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Learning goals:
• Understand basic electrical symbols
• Understand power and control circuits
• Read electrical drawings.
1. Devices and symbols
Any electrical drawing representing an electrical installation or a circuit takes the help of specific symbols to represent various electrical devices in shorthand. This provides a quick idea to the reader about a circuit or installation, and is particularly useful while troubleshooting.
Therefore, it is important to familiarize oneself with various symbols. Some of the commonly used device symbols are detailed in the following section and in FIG. 1.
2. Electrical circuits
Electrical circuits are circuits used to interconnect different electrical equipments together to enable the working of an electrical device.
Electrical schematics are commonly classified into power circuit and control circuit. A power circuit consists of the main power device (a motor, a generator, or other power devices) along with heavy power conductors, contactors, protection devices.
A control circuit consists of switches, field device contacts, timers, relay coils, relay contacts, protection devices, and light power conductors.
2.1 Power circuits
Power circuits are required for carrying power to or from heavy electrical equipments like motors, alternators, or any electrical installation.
They carry out the following functions:
• Isolation using devices such as isolators, linked switches and circuit breaks.
• Circuit control using devices such as contactors, motor circuit breakers, etc.
• Protection against overload and short-circuits using thermal overload relays, electro-magnetic relays, circuit breakers, with releases, fuses, etc.
FIG. 1 Electrical devices and symbols.
Power circuits have to carry heavy power and therefore, they consist of heavy conductors along with contactors used for switching the power on and off. Protection devices are also included in the same power circuit for resolving an overload condition or any other concerned faults.
For example, FIG. 2 depicts a Direct-on-line (DOL) starter power circuit used for a three-phase induction motor. As shown, the three-phase power input is connected to the motor through a contactor. Power is passed to the motor when the contacts (of the contactor) are in closed condition. Protection devices such as fuses, and overload relays are provided in series with power conductors to detect unhealthy conditions during operation.
FIG. 2 Power circuit for a motor.
2.2 Control circuit
A control circuit is for the automatic control of equipment, for safety interlocking, and sequencing the operations of the plant equipment and machines.
Control circuits hardware consists of relay contacts, wires, hardware timers, and counters, relay coils, etc. These consist of input contacts representing various conditions; the output coils are energized or de-energized depending on the input conditions represented by the control circuit.
Input contacts represent the binary state of the condition:
• True or false
• On or off. There are two types of contacts NO (normally open) and NC (normally closed).
• Input contact: These are contacts of relays, contactors, timers, counter, field instrument switches, pressure switches, limit switches, etc.
• Output coil: These have two states - On or Off. Output coil can be auxiliary contactor or Main contactor coil.
A few simple control circuits are shown in FIG. 3 to represent logical AND, OR, and such conditions.
1. 'AND' operation circuit FIG. 3(a) shows a simple control circuit (AND operation) with two input contacts (NO) representing two conditions that must be true to complete the circuit to switch on the output relay coil and change the state of output from 'Off' to 'On'.
2. 'OR' operation circuit FIG. 3(b) shows a circuit with three input contacts (NO) representing that at least one of the three conditions should be true to complete the circuit to switch On the relay coil and change output state from 'Off' to 'On'.
3. 'AND with OR' operation circuit FIG. 3(c) shows a control circuit, consisting of a combination of AND and OR operations. There are two parallel (OR condition) paths with two input contacts (NO) connected in series in each path representing AND conditions. The path for coil K3 will be completed when one of the path conditions comes true. The circuit then will switch 'On' the relay coil and change the output state from 'Off' to 'On'.
FIG. 3 Simple control circuits.
Example 1: Design control circuit for 'Tank water level control'. Operation sequence should be such that:
• When the water level goes below the low-level limit, open the inlet valve of water tank.
• When water level goes above high limit is detected, close the inlet valve.
Build a control circuit for the same.
As shown in FIG. 4, when the level is initially low, coil K will pickup (since both level switch NC contacts will remain as it is), thus energizing the inlet valve to open.
FIG. 4 Example of simple control circuit for water tank inlet valve
operation. Assume: both LH & LL as NC (when level is below switch).
Same NO contact is used to drive the inlet relay coil.
As the level rises above switch LL, its NC contact will open but still coil K will remain latched through latching contact K. Once the level rises above switch LH, its NC contact will open causing coil K to de-latch or de-energize and the inlet valve will close.
Now, coil K will not pickup or inlet valve will not open unless the water level drops below low-level switch LL.
3. Reading and understanding electrical drawings
Electrical drawings can represent anything from a single-line power distribution, to a power or control circuit, and are prepared using various symbols for electrical devices and their interconnections with lines representing conductors or wires used for interconnections.
To read and understand electrical drawings, it is necessary to know the following:
• Symbols used for representing electrical devices
• Their interconnections, legends, terminology, and abbreviations
• Sheet numbering and column format for each sheet
• Wire and terminal numbering (an important aspect in understanding electrical drawings). Wire and terminal numbers are quite useful during installation and termination of cables, and during fault finding and troubleshooting.
It is easy to trace the connections and continuity of wires, if the wires and terminals are numbered using thorough cross-referencing terminology. Various examples of electrical schematics are shown in this section to explain the drawing methodology in practical circuits and in the interest of simplifying the scheme the following have not been included. These are however a MUST and will be insisted upon by the regulatory agencies.
• Any power circuit has to be provided with an isolating mechanism which usually includes the fuses also in the form of a switch-fuse unit. The schematics here depict only the fuse.
An emergency switch or a push button has to be provided near a mechanism to positively isolate the electrical circuit feeding the mechanism in the case of any emergency/accident.
The NC contact of such a push button is connected in series with the other control contracts such as overload relay. The push button mechanisms are lockable and need a key to release once the push button is pressed.
3.1 Things to look for in an electrical drawing
1. The symbols shown for a device in a circuit represent its de-energized state when no power is applied. It is either a timer NO/NC contact or a relay NO/NC contact in a circuit. In addition, power devices such as circuit breakers and contactors are provided with NO and NC auxiliary contacts which are used for indicating the status of the device in signaling and interlocking circuits.
2. An electrical drawing has a sheet number and each sheet is divided into columns listed vertically as A, B, C, D and horizontally as 1, 2, 3, 4. This kind of matrix arrangement helps in quickly locating a particular device or contact in a sheet. Similarly, it is used to cross-reference a contact.
3. In order to identify different coils and their contacts a letter such as K1, K2 or C1, C2 is placed next to the circle of the coil. Contacts of the same contactor coil are shown with the same letter in the drawing.
4. Particular relay contacts may be used in different circuits at different locations. To give the reader an exact idea of where it is used, a drawing mentions a cross-reference number for each contact showing the sheet number and its matrix number.
5. In general, a heavy line is used to show high current-carrying conductors (mains supply lines, motor connection leads). In contrast, light-looking lines are used to represent low current-carrying conductors (control circuit lines).
6. Control circuit power lines are shown as L1 and L2; load (coils of relay) is connected between these two lines in series with switches, fuses, etc.
7. Conductors intersecting each other with no electrical junction in between are represented with an intersection without any dot. Conversely, conductors having an electrical junction are represented with a dot at the intersection.
8. A broken line in an electrical circuit represents mechanical action. Generally, it is associated with a push button or switch closing or opening a contact.
Sometimes these lines can also indicate in combination with suitable additional symbols, a mechanical interlocking between two or more devices such as contactors or circuit breakers.
9. Dotted lines are used to differentiate an enclosure from field devices.
10. A wiring diagram of electric equipment represents the physical location of the various devices and their interconnections.
11. In an electrical drawing, conductors are marked with cross lines and dimensions of conductors are given alongside. This is used to represent the conductor size of a particular section in a drawing.
Based on the above hints, let us consider a few common examples of electrical drawings.
Example 2: Three-phase motor with DOL starter.
This is depicted in an electrical drawing in FIG. 5 along with the power and control circuits.
FIG. 5 Typical electrical drawing of power and control circuits for
a three-phase motor with DOL starter.
The power circuit consists of a three-phase main supply with a fuse unit for protection purposes. The other side of the fuse unit is connected to a power contactor. The output terminals of the contactor are connected to an overload relay. Finally, the overload relay output terminals are connected to motor terminals. The control circuit for the motor works on a 110 V AC single-phase supply. The phase of the control supply is connected to a NC contact of the overload relay (O/L). The wire from the O/L relay contact is connected to an auto/manual mode selector switch.
In the auto mode, the motor gets a start/run command through a potential-free contact (Terminals 835-836) of a relay, which in turn is energized with a Programmable Logic Controller (PLC) output.
In the manual mode, the motor can be started with the help of a start pushbutton. When the start pushbutton is pressed, the control circuit is completed and the auxiliary control contactor (C1) coil is energized. A potential-free NO contact of the contactor (C1) is closed and keeps the contactor C1 latched when the start pushbutton is released. When the auxiliary contactor (C1) is on, the motor power circuit is completed and the motor starts and remains on until the contactor C1 is de-energized and the power circuit to the motor terminals is broken.
For the manual mode, additional interlocks to trip the motor are connected between terminals X3.1 and X3.2. The motor can be stopped with a stop pushbutton. The NC contact of a stop pushbutton breaks the control supply to the auxiliary control contactor (C1) and the motor is stopped. The neutral for the control circuit is connected with a neutral link (N/L). To indicate that the motor is ON or running, an indication lamp is connected in parallel to the contactor, which goes ON whenever the auxiliary contactor is turned on. Another indication lamp to indicate a motor trip is connected to a NO contact of the overload relay. When the motor is overloaded, the NO contact is closed and the TRIP indication lamp is turned ON, until the overload relay is reset. In the control circuit, potential-free contacts, 2 NO and 2 NC, of the auxiliary contactor (C1) are connected to various pairs of terminals such as X3:3 - X3:4 (NC), X3:5 - X3:6 (NC), 80 - 191 (NO), and X2:3 - X2:4 (NO).
The NO contact of the auxiliary control contactor is terminated at the terminals X2:3 and X2:4, and is used in parallel to the start pushbutton NO contact for latching purposes.
In addition, the contact letters 9F8-9F9 are mentioned. This shows the location of the contact in the drawing.
FIG. 6 Typical electrical drawing of power circuit for a three-phase
motor with star-delta starter
Example 3: Three-phase motor with star-delta starter
The electrical drawing in FIG. 6 depicts this power circuit.
The power circuit consists of a three-phase mains supply with a fuse unit, three contactors - line contactor, star contactor, and delta contactor. The line contactor gets its three-phase power supply from the fuse unit and the output terminals of the line contactor are connected to the overload relay. Overload relay output terminals are connected to the motor terminals - U1, V1, W1. Motor terminals U2, V2, W2 are connected through either star or delta contactors.
The star contactor and delta contactor are mutually interlocked in the control circuit to ensure only one contactor is on at a time. When the delta timer is on, the motor-winding terminals -- U2, V2, W2 -- get a three-phase supply and the motor is delta-connected.
When the star contactor is on, the motor terminals - U1, V1, W1 - are shorted and the motor is star-connected. The control circuit as shown in FIG. 7, for the motor, works on a 110 V AC single phase supply. The phase of the control supply is connected to a NC contact of the overload relay (O/L). The wire from the O/L relay contact is connected to an auto/manual mode selector switch.
FIG. 7 Typical electrical drawing of control circuit for a three-phase
motor with star-delta starter.
In an auto mode, the motor gets a start/run command through a potential-free contact of a relay, which in turn is energized with a PLC output. In the manual mode, the motor can be started with the help of a start pushbutton. When the start pushbutton is pressed momentarily, the control circuit is completed and the line contactor is energized. A potential-free NO contact of the line contactor is closed and keeps the control complete when the start pushbutton is released. When the motor is started, the star contactor is closed and the motor is started with a star connection. As the motor runs for a few seconds, the delta timer picks up, which energizes the delta contactor and de-energizes the star contactor. The motor continues to run, connected in the delta configuration, until it is stopped with the stop pushbutton or trips due to an overload or an external interlock.
As can be viewed in FIG. 7, each contactor used contacts that are given at the end of the drawing. For example, NO contact used is shown with letters 4F7-4F8 and 4F8-4F9 specify their locations in the drawing. Similarly, contact details of contactor C2 and C3 are shown.
Note: The overload relay in this circuit is actually connected in series with the phase winding of the motor in the normal running mode (i.e., delta connection). The motor rated current is normally indicated in terms of the line current which is greater than the phase current by a factor of 3 . Selection and setting of the overload relay must take this into consideration.
Example 4: Let us consider electrical drawings of an inverter drive, as shown in Figures 8 and 9. FIG. 8 shows the power circuit wiring for the motor and control circuit wiring for starting and stopping the motor. The three-phase power supply is passed through the fuses and a contactor (1K1) and is connected to an incoming choke (CH1). The output of the choke (CH1) is connected to the input terminals of the inverter drive. The inverter drive gets its main power only when the contactor (1K1) is on. The inverter drive output supply is connected to an output choke (CH2) and the output of the choke (CH2) is connected to the three-phase motor terminals. The inverter drive and the motor are earthed.
The control circuit for the inverter drive works on a 110 V AC single-phase supply. The control circuit for contactor (1K11) consists of the following series of potential-free contacts:
1. Drive OK (NO contact of 1K12)
2. Emergency stop (NO contact of 1K13)
3. Local stop pushbutton (NC contact)
4. Remote stop pushbutton (NC contact)
5. Local/remote selector switch changeover contacts
6. Start pushbutton (NO contact).
The contactor 1K11 is energized when the control circuit is completed.
The contactor 1K1 is energized, when the drive output contact is closed and additional interlocks connected between the terminals 1X 11:11 and 1X11:12 are OK. A NO contact (13-14) of 1K1 is used to turn on the indication lamp (L2) to indicate that the drive is on. An NC contact of 1K1 is used for indicating a drive trip by turning the lamp (L3) on.
Another contactor (1K12) is energized to indicate drive OK, using a 24 V DC supply through a potential-free contact drive O/P (Terminal X100: 6-7). FIG. 9 shows the wiring diagram for the inverter drive control terminals. The inverter drive has the following sets of terminals:
• X100: Contacts for drive OK status
• X101: For start/stop (13-16), reset faults (13-18) commands to the inverter drive
• X102: For remote speed reference (25-27-28) for the inverter drive and analog outputs for speed indication (34-35)
• X9: Main contactor ON (4-5) and power supply (1-2) for external use.
As shown in the figure, terminal grouping is based on different operational functions.
For example, digital contacts of drive are grouped with letter X101; whereas analog speed reference input and rpm display output are grouped with letter X102.
FIG. 8 Power and control circuit for an inverter drive
FIG. 9 Control circuit with wiring terminals for an inverter drive
4. Reading and understanding ladder logic
Once the hardwire relay logic concepts are understood then its easy to comprehend ladder logic.
The term 'Programmable Logic Controllers' (PLCs) originated from relay-based control systems. In a PLC, there is full flexibility to change the sequence of operations and interlocks for different conditions.
There are integrated circuits and internal logic in the PLC, in place of discrete relays, coils, timers, counters, and other such devices.
PLCs provide greater computational capabilities and accuracy, to achieve increased flexibility and reliability, than hard-wired relays.
The symbols and control concepts used in PLCs come from relay-based control and form the basis of ladder logic programming (FIG. 10).
FIG. 10 Comparison of relay and PLC terms
In the following sections, commonly used terminology for ladder logic is dealt with.
The terminology used in commercially available PLCs from various manufacturers may differ slightly but the concepts remain the same.
4.1 PLC terminology
PLC terminology may differ from relay terminology, but the control concepts are the same.
The following are some of the terms used in relays and PLC:
-----------------
Terms Used for Relay | Equivalent Terms in PLC
Contact input or condition Coil output or temporary working bit NO contact of relay condition Normally open NC contact of relay condition Normally closed
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As such, there is no equivalence between these terms. The term 'condition' is only used to describe ladder logic diagram programs in general and is equivalent to a set of basic instructions. The terms input/output are used for reference to I/O bits assigned to input and output signals.
In ladder logic programming, the following two types of instructions are used:
1. Instructions that correspond to the conditions of the ladder logic diagram.
They are used in instruction form only when converting a program to mnemonic code.
2. Instructions that are used on the right-hand side of the ladder logic diagram are executed according to the conditions on the instruction lines preceding them.
Most of the instructions have at least one or more operands.
4.2 Ladder logic diagram
A ladder logic diagram is so-called because the relay logic runs in parallel lines between two power lines and the whole diagram resembles a ladder.
This diagram consists of one vertical line running down the left side, with the horizontal lines branching off to the right. The line on the left is called the bus bar, while horizontal lines are instruction lines or rungs. Along the instruction lines conditions are placed, that lead to other instructions on the right side. Power flow is always from left to right. Therefore, the logical combination of these conditions from left to right side determines when and how the instructions at the right side are executed. In a ladder logic diagram, instruction lines can have multiple branches. The vertical pairs of lines are called conditions. Conditions without diagonal lines through them are called NO conditions that correspond to AND, LOAD, or OR instruction.
The conditions with diagonal lines through them are called NC conditions that correspond to AND NOT, LOAD NOT, or OR NOT instruction. Each condition has a number above/below each condition that indicates the operand bit for the instruction. Operand bit (Input/ Temporary bit) is associated with that condition.
The status of the bit determines the execution condition for the following instructions.
4.3 Basic terms used in ladder logic
Normally open and normally closed conditions Each condition in a ladder logic diagram is either 'ON' or 'OFF' depending on the status of the operand bit that has been assigned to it. A NO condition is 'ON' if the operand bit is 'ON' and it is 'OFF' when the operand bit is 'OFF'. On the other hand, a NC condition is 'ON' if the operand bit is 'OFF' and it is 'OFF' when the operand bit is 'ON'. In short, an NO condition simply follows the bit status (on => on and off => off) and an NC condition follows inverted bit status (on => off and off => on).
Execution conditions
In a ladder logic program, the logical combination of 'ON' and 'OFF' conditions before an instruction determines the conditions under which the instruction is executed. This condition is called the execution condition for the instruction. Except for the 'LOAD' instruction, all other instructions have execution conditions.
Operands
The operands designated for any of the ladder logic instructions can be I/O bits, flags, work bits or flags, timers, or counters, etc. In a ladder logic diagram, these conditions can be determined using these operands.
Logic blocks
The manner in which the conditions correspond to instructions is determined by the relationship between the conditions, within the instruction lines that connect them. Any group of conditions that go together to create a logic result is called a logic block.
4.4 Ladder logic instructions
The ladder logic instructions correspond to the conditions on the ladder logic diagram.
Ladder instructions are either independent, or are in combination with the logic block instructions, from the execution conditions, based on which the execution of all other instructions are dependent. The most common ladder logic program instructions and the symbols used are shown in the FIG. 11.
FIG. 11 Commonly used ladder logic program instructions and symbols
4.5 The 'END' instruction
The last instruction required to complete a ladder logic program is the 'END' instruction.
When the PLC CPU cycle runs through the program, it executes all the instructions up to the first 'END' instruction. After the 'END' instruction, it returns to the beginning of the program and begins the execution again. Usually, the 'END' instruction is the last instruction in the ladder logic program, but it can be placed at any point in the program, like when program debugging is undertaken. No instruction after the 'END' instruction is executed. The 'END' instruction requires no operands, and no conditions can be placed with the 'END' instruction.
4.6 Examples of simple ladder logic instructions
Examples of ladder logic instructions for simple control circuits (AND, OR, AND with OR) are shown in FIG. 12.
FIG. 12 Examples of ladder logic instructions for simple control circuits
4.7 The ladder logic diagram
The ladder logic diagram is one of the methods of programming PLCs and is covered in detail in the IEC standard 61131 Part 3. Ladder diagram is a very convenient way of representing interlocking logics which used to be configured using hard wired devices.
Current generation PLCs have other capabilities including that of PID controllers. The IEC standard therefore provides more advanced methods of programming such as Structured Text, Function Block diagram and Sequential Function chart for tasks which cannot be adequately represented using Ladder diagrams alone.
5. Wires and terminal numbering
In any electrical control panel, there are wires to which various electrical devices are connected. It is important that electrical devices in a circuit are connected accurately through wires with proper voltages and polarity.
To ensure proper connections of wires, devices as well as terminals (through which they are routed) are given unique numbers.
This practice is followed for designing, assembling, and maintenance. This helps in identifying the devices, wires, and terminals during troubleshooting.
In an electrical panel, terminals are used to connect the wires. Generally, they are grouped together and called 'Terminal Block'. They are grouped either as per their functional use or as per the device connected.
Each terminal block consists of a group of terminals with an assigned 'Terminal Block Number'. Each terminal on the block is assigned a unique 'Terminal Number'. In a panel, usually one side of the terminal is used for connecting internal wires from the devices inside the panel, and the other side is used for field or external connections.
In electrical panels, wires and cores of multi-core cables are used for interconnections.
Wires and cable cores are terminated on device terminals and terminal blocks. Wires and cable cores used for interconnection are numbered. Alphabetical symbols and numbered ferrules are used on each wire or core of the cables.
The numbering of wires should consist of the following details:
• Cable number
• Wire or core number of the multi-core cable
• Terminal block number
• Terminal number on which the wire is to be terminated.
As a wire is connected at two ends, it is quite useful to use a cross-referencing method for numbering the wires. Cross-referencing of wires or cable cores include the details of the other end of the wire where it is terminated. Details such as 'Panel Number', 'Terminal Block Number', and 'Terminal Number' of the other end of the wire are also included apart from the above-mentioned details of the termination end.
Cross-reference wire numbering and terminal numbering is shown in FIG. 13.
FIG. 13 Cross-reference numbering of wires and terminal numbering.
Though the wire numbering and terminal numbering shown in the figure is typical, in practice, there are many ways and methods of numbering wires and terminals that may be adopted. A cross-referencing number is one of the methods found useful during cable laying and termination, continuity testing, and troubleshooting.
As shown in FIG. 13, cross ferruling is used between TB1 and JB2 terminal blocks for cable C12 cores. The ferrule at TB2 terminal block gives an idea of where the other end of the core is connected.
For example, as shown in the FIG. 14, a PLC panel wiring along with cross- reference ferruling, PLC address information is also included. It is quite useful to include the PLC address in the wire ferrule number apart from the cable number, core number, terminal number, and cross-reference detail for troubleshooting. In FIG. 14, cross-referencing ferruling is used for field devices and terminal block wiring, as well as inter-terminal block wiring. Although this kind of ferruling involves lengthy ferrule numbers, the practice is certainly worth the effort while troubleshooting.
FIG. 14 Wire numbering in a PLC panel with additional details such
as PLC addresses.