Project record · Lithuania · Cable test vans & fault location
High-Voltage Silicone Test Cable for a Cable Test Van — Lithuania
A cable test van needed a replacement high voltage silicone test cable: 8.7/15 kV AC continuously, 125 kV DC short-term, fully screened, and flexible enough to wind onto a 230 mm drum barrel that gets hauled out, dragged across the ground and wound back four to six times a day. TEBAOFLEX supplied a custom screened silicone construction, 22 mm overall diameter, built to a partial discharge limit of 5 pC at 20 kV AC.
Project record
| Location | Lithuania (order placed through a Lithuanian trading partner) |
|---|---|
| End user | A supplier of cable test vans and cable fault-location, withstand-test and diagnostic systems |
| Приложение | HV connection cable on the test-van reel — DC withstand testing, fault pre-location and pinpointing on buried MV/HV cable |
| Original reference | A legacy Pirelli screened test cable type, 6/6 kV, quoted at 120 kV DC |
| Cable supplied | 8.7/15 kV AC – 40 kV AC / 125 kV DC screened silicone measurement cable |
| Общий диаметр | 22 мм |
| Дирижер | Tinned copper, Class 6 extra-flexible stranding |
| Экран | Semi-conductive conductor screen, semi-conductive insulation screen, tinned copper wire braid |
| Insulation & sheath | Silicone rubber insulation; special-grade elastomer silicone sheath, red |
| Reel duty | 230 mm drum barrel; deployed and rewound 4–6 times per working day, at different sites |
| Decided on | Partial discharge level, bend radius against the customer’s own drum, and a hand-bend sample test |
What a cable test van actually asks of its HV cable
A cable test van is a fault-location laboratory on wheels. Inside it sit a surge wave generator, a DC or VLF withstand source, a time-domain reflectometer and a bridge, and the whole lot connects to a buried cable through one flexible screened lead pulled off a reel. Fault-location workflows — TDR for low-resistance faults, arc-reflection and impulse-current methods for high-resistance breakdowns, decay for faults that only strike at high voltage, and burn-down conditioning — all push their energy down that one cable.
Electrically: it has to hold a DC withstand voltage far above anything it carries continuously, and it has to stay quiet while doing it. A test lead that discharges internally corrupts the measurement it was hired to make.
Mechanically: it lives coiled on a drum, gets dragged over kerbs, gravel and joint-bay lids, and goes back on the drum dirty. Four to six cycles a day, every working day.
Practically: it is the one component in the van a crew can damage on a Tuesday and be unable to work without on a Wednesday.
Equipment builders publish a great deal about the instruments and almost nothing about this cable. We went looking during this project and found brochures for vans, bridges, surge generators and software — and essentially no published specification for the lead that connects them to the world. That gap is why this record exists.
Why the old type code could not simply be reordered
The enquiry arrived as a legacy Pirelli type reference with a 6/6 kV rating and a 120 kV DC note against it. That code no longer maps to anything you can order: Pirelli Cables and Systems became Prysmian in 2005, and screened rubber test cables of that vintage have long since dropped out of live catalogues. A working van, meanwhile, does not care what the cable is called.
So we did what has to be done in this situation: we threw the type code away and rebuilt the specification from the duty. Rated voltage, short-term withstand, screening arrangement, partial discharge limit, overall diameter, bend radius, sheath material. Every one of those is a number the customer can check against their equipment; a discontinued type code is not.
Is the cable partial discharge free?
That was the customer’s first question, and the honest answer is that “PD-free” is not a state a cable can be sold in — it is a declared level at a declared voltage. Under IEC 60270 practice a measurement always sits above some background, so what matters is the routine-test limit. Ours is ≤ 5 pC at 20 kV AC, measured on every drum.
Five picocoulombs at 20 kV is a strict figure for a 22 mm flexible construction, and it is only reachable because of the screening. The semi-conductive layer over the conductor smooths the stranding so the insulation does not see the shape of individual wires; the semi-conductive layer over the insulation does the same job from the outside, under the braid. Remove either and the field concentrates at every irregularity — which is precisely where discharge starts and where electrical treeing follows.
| Параметр | Ценить | Why it matters on a test van |
|---|---|---|
| Rated voltage, continuous (U0/U) | 8.7/15 kV AC | The level the cable is designed to sit at indefinitely |
| Maximum voltage, phase–phase | 15.75 kV AC | Ceiling for continuous service |
| Rated voltage, short-term | 125 kV DC / 40 kV AC | Covers DC withstand and surge work during a test sequence |
| Routine test voltage | 35 kV AC, 5 min | Applied to every production length before it ships |
| Partial discharge, routine test | ≤ 5 pC at 20 kV AC | Keeps the lead from polluting the measurement it carries |
| Type test voltage | 160 kV DC, 15 min / 55 kV AC, 5 min | Design margin above the 125 kV DC working figure |
| Сопротивление изоляции | > 20 MΩ·km | Leakage low enough not to skew a DC or bridge reading |
| Общий диаметр | 22 мм | Sets the bend radius and the drum capacity |
Will a 22 mm cable wind onto a 230 mm drum barrel?
Short answer: yes, and it was verified on a sample rather than argued from a table — because the two numbers are close. The barrel the first layer winds onto is 230 mm across, which holds that layer at a 115 mm radius, about 5.2 × the 22 mm overall diameter. The working minimum bend radius we publish for this construction is 6 × D, or 132 mm. Those figures are within roughly 13% of each other, and no datasheet settles a 13% argument.
What settles it is a sample. We sent one, and it was bent by hand and wound on the customer’s own drum before any order was placed — we shoot that on video for customers who cannot get to the factory. Silicone helps here in a way that XLPE or EPR does not: it is soft, and its elastic recovery means a tight first layer does not set a permanent memory into the cable.
| Reel geometry | Figure | Expressed as × D |
|---|---|---|
| Cable overall diameter (D) | 22 мм | 1 × D |
| Drum barrel diameter | 230 мм | 10.5 × D |
| First-layer bend radius (the worst case) | 115 mm | 5.2 × D |
| Published working minimum bend radius | 132 mm | 6 × D |
| Outer layers on a filled drum | Progressively larger | > 6 × D |
Two engineering points fall out of that table. First, the innermost layer is the only one under discussion — every turn after it winds at a larger radius, so a full drum is easier on the cable than an empty one. Second, if a project genuinely has to stay inside a strict 6 × D figure on a 230 mm barrel, the fix is not a better sheath, it is a smaller conductor cross-section and therefore a smaller D. We would rather say that up front than ship a cable that lives at its limit.
Silicone dragged along the ground, twice before lunch
The customer put the real duty on the table without dressing it up: the cable is pulled off the reel four to six times a day at different locations, it rubs against the ground, it goes back on the drum, and tomorrow the same again. That question deserves the awkward answer first — ordinary silicone is a poor abrasion material. Reference texts on silicone insulation rate its abrasion resistance as fair and recommend it for static installations. If we had quoted a standard silicone compound here, the sheath would have been the first thing to fail.
What we supplied is not a standard compound. Silicone for cable sheathing splits into general-purpose grades and special grades formulated for tear strength, abrasion and repeated flexing, and this construction uses a custom special-grade sheath — the same family of compound we use on high-temperature and reeling constructions, tuned for this job. The trade-off is honest: you pay more per kilogram of compound, and in exchange the sheath survives a duty that ordinary silicone would not.
| Property in this duty | Standard silicone compound | Special-grade silicone (supplied) | PVC or PE test lead |
|---|---|---|---|
| Flexibility on a 230 mm drum | Отлично | Отлично | Poor — stiffens and takes a set |
| Abrasion from dragging | Fair — the weak point | Improved; specified for the duty | Good, but the cable is too stiff to deploy |
| Tear resistance after a nick | Low | Raised by compound and by the polyester wrap under the sheath | Moderate |
| Repeated bend life | Очень хорошо | Очень хорошо | Слабо |
| Cold weather handling (Baltic winter) | Stays flexible far below 0 °C | Stays flexible far below 0 °C | Goes rigid; risk of cracking |
| Elastic recovery after tight winding | High — no permanent memory | High — no permanent memory | Low — keeps the coil shape |
The last row is the one field crews feel. A lead that remembers the drum fights you in the joint bay; a silicone lead lies where you put it. That is worth more on a wet Tuesday than any figure on a datasheet.
Construction, layer by layer
Seven layers, and none of them are decorative. Two of them exist only to keep the electric field smooth, and one exists only to stop a nick becoming a tear.
| # | Слой | Материал | Функция |
|---|---|---|---|
| 1 | Дирижер | Tinned copper, Class 6 extra-flexible stranding | Fine strands give the cable its bend life; tinning holds off corrosion at the terminations |
| 2 | Экран проводника | Semi-conductive layer | Smooths the stranded surface so the insulation never sees a single wire as a stress point |
| 3 | Изоляция | Silicone rubber, rated for HV DC duty | Holds 125 kV DC short-term while staying soft enough to coil |
| 4 | Изоляционный экран | Semi-conductive layer | Gives the braid a smooth, continuous interface — the other half of the PD story |
| 5 | Braid screen | Tinned copper wire braid | Defines the outer boundary of the field, provides the return path and shields the measurement |
| 6 | Wrapping | One layer of polyester tape | Separates braid from sheath, stops sheath damage propagating into the screen |
| 7 | Внешняя оболочка | Special-grade elastomer silicone, red | Takes the abrasion; red so nobody mistakes it for anything else on site |
Where this construction is used besides fault location
The same cable answers a family of jobs where a flexible screened lead has to hold a high test voltage and stay electrically quiet.
HV test laboratories. DC withstand, insulation resistance and dielectric measurement, where the lead must not contribute discharge to the reading.
Factory acceptance and type testing. Routine and type tests on MV and HV cable at the manufacturing plant.
HVDC research and commissioning. DC transmission line testing and converter-station equipment verification.
Renewable grid connection. HVDC export-cable testing for offshore wind, and DC system testing at solar plants.
Difficult fault types. Linear high-resistance breakdowns at joints, flashover faults that only strike under impulse, low-insulation defects that have not broken down yet, and cases where reflection methods struggle — PVC-insulated cable, EHV metallic sheaths and steel-armoured constructions, typically worked with a bridge on the Murray principle.
Why the delivered price worked
A screened silicone test cable built in western Europe is an expensive item, and the customer said so before we did. We came in materially below the European quotation for four structural reasons, none of them a discount:
1. Manufacturing labour and machine build. Both cost less in China, and for a low-volume special construction that is run and set up by hand, labour is a large share of the cost.
2. A full in-house chain. Rubber mixing, extrusion, braided shielding, cabling and packaging all happen on our own 12 production lines, so nothing is bought in at a margin.
3. Local material depth. Compounds, tinned copper and tapes come from a dense domestic supply base with many qualified sources, which keeps input prices honest.
4. Custom is normal here, not an exception. A one-off screened silicone construction is a Tuesday for us. Our MOQ is 50 m, so nobody has to buy a kilometre to solve a metre-scale problem.
What we do not discount is the testing. Every drum gets the 35 kV AC routine test and the partial discharge measurement, because a cheap test lead that fails a withstand test in the field costs more than the cable ever saved.
What we review for a similar test-van cable
- Continuous rated voltage (U0/U)
- Short-term DC and AC withstand levels
- Required PD limit and the voltage it is measured at
- Conductor cross-section, or the test current
- Maximum acceptable overall diameter
- Drum barrel diameter and flange width
- Length per drum and number of drums
- Deployment cycles per day
- Ground surface the cable is dragged over
- Minimum ambient temperature at the test site
- Termination and connector type at each end
- Colour, printing and drum marking
The two that change a quotation most are the drum barrel diameter and the PD limit. Everything else usually has a comfortable answer.
Related constructions
- The full high voltage silicone cable ladder, 1.8 kV to 125 kV DC
- High voltage silicone wire, 3–60 kV, UL 3239 and UL 10475
- High-strength silicone cable, 180 °C, HRD / FRNC
- Flare stack ignition cable, 25 kV DC / 10 kV AC, 250 °C
- (N)TMCWOEU 26/45 kV flexible switchgear cable
- How we build high-temperature and silicone wire
- Reeling cable constructions for drum duty
Questions on this application
Is this high voltage silicone test cable partial discharge free?
No cable is absolutely PD-free; the meaningful figure is a declared limit at a declared voltage. This construction is routine-tested to no more than 5 pC at 20 kV AC on every production length, which is what a test van needs so the connection lead does not contaminate the measurement it carries.
Can a 22 mm HV test cable be wound onto a 23 cm reel drum?
Yes. A 230 mm barrel holds the first layer at a 115 mm radius, about 5.2 × the cable diameter, against a published working minimum of 6 × D. That margin is tight enough that we confirm it on a physical sample and a bend video before an order rather than on a datasheet.
Why silicone instead of EPR or XLPE for a test van lead?
Because the cable is handled, not installed. Silicone stays soft across a wide temperature range, recovers its shape after tight winding and does not take a permanent set from the drum. XLPE and EPR hold voltage well but are far stiffer, which makes daily deployment slow and hard on the crew.
Does silicone survive being dragged along the ground every day?
Standard silicone does not — its abrasion resistance is only fair, and it is normally recommended for static use. This cable uses a special-grade silicone sheath formulated for tear and abrasion, with a polyester tape under it so surface damage does not reach the braid screen.
What voltage can the cable hold during a DC withstand test?
125 kV DC short-term, alongside 40 kV AC, on top of a continuous rating of 8.7/15 kV AC. The design is type-tested at 160 kV DC for 15 minutes and 55 kV AC for 5 minutes, so the working figure sits inside a verified margin rather than at the edge of one.
Can you supply a replacement for an obsolete Pirelli or Prysmian test cable type?
Yes, and this project was exactly that. Send the old type code with whatever electrical and dimensional data you still have, plus your drum geometry, and we rebuild the specification from the duty. Legacy type codes from the Pirelli era are no longer orderable, but the construction behind them is reproducible.
What is the minimum order quantity and lead time?
Minimum order is 50 m, and normal lead time for a special construction runs 10–30 days depending on complexity and quantity. We confirm the date before you place the order, since a test van waiting on a lead is an expensive vehicle standing still.
Can we get a sample and a bending video before ordering?
Yes. Samples are free and you cover the freight. For flexibility questions like this one we also film the cable being bent by hand and wound onto a drum, because a video of the actual construction answers the question faster than a bend-radius formula does.