Protection & Circuit Breakers — Most Asked Questions
27 questions asked about protection & circuit breakers across the General and Fn 3 and Fn 5 papers, heaviest first. 3 of them are must-master.
Learn the system first
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1 What is Reverse Power Trip?
Answer
When the prime mover dies, its generator becomes a motor fed from the board — the reverse-power relay sees power flowing backwards and trips the breaker.
- A failed prime mover lets its generator motor: it draws power instead of making it, overloading the healthy machines and wrecking the dead engine.
- Reverse current in AC is too small to catch directly, so the relay watches reverse power and trips the generator breaker.
- The same device guards the prime mover itself — turbine, diesel or shaft generator — against being dragged as a load.
- Prove it on trials: check overcurrent, reverse-power and preferential trips together, and run the dedicated reverse-power trip test.
Surveyor follow-up: two generators in parallel, one engine flames out — describe exactly what the relay sees and which breaker opens.
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
2 Type of fuses?
Also asked: “Type of fuse?”, “Types of fuses Why fuses Fuses vs circuit breakers”, “wht is fuse and types”
Answer
Ships use HRC cartridge fuses — sealed, high-breaking-capacity links — because rewireable fuses are unreliable and easily abused.
- Rewireable fuses are not recommended for marine practice: less reliable than cartridge types and open to abuse such as fitting the wrong gauge of wire.
- Standard fit: HRC (high rupturing/breaking capacity, up to 80 kA) cartridge links — connector and outer cap, inner cap, body, fuse elements in filler.
- Weakness: insensitive to small overloads — an HRC link passes 20% overload for about 4 hours, so real overload protection comes from an overcurrent relay.
- Never uprate a blown fuse: the rating protects the cable, and discrimination with upstream breakers dies with the wrong link.
Surveyor follow-up: a fuse blows twice in a week with no fault found — what do you suspect besides the circuit?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
3 How u find earth fault?
Also asked: “How to check earth faults?”, “What is earth fault”, “What is earth fault, how will you detect and what are the causes”
Answer
Find earth faults by elimination: read the insulation monitor, split the board section by section, and start where moisture lives.
- Note the earth lamps/insulation monitor first, then open and close breakers section by section until the fault clears — minding which essentials you are interrupting.
- Start with galley and laundry, where faults are commonest.
- Usual hiding places: open-deck lamp fittings breathing moisture, galley ovens and hotplates, drenched laundry machines, drips over machinery, idle motors with condensed insulation, overheated or dirty aged insulation.
- Prove dead circuits with a megger before touching; a standing earth fault plus a second fault is a short circuit — find the first before it finds you.
Surveyor follow-up: earth lamp shows a dead short on one phase of the 440-V board — do you keep the ship running on the other two?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
4 Safety & environmental protection policy
Answer
The policy is the company’s signed promise: safe ships, clean seas, named duties and steady improvement.
- Objectives state safety at sea, injury prevention and pollution avoidance as the company’s first duties.
- Safe practices, safeguards against identified risks and rising skills ashore and afloat follow from it.
- Responsibility, authority and resources are written down — who provides what for the designated person to work.
- Posted, taught and audited: the policy the surveyor reads on the bulkhead must match the ship’s practice.
Surveyor follow-up: quote your company’s policy from memory — then show me where it hangs and who signed it.
Full theory: SOLAS Chapter IX — ISM Code & Safety Management Systems -
5 Health protection, medical care, welfare and social security protection
Answer
MLC health cover runs ship to shore: onboard care, owner liability, welfare access and social security behind every contract.
- Onboard medical chest, trained first-aiders and radio medical advice with shore referral without delay.
- Shipowner liability for sickness and injury costs including wages, treatment and repatriation.
- Shore welfare facilities and consular access in port for rest, contact and complaint.
- Social-security branches named in the declaration whatever the flag — the surveyor reads the cover, not the promise.
Surveyor follow-up: injured rating landed with no insurance paper — owner, P&I or flag, and who pays first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
6 What is reverse power relay?
Answer
The reverse-power relay stops a dead generator motoring: direction-sensing trip that sheds the set before the board feeds it.
- Watches power direction, not just current — AC reverse current hides too small to catch directly.
- Generator output falling below the preset sheds the breaker before busbar power drives the stator.
- Protects the prime mover stalled by fault from being dragged as a motor load.
- Prove with overcurrent and preferential trips together on schedule, not alone.
Surveyor follow-up: two sets parallel, one engine flames out — which breaker opens, and what does the relay see?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
7 Fuses and fuse ratings
Answer
Fuses rate to the cable with discrimination upward: motor tables pair relay range to diazed HRC backup.
- Size to full-load current per the motor table — relay range below, HRC backup above for short-circuit duty.
- HRC cartridge to 80 kA breaking beats rewireable wire, which crews abuse with wrong gauges.
- Small overloads belong to relays, not fuses — an HRC passes 20% over for hours before blowing.
- Prove discrimination upward so the branch fuse dies before the board breaker trips the ship.
Surveyor follow-up: branch fuse blows with the motor healthy — rating, discrimination or downstream fault?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
8 Earth fault detection
Also asked: “Earth fault finding”
Answer
Earth faults show on lamps then meters then breakers: dim lamp names the leg, resistance meter proves high impedance, elimination finds the circuit.
- Healthy lamps glow equal half-bright; the faulted leg dims or dies while sisters brighten.
- Lamps miss high-impedance faults — resistance meters with small DC injection prove those.
- Find by opening breakers section by section to essential-safe order until the fault clears.
- An earth on an earthed system equals a short — clear fast with automatic protection, not patience.
Surveyor follow-up: lamps equal with the insulation alarm sounding — high impedance or healthy, and what next?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
9 Preferential trip relay working?
Answer
Preferential trips shed hotel load in stages on overload: alarm first, then timed trips of non-essentials before the board dies.
- Overload relay watches generator current with staged time delays per load group.
- First stage drops galley, accommodation and hold ventilation; essentials never shed.
- Generator near 110% starts the sequence; near 150% the main breaker itself guards the set.
- Prove by staged-trip test with timings logged — a preferential that never trips is already a failure.
Surveyor follow-up: blackout with preferentials unfired — relay, settings or wiring, and what proves it?
Full theory: Switchboards & Protection — MSB to Emergency Bus -
10 type of circuit breaker
Also asked: “Type of breaker”
Answer
Breakers kill arcs by medium: air on the main board, vacuum or SF₆ on high voltage, MCCB and MCB downstream.
- Main LV breakers are air-break ACB with contacts parting in air and arc chutes stretching the arc.
- High voltage at 6.6 and 11 kV uses vacuum bottles or SF₆ gas — millimetres of vacuum insulate what air cannot.
- MCCB moulded cases guard feeders with thermal-magnetic trips; MCB miniature types guard final circuits.
- Size by breaking capacity, not just amps — a breaker that cannot clear the fault becomes the fault.
Surveyor follow-up: ACB on a 6.6 kV board with vacuum bottles alongside — right breaker or wrong voltage, and why?
Full theory: Switchboards & Protection — MSB to Emergency Bus -
11 Diff. B/W Relay and contactor
Answer
Relays sense and signal light; contactors switch heavy power — the relay’s contact drives the contactor’s coil.
- Relays are light-duty solenoids switching milliamps to detect faults and command switching devices.
- Contactors are heavy-duty switches breaking motor, lighting and heating loads at hundreds of amps.
- A small relay contact energises the contactor coil; the contactor’s main poles carry the load current.
- Name by duty: protection and logic live in relays, power lives in contactors with arc control on the poles.
Surveyor follow-up: contactor chattering with the relay steady — coil volts, mechanical wear or control supply first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
12 fuse and circuit breaker
Answer
Fuses melt once, breakers trip and reset: MCCB guards feeders three-pole, MCB guards final circuits single-pole.
- MCCB moulded cases replace fuses on three-phase feeders with thermal-magnetic or electronic trips plus undervoltage and shunt releases.
- MCB miniature types guard lighting and socket circuits with the same reset advantage at lower currents.
- Breakers trip all three phases together, saving motors from the single-phasing a blown fuse invites.
- Size by breaking capacity and coordination — the downstream device must clear before the upstream one trips the board.
Surveyor follow-up: one fuse blown on a three-phase motor feeder with the motor still running — keep running, stop or change under load?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
13 A 40kW,220V shunt motor has a full-load efficiency of 90 per cent, an armature resistance of 0.075 ohms and a shunt-field resistance of 5Sohms. When ‘at starting', } the starter handle is moved onto the first stud, it is
Also asked: “A 40kW,220V shunt motor has a full-load efficiency of 90 per cent, an armature resistance of 0.075 ohms and a shunt-field resistance of 5Sohms. When ‘at starting’, } the starter handle is moved onto the first stud, it is desired to limit the current, through the armature to 1.5 times the value which it has when the motor is on full load. What must be the total value of the starting resistance? If,”
Answer
Start a 40 kW shunt motor at 1.5 times full-load armature current: full-load current is about 202 A, so the starter must add about 0.67 ohm.
- Full-load line current is output divided by efficiency and voltage: 40 000 / 0.90 / 220 gives about 202 A.
- Field current is voltage over field resistance: 220 / 50 gives 4.4 A, so armature current is about 197.6 A.
- Armature current at starting is 1.5 times that, about 296.4 A, which sets the total armature circuit resistance.
- Total resistance is 220 / 296.4, about 0.742 ohm; subtract the armature resistance of 0.075 and the starting resistance is about 0.667 ohm.
Surveyor follow-up: starter set for 0.667 ohm and the motor still drawing 1.9 times full load — is the first stud, the field or the handle position at fault?
Full theory: Motor Starters — Taming 8–10× Current -
14 Contactors and Relay
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15 Difference between contractor and relay
Also asked: “diffrence in relay and contactor”
Answer
A relay decides, a contactor acts: the relay handles small control currents, the contactor switches the heavy power circuit.
- A relay is a light-duty switch — as simple as a 12 V solenoid with small contacts, used to detect faults and signal.
- A contactor is an electrically controlled switch for power circuits, similar in action but rated for far higher current.
- A small relay contact energises the contactor coil, and the contactor’s main poles carry the load current.
- Protection and logic live in relays; motors, lighting, heating and capacitor banks are switched by contactors.
Surveyor follow-up: contactor chattering with the relay proved steady — coil voltage, mechanical wear or control supply first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
16 Earth fault and how to measure
Answer
An earth fault is a leakage to earth, and it is measured either as insulation resistance or as the current flowing in the fault.
- Earth fault lamps are the old method — cheap and simple, but too insensitive to catch a high-impedance fault.
- Instrument indicators inject a small d.c. voltage and measure the current, reading insulation resistance directly in kilohms.
- They draw about 1 mA maximum against roughly 60 mA for lamps, and give both visual and audible alarm.
- On high-voltage systems the neutral is earthed through a resistor, and a CT measures neutral current or an earth fault relay trips.
Surveyor follow-up: insulation monitor reading high with one lamp dark on the same board — instrument or lamp fault first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
17 Earth fault diagram
Answer
Draw the fault path: feeder, insulation breakdown to the earthed hull, and the measuring device in that return path.
- Show the feeder from the switchboard, the insulation failure to the hull, and the return through the earthed neutral.
- On low-voltage unearthed systems the fault returns through the lamps or the insulation monitor on the return path.
- On high-voltage systems the generator neutral is earthed through a resistor, with a CT or relay in the neutral.
- Mark where the alarm and the automatic disconnection sit — the diagram is about the return path, not the supply.
Surveyor follow-up: earth fault alarm with the hull earth strap removed at the last drydock — what has changed on the diagram?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
18 Explain single phasing protection for poly phase motor
Also asked: “How to protect moter from single phasing”
Answer
Single phasing is one phase lost: the healthy phases carry more current and heat, so protection must detect the unbalance, not just the overload.
- Obvious signs are a hum louder than normal, a shaft that vibrates rather than rotates, and excess current.
- Many heater-type overload relays have no single-phase detection at all, because the total current can stay within limits.
- Special protective relays identify and isolate the affected load; smaller motors rely on overcurrent and negative phase sequence.
- Three-pole breakers must interrupt all three phases together, or a defective contact or loose connection causes the fault.
Surveyor follow-up: motor running single-phased on a breaker with one weak contact — is the relay the fault or the contact?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
19 Fuse and function and type
Answer
A fuse is the simplest protection: a element that melts at a set current and opens the circuit permanently.
- Rewirable fuses use a wire link replaced after it blows; cartridge fuses contain the element in a sealed cartridge.
- Types are chosen by current rating, breaking capacity, and the duty — fast, time-delay, or semiconductor fuses for delicate circuits.
- Its function is to protect the cable and equipment from overload and short circuit, cheaply and without any setting.
- A fuse gives no indication of why it blew, so the fault behind it must always be found before a new one is fitted.
Surveyor follow-up: second fuse on the same circuit blown this week — replace it, or find the earth fault first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
20 How Thermal relay works
Also asked: “Thermal relay working”
Answer
A thermal relay heats with motor current and bends bimetal strips until the relay trips — a thermal image of the winding.
- Three bimetal elements, each with its own expansion coefficient, sit with a heater coil in each phase.
- Over-current heats the coil, the element bends, and the relay trips the contactor feeding the motor.
- Ambient compensation balances the characteristic so a hot engine room does not cause nuisance trips.
- The differential device trips when the three currents differ, catching single-phasing and unbalance as well as overload.
Surveyor follow-up: motor tripping in a hot engine room with the ambient compensation fitted — is the setting or the ambient first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
21 In this I told that one method is by checking differential pressure
Answer
Differential pressure is how you test the earth fault system: a high reading on the lamp circuit means the insulation is down.
- Earth fault lamps were the old method — inexpensive and simple, but too insensitive to catch a high-impedance fault.
- Instrument earth fault monitors inject a small d.c. voltage and read the current, giving insulation resistance in kilohms directly.
- They draw about 1 mA maximum against roughly 60 mA for lamps, with visual and audible alarm at the set value.
- So the differential between a healthy and a faulty reading is the test, and the trend is what finds the slow decay.
Surveyor follow-up: monitor reading healthy with the ship earthed — is the monitor, the earth strap or the sensor first?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
22 Intact stability ( i confused
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23 Precaution or Cathodic protection
Answer
Cathodic protection makes the hull the cathode: a current is imposed so the local anodic cells stop corroding.
- The anodic corrosion reactions are suppressed by applying an opposing current that lowers the metal’s potential.
- That current enters at every point on the hull, so the anodic areas of each local cell become cathodes and do not corrode.
- Sacrificial anodes are less noble metals attached to the hull, consumed as they supply the protecting current.
- Impressed current uses an external source and inert anodes, giving controllable protection with the anodes not consumed.
Surveyor follow-up: hull pitted with the anodes renewed and the ICCP reading normal — is the coating, the current or the steel?
Full theory: Marine Materials, Metallurgy & Mechanical Testing -
24 Protection against pitching
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25 Purpose of air circuit breaker
Answer
An air circuit breaker interrupts the arc in air, with arc chutes stretching it until it cools — that is the main switchboard’s job.
- Contacts separate in air and the arc is drawn into the chute plates, where it is lengthened, cooled and extinguished.
- It protects and switches the incoming generator and feeder circuits, and is the standard main breaker in low-voltage boards.
- The result must clear the worst available fault current, so the breaking capacity is chosen with fault level in mind.
- Inrush from large motor starting is tolerated through the momentary withstand, while a real fault is cleared in milliseconds.
Surveyor follow-up: ACB on a board with an available fault above its breaking capacity — which device actually clears that fault?
Full theory: Switchboards & Protection — MSB to Emergency Bus -
26 What is a short circuit and how may a short- circuit develop in
Also asked: “What is meaning of short circuit?”
Answer
A short circuit is a fault where a large current bypasses the load: insulation breaks down, or conductors are shaken into contact.
- In a generator it follows insulation failure between windings or between a winding and the frame, made worse by vibration or moisture.
- In the external circuit it comes from damaged insulation, chafed cable, loose terminals that vibrate together, or mechanical damage.
- The result is a very high fault current, limited only by the source and the impedance of the path.
- Minimise it by using the correct insulation grade, protecting against vibration and water, and clearing faults fast with correct coordination.
Surveyor follow-up: repeated burning at one joint with sound insulation elsewhere — insulation, termination or vibration?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
27 motor is overloaded
Also asked: “Motor running at overload s”
Answer
An overloaded motor is drawing too much for its rating: the thermal relay heats, trips, and the cause is usually mechanical, not electrical.
- The thermal relay sees the current and trips the contactor after a delay matched to the severity of the load.
- Causes are usually drag on the driven machine, a jammed bearing, a blocked impeller or a pump running against a closed valve.
- Wrong cable or transformer settings on the supply also read as overload and trip a motor that is working perfectly.
- Find the mechanical cause before resetting — repeated resets cook windings and turn an overload into a rewinder.
Surveyor follow-up: trip on a pump with the valve shut and the current steady — is the pump, the valve or the relay setting at fault?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit
Source: 123 written papers and surveyor oral banks, 24,613 entries, merged into 17,210 topics. Ticks are stored on this device only.