Short answer: a medium voltage underground cable requires more than voltage and conductor size. Begin by dividing the route into direct burial, duct banks, tunnels or trenches, building entries, vertical sections, joint bays, and wet sections. Each section changes several duties. These include thermal inputs, water and mechanical protection, comportement au feu, support, pulling limits, collage d'écran, and accessory interfaces.
Le mot souterrain describes circuit location. It does not define one cable construction. Thermal backfill, a water-filled duct, and cleats in a ventilated tunnel create different operating conditions. Treating all three as one environment can make a valid cable unsuitable for the actual route.
Start with route segments, not a catalogue description
A useful route survey converts civil conditions into cable-system decisions. It maps soil contact, duct entries, equipment rises, passages à niveau, fire zones, and accessible sections. Those transitions often control the final specification.
| Route segment | Dominant engineering questions | Specification fields likely to change |
|---|---|---|
| Enterrement direct. | Soil thermal resistivity, moisture variation, external damage, literie, exposition chimique, and repair access. | Gaine extérieure, water barriers, protection mécanique, protection contre la corrosion, ampacity assumptions, et marquage d'itinéraire. |
| Duct bank. | Duct diameter, virages, friction, longueur de traction, spare ducts, water accumulation, and thermal interaction between circuits. | Diamètre extérieur du câble, pulling limits, sheath friction and toughness, longueur du tambour, duct arrangement, and rating model. |
| Accessible trench or tunnel. | Air temperature, ventilation, fire propagation, fumée, espacement des supports, short-circuit restraint, drainage, and access. | Fire performance, cleat system, formation, support materials, ampacity assumptions, and sheath properties. |
| Vertical section. | Poids du câble, axial movement, support load, bend geometry, and termination load. | Cleat duty, espacement des supports, attachment details, maximum unsupported length, and accessory arrangement. |
| Joint bay or termination zone. | Working space, straight length, bonding leads, moisture control, phase spacing, and test access. | Accessory interface, collage d'écran, oversheath test points, longueur du câble, and installation sequence. |
CEI 60502-2 covers fixed-installation extruded-insulation power cables from 6 kV jusqu'à 30 kV. It addresses construction, dimensions, and tests. Its scope also excludes special conditions such as submarine service. Route conditions therefore remain project inputs, even when the cable itself complies with the product standard.
Direct burial makes the surrounding soil part of the electrical design
In a direct-buried section, heat crosses the insulation, écrans, gaine, literie, backfill, and native soil. Continuous current rating is therefore not a permanent catalogue property. The calculation uses conductor temperature limit, load factor, soil temperature, thermal resistivity, profondeur d'enfouissement, circuit spacing, heat sources, and bonding losses. It may also need a soil-drying assessment.
CEI 60287-3-1 distinguishes direct burial, conduits, troughs, steel pipes, and installations in air. It also notes that national or project conditions can supersede general reference conditions. Do not transfer an ampacity table to a route with different geometry, profondeur d'enfouissement, soil, or backfill assumptions.
Thermal backfill can reduce uncertainty when the specification controls its properties. Define the design value, placement, compaction, moisture assumptions, test method, and treatment at crossings. Naming a material without a verifiable thermal requirement does not control cable rating.
Mechanical protection is a route decision
Armor is not an automatic synonym for underground service. Direct burial may justify metallic armor, non-metallic protection, slabs, ducts at crossings, or a combination. The choice depends on excavation risk, ground movement, rodents, corrosion, induced losses, local rules, and repair strategy. Single-core AC circuits need particular care because ferromagnetic armor can create unacceptable losses or heating.
An unarmored cable can suit a robust duct system. The civil design must provide the required protection, and project rules must allow the arrangement. State what protects every route segment instead of adding armor by habit.
Duct banks move risk from burial damage to pulling and heat
A duct protects against direct soil contact. It also introduces interfaces that can damage a cable before energization. The route drawing needs duct diameter and material, rayon de courbure, direction changes, chambers, articulations, direction de traction, propreté, and water management.
Cable outside diameter must leave adequate clearance after accounting for ovality, duct joints, and bends. Pulling calculations cover conductor tension, pression sur les flancs, friction, pulling-eye or stocking duty, lubricant compatibility, and winch control. A large duct does not correct an excessive bend or an unrealistic pulling section.
Water commonly enters ducts. An outer polyethylene sheath does not provide a complete water-blocking system. The design may require longitudinal blocking in the conductor or screen region. A project with radial ingress risk may also require a radial barrier. End caps, joint-bay drainage, accessory sealing, and oversheath testing remain separate controls.
Tunnels and trenches are underground locations but often air-rated installations
A tunnel often rejects cable heat to air. Ladders, plateaux, or cleats also expose the cable to nearby surfaces and services. Ventilation, ambient profile, regroupement, wall proximity, and circuit formation can dominate the rating. Fire and smoke requirements can outweigh burial impact resistance.
The support system must carry normal weight and restrain electromechanical movement during a short circuit. Cable cleats, fasteners, rails, parenthèses, and structural anchors form one load path. For single-core AC circuits, support details should avoid closed ferrous loops around one phase. Vertical sections add axial load and should not transfer cable weight into a gland or termination by default.
Fire-performance terms need precise scope. Flame retardance, faible émission de fumée, halogen acid gas limits, circuit integrity, and fire resistance use different test methods. A tunnel specification should name the required tests and acceptance criteria. The broad term “fireproof” does not define a technical duty.
Water protection has three separate boundaries
Water-related failures are easier to prevent when the specification separates three paths:
- Longitudinal migration. Water can travel along conductor strands or screen interfaces. Swellable tapes, powders, or filled conductors can limit that movement when the design includes and tests them.
- Radial ingress. Moisture can pass through or around outer layers over time. The project may require a metallic laminate, lead sheath, or another declared barrier.
- Accessory entry. Joint and termination sealing, temporary end caps, gland interfaces, and handling damage can bypass otherwise suitable cable layers.
CEI 60502-2 includes designs with longitudinal water barriers and an associated test. It also recommends considering radial water ingress risk. Longitudinal water blocking does not prove radial barrier performance.
Route conditions also change screen and bonding decisions
The metallic screen carries charging current and provides an earth-fault path for the declared duty. It also controls the electric field and participates in the bonding system. Check screen cross-section against earth-fault current and clearing time. Coordinate bonding with route length, formation, sheath voltage, pertes, link boxes, surge protection, earthing, and maintenance access.
Both-end bonding can create circulating current and additional losses. Single-point bonding can limit circulating losses but introduces standing sheath voltage and requires controlled earthing and insulation coordination. Cross-bonding can improve long-route performance but adds sectioning, transposition, link boxes, and commissioning checks. The bonding diagram must precede final approval of the cable schedule.
Le MV power cable range provides the product-family context, while an N2XS2Y construction illustrates a polyethylene-sheathed medium-voltage option. Neither product name removes the need to define water, armure, liaison, feu, et conditions d'itinéraire.
Route transitions create the highest interface risk
A circuit may leave a trench, cross a road in ducts, rise through a building, and terminate at switchgear. One construction must satisfy all compatible duties. Otherwise, the design must divide the route with approved joints and transition details.
Review bending radius, direction de traction, support, sealing, fire stopping, induced voltage, oversheath continuity, accessory space, and inspection access. The guide to MV cable bending radius and pulling tension explains the force controls at duct entries and bends.
IEEE 525-2025 frames substation cable design, installation, and protection as one system intended to minimize failures and their consequences. That system view is especially useful at building and substation entries, where civil, câble, support, feu, liaison, and termination responsibilities meet.
A route-based cable schedule is more useful than a short designation
The final schedule should preserve every approved design and installation assumption.
- Electrical basis: tension du système, U0/U(Un), fréquence, niveau d'isolation, continuous load, emergency load, conductor temperature limit, earth-fault current, duration, and short-circuit basis.
- Construction de câbles: matériau et classe du conducteur, taille du conducteur, Isolation XLPE, conductor and insulation screens, metallic screen type and area, water-blocking layers, radial barrier where required, literie, armure, gaine extérieure, and fire properties.
- Route model: segment lengths, enterrement direct, conduits, troughs, tunnel or air sections, vertical rises, passages à niveau, joint bays, and termination locations.
- Thermal inputs: soil and air temperatures, résistivité thermique du sol, profondeur d'enfouissement, backfill, duct geometry, circuit spacing, regroupement, load factor, bonding losses, and nearby heat sources.
- Installation limits: finished diameter and tolerance, cable mass, minimum bending radius by installation stage, maximum pulling tension, pression sur les flancs, longueur du tambour, tirer les yeux, and sealing method.
- Interfaces and records: joint and termination data, bonding diagram, link boxes, cleat schedule, fire stopping, plan de test, oversheath tests, identification, and as-built documentation.
The official IEC operating-condition record is a useful verification point for the distinction among direct burial, conduits, troughs, tuyaux, and air. The actual numerical inputs still come from the project and applicable local requirements.
Four checks prevent the most common specification errors
1. Confirm that every ampacity value names its environment
A rating should state the installation arrangement and thermal assumptions. Without those inputs, the current value is not auditable and cannot support a project guarantee.
2. Identify what provides mechanical protection in each segment
The answer may be armor, a duct, protective covers, controlled access, profondeur d'enfouissement, or several measures. A blank route segment is a design gap.
3. Separate water barriers from a general sheath description
List outer sheath, blocage de l'eau longitudinal, radial barrier, temporary end sealing, and accessory sealing as separate requirements.
4. Freeze civil and cable interfaces before drum lengths
Chambers, pulling directions, bend geometry, joint bays, transition points, and termination coordinates determine usable section lengths. Premature drum allocation can force extra joints or impossible pulls.
Questions answered by the route, not by the product name
Does every medium voltage underground cable need armor?
Non. Mechanical protection must match the route risk and local rules. Direct burial may justify armor or external protection, while a duct or controlled tunnel can provide a different protection system. Single-core AC armor also requires a loss and heating assessment.
Is a PE sheath sufficient for a wet duct?
Not as a complete statement. The design should separately address outer-sheath durability, longitudinal water migration, radial ingress, cable-end sealing, articulations, résiliations, and duct drainage.
Can one current-rating table cover direct burial and ducts?
Non. The heat path and installation geometry differ. Each route segment needs the applicable thermal model and project inputs, and the lowest permissible circuit rating may govern operation.
Why can a tunnel require a different cable from a buried section?
A tunnel may impose fire, fumée, cleat, ventilation, regroupement, and maintenance requirements that do not control a soil-buried section. The route must either use one compatible construction or include an engineered transition.
Which information must the design freeze before manufacture?
Approve voltage designation, conducteur, écrans, water barriers, armor and sheath, and fire requirements before manufacture. Also approve thermal inputs, liaison, diameter limits, installation forces, drum lengths, accessoires, essais, and document deliverables.
Conclusion d'ingénierie
The correct medium voltage underground cable is the result of a route study, not a catalogue label. Direct burial makes soil and backfill part of the rating system. Duct banks make pulling geometry, eau, and thermal interaction decisive. Tunnels shift attention toward fire behavior, ventilation, support, and fault restraint. Transitions combine all of these risks.
A defensible specification divides the route and records the controlling condition for each segment. It then selects one compatible system or deliberate transition points. The result remains useful from design review through commissioning.
Send the Route and Electrical Basis
For technical alignment, provide the electrical basis and each route segment. Include burial or duct geometry, soil data, tunnel conditions, exposition à l'eau, protection mécanique, liaison, joint and termination locations, and pulling direction. Add drum constraints, normes, quantité, and destination. Puissance XWA & Cable can prepare a construction and document package against the stated route basis.