BS 6622 vs BS 7835: Armoured MV Cables for UK and Middle East Projects — the Questions Engineers Ask

If you quote medium-voltage cables for projects in the UK or the Gulf, two British Standards appear in almost every specification: BS 6622 and BS 7835. Both cover armoured MV cables with thermosetting insulation from 3.8/6.6 kV up to 19/33 kV, both look almost identical on a drawing, and both are often listed side by side in the same tender. The difference sits in the materials around the core and in what happens when the cable is exposed to fire.

This guide to BS 6622 vs BS 7835 is built around the questions design engineers and estimating teams actually ask when one of these standards lands on their desk. It summarises how the standards think; it does not replace them, and it deliberately avoids reproducing their tables.

What are BS 6622 and BS 7835 for?

Both standards describe complete, ready-to-install armoured cables for medium-voltage distribution: substations, industrial plants, infrastructure, commercial developments and utility feeders. They fix the construction family, the materials that may be used, the rated voltages and the tests that prove the cable.

• BS 6622 is the general-purpose armoured MV cable: XLPE (or EPR) insulation, a metallic screen, armour and, classically, a PVC oversheath. It is the workhorse for outdoor, buried and industrial routes.
• BS 7835 keeps the same electrical heart but replaces the halogen-containing bedding and oversheath with low-smoke, halogen-free materials. It exists for places where smoke and corrosive gases in a fire would endanger people or equipment.

Put simply: BS 6622 answers "will this cable carry the load reliably for decades?", and BS 7835 adds "and if the building catches fire, will the cable make things worse?"

The same split exists at low voltage, where BS 5467 and BS 6724 play the same two roles for armoured LV cables.

Is the electrical design different between the two?

Very little. The conductor, the semi-conductive conductor screen, the XLPE insulation, the insulation screen and the metallic screen follow the same logic in both standards. As in IEC 60502-2, insulation thickness is driven by the rated voltage, not the conductor size, so moving from 6.35/11 kV to 19/33 kV changes every diameter above the insulation even when the conductor stays the same.

That means your electrical calculation, partial discharge expectations and screen sizing are largely shared. What changes is everything from the bedding outwards: the material, its density, its processing, its weight and its price.

Can a BS 7835 cable replace a BS 6622 cable — and the other way round?

This is the most common question in tenders, and the answer is not symmetrical.

BS 7835 instead of BS 6622 is usually electrically acceptable, but it is not automatically the better offer. Halogen-free compounds generally absorb more moisture and are less tough than PVC or polyethylene, so they are less suited to permanently wet, direct-buried routes. They are also more expensive. A consultant who specified BS 6622 for a buried feeder may reject an LSZH alternative on durability grounds, not fire grounds.

BS 6622 instead of BS 7835 is almost never acceptable. If the specification asks for low smoke and low acid gas emission — tunnels, metros, hospitals, airports, high-rise buildings, data centres — a PVC-sheathed cable fails the very requirement the purchaser cared about most.

A practical pattern on large projects: BS 6622 for the outdoor and buried sections, BS 7835 where the route enters buildings or enclosed spaces. That creates two cable types on the same circuit, which must be reflected in the design, the BOM and the drum schedule.

Does "low smoke halogen-free" mean the cable is fire resistant?

No — and this is one of the most expensive misunderstandings in MV tenders. A BS 7835 cable is designed to limit flame spread, smoke and corrosive gas when it is involved in a fire. It is not designed to keep a circuit working during a fire.

Circuit integrity (fire resistance) is a separate property with its own test methods, and in the British low-voltage world it is covered by standards such as BS 7846. If a tender asks for an MV cable that must keep supplying fire pumps or emergency systems during a fire, BS 7835 alone does not answer it; the requirement has to be clarified before anything is quoted. For a wider view of the material side, see our comparison of PVC, XLPE, EPR and LSZH.

Which tests prove the difference?

Both standards share the electrical test regime typical of MV cables: routine tests on every length (including voltage and partial discharge tests), sample tests on production, and type tests on the design.

BS 7835 adds the fire-behaviour evidence. In practice the cable is assessed for flame propagation, smoke density and the acidity of the gases released when the materials burn, using the familiar IEC/EN test families (IEC 60332, IEC 61034 and IEC 60754 and their EN equivalents). For a manufacturer, the consequence is that a compound change in the bedding or oversheath can invalidate fire test evidence even if nothing electrical has changed.

Insight: single-core and three-core cables are armoured differently

Both standards cover single-core and three-core constructions, and the armour follows the physics:

• Three-core cables are normally armoured with galvanised steel wire (SWA). The magnetic fields of the three phases largely cancel, so steel armour does not cause significant losses.
• Single-core cables for AC circuits use non-magnetic armour, typically aluminium wire (AWA). Steel wire around a single AC conductor would heat up through induced currents and hysteresis, cutting the current rating.

Tenders that simply say "SWA" for a single-core 33 kV cable are almost always wrong, and the correction affects weight, cost and the IEC 60287 current rating, especially when combined with the bonding arrangement.

Insight: the metallic screen is still set by the network

As with IEC 60502-2, the metallic screen — copper tape or copper wire — must carry the earth-fault current for the specified duration. BS 6622 and BS 7835 describe screen types and minimum requirements, but the actual cross-section often comes from the utility or consultant's fault data. Two "identical" 19/33 kV cables for two Gulf projects can carry very different amounts of copper in the screen.

Insight: Middle East specifications add their own layers

Projects in the Gulf frequently use BS 6622 or BS 7835 as the base and then add requirements that are not in the standard itself:

• Anti-termite additives in the oversheath.
• Lead sheath or other radial barriers for petrochemical sites where hydrocarbons may reach the cable.
• Longitudinal water blocking for buried routes with high groundwater.
• UV-resistant outer sheaths and higher ambient and ground temperatures for rating.
• Specific sheath colours, embossing and drum marking.

Each of these changes the construction and must be traceable back to a clause in the customer's specification, not just the standard.

Where do BS 6622 / BS 7835 sit against IEC 60502-2 and BS 7870-4?

The practical takeaway: a plant that already builds IEC 60502-2 cables can usually make BS 6622 and BS 7835 cables on the same lines, but must manage separate constructions, separate materials and separate type-test evidence for each.

Typical tender mistakes

1. Treating BS 7835 as "BS 6622 with a different colour sheath" and pricing it the same.
2. Offering LSZH for a permanently wet buried route without checking whether the customer accepts it.
3. Confusing low smoke halogen-free with fire resistant.
4. Steel wire armour on single-core AC cables.
5. Screen sized to the standard's minimum when the tender states a higher fault level.
6. Missing project-specific additions such as anti-termite, lead sheath or water blocking.

Tender checklist for BS 6622 and BS 7835 cables

Before designing, confirm:

1. Which standard applies to which section of the route, and whether alternatives are accepted.
2. Rated voltage U0/U (Um), not just "11 kV" or "33 kV".
3. Conductor material and size, and whether conductor water blocking is required.
4. Single-core or three-core, and the armour type that goes with it.
5. Screen type and the earth-fault current and duration it must carry.
6. Fire requirements: low smoke and acid gas only, or also circuit integrity.
7. Oversheath additions: anti-termite, UV, hydrocarbon resistance, colour and marking.
8. Installation conditions and bonding for the current rating.
9. Which existing type tests, including fire tests, cover the offered design.

How CablePLM handles BS 6622 and BS 7835

In CablePLM, BS 6622 and BS 7835 are separate calculation methods in the standards library, alongside IEC 60502-2 and BS 7870-4. You choose voltage, conductor, screen, armour and sheath; CablePLM builds the cable layer by layer — see our layer-by-layer anatomy of a power cable — and generates the bill of materials, IEC 60287 current ratings, short-circuit capability of conductor and screen, and both the engineering and customer versions of the datasheet. Every value is marked as standard data or as an estimate, so reviewers can see where a number comes from.

For tenders that mix both standards, the Tenders & Projects add-on keeps each route section linked to its cable type and specification clause, and Qualification & Quality shows which designs are already covered by your electrical and fire type tests.

Want to see your BS 6622 and BS 7835 range designed this way? Book a demo.