Electrical and Instrumentation (E&I) engineering blends two core disciplines: electrical engineering—covering power generation, distribution, protection, motors, drives, and circuit design—and instrumentation engineering—focused on sensors, transmitters, PLCs/SCADA, control loops, and automation that keep processes stable, safe, and efficient.
In modern industries, E&I engineers ensure reliable power, accurate measurement, and responsive control so plants run on-spec and on-time. Their work reduces downtime, improves product quality, and supports safety and regulatory compliance—critical outcomes for any production environment.
For fresh graduates, mastering these fundamentals builds confidence for interviews and lays the foundation for a strong technical career in E&I.
Answer: Alternating Current (AC) changes direction and magnitude periodically, while Direct Current (DC) flows in one steady direction. AC is used for long-distance power transmission; DC is common in batteries and electronics.
Answer: Ohm’s Law says the current (I) through a conductor is directly proportional to the voltage (V) across it, if temperature stays constant. Formula: V = I × R, where R is resistance.
Answer: A transformer transfers electrical energy between circuits by electromagnetic induction. It steps voltage up or down for efficient power transmission and safe use.
Answer: Real power does useful work (Watts), reactive power is stored and released by inductors/capacitors (VAr), and apparent power (VA) combines both, showing the total demand on the source.
Answer: A fuse melts once to stop excess current and must be replaced; a circuit breaker trips open and can be reset. Breakers are reusable and respond to more fault types.
Answer: Conductors (copper, aluminum) allow current easily, insulators (rubber, glass) block current, and semiconductors (silicon) conduct partially and are used in electronics.
Answer: Earthing safely directs leakage or fault current to the ground, protecting people and equipment from electric shock.
Answer: Voltage is electrical potential difference, current is the flow of electric charge, and resistance opposes current flow.
Answer: A capacitor stores electrical energy in an electric field, smooths voltage, and improves power factor in AC circuits.
Answer: An inductor stores energy in a magnetic field and resists sudden changes in current, used in filters and power supplies.
Answer: In series, components share the same current; in parallel, each branch has the same voltage. Parallel circuits keep other branches live if one fails.
Answer: Three-phase power uses three alternating currents, 120° apart, giving smoother power flow and efficiency for heavy loads.
Answer: Power factor is the ratio of real power to apparent power, showing how effectively electricity is converted into useful work. A value near 1 is desirable.
Answer: Ammeter (current), Voltmeter (voltage), Wattmeter (power), Multimeter (multiple parameters), and Clamp Meter (non-contact current).
Answer: Harmonics are voltage or current components at multiples of the fundamental frequency, causing distortion, heating, and reduced efficiency.
Answer: A relay is an electrically operated switch that opens or closes contacts to control circuits safely and automatically.
Answer: A short circuit is a direct low-resistance path causing high current; an open circuit is a break stopping current flow.
Answer: A diode allows current to flow in one direction, used for rectification, signal demodulation, and protection.
Answer: Synchronous machines rotate at constant speed with the supply frequency; asynchronous (induction) machines slip slightly below synchronous speed.
Answer: An alternator converts mechanical energy to AC electrical energy, widely used in power plants and vehicles.
Answer: RMS (Root Mean Square) value represents equivalent DC value producing the same heating effect as AC.
Answer: Devices that limit transient voltage spikes and protect electrical equipment from damage.
Answer: Load is any device consuming electrical energy, like motors, lights, or appliances.
Answer: A simplified representation of a power system using single lines and symbols to show major components and connections.
Answer: Switchgear includes circuit breakers, isolators, relays, and fuses to protect and control power systems.
Answer: A rectifier converts AC to DC using diodes, used in power supplies and battery charging.
Answer: Neutral is the return path for current in a circuit; earth is a safety connection directing leakage currents to ground.
Answer: An MCC is a panel housing motor starters, circuit breakers, and control devices to manage multiple motors.
Answer: Overloading, poor ventilation, low voltage, frequent starts, and mechanical faults cause overheating.
Answer: MCCB protects electrical circuits from overloads and short circuits with adjustable trip settings, suitable for higher currents.
Answer: Instrumentation engineering focuses on measuring, monitoring, and controlling process variables such as pressure, temperature, flow, and level to keep industrial operations safe and efficient.
Answer: A sensor detects physical changes (temperature, pressure, etc.), while a transmitter converts that sensor signal into a standardized output (like 4–20 mA) for controllers.
Answer: A control loop is a feedback system that measures a process variable, compares it to a set point, and adjusts control elements (like valves) to maintain stability.
Answer: Open-loop control acts without feedback (e.g., a simple timer). Closed-loop control measures output and corrects errors automatically.
Answer: A Programmable Logic Controller (PLC) is an industrial computer used to automate machinery and processes by executing logic-based programs.
Answer: SCADA (Supervisory Control and Data Acquisition) monitors, collects, and controls process data across multiple locations from a central system.
Answer: PID combines Proportional, Integral, and Derivative actions to correct process errors smoothly and maintain the set point.
Answer: A control valve modulates fluid flow in response to control signals, helping regulate pressure, temperature, or flow.
Answer: Pneumatic (air), Electric (motor), and Hydraulic (fluid) actuators move the valve stem to adjust flow.
Answer: A device that senses process pressure and converts it into an output signal (usually 4–20 mA) for monitoring or control.
Answer: RTDs use resistance change with temperature for accurate, stable readings; thermocouples generate a voltage when two dissimilar metals are heated, covering a wider range but less precise.
Answer: A flowmeter measures the rate or quantity of fluid moving through a pipe, using principles like differential pressure, electromagnetic, or turbine.
Answer: A standard analog signal where 4 mA represents the minimum measurement, 20 mA the maximum; 0–4 mA often indicates a fault.
Answer: It is less sensitive to noise, supports loop-powered devices, and lets 0 mA clearly show wiring or sensor failure.
Answer: A Distributed Control System (DCS) manages complex processes using multiple controllers distributed across the plant, integrated with operator stations.
Answer: Signal conditioning converts raw sensor signals (amplifying, filtering, isolating) into a usable form for controllers or recorders.
Answer: Calibration compares an instrument’s output to a known reference and adjusts it to ensure accurate readings.
Answer: HART (Highway Addressable Remote Transducer) is a digital communication method layered on top of the 4–20 mA signal for configuration and diagnostics.
Answer: Redundancy duplicates key hardware or control paths so that if one fails, another takes over, improving reliability.
Answer: Loop checking verifies the proper function of every instrument signal from the sensor through the controller to the final element.
Answer: An interlock is a logic condition preventing unsafe actions unless specified conditions are met, improving safety.
Answer: Analog signals vary continuously (e.g., 4–20 mA), digital signals are discrete on/off or binary coded.
Answer: A data logger automatically records process variables over time for analysis and compliance.
Answer: Dead time is the delay between an input change and the first observed effect on the output, affecting controller tuning.
Answer: A proximity sensor detects the presence of an object without physical contact, using inductive, capacitive, or ultrasonic methods.
Answer: Zero is the instrument reading at the lowest input; span is the range between the lowest and highest measurable inputs.
Answer: A mechanical or proximity switch that signals when an object reaches a preset position, commonly used for end-of-travel indication.
Answer: A smooth switch between manual and automatic control without a sudden jump in output, avoiding process disturbances.
Answer: An SIS is a dedicated system that detects hazardous conditions and takes action (like shutting down equipment) to prevent accidents.
Answer: Tagging assigns unique identification codes to each instrument for documentation, maintenance, and easy reference.
Answer: The rotor is the rotating part of an electric motor. It receives torque from the magnetic field of the stator, converting electrical energy into mechanical motion.
Answer: Synchronous speed (Ns) in RPM is calculated as: Ns = (120 × f) / P, where f is supply frequency (Hz) and P is number of poles. Actual rotor speed = Ns × (1 – slip).
Answer: The stator is the stationary part of a motor or generator that contains windings and produces the magnetic field that drives the rotor.
Answer: A VFD adjusts the frequency and voltage of power supplied to an AC motor, allowing precise speed and torque control, energy savings, and reduced mechanical stress.
Answer: A soft starter gradually increases voltage to a motor during start-up, limiting inrush current and torque to reduce mechanical and electrical stress.
Answer: A thyristor is a semiconductor device that conducts current when triggered by a gate signal and continues until the current falls below a holding value. It is used in rectifiers, motor controls, and phase regulation.
Answer: Slip (%) = (Ns – Nr) / Ns × 100, where Ns is synchronous speed and Nr is rotor speed.
Answer: A contactor is an electrically operated switch that controls high-current circuits, commonly used for motor on/off control.
Answer: An overload relay protects motors by sensing excess current and tripping the circuit when overheating or overloading occurs.
Answer: MCC (Motor Control Center) houses starters and protection devices for motors. PCC (Power Control Center) distributes and protects incoming power to various MCCs and loads.
Answer: A synchronous motor runs exactly at synchronous speed, locked with supply frequency, and is used where constant speed is required.
Answer: An autotransformer uses a single winding for both primary and secondary, stepping voltage up or down efficiently for applications like motor starting.
Answer: CT (Current Transformer) reduces high currents to measurable levels; PT (Potential Transformer) steps down high voltages for safe metering and protection.
Answer: A busbar is a rigid conductor (copper or aluminum) that distributes power within switchgear, panels, or substations.
Answer: Earthing (grounding) connects non-current-carrying parts to the ground, safely directing fault current and preventing electric shock.
Answer: SPD diverts transient voltage spikes away from sensitive equipment, protecting it from lightning or switching surges.
Answer: MCCB (Molded Case Circuit Breaker) protects low/medium current circuits; ACB (Air Circuit Breaker) handles higher currents and is used at main distribution levels.
Answer: A slip ring (wound rotor) motor has external resistances connected to the rotor through slip rings, allowing smooth speed control and higher starting torque.
Answer: Torque is the turning force produced by a motor, proportional to the interaction between magnetic fields of rotor and stator, measured in Newton-meters (N·m).
Answer: A capacitor bank is a group of capacitors connected to improve power factor and reduce reactive power demand on the grid.
Answer: A step-up transformer increases voltage while lowering current; a step-down transformer lowers voltage while increasing current, both maintaining power balance (minus losses).
Answer: Duty cycle defines how long a motor can operate under load compared to its rest period. It ensures the motor does not overheat during continuous or intermittent use.
Answer: Neutral provides the return path for current in AC systems, maintaining circuit balance and enabling proper operation of protective devices.
Answer: Harmonics are currents or voltages at multiples of the fundamental frequency, often caused by nonlinear loads (VFDs, rectifiers) leading to heating, distortion, and equipment stress.
Answer: An isolator is a mechanical switch that disconnects a part of the circuit for maintenance. It is not designed to interrupt live fault currents like a breaker.
Answer: Regenerative braking converts kinetic energy from the motor back into electrical energy, feeding it to the supply or dissipating it, improving efficiency.
Answer: Single phasing occurs when one phase of a three-phase supply is lost. It causes unbalanced currents, overheating, and potential damage to motors.
Answer: A dielectric is an insulating material that stores electrical energy when subjected to an electric field, commonly used in capacitors.
Answer: In a star (Y) connection, ends of each winding join at a neutral point; in a delta connection, windings form a closed loop. Star gives line voltage √3 times phase voltage; delta gives line current √3 times phase current.
Answer: A load center (distribution board) houses breakers or fuses that feed branch circuits, distributing power safely throughout a building or facility.
Answer: A Programmable Logic Controller (PLC) is an industrial digital computer used to automate machinery, processes, and control systems in real-time.
Answer: NO (Normally Open) contacts remain open until activated; NC (Normally Closed) contacts remain closed until activated. Used in inputs and outputs for control logic.
Answer: A coil represents an output in ladder logic, energizing or de-energizing devices like motors, solenoids, or indicator lights.
Answer: Timers delay or control events in PLCs. Common types: TON (On-delay), TOF (Off-delay), and TP (Pulse timer).
Answer: TON starts timing when input turns ON and activates output after preset time; TOF activates output immediately and turns it OFF after delay; TP generates a fixed-width pulse when input turns ON.
Answer: A counter counts occurrences of events. Common types: CTU (Count Up), CTD (Count Down), and CTUD (Up/Down counter).
Answer: CTU increments the count on every input pulse; CTD decrements the count on every input pulse.
Answer: CTUD is an up/down counter that can both increment and decrement counts based on two inputs (up/down signals).
Answer: Ladder logic is a graphical PLC programming language resembling relay logic, using rungs of contacts and coils to represent control circuits.
Answer: Scan cycles: 1) Input scan (read inputs), 2) Logic execution (execute program), 3) Output scan (update outputs), 4) Housekeeping (diagnostics).
Answer: Bit memory stores 1-bit data (ON/OFF) for inputs, outputs, or internal flags in PLCs.
Answer: Word memory stores multiple bits as a group (usually 16 or 32 bits) to handle integers, timers, counters, or complex data.
Answer: A rung represents a single logic statement or control operation in ladder diagrams.
Answer: SET instruction turns an output ON and latches it; RESET turns an output OFF, overriding the previous state.
Answer: Edge detection detects transitions of signals. Rising edge detects OFF→ON; falling edge detects ON→OFF transitions.
Answer: A data register stores numeric values, like integers or real numbers, used in calculations or comparisons.
Answer: Comparators compare two values using operators like =, <, >, <=, >=, <> to make decisions in the program.
Answer: Arithmetic instructions perform addition, subtraction, multiplication, division, and more on data registers or constants.
Answer: MOV copies a value from source to destination memory without affecting the original data.
Answer: AND requires all conditions to be true; OR requires at least one true; NOT inverts the signal.
Answer: Typically includes: 1) I/O configuration, 2) Main program (rungs), 3) Subroutines or function blocks, 4) Data tables.
Answer: A subroutine is a reusable block of code performing specific tasks, called multiple times from main program.
Answer: A function block is a pre-programmed PLC module for common operations like PID, counters, timers, or math functions.
Answer: Scan time is the time taken to complete one full cycle of reading inputs, executing logic, and updating outputs. Faster scan times improve response.
Answer: Absolute addressing points to a fixed memory location; relative addressing offsets from a base address for flexible programming.
Answer: I/O modules interface the PLC with the field. Input modules read sensors; output modules control actuators.
Answer: Fail-safe ensures the system defaults to a safe state (OFF or STOP) in case of faults or power loss.
Answer: Latch memory holds its state until explicitly changed, useful for start/stop logic or maintaining outputs after input removal.
Answer: PID calculates error (setpoint – process variable) and adjusts output using proportional, integral, and derivative actions for precise control.
Answer: A pulse train output generates digital pulses for controlling stepper motors, frequency converters, or pulse-driven devices.
Answer: HSC counts pulses from encoders or fast sensors at speeds faster than normal PLC scan cycles.
Answer: Synchronous counters update outputs immediately with clock pulse; asynchronous counters have a slight delay due to sequential triggering.
Answer: Shift registers store and shift data bits serially or in parallel, used for sequence control or data handling.
Answer: Ladder logic is graphical using contacts and coils; structured text is a high-level, text-based language for complex calculations and loops.
Answer: Retentive timers (RTO) maintain accumulated time even if the input goes OFF, resuming from previous value when input turns ON again.
Answer: Non-retentive timers reset to zero when input goes OFF; retentive timers retain accumulated value.
Answer: Forcing an output overrides normal PLC logic to manually turn a device ON or OFF for testing or maintenance.
Answer: Monitors PLC execution; triggers safe shutdown or alarms if scan or program execution fails.
Answer: Interlocks prevent conflicting actions, ensuring one operation completes before another starts for safety or sequence control.
Answer: Memory map shows allocation of I/O, timers, counters, registers, and flags for programming and troubleshooting.
Answer: Master initiates communication, controls data flow; slave responds and executes commands, often in networks like Modbus or Profibus.
Answer: Scan errors occur when input readings, memory access, or output updates fail, potentially causing unexpected behavior.
Answer: Redundant PLCs have backup CPUs and I/O modules to maintain operation in case of failure, increasing system reliability.
Answer: A sequencer executes a predefined sequence of outputs or actions, often used in batch processes or multi-step operations.
Answer: Analog input modules read varying signals (e.g., 4–20 mA, 0–10 V); analog output modules send variable signals to actuators like valves or drives.
Answer: P (proportional) affects response to current error; I (integral) removes steady-state error; D (derivative) predicts error trend to minimize overshoot.
Answer: Series contacts require all conditions to be true (AND logic); parallel contacts require any condition to be true (OR logic).
Answer: HMI (Human Machine Interface) allows operators to monitor, control, and visualize PLC data and process status.
Answer: Retentive memory maintains output or data even after power loss, used for counters, timers, and system state memory.
Answer: Discrete signals are ON/OFF digital inputs/outputs; analog signals vary continuously, representing real-world measurements like temperature, pressure, or flow.
Answer: PLC fault diagnostics identify errors in inputs, outputs, memory, or scan logic to ensure safe and correct operation.
Answer: Hot backup uses a secondary PLC running in parallel with the primary, taking over immediately if the primary fails, ensuring zero downtime.
Answer: Safety PLCs are designed for critical operations with redundant CPUs, fail-safe logic, and certified safety standards like SIL for emergency stops and interlocks.
Answer: SCADA connects PLCs to a central supervisory system for data collection, monitoring, alarming, and remote control.
Answer: Modbus RTU/TCP, Profibus, Profinet, EtherNet/IP, CANopen, and DeviceNet are widely used for PLC-to-PLC and PLC-to-HMI/SCADA communication.
Answer: Recipe control stores predefined process parameters in PLC memory to automatically run batch processes without manual input.
Answer: Redundant PLC networks ensure communication continues uninterrupted if a node or link fails, improving reliability in critical processes.
Answer: Motion control involves PLC commands for precise positioning, speed, and acceleration of motors, often used in conveyors, robotics, and CNC machines.
Answer: Encoders provide position or speed feedback to PLCs, enabling accurate motion control, counters, or closed-loop regulation.
Answer: Outputs that can switch at rates faster than normal PLC scan cycles, used for pulse trains, PWM, or stepper motor control.
Answer: Analog scaling converts raw input signals (e.g., 4–20 mA) to engineering units (°C, bar, liters) for meaningful display and control.
Answer: Programs that execute actions only when triggered by specific events, reducing unnecessary scanning and improving response.
Answer: Alarm management monitors process variables and notifies operators of deviations, helping prevent equipment damage or unsafe conditions.
Answer: Latched alarms remain active until manually acknowledged; unlatched alarms reset automatically when the condition clears.
Answer: Data logging stores process variables over time in PLC memory or external databases for analysis, trends, or regulatory compliance.
Answer: Remote I/O modules are installed away from the main PLC, connected via fieldbus or Ethernet, reducing wiring and extending control to distant locations.
Answer: The I/O scan reads all input signals, updates internal memory, and refreshes outputs during each PLC scan cycle.
Answer: Analog I/O redundancy uses dual channels, selecting the valid signal to maintain process continuity during sensor or module failures.
Answer: A block diagram visually represents the PLC architecture including CPU, memory, I/O modules, and communication networks.
Answer: Forcing I/O temporarily overrides the logic state of inputs or outputs for testing, maintenance, or troubleshooting.
Answer: A module that generates pulses to control stepper drives, PWM devices, or high-speed actuators.
Answer: Fast counters count high-speed events like encoder pulses faster than normal scan cycles to maintain accurate counts.
Answer: Firmware is the software embedded in PLC hardware to run the operating system, manage scan cycles, and execute user programs.
Answer: IEC 61131-3 languages: Ladder Logic (LD), Structured Text (ST), Function Block Diagram (FBD), Instruction List (IL), Sequential Function Chart (SFC), and Continuous Function Chart (CFC).
Answer: Trending graphs real-time process variables for operators to analyze performance and identify anomalies.
Answer: Recipes containing process parameters can be uploaded from PLC to PC or downloaded to PLC to execute consistent batch operations.
Answer: PLCs connected via communication protocols for centralized control, monitoring, and data exchange.
Answer: Monitors CPU operation; if the program hangs or fails, the watchdog triggers a fault to protect equipment.
Answer: Cyclic scanning executes all logic continuously; event-driven executes only on defined events, improving efficiency.
Answer: Memory retention keeps data, timers, counters, and flags even during power loss (non-volatile memory).
Answer: Adjusting output signals so that the real-world controlled device (valve, motor) responds accurately to PLC commands.
Answer: Discrete input triggered by a mechanical position switch to indicate motion end or object detection.
Answer: Special input that interrupts the main PLC scan to immediately process high-priority events like encoder pulses or emergency stops.
Answer: Operations like SET, RESET, AND, OR, XOR performed on individual bits for control logic.
Answer: Reduces noise in analog input signals using software or hardware filters for stable process measurements.
Answer: PLC automatically adjusts PID parameters based on system response to optimize control without manual calculations.
Answer: High-priority tasks can interrupt normal scan to process urgent events like safety trips or fast counters.
Answer: Software removes spurious input transitions or noise to ensure accurate digital readings.
Answer: A task scheduled to execute at regular intervals, ideal for consistent control and monitoring.
Answer: Assigns importance to tasks; high-priority tasks interrupt or preempt low-priority tasks for timely execution.
Answer: Dual communication paths to prevent network failure affecting control or monitoring.
Answer: OPC (OLE for Process Control) standard allows PLCs to exchange data with SCADA, HMI, or MES systems.
Answer: Batch control executes predefined process sequences, storing parameters like quantities, timing, and mixing instructions.
Answer: Defines motor speed, acceleration, and position over time for precise control in robotics or conveyors.
Answer: Online programming modifies running PLC; offline programming is done on PC and downloaded to PLC later.
Answer: Memory structure to store inputs, outputs, timers, counters, and registers for structured programming.
Answer: Duplicate CPUs or modules ensure system continues operation if primary fails, common in safety-critical systems.
Answer: Software-based I/O that doesn’t require physical hardware, useful for simulation or logic testing.
Answer: PLCs exchange data directly over a network to coordinate control across multiple systems.
Answer: Uses redundant CPUs, power supplies, and communication paths so that even if one component fails, process control continues without interruption.