One of the most frequent questions engineers ask before inspecting MV/HV Protection Relay Setting Calculator is: "Which standard should I cite on the certificate, and where do the pass/fail limits come from?" This reference guide maps IEC 60255 / IEEE C37 to national regulatory adoptions and provides the complete standard annex tables.
1. National & Regional Standard Equivalence Matrix
| Region / Language | Adopted National Standard & Regulatory Code |
|---|---|
| International / UK (EN) | IEC 60255 / IEEE C37 |
| Turkey (TR · Türkçe) | TS EN 60255-151 / IEEE C37.112 / TEİAŞ |
| Germany / DACH (DE · Deutsch) | DIN EN 60255-151 / VDE-AR-N 4110 |
| France (FR · Français) | NF EN 60255-151 / NF C 13-100 |
| Spain (ES · Español) | UNE-EN 60255-151 / IEEE C37.112 |
| Italy (IT · Italiano) | CEI EN 60255-151 / CEI 0-16 |
| Gulf / Middle East (AR · العربية) | IEC 60255 / IEEE C37 / SASO / DEWA / ADDC |
2. Normative Acceptance Criteria & Mathematical Basis
IEC 60255-151 & IEEE C37.112 characteristic: t = ((k / (M^a - 1)) + c) x TMS with M = If / Is. Grading against the downstream device uses the 0.30 s discrimination margin. Requires explicit TMS input (never assumed).
t_IEC: t = TMS x [ k / ((If / Is)^alpha - 1) ] — IEC 60255-151 inverse-time operating equation for fault multiple M = If / Is > 1.0.
t_IEEE: t = (TD / 7) x [ A / (M^p - 1) + B ] — IEEE C37.112 operating equation where TD/7 maps to TMS = 0..1.0.
Delta_t: Delta_t = t_upstream - t_downstream >= 0.25 s (0.30 s target) — Coordination Time Interval (CTI) accounting for CB opening time (50-80 ms), relay overshoot, and CT errors.
3. Official Standard Reference Tables
IEC 60255-151 & IEEE C37.112 Overcurrent Relay Curve Constants (IEC 60255-151 Table 1 / IEEE C37.112 Table 1)
| Curve Designation | Standard | Constant k (or A) | Exponent alpha (or p) | Constant B | Typical Engineering Application |
|---|---|---|---|---|---|
| IEC Standard Inverse (SI) | IEC 60255-151 | 0.14 | 0.02 | 0 | General radial distribution feeders & industrial incomers |
| IEC Very Inverse (VI) | IEC 60255-151 | 13.5 | 1.00 | 0 | Long lines where fault current drops rapidly with distance |
| IEC Extremely Inverse (EI) | IEC 60255-151 | 80.0 | 2.00 | 0 | Grading with HRC fuses, transformer thermal withstand & motor reacceleration |
| IEC Long Time Inverse (LTI) | IEC 60255-151 | 120.0 | 1.00 | 0 | Standby earth fault (SBEF) & neutral displacement protection |
| IEEE Moderately Inverse (MI) | IEEE C37.112 | 0.0515 | 0.02 | 0.1140 | North American industrial feeders & tie breakers |
| IEEE Very Inverse (VI) | IEEE C37.112 | 19.61 | 2.00 | 0.4910 | Substation feeders with variable source impedance |
| IEEE Extremely Inverse (EI) | IEEE C37.112 | 28.20 | 2.00 | 0.1217 | Distribution transformers & fuse-saving schemes |