1). What is a Circuit Breaker?
A Circuit Breaker (CB) is an automatic switching device that protects electrical circuits from the damage caused by overload, short circuit and earth faults.
It detects abnormal current and interrupts the electrical supply safely.
Key Functions:
- Protects electrical equipment,
- Prevents fire hazards,
- Allows manual and automatic switching &
- Can be reset after tripping.
2). What types of Circuit Breakers?
Common types of circuit breakers include:
- MCB (Miniature Circuit Breaker): Domestic and commercial applications
- MCCB (Moulded Case Circuit Breaker): Industrial distribution systems
- ACB (Air Circuit Breaker): Low-voltage high-current applications
- VCB (Vacuum Circuit Breaker): Medium-voltage systems
- OCB (Oil Circuit Breaker): Older medium/high-voltage installations
- SF₆ Circuit Breaker: High-voltage substations
- RCB/RCCB: Earth leakage protection
- ELCB: Earth leakage protection (older technology)
3). What is ACB, MCB, OCB, VCB?
| Type | Full Form | Application |
|---|---|---|
| ACB | Air Circuit Breaker | LV switchboards (up to 690 V) |
| MCB | Miniature Circuit Breaker | Homes and offices |
| MCCB | Moulded Case Circuit Breaker | Industrial feeders |
| OCB | Oil Circuit Breaker | Older substations |
| VCB | Vacuum Circuit Breaker | 3.3–36 kV systems |
| SF₆ CB | Sulphur Hexafluoride Circuit Breaker | HV/EHV substations |
4). What is the function of a Fuse?
A fuse that protects electrical circuits by melting its fuse element when current exceeds the safe limit.
Purpose
- Overload protection,
- Short-circuit protection,
- Prevents equipment damage and
- Fire prevention.
5). How does a Fuse work?
A fuse contains a low melting-point metal wire.
When excessive current flows:
- Heat is produced (I²R loss),
- Fuse element melts,
- Circuit opens and
- Current stops flowing.
Unlike a circuit breaker a fuse should be replaced after operation.
6). What is Minimum Fusing Current?
Minimum fusing current is the lowest current that causes the fuse element to melt.
Typically
Minimum Fusing Current = 1.25 to 2 times the rated current
Example:
- Fuse rating = 10 A
- Minimum fusing current ≈ 12.5 – 20 A (depends on fuse type)
7). What is a Relay?
A relay is an automatic protective device that detects abnormal electrical conditions and sends a trip command to the circuit breaker.
It protects:
- Transformers
- Motors
- Generators
- Feeders
- Transmission lines
8). What is the difference between Fuse and Relay?
| Fuse | Relay |
|---|---|
| Directly interrupts current | Sends trip signal to breaker |
| One-time operation | Reusable |
| Simple construction | Intelligent protection device |
| Fast operation | Works with circuit breaker |
| Needs replacement | Reset after fault clearance |
9). What is Reverse Power Relay?
A reverse power relay protects generators from operating as motors.
It trips the generator if power flows from the busbar back into the generator which may occur due to prime mover failure.
Typical setting:
- 2-10% of generator rated power
10). How does a Relay work?
Working principle:
Step-1: CT/PT measures electrical quantities.
Step-2: Relay continuously monitors current and voltage.
Step-3: If settings are exceeded.
Step-4: Relay energizes trip coil.
Step-5: Circuit breaker opens.
Step-6: Fault is isolated.
11). How does a Circuit Breaker work?
When a fault occurs:
- Relay detects the fault.
- Relay energizes trip coil.
- Contacts separate.
- Arc forms.
- Arc is extinguished using air, oil, vacuum (or) SF₆.
- Circuit becomes isolated.
12). What is Neutral Earthing? What are its advantages?
Neutral earthing means connecting the transformer (or) generator neutral to earth.
Advantages
- Limits fault current,
- Stabilizes system voltage,
- Protects equipment,
- Reduces overvoltage,
- Improves personnel safety and
- Enables protective relays to detect earth faults.
13). What protections are provided for Generators supplying current to Bus Bar?
Common generator protections:
- Differential protection (87G)
- Overcurrent (50/51)
- Earth fault (64)
- Reverse power (32)
- Under voltage (27)
- Over voltage (59)
- Under frequency (81U)
- Over frequency (81O)
- Loss of excitation (40)
- Negative sequence (46)
- Stator earth fault
- Rotor earth fault
- Overload
14). What protections are provided to Induction Motors?
Motor protections include:
- Overload
- Short circuit
- Earth fault
- Phase failure
- Phase reversal
- Locked rotor
- Stall protection
- Under voltage
- Over voltage
- Thermal protection
- Bearing temperature
- Winding temperature
15). How is Overcurrent Protection provided?
Overcurrent protection uses:
- Current Transformer (CT),
- Overcurrent Relay (50/51),
- Circuit Breaker.
When current exceeds the relay setting the relay trips the breaker.
16). What is an Inverse Definite Minimum Time (IDMT) Relay? How does it work?
IDMT relay operating time decreases as fault current increases.
Characteristics
- Small overload → Longer operating time
- Heavy fault → Faster tripping
This allows coordination between upstream & downstream protective devices.
17). What is Overload Trip?
An overload trip occurs when current exceeds the rated value for a prolonged period.
Example:
A 100 A motor drawing 130 A continuously will eventually trip due to overload.
Purpose:
- Prevents overheating
- Protects insulation
- Extends equipment life
18). What is the difference between Short Circuit Trip and Overload Trip?
| Short Circuit Trip | Overload Trip |
|---|---|
| Very high current | Slightly above rated current |
| Instantaneous | Time delay |
| Severe fault | Normal overload |
| Magnetic protection | Thermal protection |
| Prevents equipment damage | Prevents overheating |
19). What is Preferential Trip?
Preferential trip is a load shedding system that disconnects non-essential loads first during overload (or) low generator capacity.
Essential loads remain energized.
Used in:
- Ships,
- Power plants,
- Emergency generators and
- Hospitals.
20). What is Reverse Current / Reverse Power Trip?
Reverse current (or) reverse power trip protects generators from receiving power instead of supplying it.
It prevents:
- Generator motoring,
- Prime mover damage,
- Turbine damage and
- Diesel engine damage.
21). What is Under Voltage Trip? Why are all these trips important?
Under voltage trip disconnects equipment when voltage falls below a preset limit.
Importance of Protective Trips
- Equipment protection,
- Personnel safety,
- Prevents overheating,
- Prevents fire,
- Maintains power system stability and
- Reduces downtime.
22). What are Ammeter, Voltmeter, Wattmeter and Megger?
| Instrument | Measures |
|---|---|
| Ammeter | Current (A) |
| Voltmeter | Voltage (V) |
| Wattmeter | Power (W or kW) |
| Megger | Insulation Resistance (MΩ or GΩ) |
23). What is the difference between Ammeter and Voltmeter?
| Ammeter | Voltmeter |
|---|---|
| Measures current | Measures voltage |
| Connected in series | Connected in parallel |
| Very low internal resistance | Very high internal resistance |
| Unit: Ampere | Unit: Volt |
24). What is Megger Testing?
Megger testing measures the insulation resistance of electrical equipment using a high DC test voltage.
Common test voltages:
- 250 V – Electronic circuits,
- 500 V – LV motors and cables,
- 1000 V – Industrial LV equipment and
- 2500–5000 V – MV/HV equipment (as specified)
Applications:
- Motors,
- Transformers,
- Power cables,
- Switchgear and
- Generators.
Higher insulation resistance generally indicates better insulation condition.
25). What is the difference between CT (Current Transformer) and PT (Potential Transformer)?
| CT (Current Transformer) | PT (Potential Transformer) |
|---|---|
| Reduces high current to a standard value (typically 1 A or 5 A) | Reduces high voltage to a standard value (typically 110 V or 63.5 V) |
| Connected in series with the circuit | Connected in parallel with the circuit |
| Used for current measurement and protection | Used for voltage measurement and protection |
| Secondary should never be open circuited while energized | Secondary should never be short circuited while energized |
| Used with ammeters, energy meters and protection relays | Used with voltmeters, wattmeters, synchronizing equipment and protection relays |
