CDM Smith Interview Questions and Answers

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 devi...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 circu...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 fl...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...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...read more

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

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 ...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, electr...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 contribut...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 fu...read more

Non-linear loads are electrical devices that do not have a linear relationship between the voltage and current.

• Non-linear loads draw non-sinusoidal current waveforms.

• They can cause harmonic distortion in the electrical system.

• Examples of non-linear loads include computers, televisions, and variable speed drives.

• Linear loads have a linear relationship between voltage and current, such as incandescent light bulbs.

• Linear

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 lighti...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 harmonic...read more