Side-by-side comparison
| Parameter | Distance Relay | Overcurrent Relay |
|---|---|---|
| Measuring Principle | Measures impedance Z = V/I (proportional to distance to fault) | Trips when current I exceeds a set pickup threshold |
| Selectivity Mechanism | Zone 1 (80–85% of line) instantaneous; Zone 2 (120%) with time delay | Inverse time-current curve — downstream relay trips before upstream |
| Effect of Source Impedance | Largely independent of source strength — fault impedance is fixed by location | Sensitive — fault current varies with source impedance and fault location |
| Effect of Load Current | Directional mho characteristic separates load from fault impedance | Load current can encroach on fault current — coordination is difficult on long lines |
| Relay Characteristic | Mho (circle), Impedance (rectangle), Quadrilateral (adjustable) | IDMT (Inverse Definite Minimum Time) — SI, VI, EI curves per IEC 60255 |
| Reach / Coverage | Zone 1: 80% of line; Zone 2: 120%; Zone 3: 200% backup | Single pickup current; reach depends on system impedance at time of fault |
| Typical Application Voltage | 33 kV and above — 132 kV, 220 kV, 400 kV transmission lines | 11 kV and below — distribution feeders, industrial feeders, transformer protection |
| Pilot Scheme Integration | Permissive overreach / blocking schemes (POTT, PUTT) for end-to-end coverage | No inherent pilot capability; requires separate directional element |
| Common Relay Types | GEC LFZR, ABB REL670, SEL-321 | Siemens 7SJ62, ABB REF542, IDMT electromechanical (CDG relay) |
| Cost | High — complex measurement, requires accurate CVT and CT | Low — simpler design; IDMT overcurrent relay is inexpensive |
Key differences
An overcurrent relay trips when I > I_pickup, but on a long 132 kV line, I_pickup must be set above maximum load current (say 500 A) while the minimum fault current at the remote end during weak-source conditions may be only 600 A — the 20% margin is razor-thin and fails under mutually coupled parallel lines or varying generation dispatch. A distance relay measures Z = V/I: for a bolted three-phase fault 80 km into a 100 km line with Z_line = 0.4 Ω/km, fault impedance = 0.4 × 80 = 32 Ω, well inside the Zone 1 reach of 0.85 × 40 = 34 Ω — trip in <30 ms. Zone 2 covers the remainder plus the next line section with 200–400 ms time delay. This multi-zone graded scheme is immune to load current and source infeed variation.
When to use Distance Relay
Use distance relays (ABB REL670, SEL-321 with Mho characteristic) for 33 kV and above transmission and sub-transmission lines where load current approaches fault current range, especially on long lines with varying generation dispatch.
When to use Overcurrent Relay
Use overcurrent relays (IDMT type, ABB REF542, CDG electromechanical) for 11 kV distribution feeders, radial systems, transformer protection, and industrial LV/MV switchgear where fault current is always well above load current and source impedance is stable.
Recommendation
Choose distance relaying for any transmission line above 33 kV — the load-immune impedance measurement and multi-zone selectivity make it the only reliable choice. Choose IDMT overcurrent for radial distribution networks where current grading is straightforward and distance relay cost is unjustified.
Exam tip: Examiners ask you to define the three protection zones of a distance relay — Zone 1 instantaneously covers 80–85% of the protected line; Zone 2 (200 ms delay) covers the rest plus 20% of the adjacent line; Zone 3 (1–1.5 s delay) provides remote backup.
Interview tip: Interviewers at PGCIL and relay manufacturers ask why overcurrent relays cannot protect long EHV lines — state the load-fault current overlap problem and give the 80% Zone 1 instantaneous coverage principle of Mho distance relays as the solution.