Written by a Senior ETO — 15+ years on high-voltage ships. These are real oral-exam style questions, with the model answers examiners actually want to hear. This is free study/revision material — it is exam preparation, not a certified course or certificate.
Why most candidates fail the ETO / HV oral
It is rarely the theory. Candidates fail because they cannot give the safe working sequence cleanly, they forget to prove the voltage detector before and after, and they freeze on high-voltage safety questions. Below are 15 of the questions that catch people out. Read the model answers, then test yourself.
15 ETO / HV oral questions with model answers
1. What is the safe isolation procedure before working on HV equipment?
Work under a Permit to Work. Sequence: identify the circuit → open the breaker and rack/withdraw it → isolate and lock off (LOTO) → prove the HV voltage detector on a known live source → test the circuit dead → prove the detector again → apply earthing to discharge and hold the circuit at earth potential → display danger notices → only then start work. Common fail: forgetting to prove the tester before and after.
2. Why do we apply earthing after isolating an HV circuit?
To discharge stored capacitive charge in cables and equipment, to drain induced voltage from parallel-running cables, and to keep the conductor at earth potential so any accidental re-energisation or back-feed cannot harm the worker. Earthing is the physical guarantee of “dead.”
3. What are the main dangers of HV compared with LV?
Arc flash and arc blast (large thermal and pressure energy), flashover across an air gap without contact (so approach distance matters), stored capacitive charge, induced voltages, and step/touch potential. With HV you can be injured without ever touching the conductor.
4. Why is HV used on modern ships and what are the advantages?
For large electrical loads — big container ships, cruise, LNG and diesel-electric propulsion. For the same power, higher voltage means lower current (P = √3·U·I·cosφ), so cable cross-section, weight, copper cost and I²R losses all drop and switchgear is more compact for the power handled. Typical ship HV: 3.3 kV, 6.6 kV, 11 kV.
5. How do you test the insulation resistance of an HV motor?
Isolate, discharge and earth first. Use an insulation tester rated for the machine — typically 5 kV for HV. Measure winding-to-earth IR, take readings at 1 min and 10 min, and calculate the Polarization Index (PI = IR₁₀ / IR₁). PI ≥ 2 is generally healthy; low or falling PI suggests moisture/contamination. Always follow the maker’s values (and IEEE 43 guidance), and discharge the winding to earth after testing.
6. What interlocks are on HV switchgear and why?
They enforce the correct sequence and protect personnel: you cannot rack the breaker in/out while it is closed; the earth switch cannot close on a live circuit and the breaker cannot close onto an applied earth; the panel cannot be opened unless the circuit is isolated and earthed. Never defeat an interlock.
7. What is high-resistance neutral earthing and why is it used on ships?
The generator/transformer neutral is earthed through a resistor (NER) that limits earth-fault current to a small value. The first earth fault then does not trip the system or cause large arc damage — the ship keeps running while an alarm flags the fault for the ETO to locate. It also limits transient overvoltages. Marine HV is typically high-resistance earthed, not solidly earthed.
8. What PPE and precautions for arc flash when operating HV switchgear?
Arc-rated flash suit and hood, face shield, voltage-rated insulating gloves; operate from a safe position or remotely; keep the minimum approach distance; confirm interlocks are healthy. Treat every operation as potentially live until proven dead.
9. Describe how you rack out an HV vacuum circuit breaker for maintenance.
Open the breaker off-load and confirm “open.” Apply permit/LOTO and discharge the operating springs. Rack from “service” to “test/isolated,” then withdraw fully; confirm the shutters cover the live spouts. Earth the circuit via the earth switch, then work. Re-commission in reverse, checking interlocks at each step.
10. What protections are fitted on a ship’s HV transformer?
Differential protection for internal faults, overcurrent and earth-fault protection, and winding/oil temperature alarms and trips. Oil-filled units add a Buchholz relay (gas/oil surge on internal arcing); dry cast-resin types (common on ships) use temperature sensors instead.
11. How do you locate an earth fault on a high-resistance-earthed HV system?
The alarm tells you a fault exists while the ship keeps running. In coordination with the bridge/Chief, switch off non-essential feeders one at a time (or use the insulation-monitoring/earth-fault locating system) and watch for the alarm to clear — that identifies the faulty feeder. Then isolate and investigate that circuit.
12. What is a Permit to Work — who issues it and what does it contain?
A formal written authorisation defining the task, the isolations performed, the hazards, the precautions, the time validity and the responsible persons. Issued by a competent/authorised person (typically the Chief Engineer) and accepted by the worker. It guarantees isolation, earthing and communication are verified before work and that the system is not re-energised until the permit is closed and all personnel are clear.
13. Safety considerations for High-Voltage Shore Connection (cold ironing)?
Confirm voltage/frequency compatibility and correct phase sequence, proper earthing/bonding between ship and shore, an interlocked emergency disconnection, and procedural synchronisation/changeover. The HV shore cable must be confirmed dead and earthed during connection and disconnection, with permits and competent personnel on both sides.
14. Why prove the voltage detector before and after testing for dead?
To be certain the detector itself works. Prove it on a known live source (or proving unit), test the isolated circuit (expecting “dead”), then prove the detector again. If it still works afterwards, you can trust the “dead” reading. Skipping this is a classic oral-exam failure.
15. How is a large HV induction motor started, and why not direct-on-line?
Large HV motors draw 5–7× full-load current on start, causing voltage dip and mechanical/thermal stress. Reduced-voltage or soft starting (auto-transformer, soft starter, or VFD) limits inrush and torque; on diesel-electric/propulsion a VFD also gives speed control. DOL is reserved for smaller machines the system can absorb.
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Disclaimer: This material is for exam preparation and revision only. It is not a certified STCW course and does not replace training required by your maritime administration or flag state.