100+ Electrical Design Engineer Interview Questions and Answers

Cable sizing is calculated using the formula for voltage drop, which takes into account resistive and inductive drop. Voltage drop is limited to 5% to ensure efficient power transmission.

• Cable sizing is determined by calculating the voltage drop using the formula: VD = (I * L * R * 2) / 1000

• Resistive drop is calculated using Ohm's Law: Vr = I * R

• Inductive drop is calculated using the formula: Vi = I * X

• Voltage drop is limited to 5% to prevent excessive power loss and ensure e...read more

The input required for a single line diagram includes information about the electrical system components and their connections.

• Information about the electrical system components such as generators, transformers, switchgear, and loads

• The connections between these components including the type of connection and the ratings of the equipment

• The voltage level of the system and the type of system grounding

• The protective devices such as circuit breakers and fuses

• Any special requirem...read more

Percentage Impedance is a measure of the voltage drop in a transformer during a short circuit. It determines the ability of the transformer to withstand short circuit currents.

• Percentage Impedance is the ratio of the rated voltage of a transformer to the short circuit current at its terminals, expressed as a percentage.

• It represents the internal impedance of the transformer and is usually provided by the manufacturer.

• A higher percentage impedance means a higher voltage drop d...read more

Explanation of lighting calculation, light loss factor, utilisation factor, and minimum lux required in substation.

• Lighting calculation involves determining the amount of light required in a space.

• Light loss factor is the reduction in light output due to factors such as dirt, dust, and aging of lamps.

• Utilisation factor is the ratio of light received by a surface to the total light output of the lamps.

• Formula for lighting calculation: Total light output = Area x Required lux l...read more

Earthing calculation standards and resistances for substation and lightning protection.

• Earthing calculation involves determining the resistance of the earth electrode system.

• The standard used for earthing calculation is IEEE 80.

• The resistance in substation should be less than 1 ohm.

• The resistance for lightning protection should be less than 10 ohms.

• Factors affecting earthing calculation include soil resistivity, electrode material, and electrode spacing.

• Earthing calculation i...read more

Hazardous area classification involves categorizing areas based on the presence of flammable gases, vapors, or dusts.

• Hazardous areas are classified based on the likelihood of the presence of flammable substances.

• Temperature Class refers to the maximum surface temperature of equipment that can be used in a hazardous area.

• Hazardous areas are categorized into zones based on the frequency and duration of the presence of flammable substances.

• Examples of hazardous areas include oil...read more

1.25 FLA is taken for circuit breaker and 1.5 times for contactor to ensure protection against overloading and short circuits.

• 1.25 FLA is the maximum current that a circuit breaker can handle without tripping.

• 1.5 times FLA is taken for contactor to ensure that it can handle the inrush current during motor starting.

• This also provides protection against short circuits and overloading.

• For example, if the full load current of a motor is 10A, then a circuit breaker rated for 12.5A...read more

E-Plan, AutoCAD, and E-CAD are all software used in electrical design engineering, but they have differences.

• E-Plan is a specialized software for electrical engineering design and documentation.

• AutoCAD is a general-purpose computer-aided design (CAD) software used in various industries.

• E-CAD refers to the use of CAD software specifically for electronic design and circuit board layout.

• E-Plan focuses on electrical schematics, panel layouts, and wiring diagrams.

• AutoCAD is more v...read more

The cable size and circuit breaker rating for a 50KW motor at 30m distance from the panel depend on factors like voltage, current, and installation conditions.

• Calculate the full load current (FLC) of the motor using the formula: FLC = (Power (kW) * 1000) / (sqrt(3) * Voltage)

• Select a cable size based on the FLC, voltage drop considerations, and installation conditions

• Choose a circuit breaker rating based on the FLC and the motor's starting current requirements

• Consider factors...read more

MCB, MCCB, and MPCB are circuit breakers used for electrical protection. They differ in their current ratings and trip characteristics.

• MCB stands for Miniature Circuit Breaker and is used for low current applications up to 125A.

• MCCB stands for Molded Case Circuit Breaker and is used for higher current applications up to 2500A.

• MPCB stands for Motor Protection Circuit Breaker and is used for protecting motors from overload and short circuit.

• MCBs are classified based on their tr...read more

Types of contactors include AC, DC, reversing, and definite purpose. Maximum efficiency of transformer depends on load and design.

• AC contactors are used for alternating current circuits

• DC contactors are used for direct current circuits

• Reversing contactors are used to reverse the direction of current flow

• Definite purpose contactors are designed for specific applications

• Maximum efficiency of transformer is achieved at full load

• Efficiency is affected by design factors such as co...read more

Short circuit current from 1000kVA TF (5% Z) on a panel and determine its CB Fault current rating using IEC Standard.

• Calculate the short circuit current using the formula: Short circuit current = 1000kVA / (1.732 * 5%)

• Determine the CB Fault current rating based on the calculated short circuit current

• Refer to IEC Standard for guidelines on short circuit current calculations and CB ratings

Lux levels are calculated using the formula Lux = Lumens / Area (in square meters). Required lux levels vary depending on the specific application.

• Lux levels are calculated by dividing the total lumens of light by the area in square meters that the light is illuminating.

• For roads, recommended lux levels range from 10 to 20 lux for residential streets to 50 to 100 lux for highways.

• Workstations typically require lux levels between 300 to 500 lux for general office work, and up ...read more

The process for preparing a Single Line Diagram (SLD) involves identifying all electrical components and their connections.

• Identify all electrical components in the system

• Determine the connections between the components

• Represent the components and connections using standardized symbols

• Include power sources, distribution panels, transformers, circuit breakers, switches, and loads

• Ensure the diagram is clear, organized, and easy to understand

Sizing a cable tray involves considering the number and type of cables, environmental conditions, and support spacing. Cable trays are classified by material (steel, aluminum, fiberglass) and size (light duty, medium duty, heavy duty).

• Consider the number and type of cables to be installed in the cable tray

• Take into account environmental conditions such as temperature, moisture, and corrosive elements

• Determine the support spacing based on the weight of the cables and tray mate...read more

Motor feeder and power feeder requirements are essential for electrical design engineers to ensure proper functioning of electrical systems.

• Motor feeder requirements include selecting the appropriate motor size, voltage, and current rating.

• Power feeder requirements involve determining the load requirements and selecting the appropriate cable size and voltage drop.

• Proper coordination and protection devices such as circuit breakers and fuses must be selected for both motor and ...read more

The motor contribution to the short circuit current is significant and needs to be considered in electrical design.

• The motor's impedance affects the short circuit current level.

• The motor's starting current can cause a significant increase in short circuit current.

• The motor's location in the electrical system can affect its contribution to the short circuit current.

• The motor's size and type can also impact its contribution to the short circuit current.

• Proper coordination and p...read more

APFC stands for Automatic Power Factor Correction. Series reactor is a device used to limit current flow.

• APFC is a system that automatically corrects the power factor of an electrical network.

• It uses capacitors to improve the power factor and reduce energy consumption.

• Series reactor is a device that is used to limit the flow of current in a circuit.

• It is used to protect equipment from overvoltage and to reduce harmonics in the system.

• An example of series reactor is a line rea...read more

During a site visit for assessing existing switchgear for retrofitting circuit breakers, inputs collected include equipment specifications, condition, age, and load requirements.

• Collect equipment specifications such as voltage rating, current rating, and interrupting capacity.

• Assess the condition of the existing switchgear to determine if it can accommodate the new circuit breakers.

• Consider the age of the switchgear to evaluate potential compatibility issues with modern circu...read more

Clearance between cable tray and wet service pipe is maintained by using appropriate support systems and ensuring proper spacing.

• Use support systems such as clamps, hangers, and brackets to secure the cable tray and wet service pipe at appropriate distances.

• Ensure proper spacing between the cable tray and wet service pipe to prevent any contact or interference.

• Follow industry standards and regulations for minimum clearance requirements between different services.

• Regularly ins...read more

Yes, the use of a smartphone camera flasher is permitted while assessing old switchgears for modernization.

• Using a smartphone camera flasher can help illuminate dark areas and improve visibility while assessing old switchgears.

• It can assist in capturing clear and detailed images for documentation and analysis.

• Ensure that the flash does not interfere with any sensitive equipment or cause any safety hazards.

• Consider using additional lighting sources if needed for better assessm...read more

The voltage in a 3-phase system is typically 415 volts.

• The voltage in a 3-phase system is higher than in a single-phase system.

• In industrial and commercial applications, the standard voltage for a 3-phase system is 415 volts.

• Other common voltages for 3-phase systems include 208 volts and 480 volts.

• The voltage between any two phases in a 3-phase system is equal to the line voltage.

• The voltage between any phase and the neutral in a 3-phase system is equal to the phase voltage.

Protection schemes for transformers and generators.

• Overcurrent protection

• Differential protection

• Distance protection

• Restricted earth fault protection

• Backup protection

• Buchholz relay protection

• Thermal overload protection

• Under/over voltage protection

• Neutral grounding protection

Fault current for primary and secondary side of transformer can be calculated using Ohm's law and transformer impedance.

• Determine the transformer impedance

• Calculate the voltage drop across the impedance

• Use Ohm's law to calculate the fault current

• For primary side, use the voltage rating of the transformer

• For secondary side, use the voltage rating of the load

• Consider the type of fault (3-phase, single-phase, ground fault) and the system configuration

• Example: Primary side fault ...read more

For making LOP, the required data includes the electrical load details, power supply specifications, circuit layout, and safety regulations.

• Electrical load details such as the type of equipment, power rating, and operating voltage are necessary.

• Power supply specifications like the voltage level, frequency, and phase configuration are important.

• The circuit layout, including the arrangement of components, wiring, and connections, is required.

• Safety regulations and standards mus...read more

Eplan Electric allows generating various reports for electrical design projects.

• Reports can be generated for bills of materials, cable lists, terminal diagrams, and more.

• Reports can be customized to include specific information and formatting.

• Reports can be exported in various file formats, including Excel, PDF, and XML.

• Reports can be generated for individual pages or for the entire project.

• Reports can be scheduled to run automatically at specific intervals.

• Examples of report...read more

A rig platform is a structure used in offshore drilling operations to support drilling equipment and personnel.

• Rig platforms are typically located in bodies of water and are used for extracting oil and gas from beneath the seabed.

• They provide a stable base for drilling rigs, equipment, and crew to operate in harsh offshore environments.

• Rig platforms can be fixed (attached to the seabed) or floating (anchored to the seabed or kept in position by dynamic positioning systems).