By reducing oxygen to a level combustion cannot sustain but people can briefly survive.

The agents are mixtures of gases already present in the atmosphere - nitrogen, argon and sometimes a little carbon dioxide.

There is no chemical reaction with the fire; the mechanism is displacement.

Design concentrations sit in a deliberately narrow window between those two requirements.

The small CO2 proportion in some blends stimulates breathing and improves tolerance at reduced oxygen.

That window is why the design concentration is calculated rather than chosen.

Naturally occurring gases have no ozone depletion potential and negligible global warming potential.

That exempts them from the phase-down pressures affecting some clean agents.

For a building with a long life, that regulatory stability is a real commercial argument.

Recharge is also inexpensive, since the agents are commodity industrial gases.

There is no residue and no decomposition products of environmental concern.

It is one of the main reasons inert systems are specified in new buildings.

Because displacement requires far more agent than a chemical suppressant protecting the same room.

The gas is stored at high pressure in banks of cylinders.

Cylinder room space becomes a real design constraint rather than an afterthought.

In retrofits it is frequently the deciding factor against inert gas.

Cylinder banks also need structural support and access for maintenance.

High-pressure pipework and manifolds add to the installation cost relative to clean agents.

Yes - typically over about a minute rather than seconds, which produces much less turbulence.

That is an advantage in rooms full of delicate equipment.

Rapid gaseous discharge has been associated with disturbance to sensitive hardware.

Slower discharge also reduces the peak pressure the room must accommodate.

Pressure relief is still designed rather than omitted.

The slower fill is acceptable because the fire is being starved rather than chemically interrupted.

Design concentrations are set to be survivable for the brief period needed to evacuate - but evacuation is still the expectation.

Oxygen is reduced, so the atmosphere is not normal.

Pre-discharge alarms, delays and clear egress are required as with any system.

Prolonged exposure at reduced oxygen is not acceptable regardless of survivability.

Re-entry requires ventilation and confirmation that oxygen has recovered.

The distinction from CO2 is that a brief exposure is survivable rather than lethal.

Exactly as much - the concentration must be held for a specified period after discharge.

A leaking room loses concentration and the fire can re-establish.

Integrity is proven by fan pressurisation testing at commissioning.

Cable penetrations, ductwork and door seals degrade it over time.

Retesting after alterations is the control and is routinely omitted.

Because inert gas discharges more slowly, the room must hold the agent through a longer fill as well as afterwards.

Higher installation cost, lower recharge cost - the balance depends on how likely a discharge is.

Cylinder banks, high-pressure pipework and space requirements raise the initial cost.

Recharge with commodity industrial gases is far cheaper than a restricted clean agent.

There is no exposure to agent price rises driven by environmental regulation.

Maintenance is comparable, with cylinder pressure checks rather than weighing.

For a long-life installation the total cost frequently favours inert gas where space allows it.