EN 12845 Pre-Action Installations: Type A and Type B

How a two-stage confirmation is built in data centres, museums and archives, so that sprinkler water flows only when a fire is proven — not before.

During commissioning at a colocation data centre we created a cold-air condition with a hose beneath a K80 sprinkler directly above a server row. When the plastic outer cover deformed and the frangible element burst, we expected water on the cabinet automatically — instead the pre-action valve stayed shut, because the ceiling smoke detection zone had not operated. One of the two confirmations was missing, and the system did not release water. That is precisely why pre-action systems exist: unless the detection system and the sprinkler both say "yes, this is a real fire", the pipework stays dry.

EN 12845 describes this in its pre-action clause, recognising two types: Type A (single interlock) and Type B (double interlock). The difference lies in what opens the valve — and a wrong choice either puts millions of euros of equipment under water in an accidental discharge, or fails to deliver water in a real fire.

What the clause says

The standard frames pre-action as a safer relative of the dry pipe installation. The pipework is held under air or inert gas pressure, continuously monitored under the alarm transmission annex. The difference lies in the trip logic of the alarm valve.

Type A — single interlock

The pre-action control valve opens only on a signal from an EN 54 compliant automatic fire detection system. Sprinkler operation alone does not open the valve:

• Detection operates → the pre-action valve opens → the pipework fills with water (no discharge yet)
• A sprinkler then operates → water discharges from that head

The standard is explicit that if the detection system fails, a Type A installation behaves like an ordinary dry pipe installation: sprinkler operation drops the air pressure, the alarm valve opens anyway, and water arrives. That fail-safe behaviour makes Type A relatively forgiving.

The note matters: Type A pre-action installations are recommended only in areas where accidental water discharge would cause serious damage. So Type A goes into sensitive contents, not into LH or OH corridors.

Type B — double interlock

This type requires two independent conditions for the valve to open:

1. The EN 54 detection system gives a fire signal, or
2. A pressure drop in the pipework (sprinkler operation) is detected

The standard's text says that independently of the detectors' response, a pressure drop in the pipework causes the alarm valve to open. In practice, however, most data centre specifications require that a pressure drop alone should not open the valve without detector confirmation, achieved through a product-specific double interlock configuration.

The standard describes Type B's use as anywhere rapid fire spread is expected and a dry pipe installation is required. In cold stores and deep freeze facilities, where a wet system would freeze and delay must be minimised, Type B responds faster than a classical dry pipe system because the detection zone senses heat before the sprinkler does.

The two types side by side

Feature	Type A (single)	Type B (double)
Valve opening condition	Detection signal only	Detection signal or pressure drop
Behaviour on detector failure	Works as an ordinary dry pipe system	Pressure drop still opens the valve, though higher-tier specifications may require detector confirmation
Accidental discharge risk	Low	Lowest
Emergency manual valve	Mandatory	Mandatory
Typical application	Museums, archives, art galleries, libraries	Data centre white space, cold stores, freezers
Air or inert gas pressure monitoring	Continuous	Continuous

Data centres: why Type B becomes standard

In a white space carrying 2–3 MW of IT load, each rack holds 15–30 kW. A sprinkler opening accidentally washes cabinets across a 4–6 m radius; earthed cabinets, optical patch panels and lithium-ion UPS units are all lost instantly. So the design chain runs:

• Aspirating smoke detection — very early warning, which does not open the valve
• EN 54 point smoke detectors — opening the pre-action valve on coincidence (two detectors confirming)
• Sprinkler operation — with the valve already open after detector confirmation, water discharges from that head

The advantage of Type B in that arrangement: if maintenance staff strike a sprinkler head with a stepladder, the valve does not open because the detection zone is clear. You hear air from that head, a pressure loss alarm reaches the BMS, and you send a team. No water, no damage.

Museums, archives and libraries: Type A is generally sufficient

In those contents, fire growth is slow and the material largely cellulosic. Adding detector confirmation to a dry pipe system (Type A) largely eliminates accidental discharge. Double interlock is not needed because:

• Fire growth is slow — the interval between detection and sprinkler operation is comfortable;
• Type A's fail-safe behaviour means sprinkler operation still brings water even with a failed detector;
• Cost and complexity — the double interlock panel, extra pressure switches and dual-zone software are all operating expense.

The EN 54 detection loop: the standard's quiet requirement

The clause is clear: the detection system must be installed in all rooms and compartments protected by pre-action sprinklers and must comply with the relevant parts of EN 54. That includes concealed spaces — ceiling voids, service risers carrying the pre-action pipework, and the space beneath a raised floor. The site reality: at many commissionings there are detectors above the white space ceiling but none beneath the raised floor. Under the standard, that gap invalidates the whole pre-action logic — because nobody detects a cable fire below, and the valve never opens.

Multiple pre-action installations operating together

Where a building contains two or more pre-action installations, a risk assessment is required. Where simultaneous operation is possible:

• The stored water volume is increased by the total internal volume of all the pre-action installations;
• The time between multiple installations tripping and water discharging from the remote test valve must not exceed 60 seconds.

Example: with two 4 m³ pre-action installations, 8 m³ is added to the tank capacity. This detail is frequently skipped — the tank calculation multiplies design flow by duration and ignores pre-action internal volume. At commissioning, when both valves trip, the tank level falls faster than expected and the pump raises an NPSH alarm.

Installation size

The number of sprinklers on one pre-action alarm valve may not exceed the dry pipe installation limits — typically 500 sprinklers per valve for light hazard and 1000 for ordinary hazard. In data centres, limiting the valve to 250–500 sprinklers for faster water delivery is a common site decision, driven not by the standard's upper limit but by internal volume and delivery time.

Field error: a failed detector nobody noticed

After annual maintenance at a museum, the pre-action panel showed a detector loop fault — but because there was no BMS integration, the alarm was only on the panel in the plant room. Months later a small cascading short-circuit fire started in the reserve store; the sprinkler operated and the pipe pressure fell, but in the Type B configuration detector confirmation had been made a requirement and the detection zone was already in fault. Water never arrived. Three racks were damaged before the fire curtain operated.

The lesson: a pre-action system delivers on its promise of "water that has passed two checks" only while both confirmation loops are healthy and monitored. Connecting the installation to the BMS, sending a detector loop fault to site by SMS or email, and running weekly functional tests — emergency manual valve simulation, detection zone test — is the life-support of this system.

The NFPA 13 bridge

NFPA 13 uses the same names: single interlock and double interlock pre-action. In NFPA 13 the double interlock trip logic is stricter: both detection and sprinkler operation are required. The EN 12845 text is more flexible, treating the pressure drop as an independent trigger; but data centre specifications overwhelmingly prefer the NFPA-style dual requirement. Writing "Type B per EN 12845 with detector confirmation interlock" into the specification avoids a surprise at commissioning.

Turkish context

BYKHY references NFPA and EN 12845 for sprinkler installations, and the EN 12845 reference has become standard on data centre, bank archive, museum and most sensitive facility projects. At building inspection and fire service approval, the pre-action panel's EN 54 certificate, the detection loop supervision records and the emergency manual valve location are all reported.

Frequently asked questions

What is the basic difference between Type A and Type B?

Type A opens the pre-action valve on a detection signal alone; sprinkler operation by itself does not bring water. Type B involves both the detection signal and a pipe pressure drop, so that water does not enter the pipework until both confirmations arrive.

Type A or Type B in a data centre?

Type B (double interlock) is the field standard for data centres. A single accidental detector operation does not release water; actual sprinkler operation — real heat — is also needed, halving the risk of accidental discharge in two stages.

What happens if the pre-action detection fails?

In a Type A system, the installation reverts to ordinary dry pipe behaviour and sprinkler operation still brings water. In Type B, if the detector fault goes unnoticed, water may never arrive even when a sprinkler operates — which is why weekly supervision of the EN 54 loop and BMS transmission of fault alarms are essential.

Is an emergency manual valve mandatory?

Yes. The standard requires at least one quick-opening, manually operated valve in a suitable position, so that the pre-action valve can be opened by hand where the detection loop has failed completely or an operator sees the fire.

How many sprinklers can one control valve serve?

The number is limited to the dry pipe installation limits for the hazard class. Where multiple pre-action installations may operate together, the tank volume is increased by their total internal volume.

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