On energised electrical equipment, burning cooking oil, some flammable liquids, and where the contents cannot survive being wet.

Water conducts, so a discharge onto energised equipment is a hazard as well as ineffective.

On burning fat, water flashes to steam instantly and ejects burning oil - which is why kitchen systems use wet chemical.

Lighter flammable liquids float on water, so a water discharge can spread the fire rather than extinguish it.

Archives, data halls and collections may be destroyed by water even if the fire is stopped.

In each case a special hazard system is chosen because of what water would do, not because water would fail to put the fire out.

Clean agents interrupt combustion chemically; inert gases reduce the oxygen concentration until a fire cannot sustain itself.

Clean agents work at low concentrations and discharge very quickly, which suits sensitive electronics.

Inert gases are mixtures of naturally occurring gases and need larger volumes stored, so cylinder space becomes a design constraint.

Inert gas discharge is slower and produces less turbulence, which matters in rooms with delicate equipment.

Both are designed to be survivable for the brief period people need to evacuate.

Environmental regulation of specific clean agents differs by market and has changed over time, which affects long-term availability.

Because CO2 extinguishes by displacing oxygen, at concentrations that will kill anyone in the room.

That makes it effective on machinery and enclosures and unsuitable for normally occupied spaces.

CO2 systems require pre-discharge alarms, time delays, lock-off devices and clear warning signage.

Entry procedures after discharge are a serious matter, since the atmosphere remains dangerous long after the fire is out.

There is a documented history of fatalities associated with CO2 system discharges and with maintenance work on them.

Where a space may be occupied, an inert gas or clean agent system is the appropriate choice instead.

The agent reacts with burning fat and oil to form a soapy crust that seals the surface and stops vapour reaching the flame.

That process is called saponification, and it is the reason wet chemical is used rather than any water-based agent.

Nozzles are aimed at specific appliances, so the system is designed around the kitchen layout.

Moving or replacing an appliance can invalidate the system entirely, and it happens frequently.

Systems interlock with gas and electrical supplies so the heat source is removed on discharge.

Cooking medium changes - from animal fat to vegetable oil, for instance - have historically affected system requirements.

It is how well the room holds the agent, and most gaseous systems depend on it entirely.

A system is designed to hold a concentration for a specified period after discharge.

A room that leaks loses that concentration quickly, and the fire can re-establish itself.

Room integrity is tested at commissioning, usually with a fan pressurisation test rather than by inspection.

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

Retesting after any alteration is the control, and alterations to server rooms and plant rooms are constant.

The mixing ratio between concentrate and water - foam performance depends on it almost entirely.

Too lean and the foam blanket is thin and breaks down; too rich and the system runs out of concentrate early.

Bladder tanks, proportioners and inductors are the devices that hold the ratio across varying flows.

Concentrate type has to match the fuel - polar solvents such as alcohols destroy ordinary foam.

Concentrates have shelf lives and are tested periodically for degradation.

Certain fluorinated foam concentrates are subject to increasing environmental restriction, and the position differs markedly between markets.

Designed, installed and certified - they are engineered installations rather than products.

Agent selection, concentration, discharge time, nozzle placement and detection are all calculated for the specific enclosure.

Interlocks with ventilation, power and gas supplies are part of the design rather than add-ons.

Personnel safety measures - alarms, delays, signage, lock-offs - are mandatory elements in most regimes.

Commissioning includes integrity testing and functional testing of the whole chain.

Periodic inspection, agent weighing or pressure checks and recharge after discharge all follow schedules set locally.