Process bus

Process bus

Process bus involves positioning a merging unit close to the instrument transformer. The merging unit records the measured values of the transformers, digitizes them and sends them to one or more protection devices via a sampled measured values (SMV) data stream over fiber optical Ethernet. This digitalization of all analog and digital values close to the process supports new, flexible and beneficial solutions for substation automation systems.

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Conventional solution

The conventional instrument transformers are directly connected to the protection devices with parallel copper lines. This solution is proven, but requires a lot of wiring effort, has physical limits (accuracy, saturation) and bears the risk of open CT circuits.


Process bus solution 

This innovative solution involves positioning a Merging Unit close to the instrument transformer. The merging unit records the measured values of the instrument transformers, digitalizes them and sends them to one or more protection devices via a sampled measured values (SMV) data stream over fiber optical Ethernet.


The protection relays no longer work with analog values, but directly use the digital values from the sampled measured values data stream.


The merging unit is the interoperable interface between primary and secondary equipment for process bus solutions conforming to IEC 61869 and IEC 61850-9-2. Measured values of conventional and non-conventional transformers are converted into standardized, Ethernet-based telegrams (SMV).

Applications with SIPROTEC process bus solution

  • Collection of analog and digital feeder values close to the process
  • One central merging unit or distributed merging units for the process data acquisition of a transformer protection schemes
  • Central protection solution
  • Distributed busbar protection
  • Fault recorder for process bus
  • Centralized snchrocheck

Merging unit functionality is available for modular SIPROTEC 5 protection devices


Merging unit functionality

SIPROTEC 6MU85 merging unit server functionality is available in modular SIPROTEC 5 devices with Ethernet communication module ETH-BD-2FO


  • One sampled measured value (SMV) stream per ETH-BD-2FO Ethernet module
    - up to 32 analog values in any combination of CTs and VTs or
    - 4x CTs, 4x VTs (IEC 61850-9-2 LE)
  • Up to 4 ETH-BD-2FO modules possible
  • Reliable and redundant data transmission via PRP protocol
  • IEC 61869-9, IEC 61869-13 compliant
  • IEC 61850-8-1 GOOSE, MMS and merging unit protocol on the same Ethernet module
  • Sample and date/time synchronization via IEEE 1588v2/PTP

Process bus client functionality

Process bus client functionality is available in modular SIPROTEC 5 devices with Ethernet communication module ETH-BD-2FO


  • Support of IEC 61850-9-2 and IEC 61850-9-2LE streams
  • Support of IEC 61869 flexible streams
  • IEC 61850-8-1 GOOSE, MMS and process bus client protocol on the same module
  • Acceptance of SV sampling frequencies according to IEC 61869-9
  • Test- and Simulation Bit support
  • Up to 4x ETH-BD-2FO modules supported for physical segregation of process bus networks 
  • Sample synchronization for mixed configurations of direct connected instrument transformers and sampled measured values via IEEE 1588v2/PTP
  • Interoperability with multivendor merging units (IEC 61850 Edition 2.1 compliance recommended)
  • Mixed configurations of CIT and SMV

Benefits of process bus

  • Cost saving - Copper cable reduction, faster installation and commissioning
  • Independency - Interoperable design enables multi-vendor solutions based on IEC 61850 standard
  • Flexibility and scalability - Easier adaptation to future requirements and integration of wider range of data sources (independent signal routing)

The use of non-conventional transformers (LPIT) makes the process bus solution even more efficient.

  • Cost saving - Reduced space and 90 % reduced weight of transducers
  • Cost saving - One LPIT type for protection and measurement in all feeders because of the wide dynamic range
  • Operational safety - Danger of open CT circuits obsolete or reduce 

Benefits of Siemens process bus solution

  • Adoptable to multiple CT, VT, LPIT inputs
  • Scalable number of binary inputs and outputs
  • Expendable by a 2nd row
  • Direct “high speed” tripping of circuit breaker < 1 ms
  • Collection of additional data (temperature, pressure, tap changer positions, …)
  • Cyber security in accordance with NERC CIP and BDEW Whitepaper requirements
  • Highest availability even under extreme environmental conditions  by “conformal coating“ of electronic boards
  • Merging Unit functionality is available for all modular SIPROTEC 5 protection relays
  • Merging Unit SIPROTEC 6MU85 supports backup protection functionality and redundancy concepts

SIPROTEC 5 system for process bus

SIPROTEC 5 devices are applicable for IEC 61850-9-2 server and client solutions. This allows versatile solutions according to your requirements.


SIPROTEC 5 is an incomparably modular, flexible generation of intelligent digital field devices. SIPROTEC 5 offers a comprehensive product family with modular elements for every application and requirement that can be precisely configured to your application.


SIPROTEC 5 delivers maximum protection and flexibility throughout the entire lifecycle. Take advantage of the leading provider in the field of protection, a global partner that supports you every step of the way.


Select the right device for your process bus solution.

Merging unit – SIPROTEC 6MU85

The new SIPROTEC 6MU85 merging unit based on the flexible SIPROTEC 5 system has been designed for conventional and non-conventional instrument transformers (LPITs) and digitalizes all primary data close to the process. SIPROTEC 5 process bus provides versatile solutions and migration concepts for new and existing systems.

Rugged communications equipment for harsh environments

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