Selecting 33 kV and 34.5 kV Cable Insulation Ratings

A 33 kV or 34.5 kV system appears to fit beneath a cable marked 18/30 (36) kV, yet the phase-to-earth insulation can already be beyond its rated value before an earth fault occurs. The number that matters is the voltage applied across the insulation, including the temporary phase-to-earth rise produced by the system earthing method and fault-clearing time.

Voltage Stress as a Quantity

For a balanced three-phase system, the normal phase-to-earth voltage is the line-to-line voltage divided by √3:

Vphase-earth = Vline-line / √3

System value	Calculated phase-to-earth voltage	Selection consequence
33 kV line-to-line	33 / √3 = 19.05 kV	Already greater than the 18 kV phase-to-earth rating represented by U0 in an 18/30 (36) kV designation.
34.5 kV line-to-line	34.5 / √3 = 19.92 kV	Also greater than 18 kV, with a larger mismatch.
18 kV phase-to-earth rating	18 × √3 = 31.17 kV line-to-line	Shows why reading only the parenthetical 36 kV value gives the wrong result.

This is electric-field and thermal-aging duty, not a naming problem. A cable can have a highest-equipment-voltage marking of 36 kV while its U0 rating remains below the normal phase-to-earth voltage of the installation. The parenthetical value cannot replace the first two values in the rating.

Observed issue	Probable cause	Deciding measurement or record
18/30 (36) kV appears acceptable by its final number	Um has been treated as the insulation's continuous phase-to-earth rating	Read the complete U0/U (Um) marking and the cable datasheet
Healthy phases rise during a single-line-to-earth fault	An IT or resonant-grounded system does not hold the neutral at earth potential	Read the grounding study and earth-fault protection philosophy
Cable class changes when fault duration changes	Insulation level depends on how long elevated phase-to-earth voltage remains	Read protection settings, breaker clearing time, and backup clearing sequence

Rated-Voltage Notation and Standards Boundary

In the notation U0/U (Um), U0 is the rated voltage between a conductor and earth or metallic screen, U is the rated voltage between conductors, and Um is the highest system voltage associated with the equipment class. Selection must satisfy all three roles; the largest printed number is not a universal withstand value.

Um is the highest voltage occurring under normal operating conditions at any point on the system. It is not the sinusoidal peak obtained by multiplying an RMS value by √2. For example, calculating 30 × √2 = 42.43 kV does not establish the required Um class. Switching transients and earth-fault temporary overvoltages require their own insulation-coordination checks.

The cited material gives a maximum U0 of 18 kV. The cited scope for IEC 60840 covers extruded-insulation power cables and accessories above 30 kV, with Um = 36 kV, through 150 kV, with Um = 170 kV. Because the 33 kV and 34.5 kV cases cross the phase-to-earth capability associated with U0 = 18 kV, select the governing standard by the complete voltage designation and applicable edition, not nominal system voltage alone.

IEC 60038 is the cited reference for standardized system and equipment voltages. Its cited Series I table includes a 35 kV nominal system value and 40.5 kV highest voltage for equipment. That pairing is a warning against assigning a 36 kV equipment class merely because the nominal voltage is below 36 kV; read the utility's declared highest system voltage.

Insulation Approaches Compared

Approach	Phase-to-earth basis	Earth-fault application	Decision
18/30 (36) kV	U0 = 18 kV	Normal calculated phase-to-earth voltage is already 19.05 kV at 33 kV and 19.92 kV at 34.5 kV	Reject for these stated operating voltages unless the actual declared voltages and a manufacturer-approved rating demonstrate a different application basis.
26/45 kV	U0 = 26 kV	Provides a higher phase-to-earth insulation rating for systems where earth-fault voltage rise must be carried	Preferred technical direction from the stated choices, subject to the applicable standard, fault duration, accessories, and manufacturer data.
30/30 kV proposal	The designation presents equal conductor-to-earth and conductor-to-conductor values	Cited for an IT system when a ground fault cannot be removed automatically within one minute	Treat as a special rating proposal. Obtain the complete U0/U (Um) designation, construction standard, and manufacturer application approval before specifying it.
173% insulation level under ICEA S-93-639/NEMA WC 74	Higher insulation category for the cited 35 kV cable class	Recommended in the cited note when de-energizing a grounded section takes an indefinite time, for resonant-grounded systems, or when strength above the 133% level is wanted	Use only through the ICEA/NEMA selection route and verify the table, conductor size, insulation thickness, test voltage, and current edition.

The 26/45 kV option is the clearer recommendation when choosing only between it and 18/30 (36) kV. It raises U0 above the calculated normal phase-to-earth voltage and avoids selecting on Um alone. It still requires an earth-fault-duration check; a voltage designation by itself does not define the complete insulation coordination.

Earth-Fault Duration and Earthing Regime

On an IT system, the source neutral is isolated from earth or connected through a high impedance. During a single phase-to-earth fault, the faulted phase approaches earth potential while the two healthy phases can rise toward line-to-line voltage relative to earth. The cable screens remain near earth potential, so the healthy-phase insulation carries the increased electric field.

The number that matters is the maximum healthy-conductor-to-screen voltage multiplied by its duration and recurrence. Longer exposure increases dielectric heating, partial-discharge stress at defects, and aging at joints and terminations. Repeated uncleared faults can be more damaging than a single rapidly cleared event even when neither produces an immediate breakdown.

The cited decision threshold is one minute: if automatic protection cannot remove the grounded section within that time, the proposed IT-system solution changes to 30/30 kV. Before adopting that designation, reconcile it with the cable manufacturer's rating syntax and the governing standard. If automatic clearing occurs within one minute, the threshold alone does not approve 18/30 (36) kV; its 18 kV phase-to-earth rating remains below the calculated normal values for both stated systems.

Selection Procedure

1. Record actual system voltages. Obtain nominal voltage, maximum normal line-to-line operating voltage, and the declared highest system voltage from the network or utility specification. Use measured RMS values only as confirmation of the operating condition.
2. Calculate normal phase-to-earth stress. For the stated balanced three-phase cases, calculate VLL/√3. Compare the result directly with cable U0.
3. Classify the earthing regime. Confirm whether the network is IT, resonant grounded, resistance grounded, solidly grounded, or another arrangement. Use the grounding study rather than inferring the regime from transformer connection alone.
4. Determine earth-fault voltage and duration. Read the earth-fault study, relay functions, breaker operating sequence, backup protection, and operating policy for continued service with the first fault. Record the worst credible clearing time, including protection or breaker failure.
5. Select the standards route. Check the current project-adopted editions of IEC 60038, IEC 60502-2, and IEC 60840. If the project uses the North American insulation-level method, verify the applicable tables in ICEA S-93-639/NEMA WC 74.
6. Compare complete cable ratings. Evaluate U0, U, Um, insulation construction, test voltages, and permitted earth-fault duration. For the two principal choices, advance 26/45 kV and remove 18/30 (36) kV from consideration for the stated normal voltages.
7. Obtain manufacturer confirmation. Submit the actual maximum voltage, earthing regime, fault voltage, clearing time, and duty cycle. Request written confirmation covering the cable and its accessories as one insulation system.

Accessory and Protection Coordination

A higher-rated cable does not correct an underrated termination, joint, separable connector, surge arrester, screen break, or test adapter. Match every accessory to the selected cable's insulation diameter, conductor size, screen construction, voltage designation, and governing standard. The accessory interface is commonly the local electric-field limit.

Coordinate the cable choice with earth-fault detection. An IT system may allow continued operation after a first fault, but that operating policy creates the prolonged healthy-phase voltage that drives the insulation decision. Compare the protection pickup and delay settings with the actual breaker clearing time and the cited one-minute boundary. Include backup protection and the time needed to isolate a grounded section when automatic removal fails.

Use the system insulation-coordination study to address switching impulses and temporary overvoltages separately from Um. Read required withstand values from the governing standard and manufacturer tables; the cable's RMS voltage designation does not replace those checks.

Commissioning and Acceptance Verification

Item	Acceptance check	Where to read it
Normal voltage	Maximum phase-to-earth RMS voltage is within selected U0	Power-quality measurement and network voltage schedule
Equipment class	Cable and accessories match the declared Um	Single-line diagram, procurement datasheets, and nameplates
Earth-fault duty	Healthy-phase voltage and exposure time are within manufacturer-approved duty	Grounding study and protection coordination report
Clearing performance	Primary and backup sequences meet the design time	Secondary-injection results, breaker timing records, and event records
Construction	Installed cable, joints, and terminations match approved schedules	Drum records, accessory kits, installation records, and as-built drawings
Acceptance tests	Test method and voltage match the governing cable and accessory standard	Approved commissioning procedure and manufacturer instructions

Retain the calculation showing VLL/√3, the grounding classification, maximum clearing time, selected standard, manufacturer approval, and accessory schedule in the design record. These documents make the insulation decision traceable when protection settings or network grounding later change.

Frequently Asked Questions