A stress cone cable termination is the component of a medium-voltage cable end-fitting that gradually enlarges the cut-back insulation screen interface to spread the electric field over a longer path, preventing localized dielectric breakdown or surface flashover at the point where the cable’s metallic screen is removed. This guide explains, from first principles and with worked numbers, why stress control geometry and correct screen/sheath bonding are inseparable parts of the same termination design problem, how to estimate the field reduction a stress cone provides, and how to size single-point versus cross-bonded screen arrangements. It is written for engineers specifying or commissioning MV terminations across export markets, including the Middle East and Gulf (incl. Egypt and Saudi Arabia), North Africa, CIS, and Sub-Saharan Africa, where cable routes often combine long single-core runs with mixed termination types at switchgear, transformers, and overhead-to-cable transitions.
Why stress control and screen bonding matter together
A shielded MV cable is, electrically, a coaxial capacitor: the conductor is one electrode, the extruded or wire screen is the other, and the field between them is radial and well-behaved as long as the screen is continuous. The problem starts exactly where the screen is cut back to terminate the cable — the equipotential surface is interrupted, and the field lines that were radial now have to bend toward the exposed insulation surface. Without geometric control, this produces a sharp local concentration of tangential electric stress along the insulation surface, which is far more likely to initiate tracking or flashover than the normal radial stress the cable insulation is designed to withstand. The stress cone (capacitive or geometric) is the accessory that re-shapes this transition. Screen/sheath bonding is a separate but linked issue: once the screen is terminated, its grounding method determines what voltage appears on the screen itself during normal load and fault conditions, which in turn affects personnel safety at the termination and the design of any sheath voltage limiters (SVLs). A termination design is only complete when both the dielectric stress path and the screen bonding/grounding scheme have been checked.
The method: electric field and stress-cone formulas
1. Radial field inside the cable insulation (coaxial approximation):
E(r) = U / [ r × ln(R/r) ]
Where U is the phase-to-ground voltage across the insulation, r is the radius from the cable axis to the point of interest, and R is the outer insulation radius (under the screen). This formula gives the “designed” stress the insulation sees everywhere the screen is intact — it is highest at the conductor screen (r = inner radius) and lowest at the outer screen.
2. Surface (creepage) stress at the screen cut-back:
Once the screen is removed over a length L at the termination, a simplified first-order model treats the voltage U as dropping along that exposed creepage path:
E_creep ≈ U / L
A short, abrupt cut-back gives a very short L and therefore a very high E_creep — this is the physical reason an unprotected cut screen is prone to flashover. A stress cone works by extending the effective transition length (and by capacitively grading the voltage along it), which lowers E_creep and brings the surface stress back down toward a tolerable range.
3. Induced sheath/screen voltage on single-point bonded systems:
For single-core cables run in parallel (trefoil or flat formation) with only one end of the screen grounded, the open end develops a standing voltage due to the mutual inductance between the phase conductor and its own screen:
Vs = I × X, with X = 2πf × (2×10⁻⁷) × ln(2S/d) [Ω/m]
Where I is the phase current, f is system frequency, S is the spacing between cable centers, and d is the diameter over the metallic screen/sheath. This voltage must stay within a level the termination’s insulation and any screen voltage limiter can safely handle, and within safe touch-voltage limits set by the applicable safety code.
Worked example
Part A — Stress cone benefit (illustrative, simplified model)
Take an 11 kV cable (phase-to-ground voltage Uo ≈ 6.35 kV), with insulation inner radius r = 7 mm (over conductor screen) and outer radius R = 12 mm (under the metallic screen).
Normal radial stress at the conductor screen (this is the stress the cable is designed for along its length):
E_normal = Uo / [r × ln(R/r)] = 6350 / [0.007 × ln(12/7)] = 6350 / [0.007 × 0.539] ≈ 1.68 × 10⁶ V/m ≈ 1.68 kV/mm
Now consider the termination. If the screen were cut back abruptly with almost no stress relief, assume an effective creepage length of only L = 20 mm:
E_creep (no stress cone) = Uo / L = 6350 / 0.020 ≈ 317.5 kV/m
That is roughly 190 times the cable’s own designed radial stress — clearly unsustainable and a classic cause of termination flashover.
With a stress cone extending the effective transition length to, say, L = 150 mm:
E_creep (with stress cone) = Uo / L = 6350 / 0.150 ≈ 42.3 kV/m
The stress cone in this illustrative model reduces the surface gradient by a factor of roughly 7.5×, bringing it back into a range the insulation surface and any field-grading (capacitive or geometric) layer can handle. (This is a simplified first-order model to illustrate the principle of stress-cone action, not a substitute for a manufacturer’s finite-element field design.)
Part B — Single-point bonded sheath voltage
Three single-core cables in trefoil, center spacing S = 0.15 m, diameter over the metallic screen d = 0.05 m, carrying I = 630 A at f = 50 Hz, over a route length of 500 m, single-point bonded (grounded at one end only).
X = 2πf × (2×10⁻⁷) × ln(2S/d) ln(2 × 0.15 / 0.05) = ln(6) = 1.792 2πf = 2 × 3.1416 × 50 = 314.16 X = 314.16 × 2×10⁻⁷ × 1.792 ≈ 1.126 × 10⁻⁴ Ω/m
Vs = I × X × length = 630 × 1.126×10⁻⁴ × 500 ≈ 35.5 V
At the open (ungrounded) end of this 500 m run, the sheath will stand at roughly 35 V above ground during normal load — a level that must be checked against the termination’s insulation and the enclosure’s touch-safety design, and against the rating of a sheath voltage limiter if one is fitted. Longer routes or higher currents raise this figure roughly linearly with length and current, which is why single-point bonding is normally restricted to shorter circuits, while longer circuits typically move to cross-bonding or both-ends bonding (with circulating-current trade-offs, see table below).
Comparison: screen/sheath bonding methods
| Bonding method | Sheath condition | Circulating current | Typical standing voltage | Best suited for |
|---|---|---|---|---|
| Both-ends (solid) bonding | Grounded at both ends | Present (full circuit) — reduces standing voltage but adds I²R loss | Near zero | Short circuits where added losses are acceptable |
| Single-point bonding | Grounded at one end only | None (open circuit) | Present at open end — see worked example | Short-to-medium single-core runs, needs an SVL at the open end |
| Cross-bonding | Screens transposed and grounded at both route ends | Minimized by sheath transposition | Low, balanced across sections | Longer single-core circuits (multi-section routes) |
Stress-control method comparison (qualitative)
| Method | Principle | Typical installation context |
|---|---|---|
| Pre-molded (push-on) stress cone | Factory-formed elastomeric cone slid over prepared cable end | Fast, repeatable field installation |
| Heat-shrink stress control | Heat-shrinkable stress-control tubing graded over the screen cut-back | Flexible for varying cable diameters |
| Cold-shrink stress control | Pre-stretched tube retracted onto the cable, no heat source needed | Hazardous-area or no-flame installations |
| Hand-taped stress cone | Built up manually with semi-conductive and insulating tapes | Legacy or bespoke repair work, skill-dependent |
Selection criteria and common pitfalls
- Match the stress-control accessory to the cable insulation and screen type (extruded semi-conductive vs. wire screen vs. tape screen) — a stress cone designed for one screen construction will not grade the field correctly on another.
- Verify creepage/leakage distance, not just the stress-cone length, against the termination’s pollution/environment class, particularly in coastal, desert-dust, or high-humidity GEO regions where surface contamination lowers the effective flashover voltage.
- Decide the bonding scheme at the design stage, not at site — retrofitting cross-bonding after installation is costly. Use the induced-voltage formula above during design to check whether single-point bonding is adequate or whether a sheath voltage limiter and/or cross-bonding is required.
- Do not omit the screen-to-ground connection even on “single-point” schemes — the ungrounded end still needs a properly rated SVL and insulated support to manage the standing voltage safely.
- Coordinate the termination with upstream protection — a termination flashover looks electrically like a fault, so correct discrimination settings on the feeder relay are part of the overall system safety, alongside correct stress control. See our [protection relay](/what-is-a-medium-voltage-protection-relay-mv-protection-relay/) resources for coordination guidance.
- Verify compatibility with the connected equipment — terminations feeding into [medium voltage switchgear](/what-is-mv-switchgear-medium-voltage-switchgear/) or medium voltage transformers must match the bushing/cable-box clearances and creepage requirements of that equipment, not just the cable’s own rating.
Standards
Cable accessories including stress cones and bonding arrangements for extruded MV cables are covered by IEC 60502-4 (cables and accessories for rated voltages 6 kV to 30 kV) and IEC 60840 (extruded cables and accessories for voltages above 30 kV up to 150 kV), which set construction and type-test requirements for terminations and joints. IEEE Std 48 is the equivalent North American reference standard for test procedures and classification of cable terminations. Sheath bonding practice (single-point, both-ends, cross-bonding) is addressed in cable system design guidance referenced by these same accessory standards and by utility engineering practice for circulating-current and induced-voltage management.
Related guides
See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.
What does a stress cone actually do in a cable termination?
It extends and grades the electric field at the point where the cable’s metallic screen is cut back, reducing the concentrated surface (tangential) stress that would otherwise cause flashover or tracking along the insulation.
Is a stress cone needed on every MV termination?
Yes, in some form — factory pre-molded, heat-shrink, cold-shrink, or taped. Every shielded MV cable termination needs a way to manage the field discontinuity at the screen cut-back; omitting it is a common cause of premature termination failure.
What is the difference between single-point and cross-bonding?
Single-point bonding grounds the screen at only one end, eliminating circulating current but leaving a standing voltage at the open end (see the worked example). Cross-bonding transposes the screens between route sections so the induced voltages largely cancel, which suits longer single-core circuits.
Why does the open end of a single-point bonded cable need a sheath voltage limiter?
Because the standing voltage calculated from the induced-sheath-voltage formula can exceed safe touch-voltage or insulation limits if the circuit is long or the current is high; the limiter clamps transient overvoltages (such as during a fault) without carrying continuous circulating current.
Can a stress cone be installed incorrectly even if the right product is used?
Yes — improper cable preparation (screen cut-back length, semi-conductive layer removal, surface cleanliness) is the most common field error and can defeat the stress cone’s field-grading function regardless of accessory quality.
Does ambient pollution affect stress-cone or termination performance?
Yes — surface contamination (dust, salt, humidity) lowers the effective flashover voltage along creepage paths, which is why terminations in coastal or high-dust GEO regions often require higher pollution-class accessories or additional sheds/skirts.
Which standard should I reference when specifying MV terminations?
Use IEC 60502-4 for terminations on cables rated 6–30 kV, IEC 60840 for higher-voltage extruded cables, or IEEE Std 48 where North American test practice applies — confirm with the cable and accessory manufacturer which standard governs the specific voltage class.
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