Source: TS EN 12845+A2:2026 (EN 12845:2015+A2:2026), Clause 11.4, Table 17, Clauses 12.1.3, 14.5.1 and 16.2.3.
Pre-action installations are frequently described in the market as "double interlock systems". The term double interlock does not appear in the text of TS EN 12845+A2:2026, and there is no provision for Type B that would mean "no water until two confirmations arrive together". This article goes line by line through what Clause 11.4 says, what it does not say, and what limits the size of the installation.
The two types the standard recognises
Clause 11.4.1.1 (TSE p.71 / EN p.68) recognises only two types and defines them in its subclauses.
Type A
Clause 11.4.1.2 (TSE p.71 / EN p.68):
- This is an otherwise normal dry pipe installation in which the control valve set is brought into operation by an automatic fire detection system and not by the opening of the sprinklers.
- The air/inert gas pressure in the installation is monitored at all times (see Annex I).
- At least one quick opening manually operated valve shall be installed in a suitable position so that the pre-action valve can be brought into operation in an emergency.
- In the event of a failure of the fire detection system, the installation operates as an ordinary dry pipe system.
The note to the clause limits the field of use: Type A pre-action installations are recommended only in areas where considerable damage could be caused by an accidental discharge of water.
Type B
Clause 11.4.1.3 (TSE p.71 / EN p.68):
- This too is an otherwise normal dry pipe installation, but the control valve set is brought into operation either by an automatic fire detection system or by the opening of the sprinklers.
- Independently of the response of the detectors, a pressure drop in the pipework causes the alarm valve to open.
- Type B installations may be installed wherever a dry pipe system is required and rapid fire spread is expected. They may also be used in place of ordinary dry pipe systems with or without accelerators.
The consequence of the second bullet is this: in Type B, the opening of a sprinkler alone brings the water. Detection here does not hold the water back, it brings it earlier.
| Subject | Type A (11.4.1.2) | Type B (11.4.1.3) |
|---|---|---|
| What opens the valve | Only the automatic fire detection system | Automatic detection system or the opening of a sprinkler |
| Effect of a pipe pressure drop | Does not open the valve | Opens the alarm valve independently of the detectors |
| Monitoring of air/inert gas pressure | Explicitly required in the text | Required for all pre-action installations by Clause 16.2.3 |
| Quick opening manually operated valve | At least one, mandatory | No such provision in Clause 11.4.1.3 |
| Behaviour on detection failure | Operates as an ordinary dry pipe system | Already responds to sprinkler operation |
The size of the installation: Table 17
Clause 11.4.3 (TSE p.72 / EN p.69): the number of sprinklers controlled by a pre-action alarm valve shall not exceed that shown in Table 17 (the number of sprinkler heads in the given area).
Table 17 is headed "Maximum protected area in wet pipe and pre-action installations" (TSE p.70 / EN p.67):
| Hazard class | Maximum protected area per control valve set (m²) |
|---|---|
| LH | 10 000 |
| OH (including LH sprinklers within it) | 12 000, except as permitted in Annex D and Annex F |
| HH (including OH and LH sprinklers within it) | 9 000 |
Two points deserve attention: Table 17 is an area limit, not a limit on the number of sprinklers; Clause 11.4.3 reads that area as "the number of sprinkler heads in the given area". Also, Table 17 is not for dry pipe installations: the size of dry pipe and alternate installations is limited by the water travel time of Table 18 (LH 90 s, OH and HH 60 s, Clause 11.2.2, TSE p.70 / EN p.67). In pre-action installations this water travel requirement appears not in Table 18 but only in 11.4.1.4 b), for the case of multiple installations.
The detection system: Clause 11.4.2
Clause 11.4.2 (TSE p.72 / EN p.69) is a single sentence carrying two requirements:
- The detection system shall be installed in all rooms and compartments protected by the pre-action sprinkler system.
- The system shall comply with the relevant parts of EN 54 or, in their absence, with appropriate specifications valid where the sprinkler system is used.
In other words, leaving part of the protected area without detection means that in Type A the valve will never be triggered by that area.
More than one pre-action installation: water storage and 60 seconds
Clause 11.4.1.4 (TSE p.71-72 / EN p.68-69): where a sprinkler system contains more than one pre-action sprinkler installation, a risk assessment shall be carried out to determine whether simultaneous operation of more than one installation is possible. Where simultaneous filling is possible, the following apply:
- a) The volume of stored water supplies shall be increased by the total pre-action installation volume.
- b) The time between the tripping of more than one pre-action installation and the discharge of water from any remote test valve on those installations shall not exceed 60 s.
The remote test valve in b) is defined in Clause 15.5.2 (TSE p.112 / EN p.109): a test device connected to the hydraulically most remote point of the distribution pipe, giving a flow equivalent to the discharge of a single sprinkler.
Sprinkler orientation, sensitivity and alarm
Clause 12.1.3 (TSE p.73 / EN p.70): except where dry pendent sprinklers are used, sprinklers in dry pipe, alternate and pre-action installations shall be upright. The yoke arms of upright sprinklers shall be fitted parallel to the pipe.
Clause 14.5.1 and Table 38 (TSE p.109 / EN p.106) limit the sensitivity. In the column "Dry systems / Pre-action Type A" of Table 38, quick response sprinklers are marked "No"; standard 'A' and special response sprinklers may be used in that column. Where there are sprinklers in racks, the sensitivity of the ceiling sprinklers shall be equal to or slower than that of the sprinklers in the racks.
Clause 16.2.3 (TSE p.114 / EN p.111): each installation shall be provided with a low air/gas pressure alarm giving visible and audible warning in accordance with Annex I. Table I.1 (TSE p.158 / EN p.155) lists this in two separate rows, both of them type B:
| Alarm | Clause | Alarm type |
|---|---|---|
| Low pressure, pre-action Type A system | 11.4.1.2 | B |
| Low pressure, dry pipe and pre-action systems | 16.2.3 | B |
Two details on the pipework side
Clause 17.1.8 (TSE p.115 / EN p.112): in dry, alternate and pre-action installations, range pipes shall have a fall of at least 0.4 % towards the distribution pipe and distribution pipes a fall of at least 0.2 % towards the relevant drain valve. Range pipes shall be connected to distribution pipes only from the side or from the top.
Clause 17.1.2 (TSE p.114 / EN p.111): for dry, alternate or pre-action installations, galvanized steel should preferably be used.
Clause 20.2.2.2 a) (TSE p.122 / EN p.119) gives an acceptance measure in the weekly routine: in dry, alternate and pre-action installations the pressure in the pipework shall not fall faster than 1.0 bar per week.
Frequently Asked Questions
What is the difference between Type A and Type B pre-action in EN 12845?
Clause 11.4.1.2 (TSE p.71 / EN p.68): in Type A the control valve set is brought into operation by an automatic fire detection system and not by the opening of the sprinklers. Clause 11.4.1.3: in Type B the control valve set is brought into operation either by an automatic detection system or by the opening of the sprinklers; in addition, independently of the response of the detectors, a pressure drop in the pipework causes the alarm valve to open.
Does a Type B system need two confirmations before water enters?
No. Clause 11.4.1.3 states explicitly that independently of the response of the detectors, a pressure drop in the pipework causes the alarm valve to open. There is no definition of double interlock in the text of EN 12845.
What is the largest area one pre-action alarm valve may protect?
Clause 11.4.3 (TSE p.72 / EN p.69) refers to Table 17. Table 17 (TSE p.70 / EN p.67) gives the maximum protected area per control valve set: LH 10 000 m2, OH 12 000 m2 (except as permitted in Annexes D and F), HH 9 000 m2. The table sets an area limit, not a number of sprinklers.
Where is detection installed in a pre-action installation?
Clause 11.4.2 (TSE p.72 / EN p.69): the detection system shall be installed in all rooms and compartments protected by the pre-action sprinkler system and shall comply with the relevant parts of EN 54 or, in their absence, with appropriate specifications valid where the system is used.
What applies where there is more than one pre-action installation?
Clause 11.4.1.4 (TSE p.71-72 / EN p.68-69): a risk assessment shall be carried out. Where simultaneous filling is possible, a) the stored water volume shall be increased by the total pre-action installation volume and b) the time between the tripping of more than one installation and the discharge of water from a remote test valve shall not exceed 60 s.
References
- TS EN 12845+A2:2026 Clause 11.2.2, Table 18, Table 17 (TSE p.70 / EN p.67)
- TS EN 12845+A2:2026 Clauses 11.4.1.1, 11.4.1.2, 11.4.1.3, 11.4.1.4 a) (TSE p.71 / EN p.68)
- TS EN 12845+A2:2026 Clauses 11.4.1.4 b), 11.4.2, 11.4.3 (TSE p.72 / EN p.69)
- TS EN 12845+A2:2026 Clause 12.1.3 (TSE p.73 / EN p.70)
- TS EN 12845+A2:2026 Clause 14.5.1, Table 38 (TSE p.109 / EN p.106)
- TS EN 12845+A2:2026 Clause 15.5.2 (TSE p.112 / EN p.109)
- TS EN 12845+A2:2026 Clauses 16.2.3, 17.1.2 (TSE p.114 / EN p.111)
- TS EN 12845+A2:2026 Clause 17.1.8 (TSE p.115 / EN p.112)
- TS EN 12845+A2:2026 Clause 20.2.2.2 (TSE p.122 / EN p.119)
- TS EN 12845+A2:2026 Table I.1 (TSE p.158 / EN p.155)
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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Fixed firefighting systems — Automatic sprinkler systems — Design, installation and maintenance; TS EN 12845-2:2025 (EN 12845-2:2024) ESFR and CMSA sprinkler systems. Every figure in this article is taken from the published standard text; clause, table and page references are listed under References. General information only, not a substitute for the standard or for a design review.