Main Switchboard (MV)

What sets MV switchgear apart

Low-voltage boards at 400-690 V handle most auxiliary loads on a ship, but once installed generator output climbs past roughly 3-4 MW per set, cable cross-sections and transformer counts at LV become impractical. Cruise ships, LNG carriers, large offshore vessels and some ice-class ships generate and distribute at medium voltage instead, then step down locally to 440 V or 690 V near the consumers. The switchgear itself is a different class of equipment: breakers interrupt fault currents in a vacuum bottle or SF6 chamber rather than in open air, compartments are segregated to contain an internal arc, and every operation is mechanically interlocked because a mistake at these voltages is rarely survivable for the person making it.

Main components

Circuit breakers

Withdrawable vacuum or SF6 breakers mounted on a truck that racks in and out of the cubicle. Vacuum is the dominant technology on new tonnage; SF6 is still found on older or offshore-derived designs.

Busbar system

Copper or aluminium busbars run the length of the board, split into a port and starboard half joined by a bus-tie breaker so the board can be operated as two independent sections.

Protection relays

Numerical relays handle overcurrent, earth fault, differential and, on generator incomers, reverse power and loss-of-excitation protection, tripping the associated breaker within a coordinated time grading scheme.

Instrument transformers

Current and voltage transformers feed both the protection relays and the metering/SCADA system; their ratio must match the relay settings exactly or protection becomes unreliable.

Interlocks and earthing

Mechanical and electrical interlocks stop a breaker being withdrawn while closed, prevent the compartment door opening while live, and allow a maintenance earthing switch to be applied only once the breaker is isolated.

Selection and sizing

• Rated voltage must match the generator voltage class exactly, not just be higher than it
• Rated short-circuit breaking current, typically 25-40 kA for marine MV boards, sized against the fault level the generators and any shore or shaft-generator connection can deliver
• Busbar continuous current rating with margin for future feeders
• Internal arc classification and accessibility type, which decides how venting is routed away from operating positions
• Physical footprint and cable entry direction against the switchroom layout

Regulations and class

SOLAS Ch II-1 requires that the main source of electrical power be arranged so that essential services remain available after a single fault, which on MV ships is normally met by splitting the board into two sections with a bus-tie breaker rather than running it as one continuous bus. Class societies require type test certificates for the exact voltage and short-circuit rating supplied, generally referencing IEC 62271-200 for metal-enclosed switchgear, and verify protection relay settings during survey rather than just the physical condition of the cubicles. Internal arc containment is demonstrated through arc fault type testing so that, if an arc does occur, the pressure and hot gas vent to a safe direction instead of into the switchroom.

Typical faults

What to look for in a supplier

• Type test certificates to IEC 62271-200 that match the exact voltage class and short-circuit rating ordered, not a similar sibling model
• An internal arc test report for the enclosure size actually being supplied
• A relay settings package the ship's own electrical engineers can read, back up and edit, not one locked to the maker's service technician
• Availability of spare trip coils, closing coils and spring mechanisms as discrete parts, not only complete breaker exchange units

A bus-tie breaker left closed for operational convenience turns two independent half-boards into one single point of failure — treat it as a switch to be operated deliberately, not a permanent bridge.