Sprinkler Thermal Sensitivity: EN 12845 Table 38 and the Limit of the Standard on RTI

Thermal sensitivity determines how quickly the heat sensing element of a sprinkler operates, and in the engineering literature it is expressed by the RTI (Response Time Index). Here, however, the distinction of sources must be made clearly: TS EN 12845+A2:2026 gives no numerical limit for RTI. The standard refers the sensitivity classes to EN 12259-1 and regulates only which class may be used where. This article is limited to what the standard itself says. Source: TS EN 12845+A2:2026, Clause 14.5, Table 38, EN p.106 (TSE p.109).

1. The only thing the standard says about RTI

The NOTE to Clause 14.5.1 (EN p.106) reads as follows:

Most types of sprinkler are rated as one of the following, in decreasing order of sensitivity (see EN 12259-1): - quick response; - special response; - standard response 'A'.

That is all. An order is given, but no numbers. The RTI threshold values (for example limits expressed in m^0.5 s^0.5) are the subject of EN 12259-1 and are not part of the source set of this article. Consequently, if a requirement of the form "RTI shall be ≤ X" is to be written into a project, the basis has to be EN 12259-1 or the manufacturer's documentation, not EN 12845.

2. Table 38: which sensitivity is used where

Clause 14.5.1 (EN p.106) consists of a single sentence of rule and one table: "Sprinklers of different sensitivities shall be used in accordance with Table 38."

Source: Table 38, Clause 14.5.1, EN p.106 (TSE p.109).

Three firm conclusions follow from the table:

1. In-rack sprinklers must be quick response. Standard 'A' and Special are "No" in that column.
2. Quick response may not be used in dry systems and pre-action Type A installations. This makes sense: in these systems there is a delay between the opening of a head and the arrival of water (Table 18: 90 s for LH, 60 s for OH and HH; Clause 11.2.2, EN p.67), and a large number of heads opening early makes that delay worse.
3. All three classes are permitted at the ceiling. In the columns "all other cases" and "ceiling above in-rack sprinklers" all three are "Yes". There is no provision in Table 38 prohibiting or discouraging quick response at the ceiling.

3. The hierarchy rule between ceiling and rack

Beyond what Table 38 permits, Clause 14.5.1 imposes a ranking rule:

Where sprinklers are located in racks, the sprinklers at the ceiling shall have a sensitivity equal to or slower than that of the sprinklers in the racks. Clause 14.5.1, EN p.106

Since in-rack sprinklers must already be quick (Table 38), the ceiling sprinkler may be quick, special or standard 'A'; all three satisfy the condition "equal to or slower than quick". The purpose of the rule is to prevent the opposite situation: the ceiling opening faster than the racks. If the ceiling opens first, the curtain of water descending from the ceiling cools the rising hot gases and delays the operation of the in-rack heads; the fire then grows uncontrolled inside the rack.

The NOTE to Table 38 adds a related warning: when new sprinklers are added to an existing sprinkler installation, it may be necessary to consider the effect of different sensitivities in order to prevent an excessive number of heads from opening.

4. Interaction with other measures (Clause 14.5.2)

Clause 14.5.2 (EN p.106) turns the preservation of sensitivity into a general obligation:

• The possible interaction between sprinkler systems and other measures shall be taken into account. As a result, the thermal response of sprinkler systems shall not be impeded.
• The effective operation of other fire safety measures may depend on the most effective operation of the sprinkler equipment; in such cases the overall fire safety measures shall not be weakened. Particular attention shall be paid to this point in the case of High Hazard systems.
• The effective operation of sprinkler systems depends on suppressing or controlling the fire in its early stages. Except for those located in racks, sprinklers normally operate through the horizontal flow of hot combustion gases from the fire across the sprinklers. Nothing shall therefore interfere with this horizontal flow of combustion gases.

That last sentence is the common rationale behind the escutcheon projection prohibition of Clause 14.8 and the deflector to ceiling distance rule of Clause 12.4.2.

5. Temperature rating: an axis independent of sensitivity

Sensitivity (how fast) and temperature rating (at what temperature) are separate parameters. Clause 14.4 (EN p.105):

Sprinklers shall be selected with a temperature rating close to but not less than 30 °C above the highest anticipated ambient temperature.

The same clause also states that it may be necessary to fit sprinklers of a higher rating in unventilated concealed spaces, under skylights or glazed roofs and similar locations.

Table 37b (EN p.105) gives the colour codes corresponding to the nominal operating temperatures. For glass bulb sprinklers the liquid colour is: 57 orange, 68 red, 79 yellow, 93 green, 100 green, 121 blue, 141 blue, 163 mauve, 182 mauve, 204 black, 227 black, 260 black, 286 black, 343 black. For fusible link sprinklers the colour of the yoke arm depends on the temperature range: 57-77 uncoloured, 80-107 white, 121-149 blue, 163-191 red, 204-246 green, 260-302 orange, 320-343 black.

6. The one place where quick response is mandatory: Annex F

Annex F is entitled "Additional measures to improve system reliability and availability" and, according to F.1, applies where required by national regulations (EN p.143). F.4 "Sprinkler type and sensitivity" states:

Quick response sprinklers shall be used; however, standard 'A' and special response may be used in rooms with an area of not less than 500 m² or a height of not less than 5 m. Clause F.4, EN p.143

This is the only requirement for quick response in the standard, and it sits not in the body but in an annex that applies conditionally.

7. The link with hazard class: design density and K factor

The choice of sensitivity does not change the design density directly; the density comes from the hazard class. A few rows from Table 3 (Clause 7.1, EN p.36) for reference:

For HHS the density and area come from Table 4; for 12.5 mm/min the area of operation is 260 m², and for 20.0 mm/min and above it is 300 m² (Clause 7.2.2, EN p.38).

The sprinkler type and K factor are determined from Table 37a (EN p.104): for LH at a density of 2.25 mm/min, conventional, spray, ceiling, flush, flat spray, recessed, concealed and sidewall types with a nominal K factor of 57; for OH at a density of 5.0 mm/min, the same list of types with K 80 or 115; for HHP and HHS roof or ceiling sprinklers, conventional and spray with K 80, 115 or 160 where the density is ≤ 10 mm/min, and K 115 or 160 where the density is > 10 mm/min; for intermediate sprinklers in HHS high piled storage, conventional, spray and flat spray with K 80 or 115.

Frequently Asked Questions

Does EN 12845 give a numerical limit for RTI?

No. The NOTE to Clause 14.5.1 of TS EN 12845+A2:2026 (EN p.106) only states that sprinkler types are rated, in decreasing order of sensitivity, as quick response, special response and standard response 'A', and refers to EN 12259-1. No numerical threshold for RTI is found in the text of TS EN 12845+A2.

Which sensitivity is used in racks according to Table 38?

Table 38 (Clause 14.5.1, EN p.106): in the in-rack column only Quick is 'Yes'; Standard 'A' and Special are 'No'. In other words, in-rack sprinklers must be quick response.

May quick response be used in dry and pre-action Type A systems?

No. In the 'Dry systems / Pre-action Type A' column of Table 38 the Quick row is 'No', while Standard 'A' and Special are 'Yes'.

What sensitivity applies to the ceiling sprinklers above in-rack sprinklers?

Clause 14.5.1 (EN p.106): where sprinklers are located in racks, the sprinklers at the ceiling shall have a sensitivity equal to or slower than that of the sprinklers in the racks. In the 'Ceiling above in-rack sprinklers' column of Table 38 all three sensitivity classes are 'Yes'; the actual constraint is imposed by this equal-or-slower rule.

How is the sprinkler temperature rating selected?

Clause 14.4 (EN p.105): sprinklers shall be selected with a temperature rating close to but not less than 30 °C above the highest anticipated ambient temperature. In unventilated concealed spaces, under skylights or glazed roofs and similar locations it may be necessary to fit sprinklers of a higher rating.

Is quick response mandatory in any system?

Yes, under Annex F. Annex F (EN p.143) applies 'where required by national regulations' and F.4 requires quick response sprinklers to be used; as an exception, standard 'A' and special response may be used in rooms with an area of not less than 500 m² or a height of not less than 5 m.

References

• TS EN 12845+A2:2026 Clause 7.1, Table 3 (EN p.35-36 / TSE p.38-39)
• TS EN 12845+A2:2026 Clause 7.2.2, Table 4 (EN p.38 / TSE p.41)
• TS EN 12845+A2:2026 Clause 11.2.2, Table 18 Maximum time for water to reach the sprinkler (EN p.67 / TSE p.70)
• TS EN 12845+A2:2026 Clause 12.4.2 Distance from the deflector to the ceiling (EN p.72 / TSE p.75)
• TS EN 12845+A2:2026 Clause 14.2.1, Table 37a Sprinkler types and K factors (EN p.104 / TSE p.107)
• TS EN 12845+A2:2026 Clause 14.4, Table 37b Sprinkler temperature ratings and colour code (EN p.105 / TSE p.108)
• TS EN 12845+A2:2026 Clause 14.5.1, Table 38 Sprinkler sensitivity ratings (EN p.106 / TSE p.109)
• TS EN 12845+A2:2026 Clause 14.5.2 Interaction with other measures (EN p.106 / TSE p.109)
• TS EN 12845+A2:2026 Clause 14.8 Sprinkler rosettes (EN p.107 / TSE p.110)
• TS EN 12845+A2:2026 Annex F.1 and F.4 (EN p.143 / TSE p.146)
• TS EN 12845-2:2025 Clause 5.1.1 NOTE 2 ESFR and CMSA nominal operating temperature and response characteristics (EN p.8 / TSE p.11)

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