KEY POINTS
- MSB and MDB can describe the same main board, depending on the project.
- Switchboards distribute and protect circuits; a transformer changes voltage.
- A spare outgoing way does not prove spare capacity.
Switchboard names are useful shorthand, but they are not a universal hierarchy. To understand a board, trace its incoming supply and outgoing circuits on the single-line diagram, then check the equipment label and actual installation.
Common names and their use
| Label | Common meaning | What to confirm |
|---|---|---|
| MSB | Main switchboard | Incoming source, main protection, metering and outgoing feeders |
| MDB | Main distribution board | Whether this is the same board as the MSB or a separately defined board |
| Sub-MSB or SDB | A board serving part of the facility | Upstream feeder, area served and isolation arrangement |
| DB | Distribution board | The final circuits or smaller distribution circuits it supplies |
The project’s drawing legend and equipment schedule should resolve the naming. Do not assume an MDB must sit downstream of an MSB. Also distinguish distribution from voltage transformation: a switchboard divides and controls supply to circuits; it does not step the voltage down in the way a transformer does.
Read beyond the board name
Record the board identifier, voltage, rated current, supply source and circuits served. Review the enclosure condition, installed devices, available space and access needed for maintenance. Any proposed modification should be assessed against the assembly and component requirements, including the manufacturer’s information.
IEC 61439-1 provides general requirements for low-voltage switchgear and controlgear assemblies. A label such as “MSB-grade breaker” is not an adequate equipment specification. The design needs the relevant electrical characteristics and verification requirements.
Review a new load from source to equipment
A free outgoing way is a physical space, not a capacity assessment. Check the upstream source, busbars, feeder, protective devices and final circuit together. Cable selection also depends on installation conditions and protection, as outlined in the conductor-sizing guidance.
Keep protection coordination and fault-current assessment distinct from normal load capacity. A circuit may carry the expected operating current yet still require changes to its protection or other design details.
Prepare for an upgrade discussion
Provide current drawings, board photographs taken under safe site arrangements, equipment nameplate details and the reason for the change. Include existing test findings, obsolete components and the proposed new loads.
Ask the responsible team to compare retention, partial modification and replacement where appropriate. The work sequence, shutdown requirements and test plan should be agreed before the cutover is booked. See ETES switchboard upgrading for the service scope.
Common questions
Are MSB and MDB interchangeable?
They can be used for the same main board. Follow the project’s definitions and verify the board’s actual function and connections.
Does moving equipment only affect the nearest DB?
No. The receiving circuit and all relevant upstream equipment need to be reviewed, even if total site demand changes little.
Does an old board always need replacement?
Age is one input. Condition, supported components, required ratings, verification and the proposed use should inform the decision.
ETES services for this work
References & further reading
Primary publications used in this guide. Confirm the requirements and editions applicable to your project.
Send ETES your site location, equipment details and planned schedule. Our team can discuss the work required.
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