BD classification (BD1-BD4) — evacuation conditions in an emergency

The guide on fire-hazardous locations (§422, BE2) mentions the BD classification as a separate, independent risk axis: a space can simultaneously be BE2 (fire-hazardous due to stored materials) and have an elevated BD class (difficult to evacuate), without either classification saying anything about the other. This article covers the BD classification itself, as defined in Annex A of IEC 60364-5-51 (external influences).

Two axes: occupant density and escape conditions

The BD classification assesses a space on two independent characteristics:

Occupant density — low (few persons per unit area) versus high (many persons per unit area).
Escape conditions — easy (short, direct, spacious escape routes) versus difficult (long, narrow, winding escape routes, or escape routes running through multiple floors).

The combination of these two axes produces four classes:

Class	Occupant density	Escape condition	Typical example
BD1	Low	Easy	Low-rise dwelling, small ground-floor office
BD2	Low	Difficult	High-rise with limited occupancy per floor (for example an office tower with few persons per floor)
BD3	High	Easy	Public spaces with many visitors but direct exits — theatres, cinemas, ground-floor department stores
BD4	High	Difficult	High-rise with high occupancy and difficult evacuation — hotels, hospitals, multi-storey public buildings

Why this is relevant for electrotechnical design

A space's BD class influences, among other things:

Emergency lighting: the higher the BD class, the stricter the requirements for presence, autonomy time and illuminance of escape route lighting (see the guide on §560 emergency lighting for the illuminance requirements themselves).
Fire resistance of cabling: in a BD4 space, maintaining the function of critical circuits (for example evacuation systems, fire alarm systems) during a fire is more important than in a BD1 space, because the evacuation time is longer.
Redundancy of escape-route lighting: with difficult escape conditions (BD2/BD4), the loss of part of the lighting installation during an emergency is more critical, which more often leads to a requirement for a second, independent power source.

Difference from the BE2 classification

It is essential not to confuse BD with BE (see the BE2 guide):

BE assesses the nature of the materials and substances present (fire-hazardous, explosion-hazardous) — that is, the risk that a fire will occur and how severe it could be.
BD assesses how easily people can leave the space once an emergency has occurred — that is, the risk of the consequences for the persons present.

A chemical laboratory on the ground floor with a direct emergency exit could, for example, be BE2 (fire-hazardous substances) but at the same time BD1 (easy to evacuate). Conversely, an ordinary office on the 20th floor of a high-rise could be BD2 or BD4 (difficult escape conditions) without any elevated BE classification being involved.

Practical relevance

When drawing up an installation design or carrying out an NEN 3140 inspection of a building with a higher BD class (BD2-BD4), it is important to check that the emergency lighting, the fire resistance of critical cabling, and any redundancy of power sources are matched to the actual escape conditions of that specific building — and not automatically copied from a generic low-rise design.

Common mistakes

Confusing the BD classification with the BE classification — both are independent risk axes; a low BE class says nothing about the BD class and vice versa.
Determining the BD class based on building type alone, without assessing actual occupancy and escape routes — two high-rise buildings with an identical shape can have a different BD class depending on actual occupant density and escape-route layout.
Assessing the BD class only at new construction — a change in use or fit-out (for example more workstations on a floor) can increase the BD class of an existing space, with consequences for the emergency lighting requirements.