Selecting HORM 4 Pilot-Wire Protection for a 5 km Line

After the change, the panel must show stable pilot-wire protection with no pickup on load, external faults, switching, or pilot-cable disturbances, while an internal-zone test trips both ends correctly. A 5 km route is not automatically too long for a circulating-current scheme; the measured pilot loop resistance and intercore capacitance decide whether the circuit is usable. Before ordering or connecting anything, resolve whether the intended relay is actually HORM 4 or MHOR04, then check the selected relay manual against the installed cable and the station ground-potential-rise study.

Read the panel symptoms first

Start here. Separate a protection-zone problem from a pilot-circuit problem before changing relay settings.

Observed symptom	Likely cause or first check
Relay picks up or trips during normal load	Open, crossed, unbalanced, or incorrectly terminated pilot conductors; wrong relay pairing; excessive pilot-circuit asymmetry
Operation during an external fault	Pilot saturation or imbalance, excessive induced voltage, ground-potential rise, CT mismatch or saturation, or unsuitable isolation equipment
Failure to operate for an internal-zone test	Excessive loop resistance, excessive shunt capacitance, open pilot, reversed connection, relay mismatch, or insufficient differential operating quantity
Unstable indication after cable switching or maintenance	Wrong core identification, armour or screen bonding error, insulation damage, or an unintended earth on the pilot pair
No relay response at either end	Open circuit, incorrect test setup, disconnected relay, or a pilot pair not assigned to the same protection circuit at both terminals

A nuisance operation is not proof that 5 km is excessive. Measure the complete installed circuit. Replacing the distance scheme, changing pickup values, or adding reactors before those measurements wastes time.

Confirm the relay designation

The project information calls the device HORM 4, while a correction identifies it as MHOR04. That is not a cosmetic difference. The nameplate, instruction manual, terminal diagram, auxiliary equipment, and paired end relays must all refer to the same exact type.

1. Photograph the complete nameplate at each line terminal.
2. Record the exact type designation, rating data, serial information, and terminal references.
3. Obtain the matching instruction manual from the manufacturer’s official support channel.
4. Compare the manual’s pilot resistance, capacitance, insulation, CT, and auxiliary-equipment requirements with the installed design.
5. Stop if the two terminals use different relay variants or if the manual cannot be tied to the nameplate.

The cited pilot limits were associated with a catalogue description, but the disputed type designation means they cannot be transferred blindly to an unidentified relay. Use them as acceptance questions until the correct manual confirms them.

Understand what the copper pilots do

A circulating-current pilot-wire scheme compares quantities derived from the CTs at the two line ends. For load and faults outside the protected zone, the terminal quantities should balance and circulate through the pilot circuit without producing a trip quantity. An internal fault creates differential current and drives the operating elements.

The pilots are part of the measuring circuit. Their series resistance reduces the available circulating current, while intercore capacitance draws charging current and changes the circuit response. Unequal conductor resistance, leakage to earth, incorrect polarity, and coupling from nearby power circuits can create a false differential quantity.

Route length matters only through those electrical characteristics and the exposure created by the route. A previous application used this relay class on a combined 33 kV overhead-line and cable route totaling 20 km, so distance alone does not reject a 5 km application. That installation does not qualify this one: conductor size, cable construction, joints, route coupling, earthing, CTs, and relay variant still control the result.

Check the pilot cable limits

The installed cable is described as five-core, 1.1 kV grade, 2.5 mm2, and armoured. Record which two cores form the protection pair and how every unused core, armour, screen, and earth connection is treated. Cable nameplate voltage alone does not prove suitability for a pilot-wire protection circuit.

The catalogue values reported for the intended relay family are:

• At least 500 V withstand between pilot cores.
• A pilot loop resistance, excluding the relay, stated as 1000 ohms.
• Total intercore capacitance below 1.0 microfarad.
• A pilot circuit capable of a 5 kV rms pressure test between the two cores connected together and earth.

The wording gives 1000 ohms without explicitly saying whether it is a maximum, nominal design value, or setting-related limit. Read that definition in the exact relay manual before accepting the circuit. Do not interpret the cable’s 1.1 kV grade as proof that the installed circuit passes the separate common-mode test to earth.

Measure loop resistance end to end with the relays isolated. Measure intercore capacitance over the complete connected route, including joints and termination wiring. Perform insulation testing only at a voltage permitted for every connected cable, terminal block, isolating device, and relay accessory; disconnect electronic or sensitive equipment before applying a pressure test.

Calculate the ground-potential-rise exposure

The stated 25 kA fault level is not automatically the current flowing into either station earth grid. Determine the earth-fault current division and grid impedance for each terminal. Calculate ground-potential rise from:

GPR = I_g × Z_g

Here, I_g is the portion of fault current entering the local earth grid and Z_g is the effective grid impedance for the study condition. Obtain both from the station earthing and fault studies. Do not multiply 25 kA by a guessed earth resistance.

During an earth fault, the two substations can rise to different potentials. A metallic pilot pair joining them can carry hazardous longitudinal voltage and transfer earth potential into relay panels. Intercore insulation addresses voltage from one pilot conductor to the other; insulation to earth and common-mode protection address the terminal-to-terminal ground-potential difference. These are separate duties.

Insulating transformers are identified as required for this application class. Neutralizing or drainage reactors may also be required, depending on the calculated longitudinal voltage, cable exposure, and the selected relay arrangement. Select those devices from the relay manufacturer’s application data and the GPR study, not from the power-line nominal voltage.

Commission the complete scheme

1. Freeze the design. Confirm the exact relay type at both ends, the protected-zone boundaries, CT ratios and polarities, pilot terminals, trip circuits, and required isolating equipment.
2. Inspect the route. Check every joint, termination, armour bond, screen connection, cable entry, and core marker. Look for shared cores or earths that could unbalance the pair.
3. Prove continuity and identity. Isolate both ends, identify each conductor, test continuity, and verify that the selected pair lands on matching functions.
4. Measure loop resistance. Include the full field route and terminal wiring, but exclude the relay where the manual defines the limit that way.
5. Measure capacitance. Record intercore capacitance for the complete pilot circuit and compare it with the manual; the reported catalogue criterion is less than 1.0 microfarad.
6. Test insulation. Measure core-to-core and cores-tied-together-to-earth insulation using the manufacturer-approved method. Coordinate any high-voltage test with the rating of all connected components.
7. Verify CT circuits. Check ratio, polarity, phase association, earthing, burden, and secondary continuity at both ends.
8. Prove stability. Apply an external-fault or through-current test with the complete pilot circuit in service. Confirm no trip and compare operating quantities at both terminals.
9. Prove sensitivity. Apply internal-zone differential tests for each phase and verify correct pickup, indication, trip output, and operation at both ends.
10. Test failure states. Simulate only the pilot open-circuit, short-circuit, cross-connection, or earth conditions specifically covered by the approved commissioning procedure. Confirm alarms, blocking, and trip behavior against the relay manual.

Verify before removing distance protection

Record measured values, not pass/fail marks alone. The commissioning file needs pilot loop resistance, intercore capacitance, insulation readings, CT test results, test-current phasors, relay operating quantities, trip contacts, breaker indications, and end-to-end results.

• Confirm stable operation under normal load with the pilot circuit connected.
• Confirm stability for simulated faults outside the protected zone.
• Confirm operation for simulated internal faults on every phase.
• Verify that opening either pilot conductor produces the documented response.
• Check that armour, screens, and unused cores follow the approved earthing drawing.
• Verify trip isolation and breaker operation separately at both terminals.
• Compare every acceptance value with the manual for the exact nameplate type.

Keep the existing distance protection available until the pilot-wire scheme passes the complete end-to-end test and the protection-coordination review approves its role. A successful local relay injection does not prove the 5 km channel.

Avoid recurring pilot-wire errors

• Selecting the relay after installing the cable: cable voltage grade, conductor size, capacitance, and earth-potential duty must match the relay system and its accessories.
• Using route length as the acceptance test: accept or reject the route from measured loop resistance, capacitance, insulation, induction exposure, and GPR.
• Equating 1.1 kV cable grade with protection suitability: the circuit also faces intercore and common-mode stress.
• Applying the full 25 kA directly to the earth grid: calculate the grid-current component and current division first.
• Ignoring CT performance: saturation or unequal secondary behavior during external faults can create spill current even when the pilots are healthy.
• Testing only each terminal locally: the complete scheme includes both relays, both CT circuits, the copper pair, isolation equipment, wiring, and trip paths.
• Assuming fibre and copper channels are interchangeable: a fibre-based line differential scheme removes the metallic pilot’s resistance, capacitance, induction, and transferred-potential path, but it requires compatible relays and a communications design.

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