Bus Tie Switch Symbol
Definition: The Bus Tie Switch symbol represents a normally-open switching device used in electrical distribution schematics to interconnect two separate bus sections (Bus A and Bus B) of a switchgear or distribution system, allowing load transfer between sources or paralleling of supplies, depicted as a two-terminal switch per IEC 60617-07 and ANSI/IEEE C37 switchgear symbol conventions.
Also known as: bus coupler, bus section switch, bus tie breaker, bus interconnect switch, sectionalizer, bus tie circuit breaker, BTC.
What the Bus Tie Switch symbol means
The Bus Tie Switch symbol denotes a switching element — typically a circuit breaker, contactor, or isolator — installed between two bus sections in a distribution switchboard, substation, or motor control centre to electrically couple or decouple them. Under normal operation the bus tie is usually open (sectionalized), allowing each bus section to be fed from its own independent supply source. When one source fails or maintenance is required, the bus tie closes to restore power to the dead bus section from the healthy supply.
Bus tie switches are critical in high-availability power systems: commercial buildings, data centres, hospitals, and industrial plants use bus tie configurations to achieve N+1 or 2N power redundancy. The bus tie symbol in a one-line diagram (single-line diagram) shows the interconnection point and the switch or breaker type used to make or break the bus coupling.
How to identify the Bus Tie Switch symbol
The Bus Tie Switch symbol is drawn as a single-pole switch (an open contact arc or diagonal line) connected between two bus bars labelled Bus A and Bus B, with Bus A at the left terminal and Bus B at the right terminal. In one-line diagrams (SLDs) the bus tie breaker appears as a square or rectangle (circuit breaker symbol) bridging two horizontal bus lines, often with a normally-open indication. IEC 60617 uses the switch-symbol convention with the NO (normally open) notation; ANSI uses a square box on the bus line labelled 'TIE' or 'BT'.
Function in a circuit
A bus tie switch performs bus sectionalisation and load transfer functions. With the bus tie open, Bus A and Bus B are fed from separate independent source transformers or generators — a fault on one source affects only its bus section. Closing the bus tie connects the two bus sections in parallel, allowing both to be fed from a single source, and simultaneously requires interlocking logic to prevent both sources from being in parallel for extended periods (which could exceed transformer or switchgear fault rating). Automatic Transfer Schemes (ATS) use the bus tie to automatically restore supply within seconds of a source failure.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines switch and circuit breaker symbols used for bus tie elements. IEC 61850 governs substation automation and the logical nodes for bus coupler and bus section switch functions. IEC 60909 covers short-circuit current calculations that determine the required interrupting rating of bus tie breakers. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE C37.2 (electrical power system device function numbers) assigns device function number 89 to bus-tie breakers and couplers. ANSI/IEEE C37 series standards govern the circuit breaker or switch used as the bus tie device. Single-line diagram symbols follow ANSI Y32.2 / IEEE 315 switch and breaker symbol conventions. |
| Key difference | IEC 60617-07 uses the standard normally-open contact and switch/breaker symbols for the bus tie element with textual annotation. ANSI/IEEE C37.2 uses device function number 89 to designate bus-tie breakers on one-line diagrams and control schematics. Both conventions show the switch bridging two bus lines. |
Terminals / pins
| Pin | Name |
|---|---|
| bus_a | Bus A |
| bus_b | Bus B |
Typical values
Voltage ratings: 400 V (LV switchgear), 11 kV, 33 kV, or higher (MV/HV switchgear). Current ratings: 630 A to 6300 A typical. Fault interrupting capacity: 25–100 kA (depending on voltage level and switchgear type). Operating mechanism: motorised spring, solenoid, or manual. Interlocking: mandatory to prevent parallel operation beyond design limits.
Where the Bus Tie Switch symbol is used
- Commercial building main distribution switchboards where two utility transformer feeders supply separate bus sections with a normally-open bus tie breaker for source redundancy
- Data centre power distribution systems where two uninterruptible power supplies (UPS) feed separate bus bars with a bus tie providing automatic failover between UPS systems
- Industrial motor control centres (MCCs) with dual incoming feeders and a bus tie section breaker enabling online maintenance of one feeder without interrupting production
- Hospital essential electrical systems (EES) with automatic bus transfer to emergency generator supply via a bus tie arrangement per NEC Article 517
- Substation ring-bus or breaker-and-a-half configurations where bus tie breakers interconnect bus sections to maintain supply continuity during maintenance
- Shipboard power distribution with split-bus arrangement allowing bus coupling for propulsion loads while maintaining section independence for damage control
Example
In a commercial building electrical one-line diagram, two 1500 kVA transformers each feed their own 480 V bus section (Bus A and Bus B); a normally-open Bus Tie Switch symbol bridges the two bus sections. Under normal operation both bus sections are energised independently. When Transformer 1 loses supply, the automatic transfer scheme closes the Bus Tie Switch symbol (connecting Bus A to Bus B), restoring 480 V to Bus A from Transformer 2, with interlock logic preventing Transformer 1 and Transformer 2 from being paralleled simultaneously.
Key facts
- The bus tie switch symbol represents the coupling switch or breaker between two independent bus sections in a distribution system, normally open to keep sources separated and closed to transfer load during source failure.
- Pins: Bus A (left terminal) and Bus B (right terminal) — the two bus sections being interconnected.
- ANSI/IEEE C37.2 device function number for a bus-tie breaker is 89 (Line switch); the bus-tie circuit breaker specifically is designated in one-line diagrams with the label 'TIE' or 'BT' adjacent to the breaker symbol.
- Bus tie switches must be interlocked to prevent inadvertent parallel operation of two sources beyond the rated short-circuit capacity of the bus or switchgear.
- Automatic transfer schemes using bus ties can restore supply within 100–200 ms (motorised circuit breakers) or faster (static transfer switches) after source failure.
- IEC 61850-5 defines the logical nodes CSWI (switch controller) and XSWI (switch status) for bus coupler functions in digital substation automation systems.
- The bus tie configuration is fundamental to N+1 and 2N power system redundancy architectures used in data centres, hospitals, and critical industrial facilities.
Frequently asked questions
What does the bus tie switch symbol look like in a one-line diagram?
The bus tie switch symbol appears as a switch or circuit breaker symbol bridging two horizontal bus bars. In IEC convention it is a normally-open contact symbol between two bus lines labelled Bus A and Bus B. In ANSI/IEEE single-line diagrams it is a square or rectangle (breaker symbol) on a vertical connection line between the two bus bars, labelled 'TIE' or 'BT', often with a normally-open indication.
What does the bus tie symbol mean in a power distribution diagram?
The bus tie symbol marks the coupling point between two separate bus sections fed from independent sources. It indicates that the switch is normally open (sources separated) and can be closed to transfer load from one bus to the other when a source fails, or to parallel the buses for maintenance, subject to interlocking constraints.
What is the ANSI device function number for a bus tie breaker?
ANSI/IEEE C37.2 assigns device function number 89 to a line switch, which covers bus-tie breakers and bus-coupling switches. In practice, bus-tie circuit breakers in MV and HV switchgear are labelled with the suffix 'TIE' or annotated '89-BUS TIE' on one-line diagrams following ANSI C37.2 conventions.
Why is a bus tie switch normally open?
The bus tie is normally open to keep Bus A and Bus B on separate independent supply sources, providing source redundancy. If both buses were permanently paralleled, a fault on either source or bus would affect both bus sections simultaneously. Keeping the tie open isolates faults to one section and ensures that each bus section can be de-energised independently for maintenance without affecting the other.
What interlocking is required for a bus tie switch?
Bus tie interlocking prevents simultaneous closure of all three switches (Source 1 breaker, Bus Tie breaker, Source 2 breaker) when the two sources are not synchronised, avoiding parallel operation that could exceed fault rating. Key interlocks include: bus tie can only close if one source breaker is open (closed transition) or synchronisation check relay (ANSI 25) confirms sources are in phase before paralleling.
What IEC standard covers bus tie switch functions in substations?
IEC 60617-07 provides the graphical symbols for bus tie switches. IEC 61850-5 defines the logical nodes (CSWI, XSWI, XCBR) for switch and circuit breaker control in substation automation. IEC 60909 governs short-circuit current calculation for determining the required interrupting rating of bus tie breakers. IEC 60947-2 covers the MCCB-type bus tie breakers used in low-voltage switchgear.
What is the difference between a bus tie switch and a bus tie breaker?
A bus tie switch is a general term for any switching device connecting two bus sections; it can be an isolator (disconnect switch), a load break switch, or a circuit breaker. A bus tie breaker specifically is a fault-interrupting circuit breaker rated to interrupt the maximum fault current available at the bus. Bus tie breakers are used in MV and HV systems; lower-voltage LV switchgear may use MCCB or contactors as the bus tie device.
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