The purpose of an interlock is the same at any voltage level: no disconnector switches load, no earthing switch closes onto voltage and no sources are tied that must not be. What changes is where it is implemented. At high and extra-high voltage, with separate devices and motor operators, the logic lives in the controllers and travels between bays. In medium voltage it is inside the panel and mostly mechanical. In low voltage it is solved with cable, rod or key between breakers.
The same purpose at every level
An interlock exists to prevent three errors: a disconnector interrupting or making load, an earthing switch closing onto a live section or an earthed section being energized, and two sources being tied that must not be. Those three errors are the same on a 500 kV line and in a 480 V switchboard. IEC 61936-1 requires interlocks in installations above 1 kV to prevent incorrect operation, and in low voltage the same criterion applies to sources that must not be paralleled.
What changes with voltage level is the shape of the equipment, its physical separation and the way it is operated. And that defines where the interlock is implemented.
Extra-high and high voltage
Equipment. In a conventional air-insulated substation (AIS), each device of the bay is a separate piece of equipment: breaker, disconnectors, earthing switches and instrument transformers, several meters apart. The disconnectors have motor operators and are switched from the yard marshalling cabinet, the bay controller, the control room or the control center.
Interlocking. It is electrical or logic-based. In older installations, with auxiliary contacts wired in series with the control circuits. In current ones, in the bay controller, with one IEC 61850-7-4 CILO logical node per device. Station conditions, such as the bus earthing switch or blocking the coupler during a transfer, travel between bays by GOOSE or by wiring between cabinets.
Particular features:
- Arrangements with more devices per circuit (double bus, breaker-and-a-half, ring bus) multiply the station conditions.
- The line earthing switch depends on the remote end, which the local interlocking cannot see.
- Control from several levels requires defining which level has authority and whether control from the yard respects the interlock or not.
- It is combined with synchronism check on breaker closing.
GIS. In a gas-insulated substation the devices are enclosed and the three-position disconnector is often used, which mechanically solves the relationship between disconnector and earthing switch. The rest of the logic is in the GIS local control cubicle or in the bay controller, as in AIS.
Medium voltage
Equipment. Metal-enclosed switchgear (IEC 62271-200, IEEE C37.20 in North America), ring main units (RMU) and, in distribution, pole-mounted equipment. The breaker is usually on a withdrawable truck, or fixed with a three-position disconnector.
Interlocking. Inside each panel it is mostly mechanical and comes from the factory: the truck only moves with the breaker open, the earthing switch does not close with the truck in service and interlock-controlled compartments only open when de-energized and earthed. IEC 62271-200 requires that, if it is not mechanical, the design is safe even without auxiliary supply.
Between panels the interlock is defined by the project: two incomers and a tie, the panel and the transformer, the panel and the low-voltage switchboard. It is solved with trapped keys, wired contacts or the relay logic. The guide on MV switchgear and RMU interlocking covers it in detail.
Low voltage
Equipment. Air and molded-case circuit breakers (IEC 60947-2) inside switchboards (IEC 61439-2). The IEC 62271 series does not apply.
Interlocking. Its main function is to prevent paralleling sources: two incomers and a tie, or the utility and a standby generator. It is done with a mechanical cable or rod interlock between breakers, with a trapped key when they are far apart, and with an electrical interlock by auxiliary contacts and an undervoltage release as a complement. The mechanical one is the main barrier, because the close push button on the breaker front bypasses the control circuit. See low-voltage breaker interlocking.
Industry and solar plants: several levels in one installation
An industrial substation or a solar plant almost always brings several levels together. A high- or medium-voltage incomer, one or more transformers, MV switchgear and low-voltage switchboards, sometimes with on-site generation or a standby generator. Each piece of equipment comes with its internal interlocks, but the safety of the whole depends on the ones that cross from one level to another: access to a transformer requiring both the panel that feeds it and the low-voltage breaker to be open, or a generator unable to close onto a bus energized from the grid. Those cross-level interlocks are the ones most often left out, because each supplier delivers its own equipment and nobody designs the complete sequence.
Comparison by level and technology
| Level and technology | Where the interlock lives | Dominant type | Conditions between equipment |
|---|---|---|---|
| EHV and HV, AIS | Bay and station controller | Logic, hardwired or by GOOSE | Many, station-wide |
| EHV and HV, GIS | GIS local control cubicle and bay controller | Mechanical in the three-position disconnector, logic for the rest | Many, station-wide |
| MV, truck-type switchgear | Inside the panel | Mechanical, from the factory | Between panels, per project |
| MV, RMU | Inside the unit | Mechanical, three-position switch | With the transformer, by key |
| LV, switchboard | Between breakers | Mechanical by cable, rod or key, plus electrical | Between sources |
What stays the same at every level
- Fail-safe. When a signal or the supply is lost, the interlock must block, not allow.
- Valid position. A device in an intermediate or doubtful position gives no permissives.
- Controlled bypass. Any bypass, logic or mechanical, must be restricted, alarmed and logged.
- Extensions. When a source or a bay is added, the existing logic must be reviewed in full.
- Testing. Each condition is tested positively and negatively, by attempting the forbidden operation.
We help at any level
Our team configures and tests interlocks in medium, high and extra-high voltage substations, in industrial switchgear and switchboards and in solar plants. Field testing is done in Colombia, or elsewhere on request. We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. See secondary engineering or contact us with the voltage level and technology of your installation.
Related guides
- Mechanical and electrical interlocking in substations
- MV switchgear and RMU interlocking
- Low-voltage breaker and tie interlocking
- Bay and station interlocking: differences
- Substation communications by voltage level
References
- IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16 (interlocking).
- IEC 62271-102:2018, Alternating current disconnectors and earthing switches.
- IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
- IEC 60947-2, Low-voltage switchgear and controlgear – Part 2: Circuit-breakers.
- IEC 61439-2, Low-voltage switchgear and controlgear assemblies – Part 2.
- IEC 61850-7-4, logical nodes CILO, CSWI, XCBR and XSWI.
Frequently asked questions
Why is interlocking logic-based at high voltage and mechanical at medium voltage?
Because at high voltage the devices of a bay are separate pieces of equipment, tens of meters apart, with motor operators, and the only practical way to relate them is with signals. In medium voltage the breaker, the disconnector and the earthing switch share an enclosure and the manufacturer can link them mechanically inside the equipment.
Does a high-voltage GIS substation use mechanical interlocks?
Partly. Many GIS use three-position disconnectors, which mechanically prevent going from closed to earthed without passing through open. The other conditions, such as those of the breaker or of other bays, remain in the electrical logic of the GIS local control cubicle or the bay controller.
Which type of interlock is more reliable?
It depends on what has to be prevented. A mechanical interlock does not depend on the control supply, but it cannot see voltage or the position of distant equipment. A logic interlock sees the whole substation and is supervised, but it depends on signals and communications. That is why they are combined at the higher levels, and reliability comes from supervision and testing.