In a breaker-and-a-third arrangement each disconnector operates only with its circuit breaker open, and the disconnector of each circuit requires both breakers it hangs between to be open. The difference from breaker-and-a-half is the centre circuit of the diameter, which depends on the two inner breakers: control logic, reclosing and breaker failure must account for it when one inner breaker is out.
Breaker-and-a-third interlocking is similar to breaker-and-a-half interlocking, but the diameter has one more breaker and one more circuit. In the breaker-and-a-third arrangement two buses are tied by diameters of four breakers in series: two bus breakers (52-1 and 52-4) and two inner breakers (52-2 and 52-3). A circuit leaves between each pair, so each diameter feeds three.
As in the breaker-and-a-half, there are no bus selectors and no coupler, and no disconnector operates in parallel. What is new is the centre circuit, which hangs between the two inner breakers.
Conditions per device
Each circuit breaker has a disconnector and an earthing switch on each side. Summary of typical logic:
| Device | Opens if | Closes if |
|---|---|---|
| Disconnectors of a breaker | its breaker is open | its breaker open and the earthing switch on its side open; bus-side ones also with the bus earthing switch open |
| Diameter circuit breaker | free | its two earthing switches open |
| Earthing switches of a breaker | free | both disconnectors of that breaker open |
| Circuit disconnector (89L or 89) | both breakers it hangs between are open | both open and the circuit earthing switch open |
| Line earthing switch 57L | free | the 89L open |
| HV terminal earthing switch 57T | free | the transformer 89 and its low-voltage breaker open |
| Earthing switch of bus 1 (or 2) | free | the bus disconnectors of every diameter towards that bus open |
For the outer circuits, «both breakers» are one bus breaker and one inner breaker. For the centre circuit, they are the two inner breakers.
The full table for this arrangement, with the CILO equations and the list of test cases, can be generated with our free interlocking table tool.
What changes from breaker-and-a-half
| Aspect | Breaker-and-a-half | Breaker-and-a-third |
|---|---|---|
| Breakers per diameter | three | four |
| Circuits per diameter | two | three |
| Circuit with no breaker of its own towards a bus | none | the centre one |
| Inner breakers | one, shared by both circuits | two; each shared by an outer circuit and the centre one |
For the interlocking logic of each device, the difference is small: more rows in the table. The real difference is in the logic that depends on the state of the diameter:
- Breaker failure of an inner breaker. The 50BF of an inner breaker receives initiation from two circuits and has to open the next breaker in the diameter and, if the neighbour is a line, send transfer trip.
- Reclosing. With two breakers per circuit a leader and a follower are defined, with synchronism check. The priorities have to cover each inner breaker out of service, especially for the centre circuit.
- Stub protection. When a circuit disconnector is opened and its two breakers are closed again, the stretch between them is protected by a function enabled by the position of that disconnector.
Bay level and station level
In a single busbar, the bay is one circuit. Here the natural unit is the diameter: each circuit disconnector depends on two breakers that may be in different controllers, and reclosing, 50BF and stub depend on the state of all four. Depending on the project, there is one controller per breaker sharing the diameter positions by GOOSE, or one controller per diameter.
The bus earthing switches are station level, since they need the bus disconnectors of every diameter.
Critical switching sequences
Maintenance of an inner breaker. The inner breaker and its two disconnectors are opened, and its earthing switches are closed. The diameter is split: the outer circuit on that side depends on its bus breaker, and the centre and the other outer circuit depend on the other inner breaker. The next operation that opens the other inner breaker leaves the centre circuit without a source.
- 01Open both inner breakersThe centre circuit is de-energized
- 02Open its disconnectorWith both inner breakers open, no current
- 03Enable stub protectionThe disconnector position activates it
- 04Close both inner breakersThe diameter ties the buses again
- 05Earth the circuitWith the disconnector open and, if it is a line, the remote end open and locked out
Bus maintenance. The bus breakers of every diameter towards that bus are opened, then their bus disconnectors, and the bus earthing switch closes. The circuits stay in service through the other bus.
High and extra-high voltage
This arrangement belongs to high and extra-high voltage, where the saving in breakers compared with breaker-and-a-half matters when there are many circuits. In SF6 gas-insulated substations, the disconnector and earthing switch can be a single three-position device; the logic interlocks each position as if they were separate devices, and the manufacturer adds its internal mechanical interlocks. In any case, interlocking does not replace the five safety rules of EN 50110-1: disconnect, secure against reconnection, verify absence of voltage, earth and short-circuit, and protect against adjacent live parts.
Typical mistakes
- Centre circuit disconnector with only one inner breaker. The condition is copied from an outer circuit and leaves out the other inner breaker.
- Switching without accounting for the centre circuit. The second inner breaker is opened with the first one out, and the centre circuit loses service without being in the switching order.
- Inner breaker 50BF with a single initiation. It does not receive initiation from one of the two circuits it serves.
- Unsupervised stub disconnector position.
- Transformer earthing switch with the LV breaker closed. Voltage can back-feed from the LV side.
How it is tested
The matrix of each breaker and each circuit disconnector is covered, then the diameter logic with each breaker out of service: reclosing, 50BF with its cross initiations and transfer trip, and stub. End-to-end tests confirm the transfer trips towards the remote ends.
We can help with your interlocking logic
At Tiza Group we design diameter logic as part of secondary engineering: interlocking, stub logic, inner breaker 50BF and leader and follower reclosing, and we program it in the bay controllers and relays through remote configuration, for high and extra-high voltage. On-site testing is done in Colombia, or elsewhere on request.
Related guides
- Automatic reclosing 79
- Direct transfer trip (DTT)
- Bay-level and station-level interlocking
- Voltage selection for synchronism check in a double bus
- Breaker-and-a-half interlocking: diameter and lines
- Interlocking tests: what is checked at FAT and SAT
- GOOSE interlocking with CILO and CSWI (IEC 61850)
References
- IEC 61850-7-4: logical nodes CILO, CSWI, RREC and RSYN.
- IEC 62271-102:2018: alternating current disconnectors and earthing switches.
- IEC 61936-1:2021: power installations exceeding 1 kV AC.
- EN 50110-1: operation of electrical installations (five safety rules).
- IEEE C37.119: guide for breaker failure protection of power circuit breakers.
- IEEE C37.104: guide for automatic reclosing of circuit breakers.
Frequently asked questions
Why does the centre circuit of the diameter need more care in the logic?
Because it has no breaker of its own towards either bus: it hangs between the two inner breakers. With one inner breaker out, it depends on a single path, and opening the other inner breaker leaves it without a source. Switching sequences and reclosing priorities must account for this.
Can a circuit disconnector open with one of its breakers closed?
No. The circuit is still fed through that breaker and the disconnector would break its current with an arc. The interlocking requires both breakers the circuit hangs between to be open.
Are breaker-and-a-third interlocks bay level or station level?
Those of each breaker are bay level, but the whole diameter works as one large bay: each circuit disconnector depends on two breakers, and each bus earthing switch depends on the disconnectors of every diameter. Those conditions travel between controllers, hardwired or by GOOSE.