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Electrical diagrams are drawings in which lines, symbols, letter and number
combinations are used to represent electrical circuits.
 In some plants, electrical diagrams may also be called prints, or blue prints
Electrical drawings are valuable tools for
 Making new electrical installations
 Locating electrical problems
 Modifying existing circuits
Symbols are used to standardize the reading of electrical diagrams.
Electrical diagrams use a variety of symbols to represent component in
electrical circuit
Two tables that are helpful in understanding electrical diagrams are:
 American standard device function number
 Identifies the general function of electrical devices in terms of number
 Standard diagram abbreviation table
 Lists abbreviations that are used to identify components
Drafting Practices Using Graphical Symbols
(a) The
orientation of a symbol on a drawing does not alter the meaning of the symbol.
This is true even if the symbol is drawn backwards. A symbol is made up of all its various
(b) The weight (or width) of a line does not affect the meaning of the symbol. In some cases
a heavier line may be used for emphasis.
(c) Symbols are not drawn to scale. They can be drawn to any size compatible with the scale
of the drawing.
(d) Arrowheads can be drawn closed or open, except when showing a "protective gap" (a
gap placed between line parts and the ground which limits the maximum over-voltage
that may occur.)
(e) The standard symbol for a terminal (o) can be added to any one of the graphic symbols
where connecting lines are attached. This added terminal symbol is not a part of the
graphic symbol itself.
(f) In order to make a drawing simpler, graphic symbols for devices such as relays or
contactors may be drawn in parts. However, if this is done the drawing must show how
the parts are related.
(g) Most often, it does not matter at which angle a connecting line is drawn to meet a
graphic symbol.
(h) Broken lines with short dashes: - - - - - - , may be used to show paths or equipment that
will be added to the circuit later, or those which are connected to the circuit but are not
part of it.
(i) If details such as type, impedance, and rating are to be given, they should be drawn next
to a symbol. If abbreviations are used, they should be in accordance with the American
Standard Abbreviations for Use on Drawings. Letters that are joined together and use
parts of graphic symbols are not abbreviations.
Information found on electrical diagrams
Title block
Title block is usually located in the bottom right hand corner of an electrical
 It contains information that identifies the diagram
The notes on an electrical diagram usually give detailed information about certain
parts of the diagram and/or list other reference that may be needed
Legend identify symbols and designations that are used on electrical diagrams
 Some times, the legend is a part of the diagram sheet but, in many cases
,it is separate sheet
Suffix Letters
Suffix letters are used with device function number for various purpose. In order to
prevent possible conflict, any suffix letter used singly, or any combination of letters,
denotes only one word or meaning in individual equipment
For purpose of clarification, these suffix letter have been classified in several
Types of Diagrams
One-Line Diagram
A one-line diagram, which is also called a single-line diagram, is the most
commonly used diagram in an industrial power system.
 Interpreting the scope of a proposed installation of a power system.
 Serving as a basis to produce project drawings.
 Analyzing power system problems.
 Determining which circuit interrupters must be opened to safely isolate electrical
 A one line diagram uses
 Single lines
 Standard graphical symbols
 Standard nomenclature
 A one line diagram shows the power path of an electrical circuit or a system of circuit
 A one line diagram also shows the component devices or a parts of a power system
 The multiple conductors of power circuits and control circuits are shown as
single lines
SUB – 91, SUB – 82
A0, A1, B0, B1.C0
A2, A3, B2, B3, C2
– Substation 91, Substation – 82 respectively.
– A typical Unit Substation.
– A typical Emergency Diesel Generator Building.
– 230 kV SF6 Circuit Breakers
– 34.5 kV Circuit Breakers.
– SF6 Gas Insulated Load Break Switch
A4, A5, B4, B5
– 34.5 kV SF6 Load Break Switches
A6, B6, C6
– 4.16 kV Vacuum Circuit Breakers.
A6.1, B6.1
– 4.16kV Vacuum Circuit Breakers / Vacuum
A7, A7.1, A7.2, B7, B7.1, C.7 – 480V Air Circuit Breakers.
– 480V Emergency Automatic Transfer
Switch (EATS)
─ 480 V MCCB
T11, T12
– 34.5kV / 4.16 kV Distribution Transformers.
T21, T22
− 34.5kV / 480V Distribution Transformers.
Three-Line diagram
Three line diagram provides detailed information pertaining to threephase circuitry that is not shown on a one-line diagram
 Three-line diagrams help plant maintenance and operations personnel to
understand power system operations
 They are also used to develop metering and protective relay wiring drawing
A three-line diagram represents power system components using the
same standardized symbols as a one line diagram plus an additional
set of standardized symbols that are also used in schematic and wiring
 Unlike one-line diagram, a three-line diagram shows every
conductor of a power circuit as an individual line
Schematic Diagrams
Schematic diagrams shows circuit elements and internal connections in an arrangement
that allows a technician to interpret the function and operational logic of an electrical
control circuit
 Schematic diagrams are composed using the same standardized symbols as one-
line, three-line, and wiring diagrams.
 Schematic diagrams show all terminals and connections of functional devices.
 Figure shows a typical schematic diagram of a motor circuit and its associated
one-line diagram.
Types of Schematic diagrams
 Internal Schematic Diagrams
- show only the internal circuits
of a single physical device. Figure (a) is an example of an internal schematic
diagram. It represents the internal circuits of a protective relay.
 External Schematic Diagrams
- show the internal circuits of
physical devices but additionally show the external circuits that conduct input and
output signals into and out of the devices. Figure (b) shows the elements of the
same relay that is shown in Figure (a) but also shows the external circuit that
connect this relay to other physical devices.
 Elementary Diagrams
-- show all of the operational elements and all of
the circuits of a complete electrical control system. An elementary diagram is often used
to represent the complete control circuit of an electrical substation.
 Ladder Diagrams
-- are commonly used for understanding and designing
system control logic. A ladder diagram is a top-down logical line schematic: logical
because it moves from power input at the top through sequential operations. Figure
shows a ladder diagram that represents a generator control circuit.
Wiring Diagrams
 A wiring diagram uses standardized symbols to show the physical devices of an
electrical control panel and lines to show the wires that connect these devices to
each other.
 Wiring diagrams are used by equipment manufacturers to install wires in
electrical equipment such as switchboard and panels.
 They are also used to show the necessary interconnection wiring between
electrical equipment.
 For example, one type of wiring diagram, called an interconnection diagram,
is used to show the wiring between two or more switchboards.
 Wiring diagrams show functional devices in their correct relative physical locations.
 Standardized and non-standard symbols are used to represent these functional devices.
 Lines are used to represent single conductors.
 Multiple conductors that are bundled together or that are installed in the same channel
are shown as a single line with radial branches to show the locations where single
conductors or other bundles leave the path of the main-trunk bundle.
 Each representation of a conductor is labeled with an identifying number (conductor
Logic Diagrams
Logic diagrams show the logic for complex circuits, processes, or devices. Logic
diagrams utilize block-type and standardized logic-function symbols to represent
highly complex functions that are performed either by integrated processing
modules or by individual devices.
Logic diagrams enable the equipment users to understand the related logic
functions of devices or processes without requiring specific knowledge of their
internal operations.
 Logic diagrams utilize rectangular shaped blocks and standardized logic-function
symbols to represent highly complex functions, processes, or devices.
Each block contains a written description or a logic symbol that indicates the function of
the block.
On a logic diagram, straight lines represent the paths of process-control signals.
The points where these signal paths are illustrated as entering or leaving a block
represent the input and output signals of the block.
Logic diagrams are also used to represent the functions of an integrated control system
that comprises several physically separated devices and electrical circuits.
Numbering System Format
Major Electrical Equipment
The following numbering system shall be used for major electrical equipment in the primary
power distribution system, power supplies to process equipment, essential power distribution
systems, instrument power supplies, and supervisory systems.
The format is “XX-XX-XXXXXX” and is composed of the following parts:
 Note 1: The equipment category code shall consist of two to five letters and is a unique
code for each piece of electrical equipment.
 Note 2: The WBS (Work Breakdown Structure) unit number shall be per the
requirements of specification.
 Note 3: The voltage number is a two-digit number that identifies the voltage level as
01 - 230 kV
10 - 34.5 kV
20 - 13.8 kV
30 - 4.16 kV and 2.4 kV
40 - 480 V and below
 Note 4: The serial number is a two-digit number that starts at 01. Each
piece of electrical equipment has a unique serial number for its category
and substation. Substations do not have serial number.
 Note 5: A single letter (usually starts at A) is used to identify two or more
identical pieces of equipment in the same service, such as a shared serial
number by two transformers connected to the same double ended
switchgear or a shared serial number by two battery chargers connected to
a set of battery.
1. ESWG-83-2001A
ESWG -Essential Switchgear
83 -WBS unit number
20 -The voltage level of this equipment is 13.8 kV
01 -The first equipment in this series of this type of equipment
A -Connected to bus A
2. XFR-70-1001B
XFR -Power Transformer
70 -WBS unit number
10 -The voltage level of this equipment is 34.5 kV
01 -The first equipment in this series of this type of equipment
B -Connected to bus B
3. MCC-84-4003B1 or MCC-84-4003B2
MCC Motor Control Center
84 WBS unit number
40 The voltage level of this equipment is 480 volts
03 The third equipment in this series of this type of equipment
B Connected to bus B
1 or 2 The first or the second MCC connected to bus B
4. SUB-82
SUB Substation
82 WBS unit number
Note that substations do not have a voltage number or serial number.
Minor Electrical Equipment
The following numbering system shall be used for minor electrical equipment such
as lighting panelboards, terminal boxes and other equipment not listed as major
The format is “XXX-XX-X” and is composed of the following parts:
Note 1: The equipment category code shall consist of two to four letters and is a
unique code for each type of minor electrical equipment.
Note 2: The WBS (Work Breakdown Structure) unit numbers refer to the requirements
of specification.
Note 3: Each type of minor electrical equipment has a unique letter designation
assigned to it. All distribution panels shall be considered one type of minor
electrical equipment and shall not share common letter designation.
1. ELP-82-A
ELP -Essential Lighting Panel
82 -WBS unit number
A -Unique letter designation
2. PP-82-B
PP -Power Panel
82- WBS unit number
B -Unique letter designation