Also it would depend on what is a transformer station and what does it include and does it have any other fire protection measures built-in. Are FE intended to put out a fire in the transformer or just in the room (if it is room) for incidental fires like a trash bin?
Who is supposed to use the FE? Not for a transformer station, but I have had local fire marshal requesting foam fire extinguishers for use by fire fighting personnel in walk-in generator enclosures on roof of a building. Perhaps you can check with your local FM.
Factory Mutual's Insurance guides go from none required to full hydrant and hose plus automatic deluge systems depending on size, quantity of oil and type of facility.
For up to 10 MVA oil filled, the FM chart I have recommends portable extinguishers plus requiremetns for sprinklers depending on type of building construction. Use with caution, it is an old document.
Check with the client's insurance company and the local fire marshall. (In one township, we found he was also the local fire extinguisher salesman. Hired him and had no problems.)
Besides, some transformers (old ones) will have to be allowed to burn because cooling the flames can result to very toxic gases! Whatever your transformer is made of and all its contents will determine the type of extinguisher needed.
