Side-by-side comparison
| Parameter | Static Relay | Numerical Relay |
|---|---|---|
| Technology Basis | Analog solid-state circuits — op-amps, comparators, RC networks | Microprocessor or DSP — samples V and I at 16–64 samples/cycle, processes digitally |
| Measurement Method | Continuous analog signal processing | Discrete sampling + DFT/digital filtering for phasor extraction |
| Accuracy | Moderate — component aging shifts thresholds | High — digital measurement; calibration stable over life |
| Self-Diagnosis | None — relay failure is silent until tested | Continuous self-test — hardware watchdog, ROM/RAM check, CT/PT monitoring |
| Communication | None (older types); some have simple contact outputs | IEC 61850, MODBUS, DNP3.0, IEC 60870-5-103 — integrates with SCADA |
| Fault Recording | Not available | Built-in disturbance recorder — 1–10 s pre/post fault waveforms at 16 samples/cycle |
| Multiple Protection Functions | One relay = one function (separate IDMT OC, Earth Fault, Differential relays) | One numerical relay = OC + EF + Differential + Distance + Autoreclosure + metering |
| Setting Changes | Potentiometer and DIP switch — physical access required | Software — password-protected, remote-configurable via laptop or SCADA |
| Examples | Siemens RXB21, GEC Alsthom MCGG relays | SEL-387, ABB REF542, Siemens 7SJ80, GE Multilin 750 |
| Cost (Approximate) | ₹15,000–₹50,000 | ₹1,50,000–₹5,00,000 |
Key differences
Static relays replaced electromechanical relays in the 1970s–80s by using transistors and op-amps instead of induction discs. They are faster (10–20 ms operating time) and more reliable mechanically, but their analog measurement drifts with component aging — a 2% threshold shift in an IDMT comparator can cause mis-coordination. Numerical relays digitize current and voltage waveforms at 16–64 samples per cycle, extract phasors using DFT, and implement all protection functions in firmware. Accuracy does not degrade over time. The decisive advantage is IEC 61850 communication: numerical relays participate in station automation, inter-relay GOOSE messaging for bus protection, and remote event logging — none of which static relays can support.
When to use Static Relay
Static relays (GEC MCGG, Siemens RXB series) are still viable for upgrading existing panels where budget is constrained, IEC 61850 integration is not required, and replacement of aged electromechanical relays is the only goal.
When to use Numerical Relay
Numerical relays (SEL-387, ABB REF670, Siemens 7SJ80) are mandatory for all new substation construction at 33 kV and above, any installation requiring SCADA integration, fault analysis, auto-reclosure, or multiple protection functions in a single device.
Recommendation
Choose numerical relays for all new protection schemes — the combined functionality (OC + EF + differential + metering + communication) in one numerical relay replaces 5–6 static relay panels, and the IEC 61850 GOOSE messaging enables protection schemes that static relays cannot physically implement. Static relays only make sense as temporary retrofits in legacy panels.
Exam tip: Examiners ask you to list three advantages of numerical over static relays — write self-diagnosis, IEC 61850 communication, and built-in disturbance recording — and state that multiple functions can be implemented in a single numerical relay.
Interview tip: Interviewers at relay manufacturers and utilities ask how numerical relays extract phasors — explain DFT over a sliding window of 16 samples/cycle, state that fundamental frequency phasor magnitude and angle are computed each sample, and note that this enables both protection and power quality metering from the same hardware.