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  Blog  PE Power Tips  Fundamentals of Single-Line Diagrams in the PE Power Exam
Fundamentals of Single-Line Diagrams in the PE
Power Exam
Wasim Asghar (https://www.studyforfe.com/about-me/)
July 31, 2023
If you’re an aspiring professional engineer looking to conquer the complexities of electrical systems,
understanding single-line diagrams in the PE Power exam is a crucial skill you can’t afford to overlook.
In this blog, we’ll unravel the importance of single-line diagrams in the PE Power exam as per NCEES® guidelines
and provide a clear course roadmap according to NCEES® PE Power exam criteria and course descriptions to
master this essential aspect of your exam preparation.
According to NCEES® PE Power exam course guidelines, single-line diagrams in the PE Power exam are an integral
part of the exam, serving as visual representations of electrical systems that engineers must interpret and analyze.
They are like blueprints that guide professionals in designing, operating, and maintaining complex industrial
power systems.
This is part 1 of our in-depth study guide series of single-line diagrams in the PE Power exam. Let’s start with the
fundamentals of single-line diagrams, explore their depths in the PE Power exam, and pave the way to your
engineering success.
Disclaimer: The mentions of external drawings are purely for illustrative purposes and do not refer to any factual
documents within this context or elsewhere in the article.
Fundamentals of Single-Line Diagrams
The Single-Line Diagram (SLD) is a fundamental representation of an electrical system, providing a simplified view
of its components, interconnections, and electrical flow paths. It is a one-line drawing that depicts the power
distribution and control circuits clearly and concisely.
Before moving further, have a quick look at the following sample single-line diagrams to learn how detail-oriented,
technically deep, and insightful single-line diagrams can be:
*The information in these SLDs is based on hypothetical scenarios. For real-world applications, it is essential to
comply with relevant industry standards, such as those by the National Electrical Code (NEC), International
Electrotechnical Commission (IEC), and other regional regulations governing electrical installations and safety
standards.
Critical Components of a Single-Line Diagram
The single-line diagrams represent a complex circuit with the respective symbols or notations of the following
critical components.
Power Sources – The SLD showcases the power sources, such as generators or utility supplies, indicating their
voltage levels and connection points to the electrical system.
Electrical Equipment – Various electrical components like transformers, circuit breakers, switches, motors, and
loads are represented by standardized symbols. These symbols provide valuable information about the type and
ratings of the equipment.
Bus Arrangement – The diagram illustrates the bus arrangement, which includes bus bars for power
distribution at different voltage levels. It shows how power is routed from one location to another within the
electrical network.
Protective Devices – Protective devices like fuses, circuit breakers, and relays are depicted to highlight their role
in safeguarding the system against overcurrents, short circuits, and other electrical faults.
Metering and Instrumentation – The SLD may include metering devices (like an ammeter, voltmeter,
ohmmeter, wattmeter, etc.), instruments, and measuring points for monitoring and control purposes.
The table below showcases individual components of single-line diagrams with their purpose and use cases in
electrical and power circuits.
Symbols
Components
Purpose
Transformer
A device that transfers electrical energy
between two or more circuits through
electromagnetic induction, typically used
to step-up or step-down voltage.
Removable or drawout circuit breaker
A circuit breaker that can be withdrawn
from its housing for maintenance or
replacement purposes without disturbing
the electrical connections.
Future removable or drawout circuit
A designated space on the single-line
breaker position
diagram intended for a potential drawout
circuit breaker that may be added in the
future.
Non-drawout circuit breaker
A circuit breaker that is fixed in its
housing and cannot be withdrawn for
maintenance; it can only be replaced as a
whole unit.
Disconnect switch
A switch used to isolate electrical
equipment from the power source for
maintenance or safety purposes,
providing visible means of isolation.
Fuse
A protective device that interrupts the
flow of current in a circuit when it
exceeds a certain level, protecting
electrical equipment from damage.
Bus duct
A metal enclosure used to house bus
bars, providing a means of distributing
power to various electrical components
in a building or facility.
Current transformer
A device that converts high currents to
measurable and manageable values for
use in protective relays or metering
applications.
Potential or voltage transformer
A device that steps down voltage levels
for measurement or control purposes,
providing lower voltage outputs
proportional to the input voltage.
Ground (earth)
A connection to the earth’s surface used
to prevent electrical shock and provide a
safe path for fault currents.
Battery
A storage device that provides a source
of electrical power in the event of a
power outage or for backup purposes.
Motor
An electrical machine that converts
electrical energy into mechanical energy
to perform mechanical work.
Normally open (NO) contact
A contact that is open (non-conductive) in
its normal state and closes (becomes
conductive) when activated.
Normally closed (NC) contact
A contact that is closed (conductive) in its
normal state and opens (becomes nonconductive) when activated.
Indicating light
A visual signaling device, such as an LED
or lamp, used to indicate the status of a
particular electrical circuit or equipment.
Overload relay
A protective device that detects excessive
currents and trips the circuit to prevent
damage to electrical equipment from
overheating.
Capacitor
A passive electronic component that
stores and releases electrical energy,
often used for power factor correction or
energy storage purposes.
Ammeter
A measuring instrument used to
determine the current flowing through a
circuit.
Instantaneous overcurrent protective
A protective relay that operates instantly
relay
when the current exceeds a preset
threshold, providing rapid circuit
protection.
Emergency generator
A generator that provides backup power
in emergencies or during power outages.
Fused disconnect switch
A disconnect switch equipped with fuses,
which protect the circuit from excessive
currents and faults.
Low voltage motor control
Control equipment used for starting,
stopping, and protecting low voltage
motors.
Medium voltage motor starter
Control equipment used for starting,
stopping, and protecting medium voltage
motors.
Meter center
A distribution panel or enclosure that
houses electricity meters for multiple
circuits or tenants.
Load center or panelboard
An electrical distribution board that
receives power from the main source and
distributes it to various branch circuits
within a building.
Transfer switch
A switch that allows the transfer of
electrical power between two sources,
commonly used to switch between main
power and backup power.
Current transformer with connected
A current transformer connected to an
ammeter
ammeter for measuring and monitoring
electrical current.
Protective relays connected to current
Protective relays that use the output of a
transformer
current transformer to detect faults and
protect electrical systems.
*Remember the components listed above are commonly found in electrical systems and single-line diagrams, but
the specific layout and components may vary depending on the application and complexity of the electrical
system.
Purpose of Single-Line Devices in Power Engineering
The purpose of studying single-line devices in the PE Power exam preparation is their utmost importance in power
engineering. Apart from sharing a significant portion of the PE Power exam, they impact the following aspects of
the power engineer’s role.
Design and Planning – Single-Line Diagrams are essential during electrical systems’ initial design and planning
stages. They help engineers visualize the network’s layout, identify potential design issues, and optimize the
system for efficiency and reliability. Let’s have a quick look at some key areas that single-line diagrams impact
significantly:
Documentation – SLDs are critical documentation for electrical installations, providing a comprehensive record
of the system’s configuration and components. They are vital for future maintenance, expansions, and
modifications.
Safety and Troubleshooting – The diagram aids in safety protocols and troubleshooting procedures. Engineers
and electricians can quickly locate components and understand how the system is interconnected, allowing for
faster identification and resolution of faults.
Regulatory Compliance – Single-Line Diagrams are often required by regulatory authorities as part of electrical
system approvals and compliance assessments. They ensure that installations meet safety standards and code
requirements.
Communication – SLDs facilitate effective communication between various stakeholders involved in the
electrical system, including engineers, operators, maintenance personnel, and management. It presents
complex information in a simple, standardized format.
System Optimization – By analyzing the SLD, engineers can identify potential improvements in power
distribution, load balancing, and overall system efficiency. This leads to better resource utilization and energy
conservation.
Single-Line Diagrams in Power Engineering – Layout and
Organization
Single-line diagrams are essential for visualizing electrical systems in a simplified and easy-to-understand manner.
They represent the flow of electrical power and the connection of various components within a system. That’s why
power engineers commonly use these diagrams for circuit illustration and documentation for internal use or
support and external distribution. Below are some general layout guidelines for creating single-line diagrams:
One-Line Representation – Single-line diagrams use a single line to represent all the electrical components and
connections. This helps in reducing complexity and providing a clear overview of the system.
Unidirectional Flow – Electrical power is typically shown to flow from the top of the diagram to the bottom,
following a unidirectional flow from the source to the loads.
Logical Arrangement – Components are arranged logically, starting with the power source at the top and then
proceeding to the loads at the bottom.
Labels and Symbols – Standardized symbols represent each component uniquely, and labels are used to
identify the type and ratings of the equipment.
Breakers and Disconnects – Circuit breakers and disconnect switches are strategically placed to indicate their
protective roles and isolation points within the system.
Bus Arrangement and Voltage Levels in Single-Line Diagrams
Bus arrangement and voltage levels are crucial in determining how electrical power is distributed within a system.
The buses’ arrangement and voltage levels define the different sections and levels of the electrical network. Look
at a glance to discover key arrangements and voltage levels of different components of single-line diagrams.
Bus Duct – Bus ducts carry high currents between different electrical components or sections of the system.
They ensure efficient power distribution and reduce voltage drop over longer distances.
Transformer – Transformers are used to step up or down voltage levels. They are placed strategically to convert
high-voltage power from the utility grid to lower voltages suitable for distribution or to increase the voltage for
long-distance transmission. For instance, A 132 kV to 11 kV transformer steps down the voltage from the
transmission level (132 kV) to a distribution level (11 kV) to supply power to local distribution networks.
Voltage Transformers (VT) and Current Transformers (CT) – VTs and CTs are used to measure voltage and
current levels for metering, protection, and control purposes. For instance, A 11 kV/110 V voltage transformer
steps down the voltage for metering or protection purposes, allowing accurate measurements at a lower voltage
level.
Equipment and Device Representation in Single-Line Diagrams
The symbols used to represent equipment and devices in single-line diagrams are standardized for clarity and
consistency. Understanding these symbols is essential for interpreting the diagram. Here are some key
components and their representation:
Motor – A motor is represented by a circle with the letter “M” inside, indicating its location and connection point
in the system.
Battery – A battery is shown as a collection of cells or a rectangle with a plus and minus sign to represent the
positive and negative terminals
Fuse – A fuse is represented by a zigzag line, acting as a protective device against excessive currents. It
interrupts the circuit when the current exceeds a specific value, protecting equipment from damage. For
instance, a fuse can be placed in a circuit supplying power to a motor. If the motor experiences a short circuit or
overload, the fuse will blow, preventing damage to the motor.
Capacitor – A capacitor is represented by two parallel lines, illustrating its ability to store and release electrical
energy. For instance, capacitors are often used in power factor correction systems to improve the overall power
efficiency of electrical networks.
Circuit Breakers and Protective Devices in Single-Line Diagrams
Circuit breakers and protective devices are crucial for ensuring the safety and reliability of electrical systems. They
protect equipment and personnel from potential faults and overloads. Here’s their role in single-line diagrams:
Removable or Drawout Circuit Breaker – These circuit breakers can be withdrawn from their housing without
disconnecting the electrical connections. They facilitate easy maintenance and replacement of faulty breakers.
For instance, a draw-out circuit breaker might protect a critical process in an industrial plant. In case of a fault,
the draw-out feature allows quick replacement, minimizing downtime.
Non-Drawout Circuit Breaker – Non-drawout circuit breakers are fixed in their housings and cannot be
withdrawn. They are commonly used for less critical circuits or where regular maintenance is not required.
Disconnect Switch – Disconnect switches provide a visible means of isolation and are used to de-energize
equipment for maintenance or emergencies. For instance, a disconnect switch might be placed ahead of a motor
to isolate it from the power supply during maintenance or repair work.
Protective Relays – Protective relays are connected to current transformers and other sensors to detect
abnormal conditions such as overcurrent, undercurrent, or faults. They trigger circuit breakers to open and
protect equipment. For instance, a protective relay connected to a current transformer monitors the current
flowing to a motor. The relay trips the associated circuit breaker to prevent motor damage if the current exceeds
a preset threshold due to a fault.
By following the standard organization and understanding the role of each component, power engineers and
students can create accurate and informative single-line diagrams that effectively represent the electrical systems
they are working with. These diagrams become invaluable tools for planning, maintaining, and troubleshooting
complex electrical networks.
*Ensuring technical accuracy and safety is vital while creating or interpreting single-line diagrams. In the second
last section, we will discuss this in more detail with a range of insightful and detailed examples as per the industry
regulations and guidelines.
Safety Considerations and Documentation in SingleLine Diagrams
Safety considerations, clear documentation, compliance with regulatory standards, and appropriate safety labels
and warnings are critical when creating and using SLDs. This documentation details crucial circuit elements to
enhance safety, emergency shutdown procedures, documentation, and regulatory compliance in single-line
diagrams.
Safety Symbols and Nomenclature
The North American market, in collaboration with the Institute of Electrical & Electronics Engineers (IEEE), has
standardized drawing symbols and nomenclature for electrical drawings, including SLDs. These symbols and labels
help represent common devices, such as circuit protective devices, transformers, current transformers, etc. (IEEE
315-1975/ANSI Y32.9).
Transformers
These voltage-stepping devices come in various winding configurations and are essential components of power
systems. Properly indicating the type of transformer, its kVA ratings, cooling methods, and any surge or lightning
protection devices on the SLD is crucial for safety and documentation purposes.
In SLDs, transformers are represented by standardized symbols, with detailed information provided for each type.
For instance:
Transformer Winding Configurations – Transformers come in various winding configurations, such as deltawye (Δ-Y), delta-delta (Δ-Δ), and wye-wye (Y-Y). Each configuration has specific applications based on the system’s
voltage requirements and grounding schemes.
Transformer Ratings – Transformers are rated in kilovolt-amperes (kVA) to indicate their power-carrying
capacity. The rating is crucial for determining the transformer’s capability and suitability for the application.
Cooling Methods – Transformers can be cooled using different techniques, including natural air convection
(KNAN) and forced air cooling (KNAF). The cooling method affects the transformer’s thermal performance and is
essential for proper sizing and operation.
Surge and Lightning Protection – Surge arresters (metal oxide varistors) are often used to safeguard
transformers from voltage surges and lightning strikes. These devices are represented on the SLD and
connected to the transformer’s incoming terminations.
Circuit Protective Devices
Circuit protective devices protect power systems from overcurrents and faults. Commonly represented circuit
protective devices on SLDs include:
Circuit Breakers – Circuit breakers are automatic switches that interrupt current flow in the event of a fault.
They are represented using standardized symbols and can be of various types, such as molded-case circuit
breakers (MCCBs) or air circuit breakers (ACBs).
Fuses – Fuses are overcurrent protection devices that consist of a metal filament or conductor that melts when
subjected to excessive current. They are represented on the SLD and are available in different ratings for specific
applications.
Current Transformers (CTs)
Current transformers are used for sensing and measuring current in power systems. They are represented in SLDs
using specific symbols and can be of different types:
Bar-Type CTs – These CTs encircle the conductor whose current is to be measured. They are commonly used in
medium-voltage applications.
Donut-Style CTs – Donut-style CTs are bolted in series with the load conductors. They find applications in both
low-voltage and medium-voltage systems.
CT Polarity – The polarity marks on the conductor face toward the current source, while the polarity mark on
the CT winding represents its relationship to the H1 medium-voltage terminal for bar-type CTs or its input
orientation for donut-style CTs.
Voltage Transformers
As mentioned several times, they are used to step down the high voltages to be consumed safely by different
devices on the consumer end. They are also called potential transformers.
MV Cable Sizing and Termination Considerations
Medium-voltage (MV) cables are crucial components in power systems, and their sizing and termination must be
carefully considered in SLDs:
MV Cable Sizing – The sizing of MV cables should adhere to the NEC regulations, considering factors such as load
current, cable length, ambient temperature, and derating based on the type of raceway or duct bank used.
Termination Types – Proper termination types must be specified for higher transformer secondary currents.
Stress cone cable and porcelain terminators are options with higher short-circuit ratings, ensuring safe and
reliable connections.
Incorporating these technical insights into the single-line diagram documentation ensures a comprehensive
representation of power system components and enhances safety, efficiency, and compliance with industry
standards.
Emergency Shutdown Procedures in Single-Line Diagrams
Short-Circuit Ratings – The impedance percentage (%Z) of transformers is a critical parameter that must be
shown on the SLD. This information is necessary to calculate the required ratings of downstream equipment,
ensuring they can withstand fault currents. See the example below:
Secondary Short-Circuit Considerations – If a transformer requires an absolute minimum impedance to
prevent secondary short-circuit levels from exceeding critical values, it must be noted on the SLD and in project
specifications.
Termination Considerations – Depending on the available short-circuit ratings, medium-voltage impedance
with rubber dead-front terminations may not be suitable. Proper selection and application of terminations, such
as stress cone cables or porcelain terminators, should be specified.
Documentation and Record Keeping for Single-Line Diagrams
Detailed Device Representations – The SLD must provide detailed device representations, including symbols,
ratings, and nomenclature. This documentation aids maintenance personnel in understanding the equipment
and its capabilities.
Busway and Connection Specifications – For higher transformer secondary currents, busway flanges and nonsegregated busways should be documented. For proper illustration, the SLD should specify the rated short-time
and short-circuit withstand current values.
Compliance with Regulatory Standards in Single-Line Diagrams
ANSI and IEEE Standards – The SLD should adhere to the ANSI Y32.9 and IEEE 315-1975 standards for symbol
language and nomenclature in North America. Additionally, incorporating International Electrotechnical
Commission (IEC) symbols recognized worldwide enhances international trade.
NEC Regulations – Compliance with the National Electrical Code (NEC) for MV cable sizing, derating factors, and
other safety considerations must be ensured.
REFERENCES
NCEES® – National Council of Examiners for Engineering and Surveying
IEEE 315-1975 (Reaffirmed 1993)/ANSI Y32.9 – Standard for North American electrical drawing symbols and
nomenclature.
National Electrical Code (NEC) – Regulations for electrical installations and safety standards in the United States.
International Electrotechnical Commission (IEC) – Global standards organization for electrical, electronic, and
related technologies.
Electrical Engineering Portal
Iowa State University Press Books
Conclusion
And there you have it—the ultimate guide to single-line diagrams in the PE Power exam! We’ve uncovered the
importance of mastering these diagrams per the NCEES® PE Power exam guidelines. Remember, single-line
diagrams are not just a box to check on your journey to becoming a licensed professional engineer; they are the
roadmap to navigating complex industrial electrical systems.
Ready to take your preparation to the next level? Study for FE is your go-to platform for comprehensive PE Power
exam preparation (https://www.studyforfe.com/pe-power-exam-prep/). With expert resources, practice tests, and
personalized study plans, Study for FE will guide you throughout your preparation. Don’t wait any longer; start
studying with Study for FE (https://www.studyforfe.com/) today.
Wasim Asghar – P.E, P.ENG, M.ENG
(https://www.studyforfe.com/aboutme/)
Licensed Professional Engineer in Texas (PE), Florida (PE) and
Ontario (P. Eng) with consulting experience in design,
commissioning and plant engineering for clients in Energy,
Mining and Infrastructure.
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