CO2 extinguisher in a confined space

[nim]

Does anyone have an idea when a confined space would be unsuitable for a 2kg or 5kg CO2 extinguisher?

Occasionally we have been told by Risk Assessors that CO2 extinguishers should be removed from Lift Motor Rooms/Electrical Mains Intake rooms because CO2 is an asphyxiant and not suitable for a confined space.

On one occasion we were told by a Risk Assessor to replace a CO2 with a Water Mist.

For some Risk Assessors there seems to be a blanket ban on CO2 Extinguishers either recommending something else e.g. Water Mist and for some Risk Assessors there seems to be no issue and on occasion recommending a CO2 extinguisher.

Is there guidance anywhere?
When does a confined space become a confined space for a CO2 extinguisher?
What do we use instead of a CO2 extinguisher?
Do we use a suppression system instead?
Would you recommend another type of extinguisher? E.g. Water Mist?
Is a 2kg CO2 extinguisher more suitable than a 5kg Co2 because the volume is less?

Thanks in advance for your help

[AnthonyB]

No UK guidance, plenty in other countries.

In Germany the following guidance is in place in DGUV* Information 205-034#:
Per kg CO 2 extinguishing agent there must be at least a free floor area * of 5.5 m? are available for a person in the room. Specifically, that means:

A 2 kg CO 2 fire extinguisher requires at least 11 m 2 of free floor space.

A 5 kg CO 2 fire extinguisher requires at least 27.5 m 2 of free floor space.

If the size of the extinguisher required for the risk is greater than that permitted by the room size then it has to be mounted outside the room and signed to be discharged from outside the room only.

*The DGUV (German Social Accident Insurance) is the umbrella association of the statutory accident insurance organisations in Germany. It represents around 70 million people insured against the risks of work accidents/ occupational diseases (employees, students, schoolchildren i.e.) In the event of a work accident or occupational disease the accident insurance funds are responsible to grant comprehensive healthcare.
# The information sheet was based on a range of practical experiments on a range of typical workplace rooms and with different layouts and furnishing

Because the other EU countries have always been happy to use water and foam extinguishers that have passed the 35kV test on electrical fires up to 1000V this safety restriction on the use of CO2 doesn't cause many issues, but if adopted in the UK would cause upset as it's restricted the use of wets to keep up the use of additional CO2 extinguishers, hence why no mention. Germany in particular is pushing to reduce the indoor use of powder and replace with foam but not having the need for CO2 as well, it being reserved for risks needing a clean agent.