FM-200 has been the primary Halon replacement fire suppressant for use in these type of applications for around 13 years now. The market has developed FM-200 into the leader in this area. There are other options available but FM-200 is the predominant choice of most data center professionals.
You may be asking 'why use a clean agent' in general. The answer to this includes several different elements. First and foremost, these systems put out fires well before the smoke, fire or ensuing water based system (sprinklers) would cause any damage to the facility. Many times, owners and specifiers are not only looking for reliable fire protection, but also protection to make sure the sprinklers are not discharged unless absolutely necessary. Also, these systems are completely safe for the occupants of the room. In some cases, local authorities or insurance companies will require systems like this to be installed.
Size of the data center can be one of the decision making criteria, but more commonly, the value of the assets to be protected is used. As the size of the facility increases, typically the value of the contents and operations also increases. The need for waterless fire protection, like FM-200, becomes greater. You may conclude from this, that smaller facilities do not require a clean agent.--but not true! Again, it's the value of the contents, both financially and operationally, that should be considered when determining the appropriate spaces to protect.
When considering the use of FM-200 in the sub-floor vs. the above floor, again, I would consider the value of the contents (operational value will be most prevalent in the underfloor, as loss of cabling can lead to loss of power, leading to significant downtime). Also, you may want to consider the hazard level within these spaces. In some cases, the sub-floor area is not heavily occupied by cabling or other potential combustibles. In such a case, I would not include an extra level of protection for the sub-floor. In other cases, the data center owner my feel the cables and the power/information they bring through the underfloor are by far the most critical hazard in the space, the sub-floor is the only space protected. Most commonly, both subfloor and above floor are protected by FM-200. In this case, the same cylinder (or multiple cylinders manifolded together) can protect both spaces, by engineering the system piping network.
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