Designing MV Cable Compartments for Bending Radius, Cleats and Separable Connectors

A cable compartment is acceptable only when the specified cables can be routed, restrained, terminated, tested and replaced without violating bending radius, connector alignment, insulation clearance, internal-arc boundaries or safe-working space. A generous-looking empty box can become impossible to install once real cable stiffness, cleat brackets, screen-earth leads, test adapters and the installer’s tools are modelled.

This practical design method treats the cable set, accessory, switchgear interface, supporting steelwork and compartment as one system. It applies to bottom- and rear-entry MV metal-enclosed switchgear, both open-air terminations and screened separable connectors.

Executive rules

Start with an approved cable-and-accessory schedule, not a nominal conductor area.
Use manufacturer-specific installation and final-set bending radii; no universal multiplier is safe for every MV cable.
Model five envelopes: electrical, installation, operating/maintenance, short-circuit movement and internal-arc pressure/venting.
Provide the straight length and angular alignment required by the accessory before the first bend.
Cleats restrain the cable; the termination, bushing, gland plate and connector must not become substitute cleats.
IEC 61914 cleat performance is conditional on cable diameter, formation, spacing, mounting surface and test arrangement.
Identify the exact separable-connector interface and operating function. Screened appearance does not prove interchangeability or load-break capability.
Check single-core cable entry plates for induced heating and circulating-current paths.
Preserve IEC 62271-200 compartment, accessibility, loss-of-service-continuity and IAC features after every penetration and bracket is fitted.
Prove constructability with a scaled digital route and, for stiff/large or multiple cables, a physical installation trial.

1. Applicable standards and their boundaries

Reference	What it contributes	What it does not prove alone
IEC 62271-200:2021+AMD1:2024	MV metal-enclosed assembly, compartments, accessibility, dielectric/thermal/short-time performance and IAC	Suitability of an arbitrary cable/accessory combination
IEC 60502-2:2014+AMD1:2024 and COR1:2026	Extruded MV cable construction and tests to Um 36 kV	Compartment routing or termination workmanship
IEC 60502-4:2023 and IEC 61442:2023	Accessory type-test requirements and methods	Mechanical support of the installed cable run
IEC 61914:2021	Cable-cleat and intermediate-restraint requirements/tests	Any spacing or bracket arrangement not represented by evidence
IEC 61238-1-3:2018	Relevant MV conductor compression/mechanical connectors	The complete termination or switchgear joint
IEEE 386-2025	North American shielded separable connectors for stated voltage/current classes	Automatic interchangeability with IEC-defined interfaces

Freeze editions in the procurement specification. IEC published COR1:2026 for IEC 60502-2 in July 2026; projects using the consolidated 2014+AMD1:2024 text should confirm that the corrected text is applied. A valid accessory type test does not waive switchgear dielectric, temperature-rise, short-circuit or internal-arc evidence.

2. Build a cable-compartment input schedule

system voltage, Um, insulation level, neutral earthing and fault duration;
cable standard, make/type, conductor material/class/area, number per phase and formation;
diameter over insulation, screen, armour and oversheath, including tolerances;
minimum bending radius during installation and after setting, minimum installation temperature and pulling limit;
screen/sheath construction, bonding method, earth-lead rating and test-link arrangement;
termination or separable-connector make, part number, interface, rating and mounting orientation;
required straight length, cutback dimensions, connector stroke, torque/compression tooling and permissible terminal loads;
entry direction, trench/floor coordinates, gland/sealing arrangement and water/fire barrier;
cleat type, liner, spacing, bracket, mounting fasteners and declared short-circuit duty;
space for phasing, testing, surge arrester, CT, sensor or future cable where specified;
access rule, isolation/earthing method, LSC/IAC classification and pressure-relief route.

A drawing labelled “3 × 1C × 630 mm²” is insufficient. Cables of the same area can have materially different diameter, stiffness, screen fault rating, lug barrel, termination length and bend radius.

3. The five design envelopes

Envelope	Include
Electrical	Phase-phase/earth clearances, creepage, stress-control components, screens, barriers and transient displacement
Installation	Cable sweep, pulling head, hands/tools, cutback, compression gun, torque wrench, connector insertion and temporary supports
Operation/maintenance	Inspection, disconnection, parking, test adapter, earth application and replacement stroke
Short-circuit movement	Cable/cleat deflection, bracket reaction, conductor interaction and safe clearance after tolerances
IAC/pressure	Flaps, ducts, pressure volume, seals, partitions and debris/projectile control

These envelopes overlap in three dimensions and across time. The route used to install a connector may be unavailable after a CT, rear cover or adjacent cable is fitted. Design the installation sequence, not merely the final photograph.

4. Route the real cable centreline

Import the actual cable diameter and manufacturer radius into a 3D model. Define the cable neutral-axis centreline from trench exit to interface and add diameter/tolerance. Check:

installation radius while the cable is moving and final radius after setting;
straight length beneath the stress-control body or connector;
reverse bends and S-curves, which are harder than a single bend;
phase crossing and unequal outer-phase routes;
vertical cable weight and restoring force;
cold-installation limits and required bending former;
trench-wall, floor-edge and adjacent-panel interference;
tolerance in floor opening, panel location and cable emergence;
retermination allowance without adding an unapproved joint.

Do not measure radius to the compartment corner or use the diagonal as “available radius.” The limiting geometry is the cable centreline and inside curvature through the complete swept path. Never pull a stiff cable into alignment with the terminal bolt or connector retaining device.

5. Straight length, support and first-cleat position

The accessory maker normally declares a preparation length and a straight, stable section. Place the first cleat far enough away to permit cutback, stress-control assembly and connector movement, but close enough to intercept weight and bend reaction before it reaches the interface. The correct distance is a coupled calculation/trial, not a generic dimension.

Obtain maximum axial, lateral, torsional and bending loads/moments for the bushing and accessory.
Calculate or measure cable restoring force at the intended radius and temperature.
Include conductor thermal expansion and settlement between trench and panel.
Provide temporary installation support so the unfinished accessory is never load-bearing.
Orient brackets so clamping does not ovalise or damage the cable.
Keep cleats accessible for verified tightening and later inspection.

6. Cable cleats and short-circuit restraint

For a first approximation, force between long parallel conductors increases with the product of instantaneous currents and decreases with spacing. Peak asymmetrical current and actual conductor geometry drive the mechanical event; RMS continuous current does not. A usable design therefore needs prospective fault current, X/R-related peak, clearing/backup duration, phase formation, cable spacing and unsupported length.

Select a cleat with an IEC 61914 declaration/test applicable to cable diameter and trefoil or flat formation.
Use the tested or justified centre spacing; a stronger cleat spaced farther apart is not automatically equivalent.
Check intermediate restraints where the declared system uses them.
Verify the actual mounting surface, bracket thickness, fastener grade, edge distance and load direction.
Transfer reactions into structural members, not an unverified thin gland plate.
Account for bends, phase separation and end effects near the termination.
Check cable damage, liner compatibility and allowable compressive pressure.

The complete restraint chain is cable–liner–cleat–bolt–bracket–enclosure–anchor. The lowest-capacity link governs. Document whether the short-circuit evidence is a complete arrangement test, calculation correlated to tests, or a justified derivative.

7. Separable connectors: specify the interface and function

standard/interface designation and manufacturer compatibility statement;
voltage/current/short-circuit rating and conductor range;
screened or unscreened construction and touch-safe claim conditions;
dead-break or load-break function—never infer this from appearance;
bolted, plug-in or latch retention and required insertion/withdrawal force;
test point, capacitive indication, parking bushing and insulating/earthing caps;
orientation, minimum phase spacing and rear clearance;
earthing of conductive screen and bonding-lead route;
cleaning, approved lubricant, torque and contamination controls;
test-adapter compatibility and removal space.

“Separable” does not mean safe to separate energized. Follow the exact operating classification and manufacturer procedure. Interface mixing can produce incomplete engagement, local heating, partial discharge or loss of screen continuity even when parts appear to fit.

8. Open-air lugs and multiple cables per phase

For open-air terminations, check lug palm orientation, stud size, washers, joint materials, surface treatment, torque/preload method and anti-rotation. Keep the cable mechanically relaxed before tightening. With parallel cables, do not force unequal routes onto one pad: unequal lengths, bends and terminal pressure can cause unequal current sharing and terminal load. Provide a pad arrangement and phase sequence that permits each cable to be independently installed and supported while maintaining tested clearances.

9. Entry plate, glands, bonding and environmental sealing

For single-core AC cables, avoid a ferromagnetic closed loop around individual phase conductors; use a proven non-magnetic plate/arrangement where required.
Check eddy-current heating in plates, glands and nearby steel at rated current.
Define armour/screen bonding, insulated gland requirements and earth-lead fault duty.
Seal against water, dust, vermin and fire without defeating pressure relief or adding incompatible materials.
Maintain the assembly IP code and IAC boundary using the tested or assessed penetration design.
Provide drainage/condensation control without creating an arc-gas outlet toward personnel.
Coordinate floor opening and trench fire-stop with cable movement and future replacement.

10. Thermal and dielectric checks

Cables, screens, connectors and earth leads add heat inside a restricted volume. Consider conductor loss, connector contact resistance, induced sheath/screen currents, plate eddy currents, solar/room ambient and blocked ventilation. Temperature-rise evidence for a different cable quantity or compartment vent arrangement is not automatically transferable. Keep ventilation paths clear of cables and cleats.

Dielectric verification includes phase-to-phase/earth clearance in the worst installed position, screen cutback, stress-control cleanliness, altitude, humidity/condensation, test-adapter geometry and cable movement under fault. A screened connector reduces exposed field only when fully assembled, correctly bonded and capped.

11. Safe operation, testing and maintainability

Define how each cable is isolated, proved dead, discharged and earthed.
Provide rated test points/adapters and enough clearance for the approved test lead.
State whether cable testing requires disconnection of VTs, surge arresters, sensors or connectors.
Prevent access to adjacent live compartments according to the declared accessibility/LSC design.
Keep shutters, earth switches, interlocks and pressure flaps unobstructed.
Provide phase identification visible after assembly and unambiguous screen-earth labels.
Allow inspection of cleats, bonds and connectors without uncontrolled cable movement.

12. Verification workflow and hold points

Approve the cable/accessory/interface schedule and standards editions.
Review the 3D cable centreline, all five envelopes and installation sequence.
Verify cleat short-circuit evidence and calculate bracket/enclosure reactions.
Review thermal, dielectric, bonding, IP and IAC impacts.
Conduct a cable-routing mock-up where stiffness, quantity or access is critical.
Freeze entry coordinates, bracket drawings, torque data and special tools.
At installation, verify cable identity, temperature, cutback, connector preparation, cleat spacing and terminal-load release.
Record compression dies/indents or shear-bolt heads, torque, connector seating, bond continuity and photographs.
Perform specified insulation/sheath/phase/continuity tests within accessory and equipment limits.
Close covers only after tool/material count, barrier, seal, interlock and pressure-path inspection.

13. Frequent design failures

Failure	Engineering correction
Generic radius such as 12D	Use cable-maker values for exact construction and condition
Termination carries cable weight	Design first cleat/support and temporary installation support
Cleat chosen only by fault-current label	Match formation, diameter, spacing, mount and peak-duty evidence
Connector selected by visual fit	Freeze standard interface, function and compatibility evidence
Room for finished cable but not tools	Model installation/removal swept envelopes and sequence
Bracket added after IAC design	Assess partition, vent, projectile and pressure-volume impacts
Steel gland plate around single cores	Analyse magnetic loop/eddy heating and use proven arrangement
Future cable “space” reserved abstractly	Route and restrain the future cable in the model now

14. Procurement deliverables

approved cable/accessory/connector/interface schedule;
dimensioned compartment, entry and cable-centreline drawings;
bending-radius and straight-length source data;
terminal permissible-load data and support reaction calculation;
IEC 61914 cleat declarations/reports and bracket verification;
thermal/dielectric/short-circuit/IAC applicability assessment;
bonding, screen-earth and test schematic;
installation sequence, torque/compression schedule and special-tool list;
mock-up report, inspection-and-test plan and as-built records;
maintenance/replacement method and spare accessory list.

References

Safety note: MV cable installation, testing and compartment access are hazardous activities for qualified personnel under approved isolation, proving-dead, earthing and arc-flash controls. Manufacturer instructions and the project safety system take precedence over general guidance.