Switchgear and Protection for SSC JE Electrical: Faults, Circuit Breakers, Relays, Fuses, Lightning Arresters and Instrument Transformers
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Faults in Power Systems
- 2.Switchgear Devices
- 3.Circuit Breakers and Arc Quenching
- 4.Circuit Breaker Ratings
- 5.Protective Relays
- 6.Fuses
- 7.Protection Against Lightning and Surges
- 8.Instrument Transformers
- 9.Solved Examples
- 10.Quick Tips
- 11.Quick Revision Points
- 12.Practice Questions With Answers
- 13.How to Practise This Chapter
- 14.Frequently Asked Questions
PDF Notes: Switchgear and Protection for SSC JE Electrical: Faults, Circuit Breakers, Relays, Fuses, Lightning Arresters and Instrument Transformers
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Switchgear and protection protect the power system from faults. SSC JE Electrical asks about the types of faults, circuit breakers, relays, fuses, lightning arresters and instrument transformers. This guide on switchgear and protection for SSC JE covers all of these, with tables, solved examples, practice questions and FAQs.
Faults in Power Systems
| Type | Description |
|---|---|
| Symmetrical fault (three-phase fault, LLL or LLLG) | All three phases are affected equally; the most severe but the rarest (about 5%); the system remains balanced |
| Unsymmetrical faults | Single line to ground (LG: the most common, about 70–80%), line to line (LL) and double line to ground (LLG); the system becomes unbalanced |
| Open-circuit faults | Breaking of a conductor |
Faults are analysed by the symmetrical components method (positive, negative and zero sequence), by Fortescue. The fault level (MVA) = √3 × rated voltage × fault current.
Switchgear Devices
| Device | Function |
|---|---|
| Isolator (disconnecting switch) | Isolates a part of the circuit after the circuit breaker has opened; operated only off-load; it has no arc quenching capacity |
| Switch | Makes or breaks a circuit under normal load |
| Circuit breaker | Makes and breaks the circuit under normal and fault conditions automatically |
| Fuse | Breaks the circuit by melting when the current exceeds the limit |
| Relay | Detects the fault and sends the tripping signal to the circuit breaker |
| Earthing switch | Connects a de-energised circuit to the earth for safety |
Operating sequence: to disconnect, first open the circuit breaker and then the isolator; to connect, first close the isolator and then the circuit breaker.
Circuit Breakers and Arc Quenching
- When the contacts of a circuit breaker separate, an arc forms; it is extinguished by cooling, lengthening, splitting and de-ionising the arc.
- The arc is extinguished at a current zero in AC; restriking voltage is the voltage that appears across the contacts after the arc extinguishes, and the rate of rise of restriking voltage (RRRV) decides if the arc restrikes.
| Type | Medium | Features |
|---|---|---|
| Oil circuit breaker (bulk oil or minimum oil) | Transformer oil | Hydrogen formed from the oil decomposition cools the arc; fire risk; older |
| Air-break circuit breaker | Air at atmospheric pressure | For low and medium voltage (up to 11 kV) |
| Air-blast circuit breaker | Compressed air | Fast operation; for high voltage; noisy; no fire hazard |
| SF₆ circuit breaker | Sulphur hexafluoride gas | Excellent insulating and arc-quenching properties; for 33 kV to 800 kV; quiet; used in gas-insulated substations (GIS) |
| Vacuum circuit breaker | Vacuum | For 11 kV to 33 kV; no arc products; long life; very short arc |
| Miniature circuit breaker (MCB) | Air | Small domestic breaker for overload and short circuit |
| Molded case (MCCB) and ELCB / RCCB | – | Industrial protection and earth leakage protection |
Circuit Breaker Ratings
- Breaking capacity (MVA) = √3 × rated line voltage (kV) × rms current at breaking (kA).
- Making capacity is the peak value of the maximum current at closing; it is about 2.55 times the symmetrical breaking current.
- Short-time rating: the current the breaker can carry for a short time (1 or 3 seconds).
- Rated voltage, rated normal current and rated frequency are the other ratings.
Protective Relays
| Relay | Principle or Use |
|---|---|
| Electromagnetic attraction type | Attracted armature or plunger; instantaneous operation; used as the overcurrent and overvoltage relay |
| Induction disc (induction type) relay | Works on the induction motor principle; for the inverse-time overcurrent protection (IDMT: inverse definite minimum time) |
| Directional overcurrent relay | Operates only for the current in a certain direction; used in the parallel feeders and ring mains |
| Distance (impedance) relay | Operates when the impedance (distance) to the fault is less than the set value; for transmission line protection; types: impedance, reactance and mho relay |
| Differential relay (Merz–Price) | Compares the currents at the two ends of the protected equipment; for the protection of transformers, alternators and bus bars |
| Buchholz relay | A gas-operated relay between the transformer tank and the conservator; detects incipient faults such as the internal faults and oil decomposition (alarm for slow gas formation and trip for the heavy fault); only for oil-immersed transformers with a conservator |
| Reverse power relay | Prevents the motoring of a generator |
| Thermal relay | Overload protection of motors |
| Negative-sequence and earth fault relays | Detect the unbalance and the earth faults |
Fuses
- A fuse is the simplest protection against overloads and short circuits; it has a fuse element of tin, lead, zinc or silver; the current rating is the maximum current it can carry continuously.
- Fusing current is the minimum current at which the fuse melts; the fusing factor = fusing current / rated current (always greater than 1).
- HRC (high rupturing capacity) fuse has a sealed cartridge filled with silica sand and breaks the large fault currents without a flash; used in industries.
- The rewirable (kit-kat) fuse is used in homes; the drop-out and the expulsion fuse are used on the overhead distribution lines.
Protection Against Lightning and Surges
| Device | Function |
|---|---|
| Earth (ground) wire | A conductor above the line that protects it from the direct lightning strokes |
| Lightning arrester (surge diverter) | Diverts the surge to the earth; types: rod gap, horn gap, expulsion type and valve type (ZnO metal oxide arresters are the modern type) |
| Surge absorber | Absorbs the steep front of the travelling wave |
Instrument Transformers
| Transformer | Features |
|---|---|
| Current transformer (CT) | Steps down the current for the meters and relays; the secondary is rated 1 A or 5 A; the secondary must never be open-circuited (dangerously high voltage) |
| Potential (voltage) transformer (PT or VT) | Steps down the voltage to 110 V; the secondary must never be short-circuited |
Solved Examples
Example 1. A circuit breaker is rated 11 kV and 20 kA breaking current. Find the breaking capacity.
√3 × 11 × 20 = 381 MVA.
Example 2. Find the making current for the breaker in Example 1.
2.55 × 20 = 51 kA.
Example 3. Which relay protects the transformer against the internal faults using the gas?
The Buchholz relay.
Example 4. Which fault is the most common in a power system?
The single line to ground fault.
Quick Tips
- Memorise the order for operating the isolator and the breaker.
- Remember that the CT secondary must never be open and the PT secondary must never be shorted.
- Remember that the Buchholz relay is for transformers only.
- Remember that the making capacity is 2.55 times the breaking current.
Quick Revision Points
- Isolators are operated off-load.
- SF₆ breakers are used for the highest voltages.
- Differential relays protect transformers and alternators.
- The CT secondary must not be open-circuited.
- The most common fault is the single line to ground fault.
Practice Questions With Answers
- Which circuit breaker uses SF₆ gas?
Answer: SF₆ circuit breaker - Which relay is used for the transformer incipient faults?
Answer: Buchholz relay - Which fault is the most severe?
Answer: Three-phase (symmetrical) fault - What is the making capacity relative to the breaking current?
Answer: About 2.55 times - Which switch is operated only off-load?
Answer: Isolator - What should never be done with the secondary of a CT?
Answer: It should never be open-circuited - Which fuse has the highest breaking capacity?
Answer: HRC fuse - What is the function of a lightning arrester?
Answer: To divert the surges to the earth - Which relay uses the Merz-Price principle?
Answer: Differential relay - What is the unit of the breaking capacity?
Answer: MVA
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Electrical Engineering notes page, read the previous topic Power Systems: Transmission and Distribution.
Frequently Asked Questions
What is a circuit breaker?
A switching device that can make and break the circuit under normal and fault conditions.
What is an isolator?
A switch that isolates a section for maintenance; it is operated without load.
What is the difference between a fuse and a circuit breaker?
A fuse melts and must be replaced; a circuit breaker can be reset and used again.
What is a relay?
A device that senses the fault and gives the trip signal to the circuit breaker.
What is the Buchholz relay?
A gas-actuated relay for the transformer protection.
What is differential protection?
A protection that compares the currents entering and leaving the protected equipment.
What is breaking capacity?
The maximum fault current in MVA that a circuit breaker can interrupt.
What is a CT?
A current transformer used for measurement and protection.
Why must the CT secondary not be open?
A very high voltage appears across it and can damage the insulation.
Is this topic asked in SSC JE?
Yes. Switchgear and protection are asked in SSC JE Electrical.
What is an HRC fuse?
A high rupturing capacity fuse with a sealed cartridge for the large fault currents.
What is a lightning arrester?
A device that protects the equipment from the lightning surges.
Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.