The real workflow for a numerical feeder relay (ABB REF615/620/630, Siemens SIPROTEC 7SJ, and equivalents): three phase overcurrent stages — I> inverse-time, I>> definite-time high-set, I>>> instantaneous — plus a genuine two-stage earth fault element (I0>/I0>>) set against the actual line-to-ground fault level for your system's grounding scheme, with every pickup carried in BOTH primary amps and relay per-unit (× In), because that is how a relay is actually set and documented, not in raw primary current alone. Curve math follows IEC 60255-151 and IEEE C37.112 — the standards both manufacturers' relays are built on; this does not reproduce either vendor's proprietary setting software.
Verified against published standard
CT & Relay
Feeder Currents (primary, A)
"Max through-fault" is the largest current the relay must NOT trip fast for — downstream fault contribution, motor starting, or transformer inrush. "Max fault current" is the highest fault level genuinely within this relay's own zone.
A relay set only against the MAXIMUM fault can still be blind to a legitimate but smaller one elsewhere in its zone — leave blank to skip this check, or enter the smallest fault current the relay must actually clear (often a far-end or minimum-plant-condition fault).
Stage 1 (I>) — IDMT
Stage 2 (I>>) — Definite Time
Stage 3 (I>>>) — Instantaneous
Earth Fault (I0>/I0>>)
Ground fault current is normally near-zero under healthy conditions, so I0> is set as a sensitive % of full load current, not with the same margin logic as the phase stages above.
Enter CT ratio, relay rating, and feeder currents.
About This Calculation
A real feeder protection relay is never set in isolation — it needs three coordinated phase stages (I> inverse-time, I>> definite-time high-set, I>>> instantaneous) plus a genuinely separate earth fault element (I0>/I0>>), every pickup expressed in both primary amps and the relay’s own per-unit setting, and a check that it can actually see the smallest fault it’s meant to clear, not just the largest. This calculator follows that real workflow rather than treating relay setting as a single time-current curve calculation, matching how ABB, Siemens, and equivalent numerical relays are actually configured and documented.
Frequently Asked Questions
Why does every pickup show up in both primary amps and ×In?
The relay itself is set in per-unit (multiples of its rated current In); primary amps are what goes on the setting sheet and coordination study. Confusing the two — a real, common commissioning error — is exactly what showing both values side by side is meant to prevent.
Why is the earth fault (I0>) pickup so much more sensitive than the phase (I>) pickup?
Because the two protect against fundamentally different baseline conditions. The phase I> stage must sit above normal load current, which is always present, so it needs real margin above full load. Earth (ground) fault current is normally near-zero under healthy balanced conditions, so I0> can and should be set as a much smaller percentage of full load current — typically 10-30% — to catch a genuine single-phase-to-ground fault quickly and sensitively, exactly how a real numerical relay’s earth fault element is configured.
Why does the earth fault current depend on the system grounding type?
Because the physical fault path is completely different depending on how the neutral is grounded. A solidly grounded system lets a large fault current flow, similar in magnitude to a three-phase fault. A resistance- or reactance-grounded system deliberately limits that current to whatever the grounding resistor or reactor lets through — which is why this calculator asks for that let-through current directly rather than assuming it. An ungrounded system has no intentional return path at all, so a first ground fault produces only a small capacitive current, which is why conventional I0> protection doesn’t apply and a different scheme (sensitive/directional earth fault, or insulation monitoring) is needed instead.
What is the sensitivity check, and why does it matter?
It checks whether Stage 1 (I>) can actually detect the SMALLEST fault current the relay needs to clear — often much lower than the maximum fault used to set the other stages. A relay set only against the maximum fault can be technically blind to a legitimate, smaller fault elsewhere in its zone; this check catches that before it becomes a real gap in protection.
Why does Stage 3 (instantaneous) have no time delay at all?
It’s set safely above the maximum fault current the relay could ever see for a fault genuinely within its own protected zone, so it only operates for a close-in fault — at that point, instantaneous clearance is both safe and desirable, since there’s no coordination concern with anything further downstream.
Does this replace the relay manufacturer’s own setting software?
No — this calculates the underlying primary-amp and per-unit values using standard IEC 60255 and IEEE C37.112 equations, the same standards both manufacturers build their relays on. It doesn’t reproduce either vendor’s specific setting software or menu structure, and final settings should always be checked against that specific relay’s own setting-range limits.