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Switchboards and Power Distribution
Switchboards and Power Distribution covers the equipment that collects generator power and carries it to every consumer on a rig. That means main and emergency switchboards, medium and low-voltage switchgear panels, circuit breakers, power and distribution transformers, motor control centres and distribution boards. Used and surplus items here are usually matching panels, breakers or transformers for repairs, extensions and spares.
How rig switchgear is arranged
Generators connect to the main board through generator breakers with synchronising. On a DP unit the board is split into two or more sections joined by bus-tie breakers. Each section can run alone when the ties are open, so a single fault leaves the unit with enough power. DP class 3 units also separate the sections with fire and watertight divisions. Feeders supply drive transformers, thruster drives and step-down transformers to low-voltage boards and motor control centres. The emergency board stands apart and is fed by the emergency generator and a tie from the main board.
Configurations
- Medium-voltage switchgear at 3.3 to 11 kV with withdrawable vacuum circuit breakers, used on larger units.
- Low-voltage switchboards at 440 to 690 V with air circuit breakers and moulded case breakers, used on smaller rigs and for distribution.
- Dry-type cast-resin transformers in switch rooms, or liquid-filled units where the rules allow.
- Motor control centres with starters, soft starters or small drives for pumps and fans.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Rated voltage and current | Must match system and load | kV, A |
| Short-time withstand current | Must exceed prospective fault current | kA for 1 s or 3 s |
| Internal arc classification | Shows tested arc containment | IAC AFLR per IEC 62271-200 |
| Breaker type and frame | Decides spares and interchangeability | VCB, ACB, MCCB |
| Protection relays | Must suit your coordination study | Make, model, firmware |
| Transformer data | Must match vector group and impedance | kVA, %Z, vector group |
Standards and approvals
IEC 62271-200 governs medium-voltage switchgear and defines internal arc classification. IEC 61439 and IEC 60092-302 cover low-voltage assemblies, and IEC 60092-303 marine transformers. IEC 61892 sets the installation rules on offshore units. Class rules add requirements for the main board, such as the number of sections, preferential tripping and the arrangement of bus-ties on DP units. A used panel needs its class certificate, or class may require type tests and a factory acceptance test to be repeated.
Failure modes and inspection
- Breakers. Check operation count against the OEM mechanical endurance, contact wear indication, closing and opening times, and contact resistance. Check the vacuum interrupter integrity with a high-voltage test across the open contacts.
- Mechanisms. Check spring charging motors, racking, shutters and interlocks.
- Busbars and joints. Look for discolouration, check torque marks, and compare against any thermographic history.
- Insulation. Look for tracking or surface discharge on insulators. On medium-voltage panels, run partial discharge tests.
- Protection relays. Run secondary injection tests against the settings file.
- Transformers. Check winding insulation resistance and turns ratio. Inspect resin for cracks on dry types, and run an oil test with dissolved gas analysis on liquid-filled units.
Documentation
- Single-line diagram, panel schedules and wiring diagrams
- Class certificate and factory acceptance test report
- Type test certificates, including the internal arc test
- Breaker maintenance records and operation counts
- Relay settings files and the last protection coordination study
- Thermographic and partial discharge survey records
Integration and compatibility
Adding a panel to an existing board needs the same product family, busbar system, height and depth, and an equal or higher short-circuit rating. Check that the control voltage, interlocks and relay communication protocol match. A new transformer must match vector group and impedance, or parallel operation and the fault level change. Any change to the board on a DP unit feeds into the FMEA.
Refurbishment and spares
An OEM retrofit replaces old breakers with modern vacuum or air breakers in the existing cradles. Retrofit kits exist for several obsolete families. Unused surplus panels and breakers from newbuild projects are a good source of matching spares if the family and version match. Plan for re-testing and a class survey after any installation. ABB UniGear, Siemens and Schneider Electric switchgear are common offshore. The generators feeding the board are under Generators, and the drives it supplies under Variable frequency drives (VFD).
FAQ
Frequently asked questions
Can I extend my rig switchboard with a used panel?
Yes, if the panel is from the same product family, has the same busbar system and dimensions, and has an equal or higher short-time withstand rating. Class must approve the extension and survey it after installation, and on a DP unit the FMEA needs updating.
What tests should a used medium-voltage vacuum breaker pass?
A used vacuum breaker should pass contact resistance, opening and closing timing, insulation resistance and a vacuum integrity test across the open contacts. Compare its operation count with the OEM's mechanical endurance rating to judge the remaining life.
What does internal arc classification mean on switchgear?
Internal arc classification under IEC 62271-200 shows that a medium-voltage panel was type tested to contain an internal arc and protect people at the sides named in the rating. IAC AFLR, for example, covers front, lateral and rear access at a stated current and duration.
Why are switchboards on DP rigs split into sections?
Switchboards on DP rigs are split so that one fault cannot take out all generators and thrusters. Bus-tie breakers join the sections, and the unit can run with them open. DP class 3 also requires fire and watertight separation between sections.
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