Logical Nodes
The atomic functional units of an IEC 61850 data model — standardised function blocks like XCBR (circuit breaker), PTOC (overcurrent protection), MMXU (measurement) that compose into a substation.
Also: LN, logical node, XCBR, PTOC, MMXU
A Logical Node (LN) is the smallest standardised functional unit in the IEC 61850 data model. Every function a substation device performs — protection, measurement, control, status reporting — is expressed as one or more Logical Nodes drawn from a vendor-neutral catalogue.
Where LNs sit in the hierarchy
The data model nests:
Server → Logical Device → Logical Node → Data Object → Data AttributeA physical IED hosts a Server, which contains one or more Logical Devices, each of which contains the Logical Nodes that describe its functions. An overcurrent protection IED for a single bay might contain PTOC1, PTOC2, XCBR1, MMXU1 — overcurrent function instances, a circuit-breaker representation, a measurement function — in a Logical Device named for the bay.
Naming conventions
LN names are four-letter mnemonics drawn from the IEC 61850-7-4 catalogue. The first letter indicates the functional group:
| Prefix | Group | Examples |
|---|---|---|
L | Logical node zero (administration) | LLN0, LPHD |
P | Protection | PTOC (over-current), PDIF (differential), PTRC (trip conditioning) |
R | Protection-related | RREC (auto-reclose), RDIR (directional element) |
C | Control | CSWI (switch controller), CILO (interlocking) |
X | Switchgear | XCBR (circuit breaker), XSWI (switch) |
T | Instrument transformer | TCTR (current), TVTR (voltage) |
M | Metering / measurement | MMXU (measurement unit), MMTR (metering) |
Common Data Classes (CDCs) — defined in IEC 61850-7-3 — standardise how each kind of value (single-point status, measured value, controllable double point) is structured inside an LN’s Data Objects.
Why LNs matter for engineering
Because every function decomposes into the same standardised LNs regardless of vendor, an SCL tool from one supplier can read and reason about an IED from another. The promise of multi-vendor IEC 61850 substations rests entirely on the shared LN catalogue holding up under real engineering pressure.