Knowing the Language: The History and Practice of IEEE C37.2

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By John Tengdin, chairman, IEEE PES Substations C5 & Power System Relaying I14 Working Groups

Editor’s note: John Tengdin wrote us a letter in response to Steve Eckles’ “Detecting High Impedance Faults” article that ran in the October 2008 issue. (See the letter printed in our December 2008 issue.) We offered him the chance to write his own piece for this issue.

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At a recent substation automation conference, the audience was asked, “How many of you are familiar with a second language?” Few hands were raised. Then the second question, “How many of you know what a 52 is, or a 52a, or a 51 or a 79?” This time, lots of positive responses, as the listeners knew the topics were device numbers on elementary diagrams, but they had never thought of them as part of a notational language used to document—on an elementary diagram—the devices in a protection and control system. That language is the one defined in IEEE C37.2-2008 Standard for Electrical Power Device Function Numbers, Acronyms and Contact Designations.

How Old is IEEE C37.2?

It may surprise some to learn that this standard—or more accurately its AIEE predecessor—is one of the oldest IEEE standards in use. It was first published as AIEE No. 26 in 1928 and was used to document the control systems in the then popular automatic railway substations. Before that date, there was the proverbial Tower of Babel as each manufacturer had its own set of device numbers. The publication of AIEE No. 26 created the first documentation standard. A 1924 General Electric Co. brochure for such automatic substations included a dc elementary diagram with many function numbers that are in use today: 50 (instantaneous overcurrent relay), 52 (ac circuit breaker) and 52a (a normally open auxiliary contact on an ac circuit breaker). Of course, all of those relays were electromechanical; so, one might imagine that these control systems were simple. Not so. There was a device 7 rate of change relay and a device 82 dc load measuring reclosing relay. In combination, this pair of devices (with device 50) could distinguish between the fault resulting from a downed 600 V trolley wire—no reclosing due to the downed conductor hazard—and the slower rate of rise in trolley wire current from an overly aggressive motor man—that’s what they were called at the time—who had tried to accelerate his trolley car too rapidly (reclosing OK).

Is C37.2 Up-to-date?

During the past 80 years, as technology and protection needs changed, new device numbers were added. For example, in the 1960s, a number of unit-connected generator step-up transformers failed due to over-excitation when companies began energizing the generator field while on turning gear, and not disabling the generator voltage regulator. That’s when a volts-per-Hertz relay was developed and device 24 was added. With the advent of microprocessor-based relays, there were now many functions in one box; so “device 11—multifunction device” was added.

When work began on the update of C37.2-1996, a joint working group was formed with members from the IEEE Power Engineering Society’s Substations Committee and the Power System Relaying Committee, the IEEE Industry Applications Society’s Power System Protection Committee, and the Rural Electric Power Committee. The working group identified seven new functions that should be added; yet, only device 16—labeled “reserved for future application”—was available for use. Seven old function numbers existed, however, dating back to their use in automatic railway substations, which appeared to be candidates for retirement and reuse. These included: 10 (unit sequence switch), 22 (equalizer circuit breaker), 35 (slip ring short circuiting device) and 93 (field changing contactor). Because none of the working group members were familiar with the use of these device numbers, the joint working group employed IEEE Strategic Research and Planning Services to help it structure and conduct a simple online e-mail survey.

For each of the seven retirement candidates, the survey included the full description from C37.2 and asked for recommendations: definitely keep, perhaps keep, perhaps retire and reuse, definitely retire and reuse, or don’t know. The invitation e-mail was sent to the e-mail lists for the three regional relay conferences and several other lists, with great response. To our surprise, the survey showed that at least five of the seven device numbers are still in use. And, after further study, the working group determined that not seven but 16 new functions should be added.

Acronyms to the Rescue

That’s when the decision was made to use acronyms for the new functions.

  • AFD: Arc flash detector
  • CLK: Clock or timing source
  • DDR: Dynamic disturbance recorder
  • DFR: Digital fault recorder
  • ENV: Environmental data
  • HIZ: High impedance fault detector
  • HMI: Human machine interface
  • HST: Historian
  • LGC: Scheme logic (the function, as in a RAS; not a device like a PLC)
  • MET: Substation metering
  • PDC: Phasor data concentrator
  • PMU: Phasor measurement unit (the function)
  • PQM: Power quality monitor
  • RIO: Remote input/output device
  • RTU: Remote terminal unit/data concentrator
  • SER: Sequence of events recorder
  • TCM: Trip circuit monitor (the red light function remoted)

Serial or Ethernet Communication Devices in Substations

Previously unused device 16 is now defined as “communication networking device” with its own set of suffixes, applicable only to device 16. The first suffix letter is either “S” for a serial device or “E” for an Ethernet device. These subsequent suffix letters are used to more completely describe the device, with multiple suffix letters allowed:

  • C: Security processing function (VPN, encryption, etc.),
  • F: Firewall or message filter function,
  • M: Network managed function (e.g. configured via SNMP),
  • R: Router,
  • S: Switch (Examples: Port switch on a dial up connection is 16SS; an Ethernet switch is 16ES.),
  • T: Telephone component (example: auto answer modem).

Using that notation, Figure 1 is an example diagram of a dual-redundant, Ethernet-based relaying system. It shows all the physical connections and paths and illustrates that no single point of failure from a protective relaying perspective exists. Device 16 helps overcome a limitation in IEC 61850 Part 6 (substation configuration language). The whole concept of IEC 61850 is to deal with logical (not physical) devices and connections. In addition, Part 6 is not capable of showing switches, firewalls or network management devices. Because redundant paths are identical from a logical node perspective, redundancy is out of scope. But, as this diagram shows, those devices and redundant paths can be described using device 16 with its suffixes.

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Multifunction Devices

Since 1996, IEEE C37.2 has defined device 11 (multifunction device) as “A device that performs three or more comparatively important functions that could only be designated by combining several device function numbers. All of the functions performed by device 11 shall be defined in the drawing legend, device function definition list or relay-setting record.”

In that 1996 update, two methods of documenting the contents of a multifunction device 11 were added: the “empty box” and the “filled box” methods. Neither method, however, saw much use. The empty box method conveyed almost no information, and the filled box method was difficult to implement with current drafting software. During the balloting of the 2008 update, one balloter proposed the addition of a “list box” method he had been using. It is far superior to the 1996 methods, was accepted, and is now in the 2008 revision. The following explains an example in which an individual wants to describe the functions in a multifunction relay (device 11) whose zone of protection is line 1209. The line is connected to a breaker-and-a-half substation via bus breaker 108 and mid breaker 118. This device is the system A multifunction device for line 1209. For redundant protection, there is also a system B multifunction device 11 on line 1209, and it would have its own list box.

Figure 2 illustrates the major functions provided in this device for the protection, monitoring and control of line 1209 and its associated breakers. It also shows how remote SCADA and local control is implemented for specific device(s) (to maintain redundancy). Note that automatic reclosing (device 79) is implemented on a per-line basis and not on a per-breaker basis (otherwise it would have been listed as 79–108 or 79–118). The reclosing function is not redundant and, thus, does not have an A or B suffix.

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To maintain redundancy for the various close control functions, synch check (device No. 25) is redundant. (It has an A or B suffix). The binary I/O and communications connections—the arrows on the right side of the box—are optional, and may be implied to reduce the complexity of the diagram. “XXXX” is a placeholder for the manufacturer’s model number of this device 11. Mid-breaker 118 may also have a similar multifunction device 11 associated with its adjacent zone. It is important to know if common functions such as manual control, automatic reclosing and breaker failure for breaker 118 also reside in that device. This is readily apparent using the list box method.

The 2008 revision also includes a comprehensive cross-reference table (IEEE C37.2 function numbers to IEC 61850 logical nodes) plus a new dc elementary diagram showing coordinated trip and close fusing. (Details of all these additions, plus an excerpt from the IEC cross- reference table, may be found at

The Bottom Line

After all this, why should you care about these changes in IEEE C37.2? What’s in it for you? Well, for starters, virtually every new substation elementary diagram will be describing or using a microprocessor-based product. And, essentially, every one of those will be a multifunction device (device 11 in C37.2). In addition, you may be installing a substation communication system (serial or Ethernet) to link these new IEDs and need the new device 16 to describe the substation network. So, without having to invent something, the 2008 version of IEEE C37.2 provides the documentation tools needed to create or to understand these new applications. It’s time to start applying these extensions to an old familiar language.

Note: Please transmit any gaps or suggestions for improvement in IEEE C37.2-2008 to the C5 Working Group of the IEEE PES Substations Committee.

John T. Tengdin, P.E., has worked with Dayton Power and Light Co., General Electric Co., Honeywell Information Systems and the Tech Division of American Diversified Bank. Tengdin began work as an independent consultant in 1986 and formed OPUS Publishing as a two-man partnership specializing in substation automation and cyber security in 1999. His 2007 IEEE Fellow citation was “for leadership in Ethernet local area network based protective relaying and control in electric power substations.”

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