If you work with IEC 61850, you will deal with SCL files. They are the standard way to describe IED capabilities, substation configurations, and communication parameters.
But the first thing that confuses most engineers is this: there are six different SCL file types. ICD, SSD, SCD, IID, CID, SED — each with a different purpose, created by a different tool, at a different stage of the engineering process.
Mix them up and you will import the wrong file, overwrite a configuration, or spend hours wondering why an IED rejects your download.
This guide explains each SCL file type in plain language — what it contains, who creates it, when it is used, and how they all connect in the engineering workflow.
Table of Contents
1. What Is SCL
SCL stands for Substation Configuration Language. It is the XML-based file format defined in IEC 61850-6 for describing everything about a digital substation’s configuration.
SCL files contain:
- The substation structure (voltage levels, bays, equipment)
- IED capabilities and configurations (logical nodes, data objects, datasets)
- Communication parameters (IP addresses, GOOSE multicast addresses, report control blocks)
- Data type templates (logical node types, data object types, enumerations)
SCL makes it possible to exchange configuration data between tools from different vendors. Without it, every manufacturer would use a proprietary format, and multi-vendor interoperability would be impossible.
An IED is only considered IEC 61850 compliant if:
- It is accompanied by an ICD file describing its capabilities (or a tool that can generate one)
- It can consume an SCD file (or be accompanied by a tool that can import the SCD) to configure its communication parameters within the limits declared in its ICD file
2. The Six SCL File Types at a Glance
| File | Full Name | Contains | Created By | Edition |
|---|---|---|---|---|
| ICD | IED Capability Description | Full capability of one IED (generic, not project-specific) | IED manufacturer | Ed. 1+ |
| SSD | System Specification Description | Substation structure + required functions (no IEDs assigned) | System designer | Ed. 1+ |
| SCD | Substation Configuration Description | Complete substation — all IEDs, communication, GOOSE, reports | System configuration tool | Ed. 1+ |
| CID | Configured IED Description | Final config loaded into a physical IED | IED configuration tool | Ed. 1+ |
| IID | Instantiated IED Description | Project-specific config for one IED (derived from SCD) | System or IED configuration tool | Ed. 2+ |
| SED | System Exchange Description | Interfaces between two substations or projects | System configuration tool | Ed. 2+ |
3. ICD — IED Capability Description
What It Is
The ICD file describes the complete capability of a single IED. It lists everything the device can do — all logical devices, logical nodes, data objects, datasets, control blocks, and supported services.
It is generic. It is not tied to a specific project or substation. It says “this is what my device can do” — not “this is what my device is doing in your substation.”
Who Creates It
The IED manufacturer creates the ICD file and provides it with the device. Every IEC 61850 compliant IED must come with an ICD file.
What It Contains
- One IED section describing the device. Per the standard, the IED name in the ICD file must be set to “TEMPLATE” — it is not a real project name yet.
- DataTypeTemplates defining the data model
- Optionally, a Communication section with default parameters
- Optionally, a Substation section (with substation name also set to “TEMPLATE”) indicating what kind of process the IED is designed for. If logical nodes are bound to equipment in this template substation, it indicates predefined functionality — for example, an XCBR bound to a CBR means this function is only valid for circuit breaker control.
When It Is Used
- At the start of system engineering
- The system integrator collects ICD files from all IED vendors
- ICD files are imported into the System Configuration Tool (SCT) to build the substation configuration
Key Point
The ICD file is the starting point. It is never loaded directly into an IED. It is an input to the system engineering process.
4. SSD — System Specification Description
What It Is
The SSD file describes the substation structure and functional requirements — without specifying which IEDs will be used.
It is the functional specification: “here is the single line diagram, and here are the functions we need at each bay.”
Who Creates It
The system designer or planning engineer, using a system specification tool or SCL editor.
What It Contains
- The Substation section with voltage levels, bays, equipment, and connectivity nodes (the single line diagram in data form)
- LNode references indicating which logical node classes are needed at each bay (e.g., XCBR for circuit breaker control, PTOC for overcurrent protection). Per the standard, if a logical node is not yet assigned to an IED, the iedName attribute must be set to “None”.
- DataTypeTemplates for the referenced logical node types
- No IED section — devices are not assigned yet. If part of the system is already known, IED and Communication sections may optionally be included.
When It Is Used
- Early in the project — during system specification and functional design
- It is imported into the System Configuration Tool alongside ICD files
- The engineer then assigns IEDs to the functions specified in the SSD
Key Point
The SSD answers the question “what do we need?” The ICD answers “what can this device do?” The system integrator matches them.
5. SCD — Substation Configuration Description
What It Is
The SCD file is the master configuration file for the entire substation. It is the most important SCL file — everything is in it.
It combines the substation structure (from the SSD), all IED configurations (from the ICDs), and all communication parameters into a single file.
Who Creates It
The System Configuration Tool (SCT) generates the SCD. This is the central engineering tool used by the system integrator.
What It Contains
- The complete Substation section (single line diagram with LNode bindings to specific IEDs)
- IED sections for every device in the substation (with datasets, GOOSE control blocks, report control blocks)
- The Communication section (IP addresses, GOOSE MAC addresses, APPID, VLAN IDs for all IEDs)
- DataTypeTemplates for all data types used across all IEDs
When It Is Used
- It is the output of system engineering — the result of the complete configuration process
- It is used to generate IID and CID files for individual IEDs
- It is the reference document for the entire substation — all engineering, testing, and commissioning starts here
Key Point
The SCD is the single source of truth. If you need to know how any device in the substation is configured — its GOOSE subscriptions, its IP address, its datasets — it is in the SCD.
6. IID — Instantiated IED Description
Note: The IID file type was introduced in IEC 61850 Edition 2. It does not exist in Edition 1.
What It Is
The IID file describes the project-specific configuration of a single IED. It is used as a data exchange format in both directions between the IED configuration tool and the system configuration tool.
Going from SCT → IED tool: the IID carries system-level decisions (GOOSE subscriptions, report configurations) to the device-level tool.
Going from IED tool → SCT: the IID carries add-ons or modified values back to the system configurator, which can update the SCD.
Who Creates It
Either the System Configuration Tool (exporting from the SCD) or the IED Configuration Tool (after adding vendor-specific settings).
What It Contains
- One IED section with the instantiated configuration (datasets, control blocks, GOOSE subscriptions)
- The Communication section with that IED’s parameters
- DataTypeTemplates relevant to that IED
- Optionally, a Substation section showing the bay/equipment this IED is assigned to
When It Is Used
- The IID is passed from the system engineering phase to the IED engineering phase
- The IED configuration tool (vendor-specific) imports the IID to configure the physical device
- The IED engineer can add vendor-specific settings (protection parameters, I/O mapping) on top of the IID
Key Point
The IID bridges system engineering and device engineering. It carries the system-level decisions (GOOSE subscriptions, report configurations) to the device-level tool.
7. CID — Configured IED Description
What It Is
The CID file is the final configuration that gets loaded into the physical IED. Per the standard, the CID is essentially “an SCD file, possibly stripped down to what the concerned IED shall know.” It contains everything the device needs to operate in the substation.
If compression is applied to a CID file, the standard recommends using GZIP format (RFC 1952).
Who Creates It
The IED configuration tool (vendor-specific: ABB PCM600, Siemens DIGSI, Schneider SFT2841, etc.) generates the CID, typically from an imported IID or SCD.
What It Contains
- One IED section with the complete, finalized configuration
- The Communication section with the IED’s addresses and GOOSE parameters
- DataTypeTemplates for that IED’s data model
- All vendor-specific parameters merged with the system-level configuration
When It Is Used
- At the end of the engineering process — during commissioning
- The CID file is downloaded to the physical IED
- It is the file the IED uses to configure its MMS server, GOOSE publisher/subscriber, and report blocks
Key Point
The CID is the file that makes the IED work. ICD describes capability. CID describes the actual configuration. The IED reads the CID and configures itself accordingly.
8. SED — System Exchange Description
Note: The SED file type was introduced in IEC 61850 Edition 2. It does not exist in Edition 1.
What It Is
The SED file is used to exchange configuration data between two different substations or projects. It describes the interfaces one project exposes to another.
Who Creates It
The System Configuration Tool of one project exports an SED for another project to import.
What It Contains
- A subset of the SCD with interface definitions
- IED sections for devices that communicate with the other project
- Communication parameters for inter-project connections
- Engineering rights metadata — indicating which data belongs to which project
When It Is Used
- When two substations need to exchange GOOSE messages across an inter-substation link
- When a substation needs to communicate with an external control center project
- When different engineering teams are working on connected projects and need to share interface definitions
Key Point
The SED is the least commonly used SCL file type. You will encounter it mainly in large multi-substation projects or when separate engineering teams manage different parts of the system.
9. How the File Types Connect: The Engineering Workflow
Here is how the six file types flow through a typical project:
Manufacturer → ICD (one per IED type)
↓
System Designer → SSD (substation specification)
↓
System Configuration Tool (SCT)
Imports: ICD files + SSD
Engineer: assigns IEDs, configures GOOSE, reports, communication
Exports: SCD (complete substation)
↓
┌───────┴───────┐
↓ ↓
IID SED
(per IED) (for other projects)
↓
IED Configuration Tool
Imports: IID (or SCD)
Engineer: adds vendor-specific settings
Exports: CID
↓
Physical IED
(download CID)The flow is always: ICD → SCD → IID → CID → IED
10. Which File Type Do You Need Right Now?
| You Want To… | You Need |
|---|---|
| Know what an IED can do before purchasing | ICD from the manufacturer |
| Design the substation functional specification | SSD |
| See the complete configuration of the entire substation | SCD |
| Configure a specific IED for a project | IID (exported from SCD) |
| Download configuration to a physical IED | CID |
| Share interfaces with another project or substation | SED |
| Import a new IED into the system configuration tool | ICD |
| Verify what is actually running on an installed IED | CID (export from the IED) |
| Debug GOOSE subscription issues | SCD (check publisher and subscriber datasets) |
11. Common Mistakes with SCL File Types
| Mistake | What Happens | Fix |
|---|---|---|
| Loading an ICD directly into an IED | The IED may accept it but it will have generic (non-project-specific) settings. GOOSE subscriptions and IP addresses will be wrong. | Always use a CID generated from the project SCD. |
| Confusing ICD with CID | The IED appears to work but is not configured for your substation. | ICD = capability. CID = configured. They are not the same. |
| Editing the SCD manually without validation | One XML error can break the entire substation configuration. | Always validate against the IEC 61850-6 XML schema after any edit. |
| Using an old SCD after IED firmware updates | New logical nodes or data objects in the IED are not reflected in the SCD. | Re-import the updated ICD and regenerate the SCD. |
| Not incrementing confRev after changes | GOOSE subscribers reject messages from updated publishers. | Always increment the configuration revision number when changing datasets or control blocks. |
| Mixing Edition 1 and Edition 2 files | Schema validation errors, namespace conflicts. | Use consistent editions across all files. Convert if necessary. |
| No version control on SCL files | No way to track what changed, when, and by whom. | Use version control (Git or similar) for all SCL files. Store the SCD as the master reference. |
Summary
The six SCL file types form a clear chain:
- ICD — what the IED can do (from the manufacturer)
- SSD — what the substation needs (from the designer)
- SCD — how everything is configured (from system engineering)
- IID — what one IED needs to know (from the SCD)
- CID — what gets loaded into the IED (for commissioning)
- SED — what to share with another project (for multi-project integration)
The flow is always ICD → SCD → CID. Get this right and IEC 61850 engineering becomes manageable. Skip steps or use the wrong file type and you will spend days debugging problems that should not exist.