Discharge Obstructions in ESFR and CMSA: EN 12845-2 Clause 5.3

Source: EN 12845-2:2024 Clause 5.3 (EN p. 21-32).

In ESFR and CMSA designs, managing obstructions can be even more critical than the design tables; the Introduction of the standard also states plainly that these designs have a low tolerance of non conformities and that deviations carry the risk of leading to total system failure (EN p. 4). Clause 5.3 divides the subject into two categories: obstructions at or near the ceiling (5.3.2) and obstructions entirely below the deflector level (5.3.3).

Clause 5.3.1: a single set of rules

"The obstruction requirements and the requirements for compensating for obstructions for ceiling protection are the same for ESFR and CMSA system designs." (EN p. 21)

There is therefore no separate and looser obstruction tolerance for CMSA.

Clause 5.3.2: obstructions at or near the ceiling

In addition to the limits for obstructed and unobstructed ceilings, pendent and upright sprinklers are installed at the standard spacing in accordance with Figures 9 and 11 and Table 3, so as not to obstruct the umbrella discharge pattern of the sprinkler. Interpolation between the values of Table 3 is permitted.

The basic prohibition: no object shall be positioned closer than 300 mm to a sprinkler unless it is at least 50 mm above the horizontal plane of the sprinkler deflector.

Table 3: minimum horizontal distance of ceiling objects so as not to obstruct the umbrella pattern (EN p. 23)

There are two points to watch. First, the input to the table is not the depth of a beam but the vertical distance by which the object falls below the deflector. Second, the increase is not linear: 800 mm for 200 mm, 1 000 mm for 300 mm, 1 300 mm for 500 mm and 1 800 mm for 900 mm. A fixed multiplier rule (for example "four times the depth") gives a wrong answer for most of these rows.

The grid pattern rule: an object at or near ceiling level that stays entirely within the grid pattern of Figures 9 to 11 is not treated as an obstruction; any object that hangs into the area below the pattern is treated as an obstruction.

Exception: an object is not treated as an obstruction where it is a structural member or similar that is at least 70 % open. Otherwise the obstruction is compensated for by one of two methods:

• the obstructed sprinkler is repositioned so as to comply with the horizontal and vertical distances of Figures 9 to 11; or
• sprinklers are installed on both sides of the obstruction, at equal horizontal distances from it and at a horizontal distance of at least 300 mm from the edge of the obstruction.

Figure 11 NOTE 2 (EN p. 22): for an obstruction of width W: where W is greater than 300 mm and equal to or less than 600 mm, the underside of the obstruction is at least 900 mm above the top of the storage; where W is equal to or less than 300 mm, it is at least 450 mm above it; and where W is greater than 600 mm, additional sprinklers are required. These dimensions are measured above the top of the storage, not below the deflector.

Clause 5.3.3.1: grouped obstructions

Two or more adjacent objects are treated together as a single object (a grouped obstruction) where the horizontal distance between them is less than 3 times the width of the smallest object, but in no case less than 150 mm. Under those conditions the width of the grouped obstruction is found by adding the smallest dimensions of the two objects and the horizontal distance between them. An object is treated as a single object where it is separated from its neighbours by a distance equal to or greater than 3 times its own smallest dimension, or by at least 150 mm.

Clause 5.3.3.2.1 and Table 4: single objects below the deflector

Sprinklers are installed in accordance with Table 4 and Figure 13 so that they are not obstructed by single objects below the inner core distribution pattern of the sprinkler.

Table 4: installation guide for avoiding obstructions to the inner core distribution pattern (EN p. 24)

The upright exception: upright sprinklers can tolerate single objects up to 100 mm wide directly beneath them at any vertical distance. Where the object is up to 600 mm wide in its maximum dimension, it is positioned at a horizontal distance of at least 300 mm from the nearest sprinkler. This tolerance is not given for pendent sprinklers.

Figure 13 (EN p. 25) shows the same rule as four zones: no object is permitted in Zone I; objects up to 38 mm wide in Zone II; objects up to 150 mm wide in Zone III; and in Zone IV objects up to 300 mm wide where the object is at least 450 mm above the top of the storage and up to 600 mm wide where it is at least 900 mm above it.

Clause 5.3.3.2.2 and Table 5: obstructions directly above racks

Where the size and position of an obstruction do not call for sprinklers below it, the position of the obstruction relative to the rack flue space (transverse or longitudinal) is also checked.

Table 5: obstructions located directly above racks (EN p. 26)

Here B is the vertical distance from the object to the top of the storage (Figure 14).

Clause 5.3.3.2.3 (EN p. 27): where rack storage is affected in plan view by an obstructing object that does not meet the requirements of 5.3.3.2.2, one of two approaches is applied: sprinklers may be installed directly below the obstruction; but the recommended approach is to install additional sprinklers at the top of the rack structure, as in-rack sprinklers at all flue space intersections (face and longitudinal). Sprinklers added in this way are not added to the hydraulic design of the ceiling level sprinkler system.

Clause 5.3.3.2.4: objects wider than 0.6 m

For flat, solid, single objects wider than 0.6 m that are located below the ceiling level storage sprinklers and above the storage, additional sprinklers in accordance with Figure 16 are installed below the object (EN p. 28). The key to Figure 16 shows that the additional sprinklers are positioned at the same linear and area spacing as the ceiling sprinklers and no further from the edge of the obstruction than 50 % of the maximum permitted linear spacing.

Figures 17 to 22 (EN p. 29-31) give two options each for four scenarios: flat, solid, grouped obstructions over 0.6 m; non flat or non solid single obstructions over 0.6 m; and non flat or non solid grouped obstructions over 0.6 m. In the options either a horizontal barrier is provided or additional sprinklers are installed at a maximum of 1.2 m linear and 1.5 m² area spacing. For grouped obstructions the conditions are given as H < (3 × A) and A + B > 0.6 m.

Clause 5.3.3.3: open grating obstructions

Sprinklers are installed below open grating obstructions such as walkways or suspended ceilings; the exception is where all of the following conditions are met (EN p. 32):

• the total open plan area of the grating, including the lighting fittings, is at least 70 % of the plan area of the grating; and
• the minimum dimension of the grating openings is not less than 25 mm or the depth of the grating, whichever is the greater; and
• the grating is at least 0.9 m below the deflector of the ceiling level sprinklers; and
• it is not possible for material to fall onto the grating during a fire and obstruct the sprinkler discharge; and
• there is only one open grating between the solid ceiling and the floor; and
• the ceiling sprinkler system is such that it could protect the occupancy even without the open grating; and
• the open grating walkway is not located within any rack structure.

Additional sprinklers installed below the grating do not have to be added to the hydraulic design of the ceiling level system.

The link with the design table

Which design table the obstruction rules are applied under is also regulated separately in the standard:

• Clause 6.6.1.1 (EN p. 35): for the designs of Table 7, Clause 5.2.6.1 (horizontal air flow) and Clause 5.3.3.2.2 (obstructions above racks) do not apply.
• Clause 6.6.1.2 (EN p. 38): in the designs with special requirements of Tables 10 and 11, Clause 5.2.6.1 always applies and, where there is an obstruction above rack storage, the recommended approach of 5.3.3.2.2 is applied. In addition the ceiling is non-combustible and preferably unobstructed; where there are beams, their cross sectional area in the vertical plane has to be less than 70 % open.

Field note: the step most often skipped in obstruction analysis is assessing objects as a group rather than one by one. A cable tray and a service pipe beside it gain width as a single obstruction where the distance between them is small, and together they can cross the 0.6 m threshold.

Frequently Asked Questions

Are the obstruction rules of ESFR and CMSA different?

No. EN 12845-2:2024 Clause 5.3.1 (EN p. 21) is clear: the obstruction requirements and the requirements for compensating for obstructions for ceiling protection are the same for ESFR and CMSA system designs. CMSA therefore has no wider obstruction tolerance than ESFR.

What is the minimum horizontal distance for objects at ceiling level?

Table 3 (EN p. 23) gives the minimum horizontal distance against the maximum vertical distance of the object below the sprinkler deflector: 300 mm for 50 mm; 500 mm for 100 mm; 700 mm for 150 mm; 800 mm for 200 mm; 1 000 mm for 300 mm; 1 300 mm for 500 mm; and 1 800 mm for 900 mm. Interpolation between the values is permitted. In addition, under Clause 5.3.2 no object shall be positioned closer than 300 mm to a sprinkler unless it is at least 50 mm above the horizontal plane of the sprinkler deflector.

How wide does an object have to be before sprinklers are required below it?

Clause 5.3.3.2.4 (EN p. 28) and Table 4 (EN p. 24): additional sprinklers in accordance with Figure 16 are installed below flat, solid, single objects wider than 0.6 m (600 mm). The last row of Table 4 gives the same threshold: for an object of width exceeding 600 mm, additional sprinklers are required below the object.

Is there an exception for narrow objects below an upright sprinkler?

Yes. The provision below Table 4 (EN p. 24): upright sprinklers can tolerate single objects up to 100 mm wide directly beneath them at any vertical distance. Where the object is up to 600 mm wide in its maximum dimension, it is positioned at a horizontal distance of at least 300 mm from the nearest sprinkler. This exception is not given for pendent sprinklers.

When are two separate objects treated as a single obstruction?

Clause 5.3.3.1 (EN p. 23): two or more adjacent objects are treated together as a single object (a grouped obstruction) where the horizontal distance between them is less than 3 times the width of the smallest object, but in no case less than 150 mm. In that case the width of the grouped obstruction is found by adding the smallest dimensions of the two objects and the horizontal distance between them. An object is treated as a single object where it is separated from its neighbours by a distance equal to or greater than 3 times its own smallest dimension, or by at least 150 mm.

How are obstructions above racks dealt with?

Clause 5.3.3.2.2 and Table 5 (EN p. 26): where the width of an obstruction above a longitudinal or transverse flue space is equal to or less than 150 mm, no sprinkler is required below it, whether it affects a single sprinkler or more than one. Between 150 mm and 300 mm none is required where it affects a single sprinkler; where it affects two or more adjacent sprinklers and a flue space longer than 3 m is involved, none is required provided that the vertical distance from the object to the top of the storage (B) is at least 450 mm. Between 300 mm and 600 mm none is required where it affects a single sprinkler provided B is at least 450 mm, and sprinklers are installed below it where it affects two or more.

References

• EN 12845-2:2024 Introduction (EN p. 4)
• EN 12845-2:2024 Clause 5.3.1, 5.3.2, Figure 9 (EN p. 21)
• EN 12845-2:2024 Figure 10, Figure 11 and NOTE 2 (EN p. 22)
• EN 12845-2:2024 Table 3, Clause 5.3.3.1 (EN p. 23)
• EN 12845-2:2024 Figure 12, Clause 5.3.3.2.1, Table 4 (EN p. 24)
• EN 12845-2:2024 Figure 13 (EN p. 25)
• EN 12845-2:2024 Clause 5.3.3.2.2, Table 5, Figure 14 (EN p. 26)
• EN 12845-2:2024 Clause 5.3.3.2.3, Figure 15 (EN p. 27)
• EN 12845-2:2024 Clause 5.3.3.2.4, Figure 16 (EN p. 28)
• EN 12845-2:2024 Figures 17-22 (EN p. 29-31)
• EN 12845-2:2024 Clause 5.3.3.3 (EN p. 32)
• EN 12845-2:2024 Clause 6.6.1.1 (EN p. 35), Clause 6.6.1.2 (EN p. 38)

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