In short. The bus is where sources, loads and protection meet, so its arrangement decides how a fault or a maintenance task affects the site. Single bus is simple but exposed; a sectionalised bus contains faults; a dual or ring bus adds alternative paths; each costs more and needs more careful protection.
Common arrangements
| Arrangement | Fault effect | Maintenance | Cost and complexity |
|---|---|---|---|
| Single bus | Bus fault drops everything | Shutdown needed | Lowest |
| Sectionalised bus (with tie) | Fault drops one section | One section at a time | Moderate |
| Main-tie-main | Fault drops one section; sources interchangeable | Good | Moderate to high |
| Dual bus (A and B) | One bus lost; other continues | Full concurrent | High |
| Ring bus | Alternative path around faults | Good | High; complex protection |
See main-tie-main distribution and fault containment.
What the bus must provide
- Current rating for normal and post-failure loading, including transfers.
- Short-circuit withstand for the highest fault level, including when ties are closed.
- Insulation and clearances for the voltage and environment.
- Arc-fault containment where required.
- Space and access for maintenance.
Choosing an arrangement
- Decide the failure the site must survive: a bus fault, a breaker failure, a maintenance task.
- Check the load classes, and whether critical loads need two buses.
- Choose the simplest arrangement that meets the requirement.
- Study fault levels and protection for every configuration of ties and sources.
Protection implications
Each arrangement changes fault current paths and selectivity. A closed tie raises fault level; an open tie reduces it but isolates sections. Busbar protection, interlocking and zone selective schemes may be needed. See busbar protection and critical-power selectivity.
Interlocking
Breaker interlocks prevent unsafe or damaging combinations such as paralleling out-of-phase sources, or closing a tie onto a faulted section. Keep logic simple, hardwired where critical, and documented. See interlocking and transfer logic.
Worked illustration
A site chooses a single bus for cost. A busbar fault, or a cable termination failure inside the board, takes the whole board down, and repair needs a full shutdown. Splitting the board into two sections with a tie costs one more breaker and a section of bus. With the tie open in normal operation, a bus fault affects half the loads, and the other half continues on its own source. The extra cost is small compared with the outage it avoids.
Maintenance access
Design so one section or one breaker can be isolated while the rest stays live, with safe working clearances and lockout points. Ask who will do the maintenance and how, before finalising the layout.
Bus arrangements compared
| Arrangement | Description | Resilience |
|---|---|---|
| Single bus | One bus feeds all loads | Low: a bus fault is a total outage |
| Sectionalised bus with tie | Two sections joined by a tie | Moderate: a fault drops one section |
| Double-ended with main-tie-main | Two incomers, two sections, one tie | Good for source loss |
| Dual independent buses (A/B) | Two complete buses | High: loads with two cords stay up |
| Ring bus | Loop of breakers with multiple sources | High, complex protection |
Selection logic
- Start with the consequence of losing the bus.
- Choose the least complex arrangement that meets the continuity target.
- Check fault level with all sources in parallel.
- Make sure maintenance of any breaker or section does not force an outage.
Maintenance without outage
A bus that can only be cleaned or tested during a shutdown will eventually be neglected. Include tie breakers, bypass paths and the ability to isolate a section while the others run.
Protection implication
Bus protection should be as fast as the arrangement needs. A busbar differential or zone-selective interlocking limits the damage and the size of the outage. See busbar protection.
See main-tie-main and fault containment.
Common mistakes
- Choosing the bus layout before defining the failure to survive.
- Forgetting the higher fault level with ties closed.
- Interlocking that allows a transfer sequence which paralleled unsynchronised sources.
- No safe access for maintenance.
What to send us
Send the single-line diagram and the failures you must survive. Send your application details. See also LV switchboards.
Questions engineers ask
Is a ring bus better than a sectionalised bus?
It offers more paths but needs more complex protection. A sectionalised bus is often sufficient and easier to protect.
Should the tie be normally open or closed?
Open limits fault level and isolates faults; closed shares load but raises fault level. The choice depends on protection and the equipment’s ratings.
What is the biggest hidden risk in bus design?
Fault level under every operating configuration, including transfers, which can exceed equipment ratings.
How does bus design affect maintainability?
A well-sectioned bus lets one part be isolated while the rest stays live.
This guide gives general principles. Fault levels and ratings must be confirmed by calculation.