Short answer: shielded and unshielded medium-voltage cables are not interchangeable versions of the same product. For most fixed industrial and distribution circuits in the 5-35 kV range, a shielded cable system is the normal engineering basis. An unshielded design belongs only in a limited voltage and installation scope that the governing standard, adopted code, attrezzatura, and authority explicitly permit. UN 15 kV cable should be specified as a shielded system, not treated as a cost-reduced unshielded alternative.
The comparison becomes clearer after separating two functions that ordinary language compresses into the word shield: semiconductive screens control the electric field at insulation interfaces, while a metallic screen provides a conductive path that can be grounded and sized for system duties. Armor remains a third, separate construction decision.
First define what shielded means
Medium-voltage cable specifications use schermo E shield
| Layer or component | Typical material | Main function | What it does not prove |
|---|---|---|---|
| Schermo del conduttore | Extruded semiconductive compound | Smooths the conductor interface and reduces electrical stress concentrations around strands | It does not provide a metallic ground-fault path |
| Isolamento | XLPE, TR-XLPE, or EPR compound as specified | Provides the primary dielectric barrier | Material name alone does not define the complete cable system |
| Schermo isolante | Extruded semiconductive compound | Creates a controlled electrical interface at the outside of the insulation | It is not the metallic screen and should not carry specified fault current |
| Schermo metallico | Nastro di rame, copper wires, concentric neutrals, or an approved metallic sheath | Maintains a grounded surface and provides a defined conductive path | Its presence alone does not define armor or mechanical protection |
| Armatura | Filo d'acciaio, filo di alluminio, nastro d'acciaio, or another specified system | Addresses mechanical conditions and may take part in grounding when designed for that role | Armor does not automatically replace the required screen system |
This distinction also explains why a construction drawing is more reliable than a description such as “screened cable.” The drawing can state the semiconductive layers, copper tape overlap or wire area, separator, armatura, guaina esterna, e disposizione di legame. IL MV cable screen types guide examines the metallic options in more detail.
Two different questions hide inside the comparison
Does the insulation system include field-control screens?
Voltage stress does not distribute evenly around an irregular stranded conductor unless the insulation system controls the interface. An extruded conductor screen fills strand-surface irregularities and establishes a smooth electrical boundary. The insulation screen performs a related function at the outer insulation surface. Insieme, these layers make the insulation system predictable enough for its intended voltage class and accessory design.
These semiconductive compounds are not ordinary conductive jackets. Their formulation, extrusion cleanliness, interface quality, and compatibility with the insulation belong to the qualified cable design. Removing one layer changes the electric-field system rather than merely reducing material.
Does the cable include a grounded metallic screen?
A metallic screen creates an accessible conductive layer outside the insulation screen. When the grounding and bonding design connects it correctly, the screen holds the cable surface near ground potential, carries charging current, and provides a path whose short-circuit duty can be calculated. The required material and cross-sectional area therefore depend on the protection study, fault duration, disposizione di legame, and installation.
Copper tape and copper wires can both form metallic screens, but they are not equivalent by name. Tape geometry, wire area, protezione dalla corrosione, longitudinal water blocking, and joint continuity change the finished system. The construction schedule must state which design applies.
Where unshielded MV cable can still appear
Non-shielded power cable remains a defined construction, but its scope is narrow. ICEA lists a dedicated standard for non-shielded power cable rated 2001-5000 V, while its shielded power-cable standards extend from 5 kV into higher medium-voltage classes. UL 1072 also contains both shielded and nonshielded entries. The applicable edition and installation rule determine which construction a particular circuit may use.
Examples can occur in restricted lower-medium-voltage industrial systems, special equipment, airfield circuits, or older facilities built under a specific design philosophy. Such examples establish that the construction exists; they do not establish suitability for a new general-purpose feeder. Existing-plant continuity, trained-personnel access, raceway configuration, equipment terminations, and local rules may have shaped the original decision.
CEI 60502-2 covers extruded power cables from 6 kV through 30 kV and addresses conductor screens, insulation screens, and metallic layers as distinct construction elements. An IEC project should therefore use the cable designation and construction clauses required by its specified voltage rather than importing an informal “unshielded” label from another system.
Shielded and unshielded systems compared
| Design question | Shielded MV cable system | Unshielded MV cable system |
|---|---|---|
| Electric-field boundary | Defined by semiconductive screens and the surrounding metallic system | Depends on the specifically qualified non-shielded construction and installation geometry |
| Accessible surface potential | Metallic screen can maintain a grounded outer electrical boundary when correctly bonded | No comparable integral metallic screen path; spacing and enclosure rules become decisive |
| Voltage and standard scope | Normal basis across common MV distribution classes under the specified standard | Restricted to the voltage, costruzione, and installation that the selected standard permits |
| Termination design | Requires screen cutback and electric-stress control matched to cable dimensions | Requires an accessory or equipment interface listed or qualified for that exact construction |
| Ground-fault path | Metallic screen area and bonding can be coordinated with protection duties | Must rely on the separately designed grounding and raceway system |
| Field testing | Established shielded-cable methods can energize the conductor against the screen | Test method and electrode arrangement must suit the non-shielded design |
| Interchangeability | Compatible only with accessories and grounding designed for the shielded system | Cannot replace a shielded cable merely because conductor size and voltage marking appear similar |
Why 15 kV is not an unshielded substitution exercise
UN 15 kV feeder combines cable insulation, schermi, metallic path, terminazioni, articolazioni, messa a terra, and test procedures as one system. Deleting the screens changes the electric-field boundary and removes the electrode used by common shielded-cable tests. It also changes the way a termination controls stress at the insulation-screen cutback.
For that reason, UN 15cavo kV schedule should state the governing standard, livello di isolamento, conduttore, schermo conduttore, isolamento, schermo isolante, metallic screen material and area, guaina, armor if required, and accessory interface. Conductor size and insulation thickness alone cannot establish system compatibility.
At higher MV classes, the same principle becomes even more important. A nominal voltage label does not replace phase-to-ground voltage, metodo di messa a terra, insulation category, fault-clearing time, and accessory coordination. The final schedule must align with the network study.
Armor does not settle the shielding question
Steel wire armor can resist pulling, impatto, and installation loads in a suitable construction. Aluminum wire armor can provide mechanical protection for single-core AC cable without the magnetic-loss issue associated with ferromagnetic armor. Neither statement means armor automatically performs every electrical function of a designed metallic screen.
A grounded armor system may contribute to earth-fault continuity, but its geometry, contact continuity, corrosion condition, gland arrangement, circulating-current behavior, and short-circuit capability require separate evaluation. The specification should therefore list the metallic screen and armor on different lines. If one metallic component is intended to perform more than one duty, the applicable standard and calculations must support that combined role.
IL Cavo di alimentazione MT construction should also match the route. Duct banks, sepoltura diretta, indoor trays, tunnel, wet locations, esposizione chimica, and fire-performance zones can change sheath, blocco dell'acqua, armatura, and bonding requirements without changing the fundamental need for a coordinated screen system.
Terminations reveal whether the system has been specified correctly
A shielded-cable termination does more than expose the conductor for a lug. It ends the insulation screen at a controlled location, manages the resulting electric-field concentration, seals the cable, and connects or parks the metallic screen according to the grounding plan. Accessory selection depends on cable insulation diameter, tipo di schermo, jacket diameter, dimensione del conduttore, connector, environment, and equipment interface.
An accessory intended for a shielded 15 kV cable should not be assumed suitable for an unshielded construction. The reverse assumption is equally unsafe. Manufacturer installation instructions, dimensional windows, creepage distance, sealing method, and qualified installer procedures form part of the cable-system design.
Joints require the same discipline. The restored insulation and screen interfaces must remain concentric, clean, and continuous. Screen wires or tape connections also need the specified electrical capacity and corrosion protection. A cable datasheet without matching accessory data leaves the system incomplete.
Testing changes with the construction
Shielded cable provides a defined test electrode around the insulation. That geometry supports conductor-to-screen diagnostic and withstand methods. IL IEEE 400-2023 scope record identifies shielded insulated cable systems rated 5 kV and above as the subject of its field-testing guidance.
This does not mean that one test voltage or method fits every shielded cable. The test plan still depends on cable type, age, circuit condition, accessory technology, previous history, and the applicable IEEE 400-series method or project standard. A commissioning test and an aged-cable diagnostic have different objectives.
Non-shielded cable lacks the same integral test electrode. Applying a shielded-system procedure without an approved electrode arrangement can produce an invalid field distribution or an unsafe setup. The test method should come from the governing standard, equipment design, and responsible engineer rather than from a value copied from another cable class.
The decision belongs to the complete cable system
The following sequence prevents the word “shielded” from hiding missing project data:
- Fix the electrical basis. Record nominal and maximum system voltage, phase-to-ground duty, metodo di messa a terra, insulation category, fault level, and clearing time.
- Name the governing documents. State the cable standard, edition, adopted installation code, utility specification, and any project-specific test requirements.
- Define every layer. List conductor screen, isolamento, schermo isolante, metallic screen type and area, blocco dell'acqua, guaina, armatura, and outer protection.
- Coordinate the system interfaces. Confirm bonding, messa a terra, screen fault-current capacity, terminazioni, articolazioni, equipment bushings, pulling plan, and field-test method.
If any one of these four groups remains open, selecting between shielded and unshielded cable is premature. The cable name should be the result of the system definition, not its substitute.
Specification errors that create avoidable rework
- Using “shielded” without a layer schedule. This leaves semiconductive screens and metallic screen material open to interpretation.
- Calling armor the shield. Mechanical protection and electrical field control become mixed in one undefined requirement.
- Choosing a screen by normal load current. Metallic screen area also requires charging-current, earth-fault, legame, and protection coordination.
- Ignoring cable dimensions until accessory order. Termination and joint kits have dimensional ranges that must match the finished cable.
- Copying a 5 kV exception into a 15 kV schedule. A narrowly permitted lower-voltage construction does not extend to another voltage class.
- Writing the field-test plan after installation. The test method, access, messa a terra, and acceptance basis should be coordinated before the cable route closes.
Questions that need precise answers
Is every 5 kV cable shielded?
No universal answer applies without the governing standard, edition, installation code, and circuit conditions. Both shielded and non-shielded 5 kV constructions exist in defined contexts. A current project should state the permitted construction explicitly rather than infer it from the voltage label.
Does shielded cable always mean copper tape?
NO. The metallic element may use copper tape, copper wires, concentric neutrals, or another approved metallic system. The semiconductive conductor and insulation screens are separate layers and should also appear in the construction schedule.
Can steel wire armor replace the metallic screen?
Not by default. Armor primarily addresses mechanical conditions. Any combined electrical role requires support from the applicable cable design, grounding arrangement, ghiandole, continuity provisions, and fault-duty calculations.
Can shielded and unshielded cables use the same termination?
That assumption should not be made. A shielded termination manages stress at the insulation-screen cutback and connects the metallic screen. The accessory must match the exact cable construction, dimensioni, classe di tensione, environment, and equipment interface.
Is an unshielded 15 kV cable a practical alternative?
For normal fixed industrial and distribution service, NO. UN 15 kV cable system should use the shielded construction required by its standard and installation rules. Any claimed exception needs direct documentary approval for the exact circuit and equipment, not a general product comparison.
Conclusione ingegneristica
The useful distinction is not “more protection” versus “less protection.” A shielded MV cable defines the electric-field boundary, metallic path, grounding interfaces, accessori, and field-test geometry as a coordinated system. An unshielded MV cable is a restricted construction whose use depends on an explicit standard and installation basis.
For a technically complete cable schedule, define both screen systems, keep armor separate, and coordinate the cable with protection, legame, accessori, route conditions, and tests. That approach resolves the terminology before it becomes an installation or commissioning problem.
Send Medium-Voltage Cable Requirements
For a project-specific construction review or quotation, provide system voltage, metodo di messa a terra, applicable standard and edition, materiale e dimensioni del conduttore, conteggio dei nuclei, sistema di isolamento, required metallic screen type and area, armor and sheath, ambiente del percorso, termination interface, test requirements, lunghezza del tamburo, quantità, and destination. Potenza XWA & Cable can then prepare a construction schedule aligned with the stated cable-system basis.