🔹 LT & HT SYSTEMS
1️⃣ What is the difference between LT and HT systems?
Answer:
LT (Low Tension) systems generally operate up to 1 kV and are used for end-user distribution like lighting, small motors, and utility loads. HT (High Tension) systems operate above 1 kV and are used for bulk power transmission within plants. HT reduces current for the same power, minimizing losses, conductor size, and voltage drop. Proper insulation, clearance, and protection are critical in HT systems.
2️⃣ What are key checks before LT panel installation?
Answer:
Before installation, ensure foundation level, panel alignment, cable entry provision, earthing points, and space for maintenance. Verify panel rating, fault level, IP protection, and approved drawings. Check busbar insulation, torque tightening of bolts, and availability of interlocks. Environmental factors like dust, moisture, and ventilation must also be considered for long-term reliability.
3️⃣ What safety precautions are required during HT panel work?
Answer:
HT panel work requires permit-to-work, isolation, earthing, and discharge confirmation. Use insulated tools, PPE like arc flash suits, gloves, and safety shoes. Barricade the area, display danger boards, and ensure only authorized personnel are present. Live work is avoided unless absolutely necessary and approved by safety authority.
4️⃣ What is a VCB and where is it used?
Answer:
VCB (Vacuum Circuit Breaker) is used in HT systems for switching and protection. It interrupts current in a vacuum chamber, providing fast arc quenching and long service life. VCBs are commonly used in 6.6 kV to 33 kV systems due to low maintenance, high reliability, and suitability for frequent operations.
5️⃣ What is the purpose of CT and PT in panels?
Answer:
Current Transformers (CT) reduce high current to measurable levels for protection and metering. Potential Transformers (PT) step down high voltage to safe levels. They provide isolation between high voltage circuits and instruments. Correct CT ratio, polarity, and burden are essential for accurate protection and measurement.
🔹 TRANSFORMERS
6️⃣ What checks are done before transformer energization?
Answer:
Check oil level, BDV value, winding resistance, IR values, tap changer position, and earthing. Ensure protection relays are tested and cooling systems are functional. Verify cable terminations, clearances, and signage. Pre-commissioning tests confirm transformer health and reduce failure risk during energization.
7️⃣ Why earthing is critical for transformers?
Answer:
Earthing provides a safe path for fault current, protects personnel, and stabilizes system voltage. Transformer neutral and body earthing prevent dangerous touch potential. Proper earth resistance ensures fast operation of protection devices and minimizes damage during earth faults or lightning surges.
8️⃣ What is Buchholz relay and its function?
Answer:
Buchholz relay is a gas-operated protection device installed between transformer tank and conservator. It detects internal faults like insulation failure or overheating. Gas accumulation triggers alarm, while oil surge during major faults trips the transformer, preventing severe damage.
9️⃣ What causes transformer overheating?
Answer:
Overloading, poor cooling, high ambient temperature, low oil level, and insulation deterioration cause overheating. Loose connections and harmonics also contribute. Continuous overheating reduces insulation life and may lead to transformer failure if not corrected.
🔹 CABLES, GLANDS & TERMINATION
🔟 What are important points during cable tray installation?
Answer:
Ensure proper support spacing, alignment, and load calculation. Separate power and control cables to avoid interference. Maintain bending radius and expansion joints. Trays must be properly earthed and protected against corrosion. Route planning should allow easy maintenance and future expansion.
1️⃣1️⃣ Why cable glanding is important?
Answer:
Cable glands provide mechanical support, strain relief, and environmental sealing. They maintain IP rating of panels and prevent moisture, dust, and gas entry. Proper glanding ensures earthing continuity for armored cables and reduces risk of cable damage or short circuits.
1️⃣2️⃣ Difference between single compression and double compression glands?
Answer:
Single compression glands grip only the outer sheath, suitable for indoor use. Double compression glands grip both inner sheath and armor, providing better mechanical strength and earthing. Double compression is preferred for outdoor, hazardous, and vibration-prone installations.
1️⃣3️⃣ What precautions are taken during cable termination?
Answer:
Ensure correct lug size, crimping tool, and torque tightening. Clean insulation, avoid strand damage, and use heat shrink sleeves. Phase identification and ferruling must be clear. Improper termination causes overheating, voltage drop, and equipment failure.
1️⃣4️⃣ What is minimum bending radius of cables?
Answer:
Minimum bending radius depends on cable type and size, generally 8–12 times the cable diameter. Exceeding limits damages insulation and conductor strands. Always follow manufacturer guidelines during laying and termination.
🔹 EARTHING & LIGHTNING
1️⃣5️⃣ Why earthing system is essential in industrial plants?
Answer:
Earthing protects personnel from electric shock, stabilizes voltage, and ensures effective fault clearing. It reduces fire risk and protects equipment from surges. A well-designed earthing system improves safety compliance and system reliability.
1️⃣6️⃣ Difference between equipment earthing and system earthing?
Answer:
Equipment earthing connects non-current carrying metal parts to earth for safety. System earthing connects neutral or star point to earth for voltage stabilization. Both serve different purposes but together ensure overall electrical safety.
1️⃣7️⃣ What is acceptable earth resistance value?
Answer:
Typically, earth resistance should be below 1 ohm for substations and below 2–5 ohms for general installations. Lower resistance ensures quick fault current flow and protection device operation. Values depend on standards and plant requirements.
🔹 PANELS & PROTECTION
1️⃣8️⃣ What is an MCC panel?
Answer:
Motor Control Center (MCC) panel controls and protects motors. It includes contactors, overload relays, breakers, and control devices. MCCs allow centralized control, easy maintenance, and improved operational safety in industrial plants.
1️⃣9️⃣ What is the purpose of interlocking in panels?
Answer:
Interlocking prevents incorrect operation, such as closing breaker under unsafe conditions. It enhances safety and avoids equipment damage. Electrical and mechanical interlocks are used based on system design.
2️⃣0️⃣ What are common protection relays used in HT systems?
Answer:
Overcurrent, earth fault, differential, under-voltage, over-voltage, and distance relays are common. They detect abnormal conditions and isolate faulty sections to protect equipment and ensure system stability.
🔹 SAFETY & PERMITS
2️⃣1️⃣ What is Lock Out Tag Out (LOTO)?
Answer:
LOTO ensures equipment is isolated and cannot be energized during maintenance. Locks and tags identify authorized work. It prevents accidental energization and protects maintenance personnel from serious injuries.
2️⃣2️⃣ Why is permit-to-work required?
Answer:
Permit-to-work ensures hazards are identified, controls are applied, and responsibilities are defined. It improves coordination between teams and ensures safe execution of electrical jobs in industrial environments.
🔹 TESTING & COMMISSIONING
2️⃣3️⃣ What tests are performed on LT panels?
Answer:
Insulation resistance, continuity, functional checks, interlock testing, and protection relay testing are performed. These tests ensure safety, reliability, and readiness for operation before energization.
2️⃣4️⃣ What is insulation resistance test?
Answer:
IR test measures insulation quality between conductors and earth using a megger. High resistance indicates good insulation. Low values suggest moisture, aging, or damage requiring corrective action.
2️⃣5️⃣ Why is pre-commissioning important?
Answer:
Pre-commissioning identifies defects before operation. It ensures safety, reduces breakdowns, and improves equipment life. Proper documentation supports future maintenance and audits.
🔹 MOTORS & DRIVES
2️⃣6️⃣ What are common motor starting methods?
Answer:
DOL, Star-Delta, Auto Transformer, Soft Starter, and VFD are common methods. Selection depends on motor size, load characteristics, and system capacity to limit starting current and mechanical stress.
2️⃣7️⃣ What is the role of VFD?
Answer:
VFD controls motor speed by varying frequency and voltage. It saves energy, improves process control, reduces mechanical stress, and allows soft starting and stopping of motors.
🔹 GENERAL INDUSTRIAL PRACTICE
2️⃣8️⃣ What causes cable overheating?
Answer:
Overloading, poor termination, inadequate ventilation, and harmonic currents cause overheating. Continuous overheating damages insulation and reduces cable life.
2️⃣9️⃣ How do you ensure quality in electrical installation?
Answer:
Follow approved drawings, standards, checklists, and inspection procedures. Proper supervision, testing, documentation, and skilled manpower ensure quality execution.
3️⃣0️⃣ What is arc flash and why is it dangerous?
Answer:
Arc flash is a high-energy discharge due to faults. It causes severe burns, blast pressure, and equipment damage. Proper PPE, design, and maintenance reduce arc flash risk
🔹 SUBSTATION & POWER DISTRIBUTION
3️⃣0️⃣ What is a substation and its main components?
Answer:
A substation is a critical part of power distribution where voltage is transformed, controlled, and protected. Main components include transformers, HT/LT switchgear, busbars, protection relays, CTs, PTs, isolators, earthing systems, and control panels. Substations ensure safe, reliable power flow from transmission to distribution levels and protect equipment during faults.
3️⃣1️⃣ Difference between indoor and outdoor substations?
Answer:
Indoor substations are enclosed and suitable for limited space and controlled environments, offering better protection from weather. Outdoor substations are exposed and used for higher voltage levels, providing easy expansion and heat dissipation. Selection depends on voltage level, safety requirements, and site conditions.
3️⃣2️⃣ What is busbar and why is it important?
Answer:
A busbar is a metallic conductor used to distribute power to multiple circuits. It carries high current and ensures efficient power distribution within panels and substations. Proper sizing, insulation, spacing, and support are essential to avoid overheating, short circuits, and mechanical stress during faults.
3️⃣3️⃣ What is busbar protection?
Answer:
Busbar protection detects internal faults within the busbar zone using differential relays. It isolates faulty sections quickly to prevent equipment damage and power loss. Since busbars carry large fault currents, fast and reliable protection is essential for system stability.
3️⃣4️⃣ What is load calculation and why is it required?
Answer:
Load calculation estimates total electrical demand considering connected load, diversity factor, and future expansion. It helps in selecting correct transformer size, cables, breakers, and panels. Accurate load calculation prevents overloading, voltage drop, and unnecessary capital cost.
🔹 DG SETS & UPS SYSTEMS
3️⃣5️⃣ What checks are required before DG synchronization?
Answer:
Voltage, frequency, phase sequence, and phase angle must match grid parameters. Protection systems must be active, and communication between DG and synchronizing panel verified. Proper synchronization prevents mechanical stress, reverse power flow, and generator damage.
3️⃣6️⃣ What is AMF panel?
Answer:
AMF (Automatic Mains Failure) panel automatically starts DG sets when grid power fails and transfers load. It also stops DG when mains supply returns. AMF ensures uninterrupted power supply and reduces manual intervention during power outages.
3️⃣7️⃣ Difference between UPS and DG?
Answer:
UPS provides instant, short-duration backup without interruption, mainly for sensitive equipment. DG provides long-duration backup but with startup delay. Both are used together to ensure continuous and reliable power supply in industrial plants.
3️⃣8️⃣ What factors affect UPS capacity selection?
Answer:
Load type, VA rating, power factor, backup time, efficiency, and future expansion affect UPS selection. Environmental conditions and battery type also play a role. Proper selection avoids overload and battery failure.
🔹 HAZARDOUS AREA & INDUSTRIAL SAFETY
3️⃣9️⃣ What is hazardous area classification?
Answer:
Hazardous areas are classified based on presence of flammable gases, vapors, or dust. Zones define risk level and determine suitable electrical equipment. Proper classification prevents ignition sources and ensures safe plant operation.
4️⃣0️⃣ Why flameproof equipment is used?
Answer:
Flameproof equipment contains internal explosions and prevents ignition of surrounding atmosphere. It is designed to withstand pressure and temperature. Used in hazardous areas to ensure safety and compliance with statutory regulations.
4️⃣1️⃣ What is intrinsically safe system?
Answer:
Intrinsic safety limits energy in circuits to prevent ignition even under fault conditions. Used for instrumentation in hazardous areas. It enhances safety by design rather than containment.
🔹 TESTING, COMMISSIONING & MAINTENANCE
4️⃣2️⃣ What is relay coordination?
Answer:
Relay coordination ensures correct sequence of protection operation so that only the faulty section is isolated. It improves system reliability and minimizes power interruption. Proper settings and time grading are essential.
4️⃣3️⃣ What is contact resistance test?
Answer:
This test measures resistance at joints, breakers, and busbars. High resistance causes overheating and failure. Regular testing ensures good electrical contact and system reliability.
4️⃣4️⃣ What is tan delta test?
Answer:
Tan delta test measures insulation health of cables and transformers. Increase in tan delta indicates insulation aging or moisture. It helps in predictive maintenance and failure prevention.
4️⃣5️⃣ What is thermography in electrical systems?
Answer:
Thermography uses infrared cameras to detect hot spots in electrical equipment. It identifies loose connections, overloads, and insulation issues without shutdown. It improves preventive maintenance and safety.
🔹 SITE PRACTICE & TROUBLESHOOTING
4️⃣6️⃣ How do you handle repeated breaker tripping?
Answer:
Analyze protection logs, check load conditions, inspect cables and connections, and test relays. Identify root cause before resetting. Repeated tripping indicates serious issues and should not be bypassed.
4️⃣7️⃣ What causes voltage drop in industrial systems?
Answer:
Long cable runs, undersized conductors, high load current, and poor connections cause voltage drop. It affects equipment performance and efficiency. Proper design and cable sizing minimize voltage drop.
4️⃣8️⃣ How do you ensure electrical safety during maintenance?
Answer:
Apply LOTO, isolate supply, discharge stored energy, use PPE, and verify zero voltage. Proper communication and permits ensure safe working conditions and prevent accidents.
4️⃣9️⃣ What is root cause analysis in electrical failures?
Answer:
RCA identifies underlying causes rather than symptoms. It involves data collection, analysis, and corrective actions. RCA improves system reliability and prevents repeat failures.
5️⃣0️⃣ Why documentation is important in electrical projects?
Answer:
Documentation records drawings, test results, changes, and maintenance history. It supports troubleshooting, audits, safety compliance, and future expansion. Proper documentation ensures knowledge continuity and system reliability.