An interlock is a condition that prevents a device from being operated when the operation would be dangerous: opening or closing a disconnector under load, closing onto an earth, or earthing a live section. It is implemented mechanically, electrically, with keys, as logic in the bay controller or through GOOSE messages, and it is solved at the device, the bay or the station depending on the signals it needs.
What an interlock is
An interlock is a condition that blocks the operation of a switchyard device while other devices are not in the position that makes it safe. It does not measure currents or detect faults: it looks at positions (open, closed, racked in, racked out, earthed) and decides whether the open or close command may pass.
Interlocks exist because the devices in a substation do not all have the same switching capability. The circuit breaker makes and breaks load and short-circuit currents. The disconnector, on the other hand, may only be operated with negligible current or with no significant change of voltage across its terminals, as defined by IEC 62271-102. The earthing switch is made to connect an already de-energized section to earth. Without interlocks, a sequence error turns a routine switching operation into an arc or a bolted short circuit with people nearby.
The three operations every scheme must prevent are:
- Opening or closing a disconnector under load.
- Closing a breaker or a disconnector onto an earthed section.
- Closing an earthing switch onto a live section.
Each configuration adds its own, such as not closing a bus selector disconnector if that would tie two buses without the bus coupler closed.
Types of interlocking
| Type | How it acts | Where it is used |
|---|---|---|
| Mechanical | Cams, rods or bolts that prevent the operating mechanism from moving | Disconnector and its earthing switch, medium-voltage switchgear, withdrawable truck |
| Hardwired electrical | Auxiliary contacts in series with the coil or the motor | Motor-operated mechanisms, conventional control panels |
| Key | A key released only with the device in a given position | Industry, switchgear, transformers, access doors |
| Logic in the controller | Boolean equation in the bay controller or a PLC | Substations with numerical control |
| GOOSE | Positions of other bays received over the substation network | Station-level interlocking with IEC 61850 |
Mechanical interlocking and electrical interlocking act on the device itself and depend neither on the control supply nor on the network. Logic and GOOSE interlocking allow conditions between bays that would be costly to wire, at the price of depending on communication supervision and data quality.
Levels where it is solved
Each condition is solved at the lowest level that has all the signals it needs:
- Device: the earthing switch against its own disconnector, the truck against its breaker. Usually mechanical.
- Bay: the devices of the same bay against each other (breaker, bus and line disconnectors, earthing switches). It lives in the bay controller.
- Station: conditions that depend on other bays, such as the bus earth, the coupler during a transfer, or the bypass. They need positions exchanged between controllers, by wiring, through a station controller or over GOOSE.
- Control center: it solves no interlocks of its own; its commands go through the lower levels and respect their conditions.
The boundary between bay and station decides where each condition is programmed and what happens to it if the station controller or the network is lost.
What interlocks what in a line bay
Take a high-voltage line bay on a single bus, with bus disconnector (89B), circuit breaker (52), line disconnector (89L), line earthing switch (57L) and an earthing switch on the bus side of the breaker (57B). The usual conditions are:
| Operation | Allowed only if |
|---|---|
| Open or close 89B | 52 open |
| Close 89B | Also, 57B open and the bus earth open |
| Open or close 89L | 52 open |
| Close 89L | Also, 57L open |
| Close 57B | 89B and 89L open |
| Close 57L | 89L open and, depending on the design, no voltage on the line |
| Close 52 | Earthing switches open, breaker permissives and synchronism check where it applies |
The table of a real project depends on the configuration, on the utility’s switching philosophy and on whether there are line voltage transformers. Configurations with more than one bus add the bus selection and coupler conditions.
How the logic is expressed in IEC 61850
IEC 61850-7-4 defines the logical node CILO (interlocking) with two data objects: EnaOpn (enable open) and EnaCls (enable close). The standard calls for one instance per switching device and leaves the algorithm to the implementer. The CSWI node receives the operator’s command and passes it to the device, represented by XCBR (circuit breaker) or XSWI (disconnector), only if CILO enables it.
- 01Operator commandReaches CSWI from the HMI, the SCADA or the control center
- 02Interlock checkCSWI reads EnaCls of the CILO for that device
- 03EvaluationCILO combines its own positions with those received from other bays
- 04Permission or rejectionIf EnaCls is false, the command is rejected with its cause
- 05ExecutionWith permission, the command reaches XCBR or XSWI and the device operates
The equation is written as a sum of products: closing a disconnector is enabled if one combination of positions or another is satisfied. The most important design rule is fail-safe: an intermediate, invalid or questionable-quality position counts as “not satisfied”. The detail of the chain between bays belongs to GOOSE interlocking, where the quality of the message is part of the condition.
For a given configuration, our free interlocking table tool builds the matrix, the CILO equations and the signals that must travel between bays.
What goes wrong in practice
- Badly adjusted auxiliary contacts that indicate open before the disconnector has finished opening.
- Invalid position caused by a damaged double-point contact, which blocks every operation that depends on that device. That is the correct behavior, but it is read as a control failure.
- Bypass left active after a test or an emergency.
- Station conditions not updated when a bay was added.
- Differences between local and remote control: some designs leave the cabinet control outside the controller logic, and safety then rests only on the mechanical interlock.
When the operator sees that “the device does not obey”, the first step is to read the rejection cause in the controller before suspecting the mechanism or the motor.
How it is verified
Interlock testing checks each condition positive (allowed when it should be) and negative (blocked when each term is missing), plus the reaction to invalid positions, loss of communication and bypass mode. At the factory it is done with simulated positions; on site, with the real devices.
Do you need to define or review the interlocking table of your substation? At Tiza Group we configure and test interlocking logic in bay and station controllers, hardwired or over IEC 61850, for substations and industry at medium, high and extra-high voltage. We prepare the secondary engineering and configure controllers remotely through our remote relay configuration service; site testing is done in Colombia, or elsewhere on request. Contact us with the configuration and the equipment of your project.
Related guides
- Bay and station interlocking: what each level solves
- Mechanical and electrical interlocking in substations
- GOOSE interlocking: CILO, CSWI and data quality
- Control levels and the local/remote switch (43)
- Select before operate (SBO): how the command works
- Breaker closing permissives: what blocks the close
- Double busbar interlocking: coupler and transfer
- Breaker-and-a-half interlocking: diameter and lines
- Parallel transformers with OLTC: master-follower
- IEC 61850 test mode: Mod, Beh, Sim and the test bit
- Bus disconnector interlocking: conditions
- Line disconnector interlocking: conditions
- Line and bus earthing: interlocking
- Bus coupler interlocking: when it must not open
- Bypass and transfer disconnectors: interlocking
- Kirk and Castell key interlocking: how it works
- Interlocking in MV switchgear and RMUs
- Low-voltage breaker and tie interlocking
- Interlock bypass: release, risks and control
- Interlocking at EHV, HV, MV and LV: what changes
- Dead line and live bus closing: 25 modes
- Voltage selection for synchronism check in double bus
- Single bus interlocking: bays and bus earth
- Sectionalized bus interlocking
- Main and transfer bus interlocking
- Double bus with transfer and bypass: interlocking
- Sectionalized double bus and triple bus interlocking
- Double bus double breaker interlocking
- Breaker-and-a-third interlocking
- Ring bus interlocking
- H configuration interlocking: tie and sources
- Tee-off configuration interlocking
- Main-tie-main interlocking: secondary selective
- Primary selective and MV loop interlocking
- Medium-voltage automatic transfer: logic and blocking
- Fast bus transfer for motor buses: three methods
- Transferring protection to the transfer breaker
- Disconnect replica for bus differential protection
- Breaker-and-a-half reclosing: leader and follower
- Automatic capacitor bank control at MV and HV
- Controlled switching (point-on-wave): what it is and where it is used
- Automatic switching sequences in substations
- Automatic service restoration after a blackout
- FLISR: self-healing distribution networks
- Undervoltage load shedding (UVLS)
- Test blocks and isolating a bay for testing
References
- IEC 61850-7-4, Communication networks and systems for power utility automation – Basic communication structure – Compatible logical node classes and data object classes (CILO, CSWI, XCBR, XSWI).
- IEC 62271-102:2018, High-voltage switchgear and controlgear – Alternating current disconnectors and earthing switches.
- IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1,5 kV DC – AC.
- EN 50110-1, Operation of electrical installations – General requirements.
- CIGRE, Technical Brochure 540, Applications of IEC 61850 standard to protection schemes, WG B5.36, 2013.
Frequently asked questions
What is the difference between an interlock and a protection?
A protection acts on a fault and opens breakers. An interlock acts before a switching operation and prevents it if the position of the other devices makes it dangerous. It trips nothing: it only withholds the permission to open or close.
Is a software interlock as safe as a hardwired one?
It can be, if the logic treats any intermediate, invalid or questionable-quality position as not satisfied, and if every condition is tested both positive and negative. Many specifications combine both: a mechanical or hardwired interlock on the device itself, and bay and station logic in the controllers.
Can an interlock be bypassed?
Systems usually have a bypass mode for emergencies or testing, protected by a key or password and logged as an event. Its use is governed by the procedures of the utility and of the person in charge of the installation; a bypass left active is one of the typical causes of switching accidents.