A switchgear lineup is the backbone of any medium voltage power distribution system. It is a row of interconnected switchgear cubicles — each housing a circuit breaker, disconnect switch, or metering equipment — that collectively receive, distribute, protect, and control electrical power from incoming sources to outgoing feeder circuits.
Designing a switchgear lineup correctly is critical because it directly determines the reliability, safety, and expandability of the entire electrical installation. A poorly designed lineup leads to coordination failures, arc flash hazards, and costly downtime. This guide walks through the key decisions in lineup design, from bus configuration to cubicle selection to protection coordination.
Step 1: Define the One-Line Diagram
Every switchgear lineup starts with a one-line diagram (SLD) — a simplified schematic showing the power flow from source to load. The SLD defines how many incomers, feeders, bus couplers, and metering panels the lineup needs. Key elements to define:
- Number of incoming sources: Single incomer (one transformer), dual incomer (two transformers for redundancy), or multiple incomers (for critical facilities).
- Number of outgoing feeders: Each major load or load group gets its own feeder cubicle with a dedicated circuit breaker.
- Bus configuration: Single bus, single bus with bus coupler, dual bus, ring bus, or double bus with transfer.
- Metering and protection: Revenue metering panels, voltage transformer (VT) panels, and protection relay cubicles.
Step 2: Choose the Bus Configuration
The bus configuration is the most critical design decision because it determines how the lineup handles faults and maintenance. The four common configurations, ordered from simplest to most redundant:
Single Bus: One continuous bus bar connecting all cubicles. Simplest and cheapest. Any bus fault or maintenance requires a complete shutdown. Acceptable for non-critical industrial loads where occasional downtime is tolerable.
Single Bus with Bus Coupler: The bus is split into two sections connected by a bus coupler breaker. Each section has its own incomer. During normal operation, both sections share load. During a fault, the coupler trips to isolate the faulted section. The most common configuration for commercial and light industrial applications — balances cost and reliability.
Dual Bus: Two independent bus bars with every feeder connected to both through a transfer mechanism. The operator can move any feeder between buses without interruption. Used in utility substations and critical process plants where no feeder can afford downtime during bus maintenance.
Ring Bus: All breakers connected in a ring topology. Any single fault or maintenance event removes only one breaker from service while maintaining all connections. Highest reliability but most complex and expensive. Used in transmission substations and ultra-critical facilities.
Step 3: Select Cubicle Types
Each position in the lineup serves a specific function. The standard cubicle types in a medium voltage lineup:
Incomer cubicle: Receives power from the utility transformer. Contains a main circuit breaker (typically vacuum circuit breaker at 12-40.5 kV), disconnectors, earthing switch, current and voltage transformers, and protection relay. The incomer breaker is the first line of defense for the entire lineup.
Feeder cubicle: Distributes power to a downstream load or load group. Contains a feeder circuit breaker, cable termination, CTs for metering and protection, and a protection relay. Each feeder is independently protected and can be switched without affecting other feeders.
Bus coupler cubicle: Connects two bus sections. Contains a circuit breaker with automatic and manual operation modes. Includes interlock mechanisms to prevent both sections from being connected when one is faulted.
Metering cubicle: Houses voltage transformers (VTs), revenue meters, and power quality analyzers. Does not contain a circuit breaker — it passively monitors bus voltage and power flow. Typically placed at the incomer end of the lineup.
Bus PT cubicle: Houses bus potential transformers for protection relay voltage sensing. Required when protection relays need bus voltage reference independent of the incomer cubicle.
Step 4: Determine Ratings
Every component in the lineup must be rated for the system's electrical parameters:
- Rated voltage (kV): Must match or exceed the system nominal voltage. Standard classes: 3.6 kV, 7.2 kV, 12 kV, 17.5 kV, 24 kV, 36 kV, 40.5 kV.
- Rated current (A): Bus bars must carry the maximum continuous load current. Standard ratings: 630A, 1250A, 2000A, 2500A, 3150A, 4000A.
- Short-circuit rating (kA): Must withstand the maximum prospective fault current at the installation point. Common ratings: 16 kA, 20 kA, 25 kA, 31.5 kA, 40 kA, 50 kA.
- Short-time withstand duration: Typically 1 second, 3 seconds, or 4 seconds. Determined by protection clearing time plus a safety margin.
These ratings must comply with the applicable standards — IEC 62271 series for international projects, or ANSI/IEEE C37 for North American installations.
Step 5: Protection Coordination
Protection coordination ensures that faults are cleared by the breaker closest to the fault — not by a breaker further upstream. This is achieved through time-current coordination curves:
- Feeder breakers clear downstream faults first (fastest trip setting).
- Bus coupler breakers trip only if a feeder breaker fails to clear (delayed trip).
- Incomer breakers trip only as a last resort (longest delay), disconnecting the entire bus section.
Each breaker's protection relay must be set so its time-current curve does not overlap with adjacent breakers. This requires a coordination study — typically performed using power system analysis software and verified during commissioning.
Step 6: Physical Layout and Installation
The physical arrangement of cubicles follows industry conventions:
- Incomer cubicles are placed at one or both ends of the lineup.
- Bus coupler is positioned in the center (for split-bus configurations).
- Feeders are arranged between incomers and bus coupler, grouped by load priority or location.
- Metering and PT cubicles are adjacent to the incomer for shortest cable runs.
- Minimum aisle widths: 1000mm front (operation), 800mm rear (cable termination), per IEC 62271-200.
For metal-clad switchgear, all cubicles must be connected with continuous bus bars and include compartmentalized arc containment. For metal-enclosed types, some compartmentalization may be omitted at lower cost.
NAIJI Electric Lineup Solutions
NAIJI Electric provides complete switchgear lineup solutions from engineering to commissioning. Our standard lineup platforms include the ASN3-12 (12 kV, metal-clad, draw-out VCB), ASN550 (12 kV, compact ring main unit lineup), and custom-engineered lineups up to 40.5 kV. We support single bus, split bus, and dual bus configurations with full protection coordination studies. Contact our engineering team for a lineup design consultation.