Restricted earth-fault protection covers a defined zone between phase/neutral CTs and transformer winding terminals. Its effectiveness depends on where the neutral is grounded and where CTs are physically installed. The engineering drawing should show the protected zone, CT polarities and grounding connection explicitly; otherwise the term “REF” alone does not define what faults are included.
Draw the zone before naming the protection
For a typical grounded-star winding application, restricted earth-fault protection compares the residual of the phase CT currents with the neutral CT current using the scheme's defined polarity and scaling. The CT positions establish the protected zone. A neutral lead fault on one side of the neutral CT may be included while a fault on the other side is outside it. Show the actual conductors and earth connection so this boundary is visible rather than inferred from the label “REF.”
High- and low-impedance schemes require different design inputs
A high-impedance arrangement and a numerical low-impedance arrangement do not use identical CT requirements or setting calculations. Ratio matching, excitation data, secondary resistance and relay-specific stabilization must follow the selected design. Do not transfer a CT specification or pickup value between the two merely because both protect the same winding. The relay and CT suppliers should review one coordinated scheme drawing and the associated external-fault duty.
Grounding assumptions determine available fault current
A fault close to the neutral can have a different driving voltage and current from a line-terminal earth fault. Neutral impedance and winding arrangement therefore matter to sensitivity. Retain the manufacturer's relevant winding data and the actual grounding configuration in the protection study. REF complements other transformer protections within its defined zone; it should not be described as covering every internal fault or replacing differential, thermal or mechanical protection functions.
Technical reference framework
[1] IEEE C37.91-2021 — Protecting power transformers.
[2] IEC 60076-1:2011 — Power transformers, General.
[3] IEEE C57.105-2019 / Cor 1-2023 — Three-phase transformer connections.