MV Cable Compartments and Connectors

The cable compartment is one of the most underestimated parts of medium-voltage switchgear. On a single-line diagram, the interface may appear as nothing more than a line leaving a circuit breaker. In the actual panel, however, the designer must accommodate a large cable with a limited bending radius, a precisely matched termination, one or more current transformers, screen-earth conductors, cable cleats, surge arresters, test access, an earthing switch and an internal-arc pressure path—all inside a controlled enclosure.

A feeder can therefore be electrically correct on paper and still be impossible, unsafe or unreliable to terminate on site.

The correct engineering approach is to treat the cable compartment as a complete interface system. Its performance is limited by the weakest item in the chain:

cable → conductor connector → separable connector or termination → equipment bushing → internal connection → switchgear.

This guide explains how to specify, design, review, install, test and maintain that complete chain for metal-enclosed MV switchgear up to 52 kV. It focuses mainly on extruded-insulation cables and screened separable connectors used in IEC-based systems, while also explaining where IEEE-style loadbreak and deadbreak systems differ.

Safety note: A screened or “touch-safe” connector is not permission to touch, disconnect or work on an energized circuit. Access, isolation, voltage detection, earthing and testing must follow the switchgear manufacturer’s instructions, the connector manufacturer’s instructions and the site’s approved electrical-safety procedure.

1. What the cable compartment must accomplish

The cable compartment is not simply unused space below a circuit breaker. Depending on the switchgear design, it may perform all of the following functions:

Function	Typical equipment or design feature	Critical coordination question
Main-circuit connection	Bushings, cable pads, separable connectors or air terminations	Are voltage, current, interface geometry and short-circuit ratings compatible?
Cable support	Cleats, clamps, brackets and gland plates	Are cable weight, bending force and fault forces kept away from the bushing?
Earthing	Main earth bar, screen-earth bar, braids and bonding links	Does the arrangement match the cable bonding and protection philosophy?
Current measurement	Phase CTs, ring-core CTs, low-power sensors or CBCTs	Do all required conductors fit, with correct polarity and screen-earth routing?
Voltage indication or measurement	Capacitive test points, VDIS/VPIS, resistive or capacitive sensors	What does the device actually prove, and how will it be tested?
Overvoltage protection	Plug-in or conventional surge arresters	Is the arrester rating correct and is the earth path short and direct?
Isolation and maintenance	Earthing switch, shutters, test plugs, dead-end plugs and access interlocks	Can the cable be safely isolated, tested, earthed and returned to service?
Environmental protection	Door seals, gland seals, heaters, ventilation and drainage	Will moisture, condensation, pollution or fire sealing degrade the termination?
Arc-fault containment	Barriers, pressure-relief path and arc-resistant door	Has field-installed equipment altered the tested assembly configuration?

These functions are strongly coupled. Moving a CT may improve cable bending space but make the residual-current scheme incorrect. Adding a second cable may satisfy ampacity but overload the bushing arrangement, obstruct the pressure path or exceed the panel’s validated cable configuration. A gland seal may improve ingress protection but prevent thermal movement or damage the outer sheath.

2. The standards framework—and what each standard does not do

No single standard completes the design. The following documents address different parts of the system:

IEC 62271-200 applies to prefabricated AC metal-enclosed switchgear and controlgear above 1 kV and up to 52 kV. It covers assembly requirements and classifications such as service continuity and, when assigned, internal arc classification. The current consolidated edition is IEC 62271-200:2021+AMD1:2024.
IEC 62271-1 provides common specifications for AC high-voltage switchgear and controlgear.
IEC 60502-2 covers extruded-insulation power cables from 6 kV (Um 7.2 kV) up to 30 kV (Um 36 kV).
IEC 60502-4 specifies type-test requirements for accessories for cables from 3.6/6 (7.2) kV to 18/30 (36) kV. The current edition is IEC 60502-4:2023.
IEC 61442 specifies test methods for MV cable accessories.
IEC 61238-1-3 addresses compression and mechanical conductor connectors for power cables above 1 kV, including electrical, mechanical and short-circuit performance.
EN 50180 and EN 50181 define equipment-bushing interfaces used in many European systems. The contracted edition and the manufacturer’s declared interface must be checked.
CENELEC HD 629.1 is widely referenced for tests on accessories for extruded-insulation power cables.
IEC 60287 provides methods for calculating cable current rating and losses.
IEC 60949 and IEC 60986 address thermally permissible short-circuit current and MV cable short-circuit temperature limits.
IEC 61914 covers cable cleats and intermediate restraints, including resistance to declared electromechanical forces.
IEC 60071-1 and IEC 60071-2 cover insulation coordination.
IEC 60099-4 applies to gapless metal-oxide surge arresters for AC systems.
IEC 62271-213 applies to voltage detecting and indicating systems installed on high-voltage equipment.
IEEE Std 386 covers shielded separable insulated connector systems in its scope and establishes ratings, interchangeable construction features and tests for IEEE-market loadbreak and deadbreak systems.

A declaration that a connector complies with an accessory standard does not prove that it fits a particular bushing, cable or compartment. Likewise, a type-tested switchgear panel is not evidence that every field-selected connector, cable arrangement or surge arrester can be installed without further verification.

3. Cable data that must be frozen before the panel design is released

“One 630 mm² cable per phase” is not an adequate cable definition. Two cables with the same conductor cross-section can have different insulation diameters, screen constructions, outer diameters, weights and bending radii. Those differences can change the required connector kit and determine whether installation is physically possible.

The switchgear manufacturer and connector supplier should receive, at minimum, the following approved data:

3.1 Electrical data

System nominal voltage and highest voltage for equipment, Un and Um
Cable voltage designation, U0/U (Um)
System frequency
Maximum continuous feeder current and expected load profile
Permitted emergency or cyclic loading, if any
Three-phase short-circuit current and clearing time
Maximum single-phase-to-earth fault current and clearing time
System earthing method: solid, resistance-earthed, reactance-earthed, resonant-earthed or isolated
Required lightning impulse withstand level and power-frequency withstand level
Surge-arrester duty and temporary-overvoltage conditions, where applicable

3.2 Cable construction data

Cable standard and exact manufacturer/type designation
Number of cables per phase and phase formation
Single-core, three-core or triplex construction
Copper or aluminium conductor
Conductor cross-section and conductor class/construction
Circular, compacted circular, sector-shaped or flexible conductor, as applicable
Stranded conductor diameter
Insulation material, normally XLPE or EPR
Actual diameter over insulation—not merely a catalogue nominal value
Insulation-screen type: bonded, strippable or other approved construction
Metallic screen type and cross-section: copper wires, tape, concentric neutral or combination
Armour type and dimensions, if present
Overall cable diameter, tolerance and mass per metre
Minimum installation bending radius and minimum final bending radius
Water-blocking tapes, powders or yarns that affect preparation
Oversheath material and any special fire, oil, UV, chemical or offshore requirement

3.3 Installation data

Bottom, top or rear entry
Trench, false floor, duct or cable-ladder arrangement
Available straight length below or above the panel
Cable pulling direction and pulling-eye requirement
Cleat type, spacing and supporting structure
Ambient temperature and grouping conditions
Indoor, outdoor, humid, polluted, corrosive, floodable or offshore conditions
Fire-barrier and fire-sealing requirements
Screen-bonding method and location of link boxes or bonding links
Required cable-test method and test connection
Required CTs, CBCTs, sensors, VDIS and surge arresters

The cable data sheet should be approved before the final compartment drawing. If the cable supplier is not yet selected, the designer should use an explicitly defined envelope covering the maximum expected outer diameter, insulation diameter, bending radius and mass—not a single optimistic catalogue value.

4. Electrical rating coordination

4.1 Voltage rating: coordinate the complete insulation system

The connector must match the cable voltage designation and the switchgear’s highest voltage for equipment. For example, a cable designated 12/20 (24) kV is normally coordinated with equipment having Um = 24 kV. However, the label “24 kV connector” is not enough. The engineer must also verify:

the applicable accessory standard and test level;
the bushing-interface standard and exact interface type;
the cable’s diameter over insulation;
the system earthing and temporary-overvoltage duty;
altitude and environmental corrections where relevant;
the switchgear’s rated withstand voltages; and
any project-specific insulation-coordination study.

Do not confuse nominal system voltage, cable phase-to-earth voltage U0, cable phase-to-phase voltage U, highest cable-system voltage Um, surge-arrester continuous operating voltage Uc, and arrester rated voltage Ur. They describe different duties.

4.2 Continuous current: the smallest valid rating governs

The continuous current capability is not automatically the nameplate current of the switchgear. The current path includes:

cable conductor;
conductor lug or mechanical connector;
separable connector contact system;
equipment bushing;
internal panel conductor; and
any coupling connector, adapter or plug-in accessory.

The permitted feeder current is limited by the lowest applicable rating after installation conditions are considered.

For a three-phase load, the design current is commonly derived from:

I_b = S / (√3 × U)

or, for a load stated by active power:

I_b = P / (√3 × U × η × cos φ)

Cable ampacity should then be calculated for the real installation using an appropriate method such as IEC 60287. Important correction factors include ambient temperature, grouping, thermal environment, installation in ducts or enclosed compartments, screen bonding, harmonics and unequal sharing between parallel conductors.

For n parallel cables per phase, dividing the total current by n is only a first approximation. Equal sharing requires compatible conductor construction, equal length, equal impedance, symmetrical routing, equivalent terminations and comparable thermal conditions. A phase sequence such as ABC-ABC may not produce the same current sharing and magnetic field as an intentionally engineered arrangement. The complete formation must be reviewed.

4.3 Short-circuit duty: verify more than the conductor

The switchgear short-time withstand rating does not automatically validate the cable connection. Separate checks are required for:

conductor thermal withstand;
conductor connector or lug;
separable connector and bushing;
cable metallic screen and its earth lead;
screen-bonding links and earth bar;
CT primary path or sensor arrangement;
cable cleats, brackets and gland plate; and
any surge-arrester earth lead exposed to fault current.

An initial adiabatic check for a conductor or earthing path is often expressed as:

I²t = k²S²

or:

S = I√t / k

where S is conductor cross-section and k depends on conductor material, initial temperature, final permissible temperature and insulation system. The correct values and any non-adiabatic correction must come from the applicable standard and cable/accessory data. The equation should not be used with a remembered generic k value when the screen material, joints, earth leads or accessory temperature limit may govern.

The metallic screen is normally checked against the earth-fault current and protection clearing time, not automatically the three-phase fault level. In a resistance-earthed system, this duty may be modest. In a solidly earthed network, it may be severe. The study must also define how fault current divides between screens, earth conductors, armour and the station earthing system.

4.4 Peak electromechanical force and cable restraint

Fault current produces high forces between single-core cables. The mechanical duty depends strongly on peak current, conductor spacing, arrangement and cleat spacing. The relationship is approximately proportional to the square of the instantaneous current; consequently, a modest increase in fault level can produce a much larger force.

Cleats should be selected and spaced using the calculated fault duty and the cleat manufacturer’s declared performance under IEC 61914. The support structure is part of the system: a strong cleat on a weak bracket is not a compliant restraint.

The termination and bushing must not carry cable weight or absorb the forces produced by an unsupported bend. A fixed support should normally be provided close enough to control movement but positioned so that it does not violate the minimum bending radius or interfere with connector installation.

5. Separable connector technologies

5.1 Screened separable connector versus conventional air termination

A conventional indoor termination exposes insulated phase conductors and relies on air clearances, stress-control components and barriers. A screened separable connector uses a premoulded insulating body with a conductive outer screen connected to earth. When correctly assembled and earthed, the electric field is contained within the connector system, enabling a compact connection.

Screened connectors can offer:

compact phase spacing;
reduced sensitivity to external pollution;
defined equipment interfaces;
modular test plugs, coupling connectors and arresters;
an earthed outer screen; and
reduced maintenance compared with exposed air terminations.

Their reliability nevertheless depends heavily on cable preparation, cleanliness, correct component selection and controlled assembly. A small cut in the cable insulation or semiconductive screen, contamination at an interface, a wrong cable reducer or an incorrectly installed lug can create electric-field enhancement and partial discharge.

5.2 Outer-cone connectors

In an outer-cone system, the equipment bushing presents an external conical interface. A moulded elbow, straight connector or tee fits over that cone. Depending on the interface family, the connection may be plug-in or bolted.

Common CENELEC-style outer-cone families include:

Interface family	Common product association	Typical use	Important limitation
Type A	Commonly 250 A	Compact elbow, straight or tee connections, often up to 24 kV	The interface letter does not establish the cable range or all voltage ratings.
Type B	Commonly 400 A	Elbow connections for distribution equipment	Confirm the exact bushing dimensions, voltage class and product rating.
Type C	Commonly 630 A, with qualified 1250 A systems also available	Bolted tee connectors, coupling connectors, test rods and plug-in arresters, with products available up to 42 kV	A 1250 A connector requires a 1250 A-qualified complete contact path; a Type C shape alone does not provide that rating.

Other outer-cone interface families and manufacturer-specific arrangements exist. They should be specified only from the applicable interface standard, dimensional drawing and manufacturer evidence.

The practical rule is simple:

Never order “a Type C connector” without the exact bushing drawing, connector family, voltage designation, continuous current, cable construction, conductor range and insulation-diameter range.

5.3 Inner-cone connectors

In an inner-cone system, the equipment has a recessed socket and the cable connector plugs into the internal conical interface. Inner-cone systems are frequently used where compact construction, higher current, robust screening or modular plug-in connections are required.

“Inner cone” is a technology description, not a complete order code. The connector size, socket standard, voltage rating, current rating, contact system, cable data and mounting orientation must all match. A connector described as Size 2 by one declared system should not be assumed compatible with a different socket without documented interface conformity.

5.4 IEC/CENELEC and IEEE interfaces are different systems

IEEE Std 386 covers loadbreak and deadbreak connector systems for the voltage and current range in its scope. IEEE 386-2025 establishes requirements for systems used on power-distribution systems rated 2.5 kV through 35 kV and 900 A or less; the standard’s title identifies shielded connector classes from 15 kV through 35 kV. Products commonly include 200 A loadbreak interfaces and higher-current deadbreak arrangements.

CENELEC outer-cone Type A, B or C terminology must not be used as if it were interchangeable with an IEEE 386 interface. Even when two products have similar current and voltage labels, their geometry, contact, test point, operating method and qualification may differ.

5.5 Deadbreak and loadbreak are operating capabilities—not casual descriptions

Most bolted screened tee connectors used on IEC-style switchgear are deadbreak devices. They must be disconnected only after the circuit is de-energized, isolated, proven dead, earthed and released under the approved safety procedure.

A loadbreak connector is specifically designed and tested to make or break a defined current under prescribed conditions, normally using the intended operating tool and procedure. A connector is not loadbreak merely because it is separable or has an operating eye.

Never infer switching capability from appearance.

6. How to select the exact connector kit

The selection process should be documented as a compatibility matrix. At minimum, verify each of the following:

Selection item	Required evidence
Equipment interface	Bushing manufacturer, type, interface standard, interface family/size and dimensional drawing
System voltage	U0/U (Um), network earthing and applicable accessory voltage class
Continuous current	Qualified rating of bushing, connector, contact, coupling device and internal conductor
Short-circuit duty	Connector/bushing short-time and peak withstand evidence for the project duty
Conductor	Cu or Al, cross-section, class, shape and actual conductor diameter
Cable insulation	XLPE/EPR type and actual diameter over insulation within the reducer range
Insulation screen	Bonded or strippable semiconductive layer and approved preparation method
Metallic screen	Wire/tape/concentric construction, area and earthing-kit compatibility
Environment	Indoor/outdoor, pollution, moisture, floodability, temperature, offshore or hazardous area
Accessories	Test point, insulating plug, screen break, coupling connector, arrester, dead-end plug and earthing plug
Installation method	Required tools, crimp dies or shear-bolt sequence, lubricant, torque and trained installer

The diameter over insulation is often the decisive dimension for the cable reducer or stress-control body. Selecting by conductor cross-section alone is a common cause of incorrect kits.

Mechanical shear-bolt connectors can reduce dependence on large crimping tools, but they are not universal. Confirm conductor material, class, diameter range, sector/circular shape, centring inserts, bolt sequence and the connector’s IEC 61238-1-3 qualification. Compression connectors require the exact lug, die set, number and position of crimps, crimping direction and tool calibration.

7. Physical compartment design

7.1 Start with a scaled three-dimensional routing study

A general-arrangement drawing should show more than the termination point. The design review should include:

cable centreline and minimum bending envelope;
connector body and installation-tool envelope;
space needed to remove the connector or install a test rod;
CT and CBCT aperture, position and terminal-box access;
cable cleats and bracket locations;
screen earth and main earth-bar routing;
surge arrester and earth lead;
earthing-switch blades or moving parts;
gland plate and fire-seal zone;
door, barrier and pressure-relief clearances; and
the installer’s hand and tool access.

A bend that fits in the final condition may still be impossible to install. The connector body often has to be slid, rotated or pushed onto the bushing, and the cable cannot be bent sharply during that operation. The installation sequence must therefore be simulated, not just the final geometry.

7.2 Bending radius and termination height

Use the cable manufacturer’s stated installation and final bending radii. The installation radius can be larger because the cable is moving and under tension. Measure the bend from the cable centreline using the actual outer diameter and route.

The required termination height depends on:

cable outside diameter;
minimum bend radius;
trench or floor opening position;
connector length and orientation;
straight length required below the connector;
cleat position; and
number of parallel cables.

Do not pull the cable into alignment using the bushing fastener. The cable route should naturally meet the connector axis after it is cleated.

7.3 Multiple cables per phase

Parallel cable arrangements create four distinct problems:

Space: tee and coupling connectors extend farther from the bushing and may clash with the door, adjacent phases or barriers.
Current sharing: different route lengths and magnetic surroundings produce unequal impedance.
Thermal performance: closely grouped cables and connectors heat each other.
Fault forces: more conductors create a more complex electromechanical restraint problem.

The connector supplier should confirm the permitted number and sequence of coupling connectors. The switchgear manufacturer should confirm that the arrangement is within the assembly’s temperature-rise, dielectric and internal-arc design envelope.

7.4 Gland plates and induced heating

Single-core AC cables passing individually through ferromagnetic metal can produce eddy-current and hysteresis heating. Good practice is to use a suitable non-magnetic gland plate or arrange a common opening that encloses all phase conductors of the same circuit so that their magnetic fields substantially cancel. The exact solution must also maintain mechanical strength, ingress protection, vermin protection, fire sealing and internal-arc performance.

Do not assume every stainless-steel grade is non-magnetic under all fabrication conditions. Specify the material and validate the final arrangement.

7.5 Cable entry, sealing and drainage

The civil and electrical designs must agree on:

floor opening and trench dimensions;
water ingress and drainage;
fire-stop system and certification;
smoke and gas sealing;
spare-entry closure;
cable movement caused by thermal cycling;
induced currents in metallic frames; and
how the gland plate is removed or replaced without damaging installed cables.

Fire-stop material should not be packed around a connector body or stress-control region unless that arrangement is explicitly approved. It can alter heat dissipation, apply mechanical stress or make inspection impossible.

8. CTs, core-balance CTs and sensors

8.1 Phase current transformers

For parallel cables, the designer must decide whether all cables of one phase pass through one phase CT, whether separate CTs are used, or whether the CT is located on an internal primary conductor. The decision affects CT ratio, aperture, saturation performance, mechanical layout and protection-zone boundaries.

Verify:

CT polarity and physical orientation;
clearance between cable screen/earth parts and CT secondary terminals;
access to CT nameplate and terminal box;
cable centring where required by the CT or sensor manufacturer;
the short-circuit withstand of any primary bar; and
whether the installed cable arrangement changes the CT’s usable accuracy.

8.2 Core-balance CT routing

A core-balance CT measures the vector sum of the currents passing through its aperture. All live conductors belonging to the protected circuit must pass through the CT in a consistent direction. A neutral conductor, where part of that circuit, must be treated according to the protection design.

Metallic screens and earth tails require special attention. If the complete screened cable passes through the CBCT, screen current can influence or cancel the measurement. At a switchgear-end screen bond, the screen-earth lead is commonly routed back through the CBCT window in the direction defined by the approved CT arrangement so that its magnetic contribution is cancelled and the relay measures the intended residual current. The exact routing must follow the protection drawing and CT manufacturer’s instructions; it should never be improvised on site.

Typical errors include:

one phase cable outside the CBCT;
earth braid through the aperture in the wrong direction;
one of two parallel cables omitted;
armour or screen bonded on both sides of the CT unintentionally;
CT installed around one phase instead of all conductors of the circuit; and
cable earth conductor tied to a structural earth point before completing the intended return path.

8.3 Voltage indication and capacitive test points

A capacitive test point on a connector is a sensing feature with defined limitations. Its output depends on the coupling capacitance, indicator input and grounding conditions. It is not, by itself, a universal voltage measurement point or proof that the main conductor is dead.

Where a formal voltage detecting and indicating system is required, specify a system complying with the applicable IEC 62271-213 requirements and define:

indication location;
interface and portable indicator;
phase-comparison method;
functional test;
maintenance procedure; and
relationship to the site’s safe-isolation rules.

9. Cable screens, bonding and earthing

9.1 Why screen bonding matters

The cable metallic screen provides an earth-fault current path, controls electric field and influences induced voltage and cable losses. Its bonding method affects both safety and ampacity.

Common arrangements include:

single-point bonding, which avoids circulating screen current but can create an induced voltage at the open end;
both-end bonding, which controls standing voltage but permits circulating current and associated losses; and
cross-bonding, normally used on longer circuits to reduce net induced voltage and losses.

The selected scheme must be based on cable length, load current, formation, screen resistance, induced-voltage limits, earth-fault duty and utility practice. IEC 60287 calculations show why bonding arrangement can materially change screen losses and therefore cable current rating.

9.2 Screen-earth conductor sizing

The screen, earth braid, lug, bonding link and earth bar must withstand the earth-fault duty for the protection clearing time. Check the complete path, including the smallest braid supplied in a termination kit. A large cable screen connected through an undersized accessory earth lead does not retain the screen’s full fault-current capability.

Also verify touch voltage, induced standing voltage, sheath-voltage limiter duty where used, and the fault-current division between parallel screens and the station earth grid.

9.3 Screen-break arrangements and sheath testing

Some separable connectors provide a screen-break design that permits outer-sheath testing without dismantling the main connector. This is valuable only if the bonding links, test voltage, disconnection sequence and restoration checks are clearly defined.

The test procedure should state:

which screen and earth links are removed;
which sensitive devices or surge limiters are disconnected;
the permitted sheath-test voltage and duration;
how adjacent circuits are protected;
how the screen is discharged after testing; and
how correct bonding is verified before energization.

10. Surge arrester integration

A surge arrester should be selected from an insulation-coordination study, not added merely because space is available. Its continuous operating voltage, rated voltage, temporary-overvoltage capability, nominal discharge current, energy duty, pressure-relief or short-circuit behaviour and environmental rating must suit the network.

The protective voltage at the equipment is not only the arrester residual voltage. Lead inductance adds voltage approximately according to:

V_L = L × di/dt

During a steep surge, even a modest loop inductance can add significant voltage. Keep the connection from phase to arrester and from arrester to earth as short, straight and low-inductance as practical. Avoid large loops and do not route the arrester earth lead through a CBCT unless the protection design specifically requires it.

For plug-in arresters mounted on a tee or coupling connector, check:

interface and mechanical compatibility;
phase-to-phase and phase-to-earth space;
connector stack length and door clearance;
ability to support the arrester mass;
arrester replacement and test access;
the manufacturer’s permitted connector combinations; and
whether the full arrangement is accepted by the switchgear manufacturer.

11. Access, interlocking, service continuity and internal arc

IEC 62271-200 classifications apply to the tested and documented assembly—not to an isolated compartment viewed without its operating rules.

11.1 Loss of service continuity

The declared loss-of-service-continuity category describes conditions under which access to a high-voltage compartment may be possible while specified other parts remain energized. It does not mean the opened cable compartment is safe under every system condition.

The operating instruction must define:

which circuit must be isolated;
whether the busbar may remain energized;
which shutters or barriers remain in position;
how absence of voltage is verified;
how the cable is earthed;
which interlocks are operational; and
whether access is tool-based or interlock-controlled.

11.2 Internal arc classification

When an internal arc classification is assigned, it is supported by tests performed on a defined assembly and installation arrangement. Field changes can undermine the evidence, including:

drilling unapproved holes;
replacing a gland plate with a weaker plate;
leaving cable-entry openings unsealed;
obstructing a pressure-relief duct;
removing barriers to make cables fit;
adding unverified components near the arc-initiation zone; or
changing door fasteners or interlocks.

Any deviation should be reviewed by the switchgear manufacturer before implementation.

11.3 Earthing switch and cable testing

Confirm the earthing switch’s making capacity, short-time withstand, interlocks and connection point relative to CTs and the cable. The test procedure must identify whether test voltage is applied through a dedicated test plug, directly to a disconnected cable connector or through another approved interface.

Do not apply a cable test voltage through connected VTs, surge arresters, sensors, indicators or other components unless they are specifically rated and intended for that test.

12. A practical engineering workflow

The following workflow prevents most late site modifications.

Step 1 — Define the network duty

Issue approved values for Un, Um, frequency, load current, fault currents, clearing times, earthing method, insulation levels and surge environment.

Step 2 — Freeze the cable envelope

Obtain the exact cable data or define a conservative dimensional envelope. Include diameter over insulation, overall diameter, conductor construction, screen data, mass and bending radii.

Step 3 — Select the connection technology

Choose conventional air termination, screened outer cone or inner cone based on switchgear design, space, current, environment, maintenance and utility practice.

Step 4 — Complete the bushing-to-cable compatibility matrix

Record the bushing type, interface drawing, connector kit, conductor connector, reducer range, screen kit, continuous current, short-circuit duty and accessory combination.

Step 5 — Calculate cable rating and screen bonding

Perform the ampacity, loss, induced-voltage and screen-fault calculations for the real route and formation. Do not use free-air catalogue ampacity for an enclosed, grouped cable bend.

Step 6 — Produce a scaled routing model

Include installation movement, not only the final position. Show the cable trench, bending envelope, CTs, cleats, earth leads, arresters, barriers, door and tool space.

Step 7 — Coordinate protection and measurement

Confirm CT ratio, class, polarity, protection zone, CBCT aperture and all earth-conductor routes. Define VDIS and phase-comparison requirements.

Calculate cable-cleat duty, confirm bracket strength and validate gland plate, fire stopping, sealing and pressure-relief paths.

Step 9 — Define installation and testing before FAT

Agree who supplies and installs connectors, required training, special tools, torque records, cable test adapters, temporary earths and commissioning tests.

Step 10 — Inspect a representative feeder

For a first-of-type or difficult arrangement, use an actual cable sample or dimensionally accurate mock-up. A physical trial can reveal tool-access and bending problems that a simplified model misses.

13. Worked engineering example

Consider an incomer with the following preliminary data:

Switchgear highest voltage: Um = 24 kV
Switchgear continuous current: 1250 A
Short-time withstand current: 31.5 kA for 3 s
Cable: two parallel 1 × 630 mm² aluminium XLPE cables per phase
Cable voltage designation: 12/20 (24) kV
Metallic screen: copper-wire screen, final size to be confirmed
Entry: bottom
Proposed equipment interface: Type C outer cone
Proposed connection: tee plus coupling connector

This data is not sufficient to release the panel. The engineer should perform the following checks.

13.1 Voltage and interface

Confirm that the exact connector family is qualified for 12/20 (24) kV cable, that the bushing drawing declares the matching Type C interface and that the complete bushing/connector/contact combination is rated 1250 A. “Type C” alone does not prove 1250 A.

13.2 Cable and reducer selection

Obtain actual conductor diameter, diameter over insulation, insulation-screen type, overall diameter and tolerances from the selected cable manufacturer. Confirm both the mechanical conductor connector and cable reducer against those values.

13.3 Continuous current

A simple equal-share estimate gives 625 A per cable at 1250 A total, but the final decision requires an ampacity calculation for:

two circuits closely grouped within the panel;
the bend and cleat arrangement;
screen bonding;
ambient temperature inside the cable compartment;
unequal impedance and current sharing; and
heat from connectors and adjacent phases.

The tee, coupling connector, contact system and bushing must also be qualified for the intended total current and configuration.

13.4 Short-circuit thermal duty

Check each conductor and connector for the prospective current and 3 s duration. Assuming perfect 50/50 sharing without evidence is unsafe; tolerances and geometry must be considered. Separately calculate the copper-screen duty using maximum earth-fault current, clearing time and fault-current distribution.

13.5 Mechanical layout

Model six large single-core cables, connector stacks and required tool access. Verify the cable manufacturer’s installation bending radius. Provide fault-rated cleats and a supporting frame. Use a suitable non-magnetic gland arrangement and ensure all phase conductors of each circuit are routed in the approved formation.

13.6 CT arrangement

If phase CTs are around the cables, confirm that both parallel cables of each phase pass through the correct CT and that its aperture is practical. A single CBCT around all six large cables may be physically unsuitable; alternatives such as residual connection of phase CTs or another approved earth-fault measurement method may be needed, depending on sensitivity and protection requirements.

13.7 Testing and maintenance access

Confirm that a test rod or approved test adapter can be installed without removing barriers or overstressing the cable. Define how both parallel cables are isolated, tested, discharged and earthed. Ensure the coupling arrangement does not block the door or prevent inspection of earth leads.

The result may be a wider or deeper panel, a rear-access cable compartment, a different CT position, an inner-cone connection, a bus duct or another engineered arrangement. The correct solution is determined before manufacture—not by cutting barriers during installation.

14. Installation: where most accessory failures are created

MV separable connectors are precision insulation systems. Installation quality is as important as design quality.

14.1 Before starting

Confirm cable identity, phase, connector kit and bushing interface.
Compare actual cable dimensions with the kit selection sheet.
Review the manufacturer’s current installation instruction.
Verify installer training or authorization where required.
Check tool calibration, crimp dies, shear-bolt tools and torque wrench.
Provide a clean, dry and well-lit work area.
Inspect the bushing for damage, contamination and correct protective cap.
Confirm that the cable has been positioned and cleated without load on the bushing.

14.2 Cable preparation

Cable preparation dimensions are not generic. Use the connector manufacturer’s instruction for oversheath cutback, screen-wire treatment, semiconductive-screen removal, insulation length and conductor exposure.

Key quality controls include:

preventing knife damage to insulation;
producing a smooth semiconductive-screen edge;
removing conductive residue from the insulation;
using only approved cleaning agents and lint-free materials;
respecting the required direction of cleaning so contamination is not moved onto clean insulation;
avoiding unapproved abrasive materials;
checking insulation diameter after preparation where instructed; and
keeping prepared surfaces protected from dust and moisture.

Never hide an insulation cut by polishing it without an approved assessment. Deep scratches, embedded semiconductive material or an irregular screen edge can initiate partial discharge.

14.3 Conductor connection

For compression lugs, use the specified die, tool, sequence, number of crimps and orientation. For mechanical connectors, use the specified centring inserts and bolt-tightening sequence. Do not re-tighten a shear bolt after the head has broken unless the instruction explicitly allows it.

Aluminium conductors require the approved surface preparation and contact compound. Oxidation control, strand condition and conductor insertion depth directly affect contact resistance.

14.4 Connector assembly

Use only the supplied or approved lubricant in the stated amount and locations. Excess or incorrect lubricant can contaminate interfaces; insufficient lubricant can damage the moulded body or prevent full seating.

Check:

full engagement with the bushing;
specified clamping screw or stud torque;
correct insulating plug and conductive cap;
outer-screen continuity and earth lead;
phase spacing and connector orientation;
screen-break link position; and
absence of mechanical force from the cable.

Torque values must come from the exact product instruction. There is no safe universal torque for “a 630 A tee connector.”

15. Commissioning and test strategy

The commissioning plan should distinguish between tests of the cable, tests of the accessory and functional checks of the switchgear.

15.1 Minimum documented checks

Cable and connector identity against approved data
Visual inspection of all three phases
Installation-dimension or witness records where required
Crimp or shear-bolt record
Final torque record for specified fasteners
Connector seating and screen-earth continuity
Phase identification and phasing check
CT polarity, ratio and secondary-circuit checks
CBCT conductor and earth-tail routing
VDIS/voltage indication functional check
Earthing-switch operation and interlock check
Verification that temporary earths and test links are removed or restored as required
Final photographs before closing the compartment

15.2 Cable withstand testing

Use the contracted cable standard, utility procedure and cable manufacturer’s recommendation. The test method, waveform, voltage and duration must be suitable for the cable type, age and installed accessories.

Before applying test voltage:

isolate the cable from switchgear components not designed for the test;
disconnect or protect VTs, surge arresters, sensors and indicators as specified;
install the approved test adapter;
control and earth adjacent phases;
establish exclusion zones; and
define the discharge and re-earthing period after the test.

Do not adopt a blanket DC hipot practice for extruded MV cable systems without the applicable engineering basis and asset-owner procedure.

15.3 Partial-discharge testing

Individual connector components may receive factory AC-withstand and partial-discharge routine tests, but those tests do not validate field cable preparation. Where project criticality justifies it, an installed-system PD measurement can provide a useful baseline and may reveal defects at the stress-control region or interface.

Interpret PD results with knowledge of test voltage, noise, sensor position and propagation path. A “no PD detected” result is limited by the sensitivity and setup of the measurement.

16. Inspection, condition assessment and maintenance

Screened separable connectors are often described as maintenance-free, meaning that correctly installed internal interfaces do not require periodic dismantling. It does not mean the surrounding compartment should never be inspected.

An asset-management program may include:

visual inspection during an outage for tracking, contamination, cracking, corrosion, displaced caps or earth-lead damage;
verification that cables remain supported and do not load bushings;
thermography under representative load where surfaces are observable and the safety procedure permits;
comparison of phase temperatures and parallel-cable current sharing;
online or offline PD assessment for critical assets;
inspection of heaters, seals, drainage and condensation control;
screen-current or bonding checks where abnormal losses are suspected; and
review of fault history and through-fault duty.

Do not routinely disturb, unplug or re-torque a connector unless the manufacturer’s maintenance instruction or a diagnosed condition requires it. Unnecessary intervention can introduce contamination or mechanical damage.

17. Common failure modes and diagnostic clues

Symptom or finding	Possible cause	Engineering response
One connector hotter than the other phases	Poor lug contact, incorrect torque, conductor oxidation, overload or unequal parallel-cable sharing	Compare load and phase temperatures, inspect records, measure current sharing and follow the manufacturer’s diagnostic procedure.
Localized PD near cable entrance to connector	Semiconductive-screen cut, insulation scratch, contamination, void or wrong reducer	De-energize under an approved procedure; locate the source and replace/reterminate as required.
Tracking or surface discharge	Moisture, pollution, damaged screen, incomplete earth connection or mismatched components	Correct the environmental cause and replace damaged insulation components.
Spurious earth-fault current	Incorrect CBCT routing, screen-earth lead through the CT incorrectly, parallel cable omitted or secondary wiring problem	Compare the installed route with the protection drawing and perform primary injection or functional testing.
Repeated sheath-test failure	Outer-sheath damage, water ingress, incorrect screen-break isolation or connected limiter	Sectionalize the screen/sheath system and test using the approved procedure.
Bushing crack or connector displacement	Cable weight, excessive bend force, fault movement or assembly error	Remove mechanical load, inspect the complete interface and verify cleat design.
Corroded earth braid or screen wires	Moisture, incompatible metals, chemical environment or poor sealing	Replace affected components, correct drainage/sealing and review material compatibility.
Arrester distress or operation	Incorrect Uc/Ur, temporary overvoltage, excessive energy duty, poor earth path or end of life	Review the network and insulation-coordination study; do not replace with the same rating without diagnosis.

18. Frequent specification and design mistakes

Selecting the connector only by voltage and conductor cross-section.
Treating the interface letter as the complete electrical rating.
Ignoring actual diameter over insulation and its tolerance.
Using cable free-air ampacity for the hot, grouped compartment route.
Assuming parallel cables share current perfectly.
Forgetting the installation bending radius and connector movement envelope.
Allowing the bushing to support cable weight.
Passing individual single-core cables through a ferromagnetic plate without an induced-heating review.
Specifying a CBCT without checking the aperture and screen-earth route.
Sizing the screen earth lead without the earth-fault study.
Adding a surge arrester with long phase and earth leads.
Using a capacitive test point as an informal proof of dead.
Applying cable test voltage through connected arresters, VTs or sensors.
Removing a barrier or drilling a gland plate without reviewing IAC and IP implications.
Assuming “touch-safe” means safe to touch while energized.
Ordering connector kits before the cable manufacturer and cable construction are frozen.
Failing to provide test plugs, dead-end plugs or earthing accessories in the project scope.
Leaving responsibility for termination installation and warranty undefined.

19. Purchaser’s specification template

The following wording can be adapted for a project specification:

Each MV feeder cable compartment shall be designed for the approved project cable construction, number of cables per phase, actual cable dimensions, installation and final bending radii, cable-entry direction, fault duty and screen-bonding arrangement. The switchgear manufacturer shall provide a coordinated cable-compartment drawing showing equipment bushings, exact separable-connector interfaces, cable centrelines and bending envelopes, CTs/CBCTs, cable supports and cleats, surge arresters, earth bars, test access, gland plates, barriers and pressure-relief clearances.

Separable connectors, conductor connectors and cable reducers shall be selected against the exact equipment-bushing interface and approved cable data, including U0/U (Um), conductor material, cross-section, construction and diameter; diameter over insulation; insulation-screen type; metallic-screen construction; and environmental duty. Interface designation alone shall not be accepted as evidence of compatibility or current rating.

The complete connection path shall be rated for the specified continuous current and short-circuit duty. Cable screens, screen-earth leads, bonding links and earth bars shall be coordinated with the maximum earth-fault current and protection clearing time. Cable cleats and supports shall be selected for the calculated electromechanical duty and shall comply with the applicable IEC 61914 requirements.

Installation shall be performed in accordance with the connector manufacturer’s current instructions by trained personnel using calibrated tools. Crimp, shear-bolt and torque records, phase identification, screen-earth continuity, CT/CBCT routing, VDIS function, cable-test configuration and final inspection records shall be included in the commissioning dossier.

20. Final design-review checklist

Ratings and standards

Contracted IEC, EN/CENELEC or IEEE editions are identified.
Un, Um, U0/U (Um) and withstand levels are coordinated.
Continuous current is valid for the complete connection path.
Three-phase short-time and peak duties are verified.
Earth-fault current and duration are defined separately.
Connector, bushing, coupling device and conductor connector have documented ratings.

Cable and connector compatibility

Exact cable manufacturer and type are approved.
Conductor material, class, shape, diameter and cross-section are verified.
Diameter over insulation and tolerance match the reducer/stress-control range.
Semiconductive-screen type matches the installation method.
Metallic screen and earth kit are compatible.
Exact equipment-interface drawing and connector part number are recorded.
Required test plugs, dead-end plugs, earthing plugs and caps are included.

Mechanical arrangement

Installation and final bending radii are shown on a scaled drawing.
Connector installation and removal envelopes are clear.
Cables impose no unacceptable weight or bending load on bushings.
Cleats and support structure are verified for fault forces.
Parallel-cable formation and phase sequence are defined.
Gland plate material/openings have been reviewed for induced heating.
Trench, floor opening, drainage, sealing and fire stopping are coordinated.

Protection, earthing and accessories

CT location, ratio, class, polarity and aperture are correct.
Every required conductor passes through the correct CT/CBCT.
Screen-earth lead routing through or around the CBCT is shown explicitly.
Screen bonding and link positions match the cable study.
Screen-earth path withstand is verified.
Surge arrester rating comes from an insulation-coordination review.
Arrester leads are short and correctly routed.
Voltage indication/detection and phase comparison are defined.

Safety, testing and documentation

Cable compartment access and interlocking procedure are defined.
LSC and IAC claims match the final arrangement.
No cable or accessory obstructs the pressure-relief path.
Cable test voltage, method and connection point are defined.
VTs, sensors, arresters and indicators are protected from test voltage.
Installer qualification and special tools are included in scope.
Crimp/shear-bolt/torque records and final photographs are required.
Screen links and temporary earths have a documented restoration check.

Conclusion

Reliable MV cable termination begins long before the cable arrives on site. It begins when the network duty, cable construction, equipment bushing, connector family, protection scheme and physical route are treated as one engineered system.

The essential rules are:

Freeze the real cable data early.
Select the connector from the exact cable and bushing—not from a generic interface label.
Verify voltage, continuous current, short-circuit and earth-fault duties across the complete connection path.
Prove the cable can be installed, supported, tested and removed using a scaled routing study.
Show every screen-earth, CT and CBCT route explicitly.
Protect the switchgear’s tested barriers, gland arrangement and pressure-relief path.
Treat installation cleanliness, cable preparation and documented torque as primary insulation-quality controls.

When these principles are applied, the cable compartment becomes a controlled, maintainable interface rather than the place where unresolved project decisions are discovered during commissioning.

References and further reading

Engineering note: Always verify the contracted edition, national adoption, project specification, cable and connector manufacturers’ current data, and the switchgear manufacturer’s approved configuration before applying this guide to a supplied assembly.