The high-voltage section is where the ETO oral exam gets serious. It is a safety exam as much as a technical one, and surveyors use the ETO oral exam to find out whether you actually understand HV systems or just memorised a list. Below are the real high-voltage questions examiners ask, each with the pass-level answer and the reasoning to defend it — written by a working Senior ETO with 15+ years on high-voltage container ships.
Why do ships use high voltage instead of 440 V?
As ships and their loads grew — large container vessels, LNG carriers, cruise ships, diesel-electric propulsion — the current at 440 V became too large to handle economically. Power is voltage times current, so for the same power, raising the voltage lowers the current. Lower current means smaller cables, lower I²R losses, smaller switchgear and less voltage drop. That is why high voltage — typically 3.3 kV, 6.6 kV, and 11 kV on the largest ships — is used to move large amounts of power efficiently.
Examiner’s follow-up — “What’s the trade-off?” Higher voltage needs far greater insulation, bigger clearances, and brings arc-flash and shock hazards. You trade easier current handling for much stricter safe-working procedures.
What counts as high voltage on a ship?
By convention, high voltage is anything above 1000 V AC. Typical shipboard HV systems are 3.3 kV and 6.6 kV, and on the largest vessels 11 kV. Below that, 440 V (or 690 V on some ships) is the low-voltage system. Knowing your ship’s actual voltage matters because it sets your test voltages, clearances and PPE.
How do you isolate an HV circuit for maintenance?
This is the question that decides many orals, so answer it in order. You work under a Permit to Work, and the safety chain is always the same:
- Isolate — open the breaker and rack it out to the isolated position.
- Lock off and tag — so no one can re-energise it while you work.
- Prove dead — with an approved HV voltage detector, after proving the detector itself works.
- Earth — apply circuit main earths to discharge stored energy and hold the circuit at earth potential.
Miss one step and the circuit can become live or hold a lethal charge.
Why apply earths after you have proved the circuit dead?
Because “proved dead” only confirms that one moment in time. HV cables and capacitance hold a residual charge, and parallel runs can carry induced voltage from nearby live conductors. The earths give a permanent, visible discharge path and a guaranteed point of safety for anyone working on the busbar — so even if the circuit is accidentally re-energised, the fault trips protection instead of electrocuting you.
How do you prove an HV circuit is dead?
With an approved HV voltage detector rated for the system voltage, using prove – test – prove: prove the detector on a known live source or proving unit, test the isolated circuit at the point of work (each phase to earth and phase to phase), then prove the detector again. If it reads dead both times and still works afterwards, the circuit is dead. Never trust a single reading, and never use an ordinary multimeter on HV.
What is arc flash, and how do you protect against it?
An arc flash is an explosive release of energy when current arcs through the air between conductors or to earth — from a fault, a switching error, or a tool bridging contacts. It produces temperatures hotter than the surface of the sun, a pressure blast and molten metal, causing severe burns even at a distance. Protection is dead working under permit, switchgear interlocks, arc-rated PPE, remote or racked-out operation, fast protection, and safe clearances. The energy is instantaneous — you cannot react in time, so the defence is to never be exposed.
Why is the HV neutral earthed through a resistor?
The neutral earthing resistor limits the current during an earth fault. A solidly earthed neutral would draw enormous fault current; a fully unearthed system gives over-voltage and transient problems. The resistor limits earth-fault current to a value the protection can still detect and clear, while keeping over-voltages under control — enough fault current for the relay to “see” and trip, but limited so the fault does not cause huge damage.
What are step potential and touch potential?
During an earth fault, current flowing into the ground raises the local earth to a high voltage that falls off with distance. Step potential is the voltage between your two feet as you stand or walk in that area; touch potential is the voltage between your hand on an object and your feet. Both can be lethal without ever touching the faulted conductor — which is why equipotential bonding and keeping clear matter.
ETO oral exam: the mistakes that quietly fail candidates
Most candidates do not fail the ETO oral exam because they don’t know the answer — they fail because they cannot defend it. The common traps:
- Giving the safe-isolation steps out of order, or forgetting to earth.
- Saying “I’d prove it dead” without “prove – test – prove”.
- Treating HV like low voltage — for example, trying to pull a shocked colleague clear before the circuit is isolated.
- Memorising answers but freezing when the examiner asks “why?” one more time than expected.
Practise the way the surveyor actually asks
Reading answers isn’t the same as saying them calmly, in order, while someone watches. That is the real ETO oral exam. The material maps to the STCW Convention competence requirements for ETOs. To prepare properly:
- Grab the free pack of 15 HV oral questions with model answers — download it here.
- Test yourself with the free interactive HV quiz.
- When you want the whole map, the Complete ETO + HV Exam Prep Bundle has both handbooks, 38 full oral scenarios with the examiner’s follow-ups, and the interactive quizzes.
Written by a working Senior ETO with 15+ years on high-voltage container ships. For exam preparation — high voltage is lethal; always follow your vessel’s permit-to-work system and your flag state’s requirements.