overloading & short circuit. The fault currents generated due to these fault conditions can damage the electrical devices as well as cause fire in a building that can also pose danger to human life. The circuit breaker instantly cut off the power supply to reduce further damage.

A circuit breaker has two types of tripping units i.e. a thermal unit and a magnetic tripping unit.

The thermal tripping unit is used for protection against overloading. It uses a bi-metallic contact that bends with a change in temperature. The current flowing through the bimetallic strip heats up contact & trips the circuit breaker. The rate of bending of the bi-metallic strip depends on the amount of current. Therefore, the greater the overloading current, the faster the circuit breaker trips.

The magnetic trip unit is used for protection against short circuit current. It includes a solenoid that produced a strong magnetic field due to a high short circuit current to instantly trip the circuit breaker.

The expected behaviour of a circuit breaker depends on its design and what it is designed for. This behaviour is best described by a Trip Curve.

A trip curve is a graphical representation of the expected behaviour of a circuit breaker. Trip curves plot the interrupting time of circuit breakers based on a given current level. They are provided by the manufacturers of circuit breakers to assist users with selecting breakers that provide proper equipment protection and performance while avoiding nuisance tripping.

To avoid nuisance trips, circuit breakers need to be sized appropriately to compensate for inrush current. NEMA defines instantaneous peak inrush as the momentary current transient that occurs within half an AC cycle after contact closure.

Inrush current is what causes lights to dim in a house when a motor, such as that on a clothes dryer or vacuum cleaner starts up.

We need different trip curves in order to balance the right amount of overcurrent protection against optimal machine operation. Choosing a circuit breaker with a trip curve that trips too soon can result in nuisance tripping. Choosing a circuit breaker that trips too late can result in catastrophic damage to machine and cables.

The three most common trip curves for Miniature Circuit Breakers are B, C and D. By putting all three on one chart, we can see how the thermal portion of the curves are similar to each other, but there are differences in how the magnetic curve, and thus the circuit breaker functions.

The tripping curve tells how fast a circuit breaker will trip at a specific current. The different tripping curves classify the circuit breakers into categories where each category is used for specific types of loads. It is essential to select a circuit breaker that provides the necessary overcurrent protection.

Type B circuit breakers are designed to instantly trip when the operating current is 3 to 5 times its rated current. Their tripping time falls between 0.04 to 13 seconds. They are suitable for domestic applications where surges are very low such as lighting, small motors & resistive loads.

Type C circuit breakers trip instantly at current surges 5 to 10 times its rated current. Its tripping time lies between 0.04 to 5 seconds. As they can tolerate higher surge currents, they are used in commercial applications such as the protection of small motors, transformers, etc.

Type D circuit breakers trip instantly when operating currents reach 10 to 20 times their rated current. Its tripping time is 0.04 to 3 seconds. Such circuit breakers can tolerate the high inrush current of large motors. Therefore, they are suitable for running heavy loads in industrial applications.

According to NEC (National Electric Code), IEC (International Electrotechnical Commission) and IEEE (Institute of Electrical and Electronics Engineers), a proper size a circuit breaker is a must for all electrical circuits i.e. residential wiring and industrial or commercial installation to prevent the electrocution, hazardous fire and protection of the connected electrical equipment and appliances.

For maximum safety and reliable operation of the electrical machines, it is recommended to use the correct and suitable size of the circuit breaker according to the circuit’s current flowing through it. If we don’t use a correctly sized circuit breaker. In case of other (over or lower) sizes instead of the correct-sized circuit breaker, the circuit, cables and wire even the connected device may heat up or in case of a short circuit, it may start to smoke and burn. That’s why a correct size circuit breaker is needed for smooth operation.

Under normal conditions when the circuit current rating is lower than 80% of the circuit breaker rating, the circuit operation is normal and the circuit breaker will not trip. In case of fault or short circuit when the value of current exceeds the circuit breaker rating, It will automatically trip i.e. break the circuit from the supply.

For example, a 30 amp circuit breaker will trip at or above 30 amp no matter if is it a continuous or noncontinuous load. However, just after the current exceeds 80%, of the circuit breaker rating, even buy a small amount, the thermal element of the breaker will start to heat up and if the current does not drop below 80%, but continues to flow the breaker will eventually trip after a long period of time.

That’s why we must select 20-25% higher for circuit breaker rating than the expected, flowing current in the cables and wires to the connected device.

In order to determine the appropriate size of circuit breaker for single phase supply, it depends on multiple factors, like the type of load, cable material and even the environment temperature.

The general rule of thumb is that circuit breaker size should be 125% of the ampacity of cable and wire or the circuit which has to be protected by the circuit breaker. Let see the following solved examples:

Example# 1

Suppose, a 12 gauge wire is used for 20 amperes lighting circuit having 120V single phase supply. What is the best size of circuit breaker for that 20 A circuit?

Solution:

Circuit Current: 12A

Circuit Breaker size should be 125% of the circuit current = 125% x 20A

Circuit Breaker Size = 25A

Example #2

What is the appropriate size of circuit breaker for 2000W, single phase 120V Supply?

Solution:

Load: 2000W

Voltage: 120V (Single Phase)

Circuit Current: I = P / V

I = 2000W / 120V

I = 16.66 A.

Circuit Breaker Size: 1.2 or 1.25 to the load current.

1.2 x 16.66 A

Circuit Breaker Size = 20 A.

Circuit protection is employed to protect the wires and electrical equipment from damage in the event of an electrical overload, short circuit, or ground fault. Lightning storms, overloaded power outlets, or a sudden electrical surge may result in a dangerous situation with the potential to cause fires, equipment damage, or personal injury. Circuit protection is designed to eliminate this risk before it occurs by cutting off the power to the circuit. Circuit protection devices include fuses, miniature circuit breakers, molded case circuit breakers, supplementary protectors, motor protection circuit breakers, overload relays, electronic fuses and air circuit breakers.

Trip Curves predict the behavior of circuit protection devices in both slower, smaller overcurrent conditions, and larger, faster overcurrent conditions. Choosing the correct trip curve for your application provides reliable circuit protection while limiting nuisance or false trips.

This blog is a brief overview of circuit breakers. It is not intended to be the final answer on this topic. There is a lot more to learn, including other types of trip curves and circuit breaker coordination. With the basics now covered, one can confidently approach those topics.