IEC 61850 Logical Nodes Explained: Complete Reference for Substation Engineers

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.

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

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/LN$FC$FCDO.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.