Overcurrent Protection Configuration in SEL Relays
Key Takeaway
How to configure phase and ground overcurrent protection (50/51) in SEL relays — CT ratio, pickup, time-current curves, trip logic, and coordination.
Quick Answer
Configure 50/51 overcurrent in Quickset by setting CT ratio, phase overcurrent pickup (above max load, below min fault), time-current curve (IEEE U1-U5 or IEC C1-C5), time dial, and instantaneous pickup. Define trip logic in SELOGIC: TRIP = 51PT + 50P.
CT Ratio
Enter the CT ratio (CTR) so the relay correctly interprets primary current from secondary measurements.
Phase Overcurrent (51P)
- Pickup — 1.5-2× maximum load current
- Curve — IEEE U1 (Moderately Inverse) through U5 (Short-Time Inverse), or IEC C1-C5
- Time Dial — Shifts curve up/down for coordination with adjacent devices
Instantaneous (50P)
Set pickup at 8-12× rated current for high-magnitude faults requiring immediate trip.
Ground Overcurrent (51N/51G)
Lower pickup than phase elements. 51N uses residual (calculated) neutral; 51G uses measured ground CT current.
Trip Logic
TRIP = 51PT + 51GT + 50P + 50NSee protection elements overview and SELOGIC for details.
Frequently Asked Questions
Set above maximum load current (1.5-2×) but below minimum fault current. Time dial and curve shape coordinate with adjacent devices.
IEEE/ANSI (U1-U5: Moderately Inverse through Short-Time Inverse) and IEC (C1-C5). Custom programmable curves also supported.
51N measures residual current (vector sum of three phase CTs). 51G measures current from a dedicated ground CT in a transformer or generator neutral.