The standard behind "fire resistant cable" — except it isn't a cable standard at all. It is a test method. Understanding that distinction is what stops a life-safety circuit being commissioned with the wrong cable in it.
BS 6387 sets out a test method: it describes how to prove that a cable can keep a circuit alive while it is being burnt. It does not say what the cable is made of, how thick the insulation is, or how many cores it has. Those come from a product standard.
So a fire-resistant cable always needs two citations, not one — a product standard that defines the construction, and BS 6387 (or its international counterpart, IEC 60331) that provides the evidence. Papers citing only BS 6387 have named the exam and not the subject.
CWZThe 2013 revision withdrew the older categories (A, B, S, X and Y) and retained only C, W and Z. An old certificate citing a withdrawn category is not evidence against the current edition — worth checking if a project's paperwork is more than a decade old.
Not three grades — three different disasters. Each one reproduces a specific thing that happens in a real building fire, and each is measured over a different period.
| Category | Test conditions | Simulates |
|---|---|---|
C |
Flame at 950 °C for 3 hours, cable carrying its rated voltage. No water, no shock. | A sustained fire with no intervention — the baseline case. |
W |
Flame at 650 °C for 15 minutes, then water spray applied for a further 15 minutes. | A sprinkler system activating during the fire. Water and heat at once — the condition that cracks a rigid barrier. |
Z |
Flame at 950 °C for 15 minutes, with the cable struck mechanically every 30 seconds. | Falling debris, collapsing partitions, the impact of firefighting. The cable is mounted in an S-bend and struck with a bar. |
CWZ |
All three protocols, passed by samples from the same cable. | Not a fourth test — a statement that all three were passed. |
The pass criterion throughout is the same: the circuit must not fail. Voltage is applied through the cable under load, and a fuse in the circuit must not blow. Note the difference in durations — C runs for three hours, W and Z for fifteen minutes each. They are not comparable by severity.
It reads like a badge — three letters, all passed, sounds like the top of a scale. It is not. It means the cable was tested against all three conditions. Whether you need all three depends entirely on the route the cable takes.
| Route the cable takes | What is actually tested for |
|---|---|
| Protected riser, no sprinklers, no demolition risk | Fire alone is the governing case — C |
| Building with a sprinkler system, cable in the same zone | Water during fire matters — W |
| Route crosses areas subject to collapse or impact | Mechanical shock matters — Z |
| Shafts, tunnels, plant rooms, multi-storey escape routes | Usually more than one condition applies — hence CWZ in practice |
This is a fire engineering decision, not a procurement one. Where the specification is silent, the safe move is to ask the consultant which categories were designed for — before the order, not after the drum is delivered.
All of these also sit inside a building where the same cable run will be specified low-smoke. Expect to see both clauses on the same line item.
Fire-performance cable is the one category where an under-specified enquiry is genuinely dangerous rather than merely inconvenient. Five lines fix it.
4 C × 95 mm² ← cores and size
XLPE / SWA / LSZH ← the product construction
600/1000 V ← voltage class
to BS 6724 ← the product standard
circuit integrity to BS 6387 CWZ ← the test standard and categories
01 · THE TEST STANDARD INSTEAD OF THE CABLE
A purchase order reading only "BS 6387 CWZ" has not described a cable. Armour, sheath and cores are all still open, and those are the expensive variables.
02 · A REPORT THAT DOESN'T COVER YOUR SIZE
Fire test reports are issued for a tested construction at a tested size. A report for a small multi-core does not automatically cover a 4 × 95. Always ask what construction and what size the report actually covers.
03 · AN OLD EDITION
Certificates against the pre-2013 categories are citing withdrawn classes. On a project with a strict consultant, that is a rejection at delivery.
Third-party fire tests are arranged per project. Raise the requirement at tender stage and you will get an honest answer on what is achievable and in what timeframe — which is information you need before committing a delivery date.
No. It is a test standard — it describes how to test whether a cable maintains circuit integrity under fire, not what the cable is made of. A fire-resistant cable needs a product standard for the construction and BS 6387 (or IEC 60331) for the evidence. A quotation naming only BS 6387 has named the test.
No, and this is the most common misreading. C, W and Z are different test conditions, not grades. C tests fire alone; W adds water; Z adds mechanical shock. CWZ means all three were passed. Whether you need all three is a property of the route, not of the cable being superior.
The British and international versions of the same test. The 2013 revision of BS 6387 moved closer into line with IEC 60331, but a report to one is not a report to the other. If the specification names one, ask for that one — do not accept the other as equivalent without the consultant's agreement in writing.
C — 950 °C flame for 3 hours, cable under voltage, no water or shock. W — 650 °C flame for 15 minutes, then water spray for 15 minutes, simulating sprinklers. Z — 950 °C flame for 15 minutes with the cable struck every 30 seconds, simulating falling debris. Note the durations differ, so the three are not directly comparable by severity.
Yes, and on life-safety circuits it usually has to be. They are separate properties with separate documents: circuit integrity to BS 6387 or IEC 60331, emissions to IEC 60754 and IEC 61034. A cable tested to both exists — but only if both were specified in the first place.
BS 8491 is the assessment method for large-diameter power cables used in smoke and heat control systems and similar active fire safety applications. Where a project cites it rather than BS 6387, that is usually a sign the cable is part of a smoke extraction circuit rather than an alarm circuit. Send the citation exactly as written.
The emissions half of the same conversation. Low smoke is not fire resistance.
BS 5467The standard armoured LV power cable, and how to read its designation.
IEC 60502-1Why a standard that is cited everywhere decides almost nothing on its own.
Send the citation exactly as the specification writes it — product standard and test categories. You will get a straight answer on feasibility, and on what evidence can be arranged and when.