TST Engineering Services • Module 28
Marine High Voltage Systems
Exam-ready oral revision covering HV configuration, neutral earthing, protection, breaker operation, permits, cable/busbar/transformer work and safe isolation.
Give answers in order: define → purpose → hazard → protection → safe action. Never invent relay settings; state the principle and refer to vessel-specific data.
Why ships use high voltage
Core definition
HV is generally any voltage exceeding 1,000 V AC. Common marine distribution levels are 3.3 kV, 6.6 kV and 11 kV.
Why HV?
For the same power, higher voltage gives lower current. This reduces cable size, voltage drop, I²R losses and switchgear stress.
Big-load examples
Propulsion motors, bow/stern thrusters, cargo pumps, large compressors and large transformers are typical HV consumers.
Oral line
“At 440 V, megawatt loads draw impractically high currents; HV makes the installation practical and controllable.”
Insulated neutral, earthed neutral and NER logic
Insulated neutral
First earth fault normally gives alarm without immediate supply interruption. The fault must still be traced quickly because a second earth fault can become a short circuit.
Earthed neutral
Earth fault current has a return path and protection can automatically disconnect the affected equipment. Overvoltage risk is lower.
High resistance earthing
NER limits earth fault current while still allowing detection. Commonly used on marine 3.3 kV and 6.6 kV systems.
Generator, feeder and switchboard protection
Thermal monitor
Protects generator windings against overheating and normally alarms/trips depending on severity.
Differential
Compares current into and out of windings; difference indicates internal fault and needs fast action.
Negative phase sequence
Phase imbalance causes damaging heating and is treated sensitively on generators.
Voltage restrained O/C
Maintains fault sensitivity when terminal voltage collapses under heavy short-circuit conditions.
Reverse power
Prevents motoring of a generator if prime mover input is lost.
Preference trip
Removes non-essential loads in stages to protect essential supply and avoid blackout.
Lock-out relay
Requires deliberate reset after investigation; do not reset until cause and damage are known.
Field suppression
For serious generator faults, excitation is removed and prime mover may shut down to limit fault energy.
Fuses, ACBs, vacuum and SF6 breakers
HV fuses + striker
Fusible element melts; quartz helps extinguish the arc; striker gives indication and can trip other switchgear to prevent single-phasing.
Air circuit breaker
Common at LV. At high voltage/high current, arc size makes simple air interruption impractical.
Vacuum interrupter
Ceramic sealed bottle with moving/fixed contacts and bellows. High dielectric recovery, short arcing time, low erosion.
SF6 interrupter
SF6 gas insulates and quenches the arc; puffer action blows gas through the arc path as contacts part.
SF6 pressure
Gas pressure is monitored; low pressure alarms, and critically low pressure can block/trip depending on design.
Withdrawable gear
Connected, test and isolated positions separate main contacts and allow shutters/interlocks to protect busbar contacts.
Permit, sanction to test and limitation of access
Permit to Work
Issued by an authorised person to a named competent person. Defines apparatus, scope, isolation, earths and work boundary. Not transferable.
Sanction for Test
A safety document/control used when normal Permit-to-Work conditions must be suspended or stood down for testing. It places the apparatus under authorised test control, such as when earths are temporarily removed for insulation or functional testing. After the test, the circuit is re-earthed, made safe and handed back before normal work continues.
Limitation of Access
Written limits for work in the vicinity of HV equipment but not on exposed HV apparatus.
Full safe isolation and working procedure
- Define the exact job, apparatus, work boundary and operational impact.
- Confirm competent and authorised personnel; brief the team and bridge/ECR.
- Review current single-line diagram, switching plan, permit and all possible sources/backfeeds.
- Identify primary, secondary, VT, control, UPS, converter and stored-energy sources.
- Carry out switching in the approved sequence; open, trip, withdraw/rack out as required.
- Secure against reconnection using locks, tags, key safes and safety locks.
- Apply danger/caution notices and barriers; control adjacent live parts.
- Use approved HV tester: prove tester, test the circuit, re-prove tester.
- Discharge and apply circuit main earths / additional earths at all required points.
- Issue permit; walk the job with the competent person and confirm scope.
- For testing, stop the normal permit work, suspend or stand down the Permit-to-Work condition and issue a Sanction to Test. Remove earths only as required for the authorised test, then re-earth, make safe and formally hand back before normal work continues.
- On completion, remove tools, refit covers, withdraw personnel, cancel permit and restore using de-isolation plan.
Cables, transformers, busbars and ring main units
HV cables
Check records, visually trace, positively identify, prove dead, earth, and use approved signal injection/spiking or cutting method when required.
Transformers
Transformers work both ways. Isolate HV and LV sides, voltage transformers and all points capable of backfeed.
Busbars
Isolate all feeds and connected equipment, withdraw switches, lock shutters, prove dead and earth the section. Separate documents for separate sections.
Ring main units
Remote ends may have to be isolated first. Follow the one-shot label, diagram and maker procedure exactly.
Ready-to-say examiner responses
Why HV?
“Because large ship loads at 440 V would require excessive current. Raising voltage reduces current, cable size, voltage drop and losses while making switchgear and distribution practical.”
First earth fault?
“On an insulated system it may not trip immediately, but it must be treated seriously because a second earth fault can create a phase-to-phase fault through earth.”
Safe isolation?
“Plan, identify sources, switch, lock, prove dead, earth, permit, work, test under sanction if required, restore under controlled de-isolation.”
Breaker reset?
“I would not reset blindly. I would record the relay indications, identify the cause, assess damage, and only reset after competent investigation and permission.”