Behind the scenes of GS: nothing left to chance

The AS (Alarm Systems) Section in the GS-ASE Group is, as its name suggests, in charge of the various alarm systems spread across CERN’s many sites. Its mission? To install, manage and maintain more than 26,000 alarms of all types located both above ground and in the tunnels.


Among these systems, the best known are of course the heat and/or smoke detectors, which quickly raise the alarm in the event of a fire. CERN has 8500 of these devices in total. In combination with these, evacuation alarms are also found all over the Laboratory, including some 1800 break glass call points for 2000 sirens. In the LHC tunnel, the evacuation alarms are connected to 200 Oxygen Deficiency Hazard (ODH) sensors, but this is not the only way of triggering an alarm. “The Fire Brigade permanently monitors the evolution of safety conditions in the LHC tunnels,” says Henrik Nissen, who is responsible for “Alarm Transmission” in the GS-ASE-AS Section. “If necessary, they can also trigger the evacuation sirens.” Other types of detectors, such as for monitoring the emission of explosive or toxic gases, are also in place in certain specific areas.

A fire detection system using suction (via pipes indicated by red arrows) was recently installed at Point 7 of the LHC.

Of course, the unique nature of CERN has also required the development of specially designed alarm systems. The GS-ASE-AS Section has therefore worked closely with the members of the four main LHC experiments to develop the so-called “Sniffer” systems, which are able to monitor both fire and gas in the very heart of the particle detectors. “We have developed sensors equipped with pumps and long hoses,” explains Henrik Nissen. ”The hoses are inserted right into the heart of the particle detectors, where they continuously suck up air. This is then analysed by the sensor, which is located a few hundred metres away.”

Each type of alarm is connected to a detection unit, which is then connected to a transmission unit. From here, the information – for example, which type of alarm has been activated in which building – is transmitted to the Fire Brigade’s Safety Control Room (SCR) and to the CERN Control Centre (CCC). “The information is transferred via two channels,” explains Henrik Nissen. “The first channel is a basic electrical (wire) network which, by its very nature, ensures a very high level of reliability. The second channel is a computer network which, although it allows more precise information to be transferred, is not as reliable as the first.” All of the alarms essential for the safety of people and equipment (level 3 alarms), as well as vital technical alarms (for cryogenics, for example) always use both channels. This redundancy ensures that the information is transmitted whatever happens.

On the maintenance side, each of the 11,000 level 3 alarms is tested every year. This is a mammoth task which requires the expertise of seven people working full time in close cooperation with CERN’s Fire Brigade.

Test platform for detecting gas (including ODH). The bottles at the bottom of the image contain different types of gas used for tests.  The Fire Brigade’s Safety Control Room, which receives level 3 alarms.


by Anaïs Schaeffer