Bart wrote:
I don't know the answer but I'm hoping you are asking out of curiosity and not because of an ongoing situation

No. We were within an inch of having the requirement to carry a fire extinguisher removed from our conditions the other night until on of the Councillors said that we should keep them to use on a person who is on fire.
I sent an email to our local fire authority and I have just received the following reply.
"We would always recommend the ‘stop, drop and roll’ method for a person on fire, with someone else attempting to smother the flames with a coat for example. If a water extinguisher was to hand, it could be used and would have a cooling effect, however if the person had used an accelerant such as petrol, a water extinguisher could make things worse.
We would not generally advise that you use a dry powder extinguisher on a person, as they can cause respiratory problems for the individual and dry powder would have no cooling effect."