Decoding cable type designations — VOB, XVB, YMvK and the "as"/"mb" suffixes

The ferrule crimping guide and the cable lug crimping guide both assume the correct cable type has already been chosen. This article covers the step before that: how the letter-and-digit codes on a cable drum or in a technical drawing are actually read.

The basic principle: each letter describes a layer or property

Dutch cable naming (partly historical, rooted in the VDE tradition and now largely incorporated into NEN-EN 50525) builds a name from individual letters, each describing a layer, material, or property of the cable — from the inside (the conductor) outward.

The most common types

Code	Construction	Typical use
VOB	A single solid copper core with PVC insulation, no sheath	Fixed wiring inside an empty conduit/trunking or a socket-outlet connection; must not be run exposed or unprotected
XVB	Multiple VOB-like cores (but with XLPE instead of PVC insulation) gathered under one common PVC outer sheath	Fixed final-circuit wiring between the distribution board and a socket outlet/light point — the most common cable in Dutch domestic installations
YMvK	Cable with copper cores, PVC core insulation, braided or solid PVC sheath, rated up to 600/1000 V	Fixed installations in dwellings, commercial premises and areas with heightened fire-safety requirements; standard flame-retardant
YMvKas	YMvK with an additional armoured (steel) sheath around the core	Direct burial or situations with elevated risk of mechanical damage (rodents, excavation work)

In this series, the letter Y denotes PVC core insulation, X denotes XLPE core insulation (cross-linked polyethylene, higher temperature resistance than PVC), M indicates the presence of a sheath, the lowercase v denotes the PVC sheath material (vinyl), and K denotes a multi-core cable.

The "as" and "mb" suffixes

"as" (appended to the base name, as in YMvKas) means the cable is suitable for direct burial in the ground — the armoured sheath provides the mechanical protection required for this (see also the underground cables & burial depth guide for the associated depth requirements).
"mb" (flame-retardant) indicates the cable is made of materials that do not continue burning on their own once the ignition source is removed — similar to, but not the same as, a cable's CPR class (see the cable fire classification (CPR) guide for the formal European classification system, Aca through Fca).

Note: "flame-retardant" (mb) is a historical, informal designation and not a formal CPR fire class. For an installation with specific fire-safety requirements (for example an escape route), the CPR class on the cable label must always be checked, not just the traditional type designation.

Why VOB is never applied bare and unprotected

A common exam question: why can't a VOB core be run directly, without sheath or conduit, inside a wall or exposed in a room? The answer: a VOB core has only core insulation, no additional mechanical protective layer (sheath). Without a surrounding conduit, trunking or cable tray, the core is vulnerable to mechanical damage, and there is no second barrier should the core insulation itself become damaged.

Practical relevance

When assessing an installation or ordering cable, the type designation should be read as a sequence of meaningful letters, not as an arbitrary product name — this prevents, for example, a YMvK cable (without an armoured sheath) being applied where YMvKas (with armoured sheath, suitable for burial) is actually required.

Common mistakes

Applying VOB without conduit or trunking — this type is intended solely for use inside a protective enclosure.
Using YMvK where YMvKas is required (for example directly buried) — without the armoured sheath, the required mechanical protection is missing.
Confusing "mb" (flame-retardant) with a formal CPR fire class — for formal fire-safety requirements, the CPR class (Aca–Fca) is what counts, not the traditional "mb" designation.
Confusing XVB and YMvK — both are sheathed multi-core cables, but XVB uses XLPE-insulated cores while YMvK uses PVC-insulated cores; this affects the cable's temperature and current-carrying capacity.