ANSI numbers, defined by IEEE C37.2, identify each function of a power system device with a number and sometimes letters: 50 is instantaneous overcurrent, 51 is time overcurrent, 52 is the circuit breaker and 87 is differential. With this code you can read the drawings, settings and records of any relay from any manufacturer.
What ANSI numbers are for
A protection drawing can have dozens of devices from different manufacturers. So that any engineer reads it the same way, the industry uses a short code: a number that says what each device does. That code comes from the standard IEEE C37.2, which assigns each function a number, and sometimes letters that make it more specific.
If you are looking for a specific number, the ANSI number table is at hand. So «50» is instantaneous overcurrent, «51» is time overcurrent, «52» is the circuit breaker, «87» is differential and «86» is the lockout relay. Whoever knows the code can read a single-line diagram, a settings list or an event record without knowing the relay brand.
The standard is called IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations. It is the one used on substation and industrial drawings, even though the name «ANSI» comes from its historical origin.
How a code is read
A code has three possible parts: the number, the suffixes and the context in which it appears.
- The number says the function. There are protection numbers, such as 21 (distance), 27 (undervoltage), 59 (overvoltage), 79 (reclosing) or 81 (frequency); switching numbers, such as 52 (circuit breaker) and 89 (disconnect switch); and control or auxiliary numbers, such as 86 (lockout) or 94 (tripping).
- The suffixes make the function or the protected equipment more specific. The most common are N (neutral or residual), G (ground or generator), T (transformer), B (bus), L (line), M (motor) and O/U (over or under). So 87T is the transformer differential, 87B the bus differential, 87L the line differential and 51N the ground overcurrent.
- Combinations join functions that work together, such as 50/51 or 27/59.
- Number 87. Differential function: compares currents entering and leaving the zone
- Suffix T. The protected zone is a transformer
- Result. Transformer differential, with ratio and vector group compensation
Some tricky suffixes and acronyms
50BF is breaker failure. «BF» comes from breaker failure. It is a good example that the suffix is not always a single letter. The auxiliary contacts of the circuit breaker are written 52a and 52b: the «a» is closed when the breaker is closed, and the «b» when it is open.
The letters X, Y and Z designate auxiliary relays, not protection functions. And there are widely used acronyms that are not numbers in the standard: SOTF (switch-onto-fault) is one of them, and the AFD for arc flash detection is a manufacturer acronym. They appear next to the numbers but do not replace any of them.
One relay, many functions
In the era of electromechanical relays, each number was a box. Today a numerical relay groups dozens of functions in one device, and each one is enabled, set and tested separately.
That is why on modern drawings a single relay symbol carries several numbers. A common practice is to list the enabled numbers inside the symbol, or to use 11 for the multifunction device and define the functions in a separate table. When reviewing a setting, the right question is not «which relay is it?» but «which functions does it have active and what zone does each one protect?».
Which function covers which fault in a generator is summarized in Generator protection: which function covers which fault; the protection of a capacitor bank, which uses several of them, is in Capacitor bank protection.
From the ANSI number to the IEC 61850 logical node
In a substation with IEC 61850, each protection function is modeled as a logical node with a four-letter name. The name of the function on the drawings (the ANSI number) and the name in the communication (the logical node) are two views of the same thing.
| ANSI number | Function | Usual logical node |
|---|---|---|
| 21 | Distance | PDIS |
| 25 | Synchronism check | RSYN |
| 27 | Undervoltage | PTUV |
| 50 | Instantaneous overcurrent | PIOC |
| 51 | Time overcurrent | PTOC |
| 52 | Circuit breaker | XCBR |
| 59 | Overvoltage | PTOV |
| 79 | Reclosing | RREC |
| 87 | Differential | PDIF |
The correspondence is stated only where it is well established. There are functions, such as the lockout relay 86, that have no associated logical node, and one should not be assumed. The protection nodes and their use are in the guide on IEC 61850 protection logical nodes.
Where it appears and why it matters
The ANSI number appears in four places that are worth connecting:
- Single-line diagrams and functional schematics, where they say what each relay protects.
- Settings lists and coordination studies, where each function has its value and its curve.
- Event and fault records, where operations are named by function («51N trip»).
- Commissioning tests, where each function is checked separately.
When these documents use the same code, it is easy to follow a fault from the drawing to the record. When they do not, or use it wrongly, interpretation errors appear that are paid for in the field. A function placed under the wrong number on the drawing usually means a function that is wrongly set or wrongly tested.
In protection and control drawings, each function appears with its ANSI number in the device list; see Secondary design: protection and control drawings.
What to keep in mind
- Function names have editions. Those of the 2008 edition remain the commonly used reference, and the standard has had a later revision; some names of numbers 7, 11, 16, 54 and 61 changed in 2008.
- A number is not enough to know how a function is set. The 51 of a feeder and the 51 of a motor share the number but not the setting criterion.
- The number table helps to read, not to set. Setting depends on the system, the equipment and the protection coordination study.
To understand what each function does, start with the fundamentals of electrical protection. The most common functions have their own guide: overcurrent 50/51, differential 87, distance 21, breaker failure 50BF, reclosing 79 and frequency 81.
Calculate it here
To look up a number, a suffix or an acronym from a drawing, use our ANSI/IEEE C37.2 number table.
We can help with your protection
At Tiza Group we prepare and review protection and control engineering: functional drawings, settings lists and tests by function. See our secondary engineering service and our remote relay configuration service.
Related guides
- Fundamentals of electrical protection
- IEC 61850 protection logical nodes
- Generator protection: which function covers which fault
- Secondary design: protection and control drawings
- Capacitor bank protection
References
- IEEE C37.2, device function numbers, acronyms and contact designations for electrical power systems.
- IEC 61850-7-4, compatible logical node classes and data classes.
Frequently asked questions
Does a relay have only one ANSI number?
No. A numerical relay brings together many functions, and each one has its number. On drawings it is shown with the list of numbers it has enabled or with 11, which designates a multifunction device. What is set and tested is each function, not the relay as a box.
What is the difference between the ANSI number and the IEC 61850 logical node?
The ANSI number names the function on drawings and documents. The logical node, such as PTOC or PDIF, is its model within IEC 61850 communication. Many functions have a well-established logical node, but some have none, and a correspondence should not be assumed.
Are all the acronyms I see on a drawing ANSI numbers?
No. There are manufacturer or industry acronyms, such as SOTF for switch-onto-fault or AFD for arc flash detection, that accompany the numbers but are not in the table. When in doubt, check the relay manual and the drawing legend.