Restricted Earth Fault: Proving the Transformer Neutral CT Zone

1. Start with the physical zone

Restricted earth fault protection earns its sensitivity by comparing currents at the boundaries of one grounded winding. It needs a correct neutral-current measurement and a correct residual from the phase CTs. Adding an REF element to a transformer relay without drawing those boundaries leaves uncertainty about which bushings, neutral leads and connecting conductors are actually protected. REF complements transformer differential protection; it does not replace phase-fault protection. The SEL application paper explains why ground faults near the neutral or limited by a resistor can escape the sensitivity of phase differential elements.

2. Collect inputs before choosing the scheme

Draw the phase CT locations, neutral CT, winding star point, grounding resistor and every neutral-to-earth connection. Record CT ratios, selected taps, polarity marks, secondary ratings, excitation data, lead resistance, relay input ratings and the minimum internal and maximum external fault cases. A neutral conductor routed around its CT creates an unmeasured path. An earth connection placed on the wrong side changes the zone rather than merely changing a setting. Confirm the applicability of grounded-neutral REF with the Schneider explanation. Obtain the actual relay algorithm: directional, biased differential and high-impedance implementations have different requirements.

3. Illustrative ratio reconciliation

Assume phase CTs of 1000/1 A and a neutral CT of 200/1 A. An external earth fault carries 100 A primary residual through the phase boundary and 100 A through the neutral return. The relay sees 0.100 A residual and 0.500 A neutral current. Their raw secondary magnitudes are unequal, although the primary currents balance.

On a common primary-current base both become 100 A. A relay supporting independent ratio compensation can reconcile them; a high-impedance scheme cannot simply inherit this wiring. Direction still depends on CT orientation and the relay convention. This illustration assumes ideal CTs and no parallel return. It establishes neither pickup nor percentage winding coverage.

4. Make the scheme choice explicit

For resistance-grounded windings, review external phase-to-phase-to-ground faults as well as single-phase earth faults. Large phase currents can saturate phase CTs while neutral current remains resistor-limited. The SEL paper discusses algorithm-specific security for this case. Do not infer external-fault stability from a balanced steady-state injection alone.

5. Prove the zone and the trip path

Commission against an approved isolation and test plan. Record which current direction represents an external fault, demonstrate restraint for that case, then reverse the appropriate simulated boundary condition to represent an internal fault. Repeat through the installed test blocks so terminal labels, ratio compensation and input assignment are included. Test loss of a current channel and the intended alarm or blocking response separately from an actual internal fault. CT secondaries must not be opened while primary current may flow; Schneider's shorting-block guidance identifies that hazard. Leave no ambiguous temporary test connection in the final drawing.

6. Specify evidence that survives handover

Request the signed zone drawing, CT certificates, relay model and firmware, configuration export, current-direction convention, test records and trip matrix. State whether neutral CT installation includes the full neutral connection or only the resistor branch. Require a justified sensitivity study and a stability assessment using project fault currents and CT performance. Compare the long-cable burden calculation with the actual neutral circuit. A test report should identify the operating bits and output contacts, not only say “REF passed.” Changing grounding topology later should reopen this review.

7. Questions that prevent misapplication

Can REF detect a phase-to-phase fault?

Its intended measurement is earth-fault current within the restricted grounded-winding zone. Retain the appropriate phase protection.

Does a smaller neutral CT always improve protection?

It can improve secondary signal level, but input range, fault withstand, saturation and ratio compatibility still govern the design.

Does an external-fault injection prove winding coverage?

No. It checks stability for the injected condition. Coverage needs minimum internal-fault analysis, the actual grounding model and the chosen relay characteristic.

8. Primary references

• SEL — REF
• Schneider Electric — 64REF
• Schneider Electric — CT