HOW DO YOU CALCULATE VIEWING DISTANCES FOR EXIT SIGNS MOUNTED GREATER THAN 2.5M?

BS5499-4, Clause 4.6c makes it very clear that “mounting height greater than 2.5m may be used, e.g. in large open spaces or for operational reasons, but such signs should be both conspicuous and identifiable and therefore large signs might be necessary”.

To assist evacuees with the prediction of the location of successive signs, the following principles should be applied:-

  1. Sign mounting heights are measured to the base of the sign.
  2. All signs should be mounted so that they are readily visible.
  3. Signs above doors or open spaces should be mounted between 2m and 2.5m from floor level, measured to the base of the sign and be sited as close to the centre line of the escape route as practicable;
    NOTE 1 This mounting height is intended to ensure that signs can be readily seen, e.g. over the heads of people.
  4. Signs sited on walls should be mounted between 1.7m and 2m, from floor level measured to the base of the sign;
    NOTE 2 This mounting height is intended to ensure the signs are within the immediate field of vision.
  5. Mounting heights greater than 2.5m may be used, e.g. in large open spaces or for operational reasons, but such signs should be both conspicuous and identifiable and therefore larger signs might be necessary.
  6. Signs should be sited at the same height throughout the escape route, so far as reasonably practicable.

BS EN 1838:2013 makes it very clear that “you should avoid mounting signs more than 20 degrees above the horizontal view”

It would be good practice to reduce the viewing distance according to BS5499-4 Annex C

Using our previous technical blog about “Legend Viewing Distances” for signs mounted less than 2.5m

Viewing Distance (mm) = Z0 x h (mm)
Viewing Distance (mm) = 200 x for a typical sign height of 130mm
Viewing Distance (mm) = 26,000 mm
<2.5m Mounting Height Viewing Distance (m) = 26m

Example for 3m mounting height using an average eye level of 1.5m.

Firstly we need calculate the sign level above eye level, this is:

ΔH = Mounting Height (m) – Eye Level Height (m)
ΔH = 3m – 1.5m
ΔH = 1.5m

Onto the radius of viewing distance reduced for a 3m mounting height this is calculated by:

r= √ [ (0.5 x Z0 H)2 – (ΔH)2 ]
r= √ [ (0.5 x Viewing Distance)2 – (sign level above eye level)2 ]
r= √ [ (0.5 X 26)2 – 1.5 2 ]

r= √ [132 – 1.52 ]
r= √ (166.75)
r= 12.9m

Therefore the viewing distance normal to the sign for a 3m mounting height would be:

3m Viewing Distance = 2 x Radius of viewing distance (r)
3m Viewing Distance = 2 x 12.9m
3m Viewing Distance = 25.8m

The typical viewing distance for the sign mounted <2.5m calculated at 26m compared to the 3m viewing distance of 25.8m you will see a loss of 0.2m distance.

Something to consider……Viewing distances should then be reduced if the sign is to be viewed predominantly from the left or right of its centre.

For example, if you (the observer) are viewing from 30 degrees to the left or right of centre, the effective viewing distance is reduced by the cosine of the angle

3m Viewing Distance @ 30° = Viewing distance (I) Cos α
3m Viewing Distance @ 30° = I Cos α
3m Viewing Distance @ 30° = 28.8 Cos 30
3m Viewing Distance @ 30º = 22.3m

To improve conspicuousness of the exit sign for mounting heights above 2.5m we would suggest using a bespoke solution.
Please contact one of our Sales Managers for more details
Contact | Zemper

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