How to Read and Interpret Single Line Diagram – Part Two

Today, we will explain the following:

  • Voltage Regulators Data,
  • Transformers Data,
  • Switchgear data.


Interpreting Single-Line Diagram – Part Two

The interpretation of single-line diagrams is explained in this section under the following major subject headings:
  1. Informational Elements
  2. Details Variations
  3. Relationships between one-line diagrams and associated reference documents.


D.1- Informational Elements
The following informational elements of a single-line diagram are explained in this section:
  1. Point of connection to the utility company,
  2. Identification of buses, substations, generators, motors and other equipment,
  3. Ring Main Units data,
  4. Automatic voltage regulator data,
  5. Power transformer data,
  6. Switchgear data,
  7. Protective device ratings,
  8. Instrument transformer data,
  9. Metering, relaying, and control device data,
  10. Available short circuit current data,
  11. Switch or breaker status,
  12. Key interlock systems,
  13. Protective relay trip logic,
  14. Generator data,
  15. Load bank data,
  16. Connected load data,
  17. Cable ratings,
  18. Bus ratings,
  19. Future installation.


We will use the below single-line diagram throughout these articles for explaining how to interpret this type of electrical diagrams. You can download a PDF copy of this single-line diagram by following the link.


D.1.4 Voltage Regulators Data




The automatic voltage regulator (AVR) is a device designed to regulate voltage automatically – that is, to take a fluctuating voltage level and turn it into a constant voltage level
Why would we need a voltage Regulator?
Variation of voltage can have detrimental effects on Utilities and their customers so; we use AVR to prevent customer complaints, loss of revenue due to sub-normal voltage, and increased costs due to higher line losses.
Generally, AC automatic Step-Voltage Regulators applied in the utility’s distribution systems are two distinct types as follows:
  1. Medium-voltage (mechanical)
  2. Low-voltage regulators (mechanical or electronic).
The difference in their operation and design clearly demonstrate that their applications are not the same as follows:
  1. The medium-voltage regulators, which are primarily used by the electric utility to compensate for the voltage drop in the feeders or distribution systems, the term step-voltage regulator is often used to refer to utility AVR.
  2. The Low-voltage regulators are intended to protect end-user devices from overvoltage and under-voltage conditions.


AVR Regulation range

  • A voltage regulator may also perform a voltage step up or step down function whereby the nominal incoming voltage is transformed to a different output voltage level.
  • Utility step-voltage regulators usually allow a maximum voltage regulation range of ±10% of the incoming line voltage in 32 steps of 5/8% or 0.625%. That makes 16 steps each for buck and boost – 5/8% x 16 steps = 10%. For a voltage regulator with step up or step down capability, the input and output voltage ranges are usually applied to the input and output voltages.

AVR units connection Arrangements with supply and loads:

Automatic voltage regulators can be designed for single-phase or three-phase AC applications as per the following arrangements:
A- Single phase AVR with 3-phase Supply& Three phase Loads
It is common for utilities to use single phase automatic voltage regulators ganged together to provide voltage regulation for three phase. These are often “can-type” units pole-mounted outdoors.
B- Single phase AVR with 3-phase Supply & single phase Loads
Single phase automatic voltage regulators may also be used where a three phase source is used to supply three single phase loads.
C- Three phase AVR with 3-phase Supply & single phase Loads
Most three phase AVRs may also be used to feed single phase loads.
  • For three phase loads, it is usually more cost effective to use a three phase AVR.
  • A three phase automatic voltage regulator might regulate all three phases together or it might regulate each phase independently, depending on the design of the AVR.
  • When dealing with three phase power, it is not uncommon to find that one phase has a “high” voltage level while another has a “low” voltage level. In this situation, being limited to regulating the voltage level of all three phases together, up or down the same amount, may not produce satisfactory results.
  • Independent phase regulation is often the preferred method since it typically provides better phase-to-phase voltage level balance. Large differences in voltage levels from phase-to-phase can cause premature failure of electrical devices due to overheating or vibration.



Single phase AVR units power connection
The single phase AVR units can be connected as a single phase or 3- phase circuit as per the following connection diagram:
3 nos. of single phase AVR units can be connected as Delta or Y bank as shown in the above connection diagram.
  • Before connecting AVR units, proper grounding, surge arrestors and bypass switching devices suitable for the line current must be used.
  • When AVRs are Y- connected, the neutral of the regulator bank must be effectively connected to the system neutral, preferably by the fourth wire.


AVR data:
  • Type: pole mounted or station pad mounted,
  • Voltage Range,
  • KVA rating,
  • BIL rating,
  • Regulation percentage.


On Fig.1, the following information is noted:


  • Type: pole mounted
  • Voltage Range: 13.8 KV
  • KVA rating: 125 KVA
  • Regulation percentage: ±10%





D.1.5 Transformers Data






A transformer is a device that converts electrical power in an AC system from one voltage or current into another voltage or current.
Principle of operation:
  • Transformers operate on the principle of induction as shown in below figure. When the magnetic field of a conductor that carries a current (the primary coil) moves across another conductor (the secondary coil), a voltage is produced in the second conductor by induction.
  • Transformers can "step-up" or "step-down" voltage, depending on the ratio of primary to secondary coils. Step-up means that the output voltage (secondary) is higher than the input. Step-down means the output voltage is less than the input voltage.
Method of classifications:
Transformers are identified with symbols according to their function as per the above figure and are classified according to:
  1. Method of cooling
  2. Number of phases
  3. Purpose
  4. Insulation between windings
  5. Method of mounting
  6. Service
For more information about transformer construction, classifications and types, please review our course “EP-3: Electrical Procurement – Transformers Course
Where the following topics were/will be explained:
  • Transformer design,
  • Transformer types,
  • Transformer components,
  • K-factor Transformer,
  • Transformer accessories,
  • Transformer paralleling,
  • Transformer protection,
  • Transformer ratings,
  • Transformer nameplate data,
  • Transformer testing,
  • Transformer troubleshooting,
  • Transformer dictionary.
Power Transformer Data
For each power transformer symbol that appears on a one-line diagram, the following information is printed next to the symbol:
  • A note that distinguishes whether the single transformer symbol represents a bank of three single-phase transformers, a three-phase transformer, or a single-phase transformer,
  • The kVA ratings with corresponding designations of cooling class,
  • The primary and secondary voltage ratings,
  • The percent impedance,
  • A winding polarity diagram.
On the below Figure, the following information is noted:
  • The circuit is 3-phase, 4 way having 60 Hz frequency.
  • There is a transformer symbol with the following data 3.750 MVA identifies this transformer as having a capacity of 3.750 MVA
  • The high-voltage winding is rated 13.8 line to- line kilovolts (kV) and The low-voltage winding is rated 380Y line-to-line volts / 220 line-to-neutral volts.
  • The transformer is connected high-voltage delta, low-voltage wye, and the low-voltage neutral terminal is grounded.
On the below Figure, the following information is noted:
  • The circuit is 3-phase, 4 way having 60 Hz frequency.
  • there is a transformer symbol with the following data 15/20 MVA OA/FA, identifies this transformer as having a capacity of 15 MVA when using its class OA (oil and air) cooling equipment, and a 20 MVA capacity when using its FA (fans and air) cooling equipment.
  • The high-voltage winding is rated 69 line to- line kilovolts (kV) and The low-voltage winding is rated 13.8 line-to-line kilovolts.
  • The note, Z=7.6%, indicates that the impedance of the transformer is 7.6%. Unless otherwise noted, the impedance shown on a one-line diagram is based on the transformer’s OA rating.
  • The polarity diagram indicates that the transformer is connected high-voltage delta, low-voltage wye, and the low-voltage neutral terminal is grounded.



D.1.6 Switchgear data



Switchgear is a generic term. The industry uses it to cover “assemblies of switching and interrupting devices, along with control, metering, protective and regulating equipment.”
  • The switchgear is used mainly for:
  • Electrical protection,
  • Safe isolation from live parts,
  • Local or remote switching.
Switchgear can be divided to many types according to many classification factors like these included in the below image.
The most important classifications are as follows::
1- According To Their Voltage Level:
  • Medium voltage switchgear: range of 1000 volts to 38 kV.
  • Low Voltage switchgear: below 1000 volts.
2- According To Their Location:
  • Indoor switchgear,
  • Outdoor switchgear.
3- According To Their Function:
  • RMU,
  • MV Distributors,
  • LV Distributors,
  • MCC (motor control center),
  • Load center (MDB, SMDB& DP),
  • Control and/or monitoring,
  • PFC (Power Factor correction),
  • Synchronization,
  • Charging,
  • ATS automatic Transfer Switches,
  • Lighting control,
  • Feeder pillars.


Construction of Switchgear
First: Medium Voltage Switchgear
A typical medium voltage switchgear assembly has:
  • Removable medium voltage circuit breakers
  • Separate compartments for main bus
  • Separate compartments for incoming/outgoing line connections
  • Separate compartments for circuit breakers, control equipment and other auxiliary equipment
  • Insulated main bus and connections
  • Metal barriers separating each vertical structure and each compartment within each structure.
Metal-Clad Medium Voltage Switchgear
Metal-Clad  vs. Metal-Enclosed Medium Voltage Switchgear
  • Metal-Clad Medium Voltage Switchgear: the structures (and compartments within each structure) are physically separated from each other by grounded metal barriers.
  • Metal-Enclosed Medium Voltage Switchgear: (often associated with low voltage equipment) encloses the equipment in separate metal vertical structures. However, compartments are not separated from one another with metal barriers.

Second: Low Voltage Switchgear
The low voltage switchgear can all be used as service entrance electrical distribution equipment or as a load center or that is feeding power to a number of smaller circuits. It is divided to:
  1. switchgear,
  2. switchboards,
  3. Panelboards.


1- Switchgear
Low voltage switchgear provides centralized control and protection of low voltage power equipment and circuits in industrial, commercial and utility installations involving transformers, generators, motors and power feeder circuits.
Low voltage switchgear
Low voltage switchgear features the following components:
  • Low voltage drawout power circuit breakers,
  • Circuit breaker compartments,
  • Primary and secondary power connections,
  • Secondary control compartments,
  • Structures,
  • Busbars (main and section)
  • Customer termination areas.
Switchgear is generally installed at the highest level of the power system. Cables or conduits can be used to feed power from the switchgear into other switchboards, panelboards or main loads.


2- Switchboards
For larger scale buildings or sites, a large single panel, frame, or assembly of panels can be used for mounting the overcurrent switches and protective devices, buses and other equipment. These floor-mounted, freestanding solutions are known as switchboards. Switchboards are most often accessible from the front, mounted on the floor and close to the wall.
The primary components of a switchboard include:
  • The frame,
  • Bus,
  • Overcurrent protective devices,
  • Instrumentation,
  • Enclosures
  • Exterior covers.
There are four main structure types common to all switchboards, but all switchboards do not use all of these structure types:
A- The Main Switchboards Structure: 
It contains the main disconnects or main lugs. It often contains surge protection, utility and/or customer metering equipment.
B- The Pull Structure:
It is a blank enclosure containing empty space through which cabling can be pulled. It is commonly used with service entrance switchboards where the utility feed comes through the floor. Service can be fed from the top without any exposed conductors.
C- A Distribution Structure: 
It divides and sends power to branch circuit protection devices and then to branch circuits to power downstream loads. Power moves from the incoming structure to the distribution structure via cross bus.
D- The Integrated Facility System (IFS) Switchboard Structure:
It includes panelboards, dry-type transformers, transfer switches and blank back pans for field mounting other equipment. The IFS is helpful when panelboards and dry-type transformers are used in the same room as switchboards as it can reduce the need for linear wall space and area required for equipment. A key benefit of the IFS is that it significantly reduces the installation and wiring time and the number of pieces of equipment to be handled.
Integrated Facility System (IFS) Switchboard


3- Panelboards
A panelboard is a component of an electrical distribution system that divides an electrical power feed into branch circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure. In essence, panelboards are used to protect against electrical overloads and short circuits while distributing electricity throughout a building or facility.
The main components of a panelboard typically include:
  • The enclosure,
  • Interior,
  • Circuit protection devices,
  • Labels,
  • Deadfront and trim,
  • Filler plates.
Panelboards can be installed using one of two common approaches; flush mounted or surface mounted. When flush mounted, the panelboard is placed in a recessed area between the wall studs. When surface mounted, the panelboard projects out from the wall.
Panelboards are often categorized by their general application to:
  1. lighting and appliances
  2. power.
Lighting and appliance panelboards contain overcurrent protection and a means to disconnect lighting, appliances, receptacles and other small load circuits. All other panelboards are used for power and may also feed other panels, motors and transformers in the building’s or site’s overall power distribution systems.


Switchgear Rating Data
The switchgear rating data that must appear on the Single-Line diagram will be as follows:
  • Operation voltage,
  • Busbar Rating,
  • Short circuit capacity,
  • Nos. of phases,
  • Nos of wires,
  • System frequency.
In our example, the following data are provided for Low voltage switchgear MCDS:
  • Operation voltage = 380/220 V
  • Busbar Rating = 6000 A
  • Short circuit capacity = 100 KAIC
  • Nos. of phases = 3
  • Nos of wires = 4

As per the above switchgear definition, it can be break up to different parts as follows:

  • “Control devices” check and/or regulate the flow of power.
  • “Switching and interrupting devices” are used to turn power on or off.
  • “Metering devices” are used to measure the flow of electric power.
  • “Protective devices” are used to protect power service from interruption, and to prevent or limit damage to equipment.

In the next article, we will explain the data required on a single lie diagram for each switchgear part from the above list. So, please keep following.

The previous and related articles are listed in the below table:

Subject of Previous Article
1- Overview for the articles/courses that give a preliminary explanation for the different Types of Electrical drawings.
2- Electrical Drawings Glossary.
3- Resources used to Read and Interpret Electrical Drawings.
4- Electrical Symbols and Abbreviations
5- Electrical Abbreviations
6- Device Function Numbers
7- Drafting Practices Using Graphical Symbols and abbreviations
 8- Basic Elements of Electrical Drawings
9- Types of Electrical Drawings
9.1- Electrical Diagrams
9.1.1- Single-Line /One-Line Diagram
A- Characteristics of Single-Line Diagram
B- Purposes of Single-Line Diagram
C- Arrangement of Components on Single-line Diagrams
D- Interpreting Single-Line Diagrams
D.1- Informational Elements
D.1.1 Point of Connection to the Utility Company
D.1.2 Identification of buses, substations, generators, motors and other equipment
D.1.3 Ring Main Units data

Back To 
Electrical Shop Drawings Course


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