Protection Relays and ANSI Codes: 50/51, 27/59 and 87 Functions
protection relay ANSI codes: ANSI codes are the common language of protection functions: 50 instantaneous overcurrent, 51 inverse time overcurrent, 50N/51N earth fault, 27/59 under and overvoltage, 87 differential. Relay selection is decided together with the CT class and the trip circuit.
Contents
In medium voltage panels and large low voltage switchboards, protection is no longer built from separate electromechanical relays but from numerical relays that gather many functions in a single case. Catalogues list those functions with ANSI/IEEE C37.2 numbers. Reading the numbers is the fastest way to see which fault types are actually covered, because a panel offering 51 but no 51N leaves earth faults unprotected.
What a protection relay does: measure, decide, trip
A numerical relay samples the signals from current and voltage transformers, filters the fundamental to compute the true value and compares it with the thresholds in the setting file. When a threshold is exceeded, the timing function starts and at the end of the delay the trip output energises the breaker trip coil. Every link of this chain must be verified separately: a measuring error, a wrong ratio, a broken trip circuit or a discharged spring mechanism makes protection look operational while being ineffective.
The most frequently used ANSI function codes
A typical set for a distribution feeder starts with 50/51 and 50N/51N. Motor feeders add 49 thermal model, 46 phase unbalance and 48/51LR starting supervision. Incoming panels use 27 undervoltage and 59 overvoltage, while generator paralleling requires 81 frequency, 32 power direction and 25 synchronism check. Transformer and busbar protection use 87T and 87B differential functions, and arc protection schemes add fast busbar protection combining a light and a current criterion.
Overcurrent curves: inverse time or definite time?
Definite time protection trips with a fixed delay once the threshold is exceeded and is simple to set. Inverse time protection operates faster as the current grows; the standard inverse, very inverse and extremely inverse curves of IEC 60255 approximate the thermal behaviour of loads and cables. When building selectivity, the downstream stage receives a faster curve, the upstream stage a slower one, and a grading margin covering breaker operating time and tolerances is left between them.
Matching relay, current transformer and trip circuit
| ANSI code | Function | Typical setting range | Application |
|---|---|---|---|
| 50 / 50N | Instantaneous overcurrent and earth | 2 – 10 × In, 0 – 100 ms | Transformer outgoing and busbar |
| 51 / 51N | Inverse time overcurrent and earth | 1.05 – 2 × In, IEC curves | Distribution feeder and motor |
| 27 / 59 | Under and overvoltage | 0.8 Un / 1.15 Un | Incoming panel and generator |
| 87T / 87B | Differential protection | 10 % – 30 % slope | Transformer and busbar differential |
Correct measurement depends on the CT ratio, accuracy class and burden capability; a protection class such as 5P or 10P with a sufficient accuracy limit factor must be chosen. On the tripping side the continuity of the auxiliary supply is critical, so the battery system and the trip circuit supervision function are planned together. The table below summarises the typical setting ranges and applications of the most used functions.
Setting file, testing and commissioning
During commissioning the setting file is managed as a single reference document with version number, date and approving engineer recorded. Secondary injection verifies thresholds and times, while primary injection or a CT ratio test checks the measuring chain. The trip circuit is proven with the actual breaker, and event records and waveform captures are read to confirm that the relay clock is synchronised with system time.
Key takeaways: protection relay ANSI codes
- Function list — 50/51 and 50N/51N for feeders, 49 and 46 for motors, 27/59 for incomers form the minimum set.
- Curve choice — Selectivity is built with inverse time curves and a grading margin between stages.
- Measuring chain — CT ratio, 5P/10P class and burden are verified together with the relay.
- Verification — Secondary injection, trip circuit testing and setting file version control are mandatory.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
What is the difference between functions 50 and 51?
Function 50 is instantaneous overcurrent and clears currents above its threshold with a very short delay, which suits close-up short circuits. Function 51 is inverse time, becomes faster as current grows and preserves selectivity for overloads and remote faults.
Is a separate current transformer needed for earth fault protection?
If small earth currents must be measured with high sensitivity, a core-balance ring CT is preferred. Otherwise the vector sum of the three phase currents is calculated inside the relay, but sensitivity is limited by ratio errors.
How often should numerical relay settings be verified?
A full test is performed at commissioning, after which secondary injection is usually repeated every two to five years according to the maintenance plan. Settings are recalculated whenever the network or the load structure changes.
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Let us review the function list, curve settings and CT compatibility of your panel protection together; the Devpan engineering team performs the coordination study with your project data. Related Devpan solution: medium voltage switchgear.
