ANSI 50/51 Siemens — Overcurrent Protection 50/51 Trip
At a glance
- Severity
- FAULT STOP
- Domain
- CABINA_ELETTRICA
- Reference standard
- CEI 0-16, IEC 60255-151, IEC 61869-2
- Product / context
- 7SJ82 — Overcurrent time protection
What it means
The overcurrent protection has detected a current above the set threshold. Function 50 (instantaneous) trips for bolted faults with very high currents (typically 6-10 times rated current). Function 51 (inverse time) trips for overloads with times proportional to the overload magnitude, following standardized curves (Normal Inverse, Very Inverse, Extremely Inverse). The trip causes MV circuit breaker opening and facility blackout.
Probable causes
- Short circuit in MV cable or transformer (function 50)
- Progressive overload from simultaneous large motor starting (function 51)
- Insulation failure in MV switchgear (ground fault evolving into phase-to-phase short)
- Protection settings error after maintenance intervention
- Current transformers (CTs) with wrong ratio or saturated
Reference values
| funzione 50 istantanea | Instantaneous trip for current > threshold (typical: 6-10x In), time < 50ms |
|---|---|
| funzione 51 tempo inverso | Inverse time trip per IEC curve |
| curva very inverse | t = (13.5 x TMS) / ((I/Is)^1 - 1) |
| curva extremely inverse | t = (80 x TMS) / ((I/Is)^2 - 1) |
| curva normal inverse | t = (0.14 x TMS) / ((I/Is)^0.02 - 1) |
| coordinamento cei 0 16 | Selectivity with ENEL general protection: minimum trip time 120ms downstream |
| TA rapporto tipico | 200/5A or 400/5A class 5P20 |
Where to start
Work per CEI 11-27 (qualified person only). Before any intervention apply the 5 golden rules: disconnect, secure against reclosing, verify absence of voltage, earth and short-circuit, delimit the work area. Read from the protection relay which function operated: 50 (instantaneous) or 51 (inverse time)? Record the fault current and trip time.
Full guided diagnosis in the app
The diagnostic tree for this fault has 7 guided steps with measurements, checks and resolutions. You'll find it in ElectraIQ, one of the five IndexOS apps: free, works offline.
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