Key takeaways
- IEC 61850 logical nodes in full: all 13 LN groups, the ones you meet daily like XCBR, PTOC, MMXU, CSWI, and GGIO, plus data objects and object references.
- Focus protocol: IEC-61850 — see the reference page for frame format, OSI layer, and port/ethertype details.
- Related topics: substation communication, GOOSE messaging, Sampled Values, MMS protocol.
Every piece of data in an IEC 61850 system lives inside a logical node. A circuit breaker position is inside XCBR. An overcurrent trip signal is inside PTOC. A voltage measurement is inside MMXU. A GOOSE control block is inside LLN0.
This article is part of the broader IEC-61850 series. If you're new to this protocol family, start with What Is IEC 61850? The Language of Power System Functions.
Logical nodes are the building blocks of the IEC 61850 data model. They represent functions — not physical devices. A single IED can contain dozens of logical nodes. A single logical node can appear in GOOSE messages, MMS reports, SCL files, and Wireshark captures.
If you do not understand logical nodes, you cannot read an SCL file, configure a GOOSE subscription, decode an MMS object reference, or troubleshoot any IEC 61850 system.
This guide covers what logical nodes are, how they are organized into groups, the most important LN classes you will encounter in substations, and how to read logical node references in practice.
- 1. What Is a Logical Node
- 2. Where Logical Nodes Sit in the Data Model Hierarchy
- 3. The 13 Logical Node Groups
- 4. System Logical Nodes (Group L)
- 5. Protection Logical Nodes (Group P)
- 6. Switchgear Logical Nodes (Group X)
- 7. Measurement Logical Nodes (Group M)
- 8. Control Logical Nodes (Group C)
- 9. Protection-Related Logical Nodes (Group R)
- 10. Automatic Control Logical Nodes (Group A)
- 11. Sensor and Monitoring Logical Nodes (Group S)
- 12. Transformer Logical Nodes (Groups T and Y)
- 13. Interfacing and Archiving Logical Nodes (Group I)
- 14. Generic Logical Nodes (Group G)
- 15. Common Data in Every Logical Node
- 16. How to Read a Logical Node Reference
- 17. Logical Nodes in SCL, GOOSE, and MMS
- 18. Quick Reference: 50 Most Used Logical Nodes
- Summary
Practical tool: Use our IEC 61850 to parse raw packets and inspect field-level details while you read.
1. What Is a Logical Node
A logical node is a named container for a specific function inside an IED. It groups together all the data objects related to that function.
For example, the logical node XCBR represents a circuit breaker. It contains data objects like:
- Pos — the current position (open/closed)
- OpCnt — operation counter
- CBOpCap — circuit breaker operating capability
- BlkOpn — block opening command
- BlkCls — block closing command
The logical node does not care which physical device it runs on. XCBR could be inside an ABB relay, a Siemens relay, or a Schneider bay controller. The data model is the same everywhere — that is the whole point of IEC 61850.
2. Where Logical Nodes Sit in the Data Model Hierarchy
The IEC 61850 data model is a strict hierarchy:
IED → Access Point → Server → Logical Device → Logical Node → Data Object → Data Attribute
Example path: IED1/S1/Server/PROT/PTOC1.Op.general
| Level | Example | Meaning |
|---|---|---|
| IED | IED1 | Physical device |
| Logical Device | PROT | Container for related LNs (e.g., protection functions) |
| Logical Node | PTOC1 | Overcurrent protection, instance 1 |
| Data Object | Op | Operate (trip output) |
| Data Attribute | general | General trip indication (boolean) |
Multiple instances of the same LN class are distinguished by an instance number: PTOC1, PTOC2, PTOC3, etc.
3. The 13 Logical Node Groups
Every logical node name starts with a letter that identifies its group. The groups are defined in IEC 61850-7-4.
| Group | Letter | Description | Examples |
|---|---|---|---|
| System | L | System-level LNs present in every IED | LLN0, LPHD |
| Protection | P | Protection functions | PTOC, PDIS, PDIF, PTOV, PTUF |
| Switchgear | X | Circuit breakers, disconnectors | XCBR, XSWI |
| Measurement | M | Metering and measurement | MMXU, MMTR, MSQI, MHAI |
| Control | C | Supervisory control | CSWI, CILO, CPOW |
| Protection-Related | R | Functions supporting protection | RREC, RPSB, RSYN, RBRF |
| Automatic Control | A | Automatic control functions | ARCO, ATCC, AVCO, ANCR |
| Sensor/Monitoring | S | Sensors and condition monitoring | SIML, SIMG, SPDC, STMP |
| Instrument Transformer | T | CT and VT models | TCTR, TVTR |
| Power Transformer | Y | Transformer and related functions | YPTR, YLTC, YPSH |
| Further Equipment | Z | Other power system equipment | ZBAT, ZGEN, ZMOT, ZCAP |
| Interfacing/Archiving | I | Interfacing and data archiving | IHMI, ITCI, IARC |
| Generic | G | Generic function references | GGIO, GAPC, GSAL |
4. System Logical Nodes (Group L)
These are mandatory in every IED. They provide system-level information.
| LN Class | Name | Purpose |
|---|---|---|
| LLN0 | Logical Node Zero | The "root" LN of each logical device. Contains datasets, GOOSE control blocks, report control blocks, and log control blocks. Every logical device has exactly one LLN0. |
| LPHD | Physical Device Information | Represents the physical IED hardware. Contains nameplate data, health status, power-up counter, and watchdog information. Every server has exactly one LPHD. |
LLN0 is the most important LN in practice because all GOOSE and report control blocks are defined inside LLN0. When you configure a GOOSE publisher or a buffered report, you are working inside LLN0.
5. Protection Logical Nodes (Group P)
The largest group. Each LN represents one protection function.
| LN Class | Name | IEEE Device # | Purpose |
|---|---|---|---|
| PTOC | Time Overcurrent | 50/51 | Overcurrent protection (definite time and inverse time) |
| PIOC | Instantaneous Overcurrent | 50 | Instantaneous overcurrent protection |
| PDIS | Distance Protection | 21 | Impedance-based distance protection |
| PDIF | Differential Protection | 87 | Current differential protection (transformer, line, bus) |
| PTOV | Time Overvoltage | 59 | Overvoltage protection |
| PTUV | Time Undervoltage | 27 | Undervoltage protection |
| PTOF | Time Overfrequency | 81O | Overfrequency protection |
| PTUF | Time Underfrequency | 81U | Underfrequency protection |
| PTTR | Thermal Overload | 49 | Thermal protection |
| PDOP | Directional Overpower | 32 | Directional power protection |
| PHIZ | Ground Detector (high impedance) | 87N | High-impedance ground fault protection |
| PSCH | Protection Scheme | — | Protection communication scheme (permissive, blocking, etc.) |
| PPAM | Phase Angle Measuring | 78 | Phase angle measurement for protection |
Key Data Objects in Protection LNs
Every protection LN typically contains:
- Op — operate (trip output): the key data object. Op.general = TRUE means the function has tripped.
- Str — start: the function has detected a fault but not yet tripped.
- StrVal — the measured value that caused the start.
- Settings like SetA, SetB — setting groups for different operating conditions.
6. Switchgear Logical Nodes (Group X)
| LN Class | Name | Purpose |
|---|---|---|
| XCBR | Circuit Breaker | Models a circuit breaker. Contains position (Pos), operating capability, operation counter, and block commands. |
| XSWI | Disconnector / Switch | Models disconnectors, earthing switches, and load break switches. |
Key Data Objects
- Pos — position (DPC: double point control). Values: intermediate (00), off/open (01), on/closed (10), bad state (11).
- OpCnt — operation counter.
- BlkOpn / BlkCls — block open / block close commands.
- CBOpCap — circuit breaker operating capability.
XCBR is one of the most frequently referenced logical nodes. It appears in GOOSE messages (breaker position), control commands (open/close), and interlocking logic.
7. Measurement Logical Nodes (Group M)
| LN Class | Name | Purpose |
|---|---|---|
| MMXU | Measurement | Three-phase electrical measurements: voltage, current, power, power factor, frequency. The most commonly used measurement LN. |
| MMTR | Metering | Energy metering (kWh, kVArh). Accumulates energy values over time. |
| MSQI | Sequence and Imbalance | Positive, negative, and zero sequence quantities. Used for unbalance monitoring. |
| MHAI | Harmonics and Interharmonics | Harmonic analysis: individual harmonic magnitudes and THD. |
| MSTA | Metering Statistics | Statistical analysis of measurements (min, max, average). |
Key Data Objects in MMXU
- TotW — total active power (watts)
- TotVAr — total reactive power
- TotVA — total apparent power
- TotPF — total power factor
- Hz — frequency
- PhV — phase-to-neutral voltages (phsA, phsB, phsC)
- A — phase currents (phsA, phsB, phsC)
- PPV — phase-to-phase voltages
8. Control Logical Nodes (Group C)
| LN Class | Name | Purpose |
|---|---|---|
| CSWI | Switch Controller | Controls a switch (XCBR or XSWI). Receives open/close commands from operators or automation. One CSWI per controlled switch. |
| CILO | Interlocking | Implements interlocking logic. Prevents unsafe switching operations (e.g., opening a disconnector under load). |
| CPOW | Point-on-Wave Switching | Controlled switching at specific point on the voltage/current waveform to minimize transients. |
CSWI is the LN that operators interact with. When a dispatcher sends an "open breaker" command from SCADA, the command goes to CSWI, which verifies interlocking (via CILO) and then sends the operate command to XCBR.
9. Protection-Related Logical Nodes (Group R)
These support protection functions but are not protection functions themselves.
| LN Class | Name | Purpose |
|---|---|---|
| RREC | Auto-Recloser | Automatic reclosing after a trip. Controls reclosing time, number of shots, and lockout. |
| RBRF | Breaker Failure | Breaker failure protection. If the breaker does not open within the expected time after a trip command, RBRF trips backup breakers. |
| RSYN | Synchronism Check | Checks voltage magnitude, phase angle, and frequency before closing a breaker to synchronize two systems. |
| RPSB | Power Swing Blocking | Detects power swings and blocks distance protection from tripping during stable swings. |
10. Automatic Control Logical Nodes (Group A)
| LN Class | Name | Purpose |
|---|---|---|
| ATCC | Tap Changer Controller | Automatic voltage regulation via transformer tap changer. |
| ARCO | Reactive Power Control | Automatic reactive power compensation control. |
| AVCO | Voltage Control | Automatic voltage regulation. |
| ANCR | Neutral Current Regulator | Controls neutral current (e.g., for Petersen coil tuning). |
11. Sensor and Monitoring Logical Nodes (Group S)
| LN Class | Name | Purpose |
|---|---|---|
| SIML | Insulation Medium Level | Oil level monitoring in transformers. |
| SIMG | Insulation Medium Gas | Dissolved gas analysis (DGA) monitoring. |
| STMP | Temperature | Temperature measurement. |
| SPDC | Partial Discharge | Partial discharge monitoring. |
12. Transformer Logical Nodes (Groups T and Y)
Group T — Instrument Transformers
| LN Class | Name | Purpose |
|---|---|---|
| TCTR | Current Transformer | Models a current transformer. Used with merging units on the process bus. |
| TVTR | Voltage Transformer | Models a voltage transformer. |
Group Y — Power Transformers
| LN Class | Name | Purpose |
|---|---|---|
| YPTR | Power Transformer | Represents the power transformer itself. |
| YLTC | Load Tap Changer | Models the tap changer mechanism. |
| YPSH | Power Shunt Reactor | Shunt reactor model. |
13. Interfacing and Archiving Logical Nodes (Group I)
| LN Class | Name | Purpose |
|---|---|---|
| IHMI | HMI Interface | Human-machine interface parameters. |
| ITCI | Telecontrol Interface | Interface to telecontrol protocols (e.g., IEC 104, DNP3). |
| IARC | Archiving | Data archiving and logging. |
14. Generic Logical Nodes (Group G)
| LN Class | Name | Purpose |
|---|---|---|
| GGIO | Generic Process I/O | Generic digital and analog I/O points that do not fit any specific LN class. The "catch-all" LN. Used for auxiliary contacts, LEDs, binary inputs/outputs. |
| GAPC | Generic Automatic Process Control | Generic PID controller or automation function. |
| GSAL | Generic Security Application | Security-related functions per IEC 62351. |
GGIO is extremely common. Whenever a device has I/O points that do not belong to a standard protection or measurement function, they go into GGIO.
15. Common Data in Every Logical Node
Every logical node (except LPHD) inherits these mandatory data objects:
| Data Object | CDC | Meaning |
|---|---|---|
| Mod | INC | Mode control — on, off, test, test/blocked |
| Beh | INS | Behaviour — reflects the current operating mode |
| Health | INS | Health status — ok, warning, alarm |
| NamPlt | LPL | Nameplate — vendor name, software revision, configuration revision |
These four data objects are always present. They let you check if a logical node is active (Beh = on), healthy (Health = ok), and what mode it is running in (Mod = on vs test).
16. How to Read a Logical Node Reference
An IEC 61850 object reference follows this pattern:
LogicalDeviceName/LogicalNodeNameFunctionalConstraintFunctionalConstraint FunctionalConstraintDataObject.DataAttribute
Example
PROT/PTOC1$ST$Op.general
| Part | Value | Meaning |
|---|---|---|
| Logical Device | PROT | Protection logical device |
| Logical Node | PTOC1 | Overcurrent protection, instance 1 |
| Functional Constraint | ST | Status information |
| Data Object | Op | Operate (trip) |
| Data Attribute | general | General trip (boolean) |
Common Functional Constraints
| FC | Meaning |
|---|---|
| ST | Status |
| MX | Measured values (analog) |
| CO | Control |
| CF | Configuration |
| SP | Setting group (setpoints) |
| SG | Setting group parameters |
17. Logical Nodes in SCL, GOOSE, and MMS
In SCL Files
Logical nodes appear in the IED section of SCL files:
xml
<LN lnClass="PTOC" inst="1" lnType="PTOC_Type">
<DOI name="Op">
<DAI name="general" />
</DOI>
</LN>In GOOSE Messages
GOOSE datasets reference data objects inside logical nodes:
xml
<FCDA ldInst="PROT" lnClass="XCBR" lnInst="1" doName="Pos" daName="stVal" fc="ST" />This means: "Include the status value of the Position data object from XCBR instance 1 in the PROT logical device."
In MMS
An MMS client reads data using the full object reference:
IED1PROT/PTOC1$ST$Op → reads the trip status of overcurrent protection
18. Quick Reference: 50 Most Used Logical Nodes
| LN | Group | Function |
|---|---|---|
| LLN0 | L | Logical device root — datasets, GOOSE, reports |
| LPHD | L | Physical device information |
| XCBR | X | Circuit breaker |
| XSWI | X | Disconnector / switch |
| CSWI | C | Switch controller |
| CILO | C | Interlocking |
| CPOW | C | Point-on-wave switching |
| PTOC | P | Time overcurrent (50/51) |
| PIOC | P | Instantaneous overcurrent (50) |
| PDIS | P | Distance protection (21) |
| PDIF | P | Differential protection (87) |
| PTOV | P | Time overvoltage (59) |
| PTUV | P | Time undervoltage (27) |
| PTOF | P | Time overfrequency (81O) |
| PTUF | P | Time underfrequency (81U) |
| PTTR | P | Thermal overload (49) |
| PDOP | P | Directional overpower (32) |
| PHIZ | P | High-impedance ground fault (87N) |
| PSCH | P | Protection scheme |
| PPAM | P | Phase angle measuring (78) |
| PFRC | P | Frequency rate of change |
| RREC | R | Auto-recloser (79) |
| RBRF | R | Breaker failure (50BF) |
| RSYN | R | Synchronism check (25) |
| RPSB | R | Power swing blocking (68) |
| RDRE | R | Disturbance recorder |
| RADR | R | Analog disturbance recorder |
| MMXU | M | Three-phase measurements |
| MMTR | M | Energy metering |
| MSQI | M | Sequence and imbalance |
| MHAI | M | Harmonics |
| MSTA | M | Metering statistics |
| ATCC | A | Tap changer controller |
| ARCO | A | Reactive power control |
| AVCO | A | Voltage control |
| ANCR | A | Neutral current regulator |
| TCTR | T | Current transformer |
| TVTR | T | Voltage transformer |
| YPTR | Y | Power transformer |
| YLTC | Y | Load tap changer |
| SIML | S | Insulation medium (oil) level |
| SIMG | S | Insulation medium gas (DGA) |
| STMP | S | Temperature monitoring |
| SPDC | S | Partial discharge |
| ZBAT | Z | Battery |
| ZGEN | Z | Generator |
| ZMOT | Z | Motor |
| GGIO | G | Generic process I/O |
| GAPC | G | Generic automatic process control |
| IHMI | I | HMI interface |
Summary
Logical nodes are the language of IEC 61850. Every function in a digital substation is modeled as a logical node with standardized data objects.
The key points:
- The first letter tells you the group: P = protection, X = switchgear, M = measurement, C = control
- LLN0 holds all GOOSE control blocks, datasets, and report control blocks
- XCBR and CSWI are the most referenced LNs for breaker control
- PTOC and PDIS are the most common protection LNs
- MMXU is the standard measurement LN for voltage, current, and power
- GGIO is the catch-all for anything that does not fit a specific class
- Every LN has Mod, Beh, Health, and NamPlt as mandatory data objects
- Object references follow the pattern: LD/LNFCFCFCDO.DA
Once you know the logical node names, you can read any SCL file, any GOOSE dataset, and any MMS object reference. They are the vocabulary of IEC 61850.
IEC-61850 Related reads in the same family: