ETO · STCW III/6 · Concept refresher

Chapter briefs for ETO.

Eleven topics, each in one screen: what the assessment asks, the relationships worth carrying, the mistakes that cost marks, and a recall list to run through before you sit.

CertificateETO · STCW III/6
Topics11 chapter briefs
Read time≈ 6 min each
01 Fundamentals 02 Generation 03 High voltage 04 Motors & drives 05 Automation 06 Power electronics 07 Nav & comms 08 Instrumentation 09 Troubleshooting 10 Regulations 11 Assessment
01 / 11DC, AC, three-phase

Electrical Fundamentals

Everything else in the ETO syllabus is this, applied. Ohm's law, the three-phase relationships and the power triangle carry more marks than any single machine.

What the paper asks

Series and parallel circuits; Kirchhoff's laws; AC waveforms, RMS and peak; inductance, capacitance and reactance; three-phase star and delta relationships; power factor and the power triangle; and per-unit and cable calculations.

The concepts, in order

The power triangle explains the whole ship's electrical system. Real power in kW does the work, reactive power in kVAr magnetises the motors and transformers, and apparent power in kVA is what the cables and generators must carry. Power factor is the ratio of the first to the last.

Star and delta are not interchangeable. In star the line voltage is √3 times the phase voltage and the currents are equal; in delta the voltages are equal and the line current is √3 times the phase current. Every three-phase calculation starts by establishing which connection is in use.

Reactance is frequency dependent. Inductive reactance rises with frequency, capacitive reactance falls. That is why a motor drawing acceptable current at 60 Hz behaves differently on a 50 Hz supply, and why harmonics from converters overheat equipment that is not overloaded.

Poor power factor costs current. The same useful kilowatts at a lower power factor mean more amps in the cables, more copper loss, and generators that reach their current limit before their power limit.

Where marks are lost

  • Using peak voltage where RMS is meant. Ratings are RMS; peak is √2 times larger.
  • Applying the √3 factor twice. It belongs either to the voltage or to the current, depending on the connection.
  • Confusing kW and kVA. Generator ratings are usually in kVA at a stated power factor.
  • Forgetting that cable resistance changes with temperature. A hot cable drops more volts.
60-second recall
  1. P = √3·V_L·I_L·cos φ.
  2. Star: V_L = √3·V_ph. Delta: I_L = √3·I_ph.
  3. RMS = peak/√2.
  4. Low power factor means more current for the same kW.
  5. X_L rises with frequency, X_C falls.
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02 / 11Alternators & boards

Generation & Distribution

Two independent controls — the governor and the AVR — and everything that goes wrong with load sharing comes from confusing them.

What the paper asks

Alternator construction and excitation; AVR operation; synchronising conditions and procedure; real and reactive load sharing; droop; preferential trip; protection and discrimination; and blackout recovery.

The concepts, in order

The governor sets kW; the AVR sets kVAr. Real power sharing is adjusted by the governor's speed setting, reactive sharing by the AVR's voltage setting. Trying to correct a circulating current with the governor moves kilowatts instead and leaves the problem untouched.

Synchronising needs four conditions. Equal voltage, equal frequency, correct phase sequence, and the correct phase angle at the instant of closing. Closing out of phase produces a violent torque transient through the coupling and can shear a shaft.

Droop makes sharing stable. A governor with a few per cent droop lets two machines share load in proportion without hunting. In isochronous mode with load sharing lines, the control system does the same job electronically.

Preferential trip buys time. As the running sets approach overload, non-essential loads are shed in a set order with short delays — air conditioning and ventilation first — before the generator's own overload protection acts and causes a blackout.

Where marks are lost

  • Adjusting the governor to correct reactive load sharing.
  • Skipping the phase sequence check after maintenance or re-cabling.
  • Closing the breaker with the synchroscope moving fast. It must creep slowly in the fast direction.
  • Treating preferential trip as a fault. It is the system working as designed.
60-second recall
  1. Governor = kW. AVR = kVAr.
  2. Voltage, frequency, phase sequence, phase angle.
  3. Droop typically 3–5 % for stable sharing.
  4. Preferential trip sheds non-essentials before the set trips.
  5. Emergency generator on load within 45 seconds.
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03 / 11Safety discipline

High Voltage Systems & Safety

The theory is familiar; what is examined is the discipline. High voltage forgives nothing, and the procedure exists because people have been killed skipping steps.

What the paper asks

Why HV is used; neutral earthing philosophy; switching and isolation procedure; proving dead, discharging and earthing; permits and the authorised person; insulation testing and polarisation index; and arc flash risk.

The concepts, in order

Higher voltage means lower current for the same power — smaller cables and lower losses. Above a few megawatts the economics are decisive, which is why large passenger ships and diesel-electric vessels use 6.6 or 11 kV.

HV is usually earthed through a resistor, LV is insulated. The LV philosophy keeps supply on through a single earth fault; the HV philosophy limits the fault current to a value that protection can detect and clear selectively. Continuity is traded for the ability to find and clear a fault before it escalates.

The access sequence is fixed. Isolate, lock off, prove the tester, prove dead, prove the tester again, discharge, apply earths — and only then work. Cable capacitance holds a lethal charge after the breaker opens, which is why discharging and earthing are separate steps.

Insulation testing tells you the condition. A 5 kV megger for HV machines, and the polarisation index — resistance at ten minutes divided by resistance at one minute — indicating whether the winding is dry and clean. A value below about two suggests moisture or contamination.

Where marks are lost

  • Proving dead without proving the tester before and after.
  • Treating switching off as isolation. Isolation is proven, locked and earthed.
  • Applying LV insulated-neutral reasoning to HV.
  • Ignoring arc flash. The energy at HV switchgear is a burn hazard even without contact.
60-second recall
  1. Isolate, lock, prove tester, prove dead, discharge, earth.
  2. HV above 1 kV; typically 3.3, 6.6 or 11 kV on board.
  3. HV resistance earthed; LV insulated.
  4. Polarisation index ≥ 2.
  5. Only an authorised person switches HV.
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04 / 11Starting & speed

Motors, Starters & Drives

The induction motor turns most of the machinery on the ship. Knowing how it starts, how its speed is changed and how it fails covers most of a working day.

What the paper asks

Induction motor construction and slip; torque–speed characteristics; starting methods and their starting current; star-delta and soft starters; variable frequency drives and the volts-per-hertz principle; motor protection; and insulation classes.

The concepts, in order

Slip is what makes torque. The rotor must turn slower than the rotating field for current to be induced in it, so an induction motor can never reach synchronous speed. Full-load slip is typically two to five per cent.

Starting current is the problem, not starting torque. Direct-on-line starting draws six to eight times full-load current, which is acceptable on a small motor and unacceptable on a large one because of the voltage dip it causes across the whole board.

Star-delta reduces both current and torque to a third. That is the catch: it only works for a load that can be started unloaded or lightly loaded. A soft starter ramps voltage more gracefully; a variable frequency drive avoids the problem entirely.

A VFD holds volts per hertz constant. Reducing frequency to reduce speed while holding the ratio keeps the flux — and therefore the torque capability — constant. Above base speed the voltage cannot rise further, so torque falls off.

Where marks are lost

  • Star-delta on a load that starts under full torque. The motor will not accelerate.
  • Assuming a VFD reduces starting current only. It also removes the mechanical shock.
  • Ignoring bearing currents on VFD-fed motors. Insulated bearings or shaft brushes are the answer.
  • Reading a motor's rating plate current as its starting current.
60-second recall
  1. N_s = 120f/p; slip = (N_s − N)/N_s.
  2. DOL starting current 6–8 × full load.
  3. Star-delta: one third of the current and the torque.
  4. VFD holds V/f constant below base speed.
  5. Insulation classes: B 130 °C, F 155 °C, H 180 °C.
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05 / 11PLC & loops

Automation, PLC & Control

A control question is always the same three questions: what is measured, what the controller does with the error, and where the valve goes when everything fails.

What the paper asks

PLC hardware, scan cycle and ladder logic; digital and analogue input and output; P, PI and PID action; cascade and feed-forward; boiler level and three-element control; fail-safe design; and alarm and monitoring systems including UMS requirements.

The concepts, in order

A PLC executes a scan cycle. Read inputs, solve the logic, write outputs, repeat. Scan time matters when a fast event could be missed between scans, which is why safety functions use hard-wired trips rather than relying on the program.

Ladder logic reads like a relay circuit. Rungs, normally open and normally closed contacts, coils, latches and timers. Being able to read a rung and say what it does — and to spot a latch that never resets — is the practical skill.

The three control terms do different jobs. Proportional responds to the size of the error and leaves an offset; integral removes that offset by accumulating error; derivative responds to rate of change and anticipates, at the cost of amplifying noise.

Fail-safe direction is a design decision. On loss of signal or air, a fuel valve fails closed and a cooling valve usually fails open. Naming the direction and the reason is worth more marks than describing the actuator.

Where marks are lost

  • Claiming proportional action removes offset. Only integral does.
  • Missing shrink and swell in a boiler level question. It is why three-element control exists.
  • Sharing a transmitter between control and shutdown. Safety systems must be independent.
  • Adding derivative to a noisy fast loop. The valve hunts.
60-second recall
  1. Scan: read, solve, write.
  2. P leaves offset, I removes it, D anticipates.
  3. Three-element level: level, steam flow, feed flow.
  4. State the fail-safe direction and why.
  5. Safety systems are independent of control systems.
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06 / 11Converters & UPS

Power Electronics

Nearly every load on a modern ship is fed through a converter of some kind, and every converter distorts the supply it draws from.

What the paper asks

Diodes, thyristors, IGBTs; rectifier and inverter circuits; DC link and PWM; UPS types and battery systems; battery charging and maintenance; harmonics, their effects and their mitigation; and converter fault finding.

The concepts, in order

A drive is three stages. A rectifier turns AC into DC, a DC link smooths and stores it, and an inverter switches that DC to produce a variable-frequency AC output. Faults localise to one of the three, which makes systematic diagnosis possible.

PWM makes an AC waveform out of switching. The inverter switches the DC link at high frequency, varying the pulse width so the average follows a sine wave. The motor's inductance does the smoothing, which is why the current is far more sinusoidal than the voltage.

UPS types differ in transfer. Off-line switches to battery on failure with a short break; line-interactive regulates as well; on-line runs the load through the inverter continuously and has no break at all. Which one is fitted depends on what the load tolerates.

Harmonics heat equipment without overloading it. Non-sinusoidal current from converters causes extra losses in transformers, cables and generators, and can overload a neutral conductor. Mitigation is by filters, multi-pulse rectifiers, or phase-shifting transformers.

Where marks are lost

  • Measuring distorted waveforms with an averaging meter. Use a true-RMS instrument.
  • Diagnosing a drive fault without isolating the stage. Rectifier, DC link or inverter.
  • Assuming a machine running cool is unaffected by harmonics. Check temperature and THD.
  • Working on a DC link without allowing the capacitors to discharge.
60-second recall
  1. Rectifier → DC link → inverter.
  2. PWM: the inductance does the smoothing.
  3. On-line UPS has no break; off-line has a short one.
  4. Harmonics heat without overloading.
  5. True-RMS meter on any distorted waveform.
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08 / 11Measure & calibrate

Instrumentation & Calibration

A reading is only as good as the instrument and its installation. When a value is disputed, the first question is how it is measured — not what it says.

What the paper asks

Pressure, temperature, level and flow measurement principles; thermocouples and RTDs; 4–20 mA loops; transmitter calibration and span and zero; installation errors; and calibration records and traceability.

The concepts, in order

4–20 mA has a live zero for a reason. Zero measurement is 4 mA, so a broken loop reads 0 mA and is instantly distinguishable from a genuine zero. Current transmission is also immune to volt drop along the cable.

Thermocouples and RTDs work differently. A thermocouple generates a small voltage from the junction of two dissimilar metals and needs cold junction compensation; an RTD such as Pt100 changes resistance with temperature and is more accurate over a smaller range, but needs three or four wire connection to cancel lead resistance.

Zero and span are separate adjustments. Zero shifts the whole line, span rotates it. Calibration means checking at several points across the range, not just at one, and recording the as-found and as-left values.

Installation errors masquerade as instrument faults. Impulse lines trapping air or condensate, a thermowell not in contact, a level transmitter referenced to the wrong liquid density. Check the installation before condemning the transmitter.

Where marks are lost

  • Treating 0 mA as a zero reading. It is a broken loop.
  • Adjusting span before zero. Set zero first, then span, then re-check zero.
  • Two-wire RTD on a long run. Lead resistance adds directly to the reading.
  • Recording only the as-left value. The as-found value is the evidence of drift.
60-second recall
  1. 4 mA is zero; 0 mA is a fault.
  2. Pt100 = 100 Ω at 0 °C, ≈ 0.385 Ω/°C.
  3. Thermocouple needs cold junction compensation.
  4. Zero first, then span, then re-check zero.
  5. Record as-found and as-left.
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09 / 11Systematic method

Maintenance & Troubleshooting

The examinable skill is method: narrowing a fault by measurement rather than by replacing parts until it stops.

What the paper asks

Insulation resistance testing and interpretation; earth fault location on an insulated system; systematic fault finding; use of test instruments; motor and cable testing; preventive maintenance and condition monitoring; and record keeping.

The concepts, in order

Insulation resistance is a trend, not a pass mark. A single reading means little; the same machine's reading falling over months means a great deal. Temperature and humidity affect the value, so record them alongside.

Earth fault location on an insulated system is a process of elimination. With the earth fault lamps or monitor indicating, breakers are opened one at a time — with the operational consequences considered first — until the indication clears. The fault must be found: it is the second earth fault that causes the damage.

Systematic fault finding beats replacement. Confirm the symptom, establish what has changed, divide the system in half and test at the midpoint, and keep halving. Replacing components in hope is expensive and hides the real cause.

Condition monitoring finds faults before failure. Thermography on switchboards and connections, vibration on rotating machines, and insulation trending — each detects a different developing fault, and all three depend on having a baseline.

Where marks are lost

  • Meggering a circuit with electronics still connected. The test voltage destroys them.
  • Ignoring a single earth fault because nothing tripped.
  • Comparing IR readings taken at different temperatures.
  • Fixing the symptom without establishing what changed.
60-second recall
  1. IR is a trend — record temperature and humidity.
  2. Disconnect electronics before insulation testing.
  3. One earth fault is an alarm; two is a short circuit.
  4. Halve the system, test, halve again.
  5. Baseline first, then monitor.
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10 / 11STCW & class

Regulations, Safety & Documentation

What the certificate requires, what class inspects, and what a permit to work is actually protecting against.

What the paper asks

STCW Regulation III/6 and the competence tables; class survey requirements for electrical installations; permits to work and isolation; SOLAS requirements for emergency power and lighting; PMS records; and the ETO's role in a survey.

The concepts, in order

STCW III/6 defines the competences. The certificate is issued against demonstrated competence in the Code Table A-III/6 areas — monitoring electrical and electronic systems, maintenance, controlling operation, and contributing to safety — recorded in an approved training record book.

Class surveys the electrical installation. Emergency source of power and its automatic start, emergency lighting, main and emergency switchboards, insulation levels, and the protection arrangements. The survey looks for records as much as for condition.

Emergency power has defined requirements. The emergency generator must start automatically and be on load within 45 seconds of main power failure, with autonomy of 18 hours on a cargo ship and 36 on a passenger ship — and it must be outside the machinery space boundaries.

Permits protect against stored and restored energy. The isolation must be proven, locked and tagged, and the person who applied it is the person who removes it. The commonest fatal failure is somebody else energising a circuit that looks finished.

Where marks are lost

  • Removing someone else's lock or tag.
  • Testing the emergency generator without confirming the automatic start function.
  • Assuming the certificate covers competence not in the record book.
  • Presenting a PMS record that does not match the equipment.
60-second recall
  1. STCW III/6 with an approved training record book.
  2. Emergency generator on load within 45 seconds.
  3. 18 hours cargo, 36 hours passenger autonomy.
  4. One person's lock, one person's key.
  5. Records are what the surveyor examines first.
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11 / 11Orals & practical

ETO Orals & Practical Assessment

The assessor is deciding whether to trust you alone with a live switchboard. Answers need the procedure, the reason, and the safety step you would not skip.

What the paper asks

Demonstrating competence against the STCW tables; scenario questions on blackout, earth fault, motor failure and HV work; use of the training record book; and how you satisfy yourself that equipment is safe to work on and safe to return to service.

The concepts, in order

Answer with a sequence, and name the safety step. Isolate, prove dead, work, test, restore, record. An assessor listens for the isolation and the proving above everything else, because that is the step that kills people when it is skipped.

Blackout is a standard scenario. Confirm the emergency generator has started and taken load, restore steering and essential services, then restore the main board in a considered order — and find the cause before the second attempt.

An earth fault question is about consequence. One earth fault on an insulated system does not trip anything; it must be located and cleared before a second fault on another phase turns it into a short circuit through the hull.

Say how you know something is fit to return to service. Continuity, insulation, polarity, function tested; guards and covers replaced; isolation removed by the person who applied it; and the record made. Never “I would just check it”.

Where marks are lost

  • Describing work on equipment without describing the isolation.
  • Restoring after a blackout without finding the cause.
  • Treating a single earth fault as harmless.
  • Guessing a regulatory figure rather than saying where you would verify it.
60-second recall
  1. Isolate, prove dead, work, test, restore, record.
  2. Blackout: emergency set, steering, essentials, then find the cause.
  3. Find the earth fault before the second one finds you.
  4. Continuity, insulation, polarity, function before return to service.
  5. Never invent a figure.
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