Cables, Insulation & Earthing — Most Asked Questions
22 questions asked about cables, insulation & earthing across the General and Fn 3 and Fn 5 papers, heaviest first.
Learn the system first
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1 Earth neutral system
Answer
Ships sail insulated-neutral: the first earth fault only alarms and she keeps running; earthed neutral trips it at once.
- Most ships run three-wire insulated neutral with alternator star points off earth — a single earth fault draws almost no current.
- First fault just lights the earth lamps and alarms; find it by opening breakers section by section, galley and laundry first.
- Earthed neutral instead limits overvoltage and auto-locates and disconnects the fault — safer for high voltage, harsher on continuity.
- High-voltage systems therefore earth through resistors to cap fault current at generator full load, watched by earth-fault relays.
Surveyor follow-up: earth lamp dead on one phase of the 440 board — keep running or stop, and what is your first breaker move?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
2 Ans a) WHAT IS MLC 2006? The MLC 2006 has become a global legal instrument that will be the fourth pillar of IMO.MLC stands for Maritime Labour Convention. It was adopted on 23 February 2006 and came into force on 20 Aug
Also asked: “Ans a) WHAT IS MLC 2006? The MLC 2006 has become a global legal instrument that will be the fourth pillar of IMO.MLC stands for Maritime Labour Convention. It was adopted on 23 February 2006 and came into force on 20 August 2013. It is applicable to all ships above 500 GT. Purpose of MLC”
Answer
MLC 2006 is the fourth pillar beside SOLAS, MARPOL and STCW: adopted February 2006, in force August 2013, setting minimum seafarer rights with teeth.
- Seafarers need special protection — MLC balances the industry for employers and crew alike.
- Sets minimum bars to work on a ship and strengthens enforcement at flag and port-state levels.
- Covers agreement, wages, hours, leave, accommodation, food, health and repatriation in one text.
- Long-term payoff for compliant owners through stable crews and fewer detentions.
Surveyor follow-up: fourth pillar beside which three conventions, and what date did it enter force?
Full theory: STCW: The Convention & Its Eight Chapters -
3 Copies of the SEA and Collective Bargaining Agreement (CBA) (if applicable) must be available onboard
Also asked: “• Copies of the SEA and Collective Bargaining Agreement (CBA) (if applicable) must be available onboard”
Answer
The signed agreement and any collective bargain ride the ship: originals aboard for inspection with copies ashore.
- Every seafarer holds a signed employment agreement with wages, leave and repatriation terms.
- Where a collective agreement applies, its copy rides beside the SEA for the same inspection.
- Originals aboard for flag and port-state checks; unsigned or missing papers fail manning.
- Piracy and notice clauses extend cover past expiry — the file proves protection never lapsed.
Surveyor follow-up: CBA applied but no copy aboard — compliant or not, and where should it be?
Full theory: STCW: The Convention & Its Eight Chapters -
4 Class of insulation
Also asked: “insulation nd classes of insulation”, “Classes of insulation details”
Answer
Insulation classes rank heat endurance from Y coolest to C hottest: B and F rule shipboard motors.
- Classes run Y, A, E, B, F, H and C with allowable temperature rising each step above 40-degree ambient.
- B and F dominate marine motors; H appears in hot spots needing extra thermal headroom.
- Ten degrees past the class limit halves insulation life — heat kills faster than load.
- Pick by duty and space: hot engine flats earn higher classes with ventilation to match.
Surveyor follow-up: Class B motor running at Class F temperature — rewind, derate or ventilate, and what proves it?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
5 How will u check insulation
Also asked: “how will u know insulation condition”
Answer
Check insulation dead with a megger, not a multimeter: isolate, discharge, test phase-to-phase and to earth.
- Isolate and discharge the circuit with men-at-work tags before probes touch metal.
- Megger to 500 volts for low-voltage gear — multimeters cannot stress insulation into showing faults.
- Dry the motor with air and heaters if damp, varnish windings if readings sag, retest to steady values.
- Start rebuilt motors light with current watched for hours before full load.
Surveyor follow-up: megger low after a humid port stay — dry, varnish or renew, and what reading releases it?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
6 Insulation resistance test
Answer
Megger at 500 volts in megaohms: conductor-to-conductor and conductor-to-earth with health judged on trend.
- Hand-cranked or battery-electronic source stressing insulation far past multimeter volts.
- Best single guide to equipment health — falling trend fails before the machine does.
- Test cables, windings and switchgear wiring dead with guards proved and discharges done.
- Log hot and cold values with temperature noted; damp and heat both drag readings down.
Surveyor follow-up: 0.5 megaohm on a 440-volt motor — run, dry or renew?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
7 When there is a rise in temperature say due to fire, the metal-strip end of A will move to close the gap C at a rate that is faster than B (since it is better insulated) to move for maintaining the gap C
Answer
Differential strips sense fire by speed: thin exposed metal bends fast, lagged metal follows slow — contact on rapid rise only.
- Two bimetallic strips in parallel deflect the same way, thin exposed one fastest.
- Slow daily temperature swings bend both together so the alarm gap holds open.
- Fast fire heat bends the exposed strip ahead until contacts meet and alarm.
- Maximum-temperature backup closes the same contacts when heat soaks both strips to the limit.
Surveyor follow-up: alarm on a hot still day with no fire — rate-of-rise, fixed-temp or siting, and what proves it?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
8 Which annexes applicable on board
Answer
Annexes bite by ship and trade: oil, sewage, garbage and air always; chemicals and packaged harm only in those trades.
- Annex I oil and its record book on every ship with machinery spaces — tankers add cargo rules.
- Annex IV sewage past 400 gross tons or 15 persons; Annex V garbage and Annex VI air across the fleet.
- Annex II bulk chemicals only when carrying them; Annex III packaged harm only in that trade.
- Answer with your last ship: type, tonnage and cargo first, then the annexes that applied.
Surveyor follow-up: your ship loads a chemical parcel next voyage — which new annex wakes up?
Full theory: MARPOL 73/78: The Convention & Its Six Annexes -
9 what is insulation resistance
Also asked: “What is mean by insulation resistance value”
Answer
Insulation resistance is the leakage defence in ohms: radial resistance to stray current, falling with length, heat and damp.
- Healthy current runs along the core; leakage creeps radially through insulation to earth.
- Longer cables leak more — resistance falls as length and wet heat rise.
- Low-voltage gear expects megaohms, high-voltage tens of megaohms and up.
- Megger dead, trend the readings — a falling curve condemns before the fault flashes.
Surveyor follow-up: readings halved since last month with load steady — heat, damp or damage, and what next?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
10 , Discuss the suitability and limitations of the following insulating materials for use on board ships and state suitable application in each case. Particular reference should be made to the influence of environment, and
Also asked: “4, Discuss the suitability and limitations of the following insulating materials for use on board ships and state suitable application in each case. Particular reference should be made to the influence of environment, and its effect, on deterioration in service”
Answer
Insulation choice is a trade between temperature, moisture and flame: the class sets the heat, the material sets the fire risk.
- Rubber (natural and synthetic) is flexible, oil-resistant and repairs easily, but ages in heat and ozone and is poor at high temperature.
- PVC is tough, flame-retardant and cheap, but stiffens when cold and gives off fumes in a fire, so it is avoided in accommodation.
- Impregnated paper and mica handle heat and flame well, but they absorb moisture and are used only in dry, warm or enclosed positions.
- Silicone and glass-mica combinations carry the highest classes for generators and heavy machinery, at higher cost and careful handling.
Surveyor follow-up: PVC cable found in the accommodation with a note to replace it — what is the fire argument, not the cost?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
11 A 2-core cable, cach core of which is 300 m long and of uniform cross-sectional area of 150 mm? is fed from one end at 240V. A load of 200A is taken off from the centre of the cable and a load of 100A from the far end. C
Also asked: “A 2-core cable, cach core of which is 300 m long and of uniform cross-sectional area of 150 mm? is fed from one end at 240V. A load of 200A is taken off from the centre of the cable and a load of 100A from the far end. Calculate the voltage at each load”
Answer
Work the two halves: core resistance about 0.035 Ω, so 7.0 V drops to the centre load and 3.5 V beyond it — 10.5 V in all.
- Core resistance is resistivity times length over area: for 300 m of 150 mm² copper that is about 0.035 Ω per core.
- The first 300 m carries the full 200 A to the centre tap, so the drop there is 200 × 0.035 = 7.0 V.
- The second 300 m carries only the 100 A end load, so that drop is 100 × 0.035 = 3.5 V.
- Total drop is 10.5 V, leaving about 229.5 V at the far end and 233 V at the centre load — a 4.4% drop, near the practical limit.
Surveyor follow-up: 10.5 V drop computed but the far-end lamp reading 224 V — is the resistivity, the length or a joint the difference?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
12 Grade of insulation
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13 HV system neutral point earthed? Why ? Ner ?
Answer
On high voltage the neutral is earthed through a resistor: it limits the earth fault current to something a protection can clear.
- Without an earthed neutral a single earth fault on a high-voltage system would produce almost no fault current.
- An ungrounded system would then let the healthy phases rise to line voltage against earth, which is dangerous to people.
- The earthing resistor sets that fault current to a defined value — high enough to operate, low enough not to damage — and the earth fault relay trips.
- So the earthing is what gives both fault location and a safe earth voltage potential, which is why HV ships earth and LV ships insulate.
Surveyor follow-up: 6.6 kV system with the neutral resistor open and a healthy board — which protection is now blind?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
14 Insulation class and its temp's
Also asked: “Insulation class in detail (what to know temp”
Answer
Insulation classes are temperature ratings: each letter is the temperature the material can carry without ageing.
- Class A operates to 105°C, Class B to 120°C, Class E to 120°C and Class F to 155°C.
- Class H materials — silicone, mica, glass, elastomer and bonded combinations — carry up to 180°C.
- Anything operating above 180°C, proven by experience or accepted test, is a class above H.
- Pick the class for the hottest place the cable actually sees, not the ambient — a cable on a manifold is not a 70°C cable.
Surveyor follow-up: Class F cable run past a hot cylinder head — is the rating, the derating or the routing first at fault?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
15 Insulation material of tank
Answer
Cable insulation is a class of material chosen for the temperature it must carry — rubber, PVC, mineral, silicone or epoxy by the class letter.
- Class A to 105°C, B and E to 120°C, F to 155°C, and H materials like silicone and mica to 180°C.
- Marine practice favours fire-resistant mineral or silicone-sheathed types in accommodation and machinery spaces where fire load is high.
- The rating is the temperature the material carries without ageing — the class letter, not the conductor size, sets the ampacity.
- Choose for the hottest place the cable actually sees, and derate for the ambient on top of that.
Surveyor follow-up: Class F cable run past a 200°C manifold at full load — the rating, the derating or the routing first to fail?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
16 Motor insulation low , what all will you do
Answer
Low insulation is usually damp: dry it, heat it, re-varnish it — and only replace the windings when drying does not bring it back.
- Dampness is the most common cause of falling insulation resistance, and it precedes earth faults and winding shorts.
- Clean and dry the motor with clean, dry compressed air at low pressure, keeping dirt out of the windings.
- Warm the machine with heaters to raise the resistance, and re-measure — a rising trend is the answer that matters.
- If drying does not recover it, varnish or rewind the windings, and keep heaters working while the motor is idle.
Surveyor follow-up: insulation at 0.5 megohm after a week of heaters with no improvement — varnish, rewind or re-measure?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
17 Neutral earthed, insulated earthed
Answer
Earthed neutral gives a big fault current and fast clearance; insulated neutral gives none until the fault is large — so ships choose insulated.
- Earth fault current is high with an earthed neutral, but the over-voltages, especially from earth faults, are lower.
- Earthed systems give automatic earth fault location and disconnection, which is desirable on a high-voltage system.
- An insulated neutral gives no earth fault current on a single fault, so the system keeps running and continuity is protected.
- Standard marine practice is three-phase three-wire with the alternator neutral points insulated, unlike the earthed systems ashore.
Surveyor follow-up: LV board with an earthed neutral on a ship — which convention does that follow, and what does it cost in fault current?
Full theory: Switchboards & Protection — MSB to Emergency Bus -
18 Shaft earth method what will happen is not done and the unti of current and voltage measured across it
Answer
Shaft earthing carries the galvanic current away in a measured, low-resistance path — remove it and the bearings take the spark erosion.
- Galvanic action between the steel hull and the propeller shaft, with seawater as electrolyte, sets up the potential.
- That potential is transferred to the main crankshaft, where dissimilar metals let erosion occur between white metal and journal.
- Spark erosion can only occur if that current is not grounded — the shaft earth is the controlled exit for it.
- Check it by measuring the current and voltage across the shaft earth: a high reading is the wear you are about to find.
Surveyor follow-up: shaft earth removed for a month with white metal fragments found — what was the current doing?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
19 The insulation reading of new motor
Answer
A new motor should read high from the first megger test — the number is the baseline every later reading is compared against.
- The reading is taken at 500 V for low-voltage windings, with the machine disconnected, dry and at a known temperature.
- A new or rewound machine typically reads in the hundreds of megohms; anything near 1 megaohm is already suspect.
- Record it on the commissioning sheet as the reference, because it is the trend that condemns and not the number.
- If it reads low from the factory, it is damp or the varnish was not cured — the fix is drying, not accepting it.
Surveyor follow-up: new motor reading 2 megohm at handover with a 1 megaohm limit — accept, dry, or send it back?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
20 What is insulation
Answer
Insulation is the material that keeps conductor current in its conductor: it defines the class, the ampacity and the safety of every circuit.
- It is a dielectric that resists current between conductors, and from conductor to earth and to the ship’s steel.
- Its class letter sets the temperature it can carry, which in turn sets how much current the cable may take.
- Insulation resistance in megohms is the number that measures whether it is still doing that job.
- Insulation fails by ageing, heat, damp and abrasion — and every failure starts as a fall in that resistance figure.
Surveyor follow-up: cable with sound insulation but a chafed sheath — is the conductor protected, and what do you find at the bend?
Full theory: Marine High Voltage (HV) Installations — 3.3 kV, 6.6 kV & 11 kV Switchgear and Safety -
21 What is shaft earthing. Why is it needed? How to check?
Also asked: “• what is shaft earthing why it is used”
Answer
Shaft earthing gives the propeller shaft a controlled path to the hull so galvanic current does not eat the bearings instead.
- Galvanic action between the steel hull and the bronze propeller, with seawater as electrolyte, puts a potential on the shaft.
- Without a ground path that current discharges as sparks between the white metal bearings and their journals — spark erosion.
- The earthing is a monitored connection between the shaft aft and the hull, with the current and voltage read across it.
- A high reading or a broken connection is the finding — prove it alongside and look for white metal fragments at the bearings.
Surveyor follow-up: shaft earth current high with the reading taken under way — is it the gear, the propeller material or the earth itself?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
22 Which code is applicable on your ship
Answer
Name the code for the ship’s service: SOLAS for safety, MARPOL for pollution, and the class rules behind the statutory certificates.
- SOLAS applies to the safety of life at sea — construction, fire protection, life-saving, navigation, radio and the management chapters.
- MARPOL applies to pollution, with the ballast water convention and the special area rules sitting beside it.
- The flag administration issues the statutory certificates, and the classification society supplies the rules the surveyor applies.
- So the answer names the convention, the chapter and the certificate it produces — that is the level the surveyor wants.
Surveyor follow-up: a tanker quoting a class code where the convention applies — which code governs the discharge she is making?
Full theory: Maritime Codes & Conventions: The Complete List
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