What to check on a Power Cable (3.6/6 kV).
20 models from NKT Marine in Power Cable (3.6/6 kV). Below: the full model list.
What sets 3.6/6 kV cable apart
Standard 0.6/1 kV shipboard cable becomes impractical once a consumer draws real power over any distance: the conductor cross-section needed to keep voltage drop and I²R losses within limits grows too large to bend around a cable tray. Stepping the system voltage up to 3.3 kV or 6.6 kV (the cable itself is rated 3.6/6 kV, meaning 3.6 kV phase-to-earth and 6 kV phase-to-phase) lets the same power flow through a fraction of the copper. This is why bow thrusters, large propulsion motors and generators above roughly 1-2 MW are wired at medium voltage while lighting and small motors stay on the low-voltage system.
The cable is not simply a low-voltage cable with thicker insulation. At medium voltage the electric field concentrates at any irregularity in the insulation surface, and a void or a sharp conductor edge can trigger partial discharge that erodes the insulation from the inside over years. Marine MV cable construction exists specifically to control that field.
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Main components
Conductor
Stranded copper, with a semiconducting layer extruded directly over it to eliminate air gaps and even out the field at the conductor surface.
Insulation
Cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR) is standard for marine MV cable; EPR tolerates more flexing and is common where the cable sees vibration or repeated movement, XLPE gives a slightly better electrical loss figure.
Insulation screen and metallic screen
A semiconducting layer over the insulation, followed by a copper wire or tape screen, confines the electric field inside the cable and provides the earth-fault return path. This screen must be earthed correctly at both ends, or induced sheath currents become a shock and fire risk.
Bedding, armour and outer sheath
A halogen-free, flame-retardant outer sheath is standard for shipboard use. Armour, where fitted, protects against mechanical damage in exposed runs.
Selection and sizing
- Voltage rating matched to the ship's MV system, commonly 3.3 kV or 6.6 kV nominal, with the cable rated 3.6/6 kV or 6/10 kV to give insulation margin.
- Continuous current rating, derated for grouping with other cables in the same tray and for the ambient temperature of the space.
- Short-circuit withstand of conductor and screen, checked against the fault current and clearing time of the protective relay.
- Flexibility class where the run crosses a mechanical connection, for example into a thruster room.
- Fire performance: flame-retardant to the applicable bunched-cable test as a minimum, low smoke and halogen-free for accommodation and escape route runs.
Regulations and class
SOLAS Ch. II-1 sets the general requirement for electrical installations and cable fire protection; class rules, built on the IEC 60092 series for shipboard cable construction and testing, add the detailed construction, type-approval and routing requirements. MV cable runs additionally need segregation from other services and, per class rules, physical separation or barriers where they pass through machinery spaces, since a MV fault carries far more energy into an arc than a low-voltage one. Periodic insulation resistance testing, and on larger installations partial-discharge testing, are typically required at survey intervals set by class.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Partial discharge damage | Voids or contamination at the conductor or insulation screen introduced during termination | Slow insulation erosion, eventual flashover, often years after installation |
| Screen overheating | Screen earthed without accounting for induced circulating current | Local heating, accelerated ageing of the adjacent insulation |
| Water treeing | Moisture ingress through a damaged sheath into XLPE insulation | Progressive dielectric weakening, eventual breakdown under a switching surge |
| Termination flashover | Incorrect stress-cone or heat-shrink termination for the voltage class | Immediate fault, often at first energisation or during a switching transient |
What to look for in a supplier
- Type approval from a recognised classification society for the specific construction and voltage class ordered, not just a general marine cable certificate.
- Factory partial-discharge test records for the actual production batch, not a type-test report alone.
- Documented compatibility between the cable and the termination/jointing kits it will be installed with; mismatched MV systems are a common site-installation failure.
- Experience with the specific fire-performance standard called up for the installation.
Treat MV cable terminations as the highest-risk step in the whole installation: a rushed stress-cone job is invisible from the outside and often does not fail until the cable has been in service for months.
