The high pressure self protection system of piping and spray heads is very similar to the systems which are very commonly found on French brush trucks. Typically these have a separate 200 or 300 litre water supply so they are still available even if the main water tank is already drained. It is easy to see that in a tunnel fire the truck may have difficulty retreating from a fire after it has emptied its main tank, with potentially serious consequences if it cannot use its self protection system. Some tunnel pumpers also have an extra compressed air supply in the cab for the same reason, and/or a dedicated air supply to the engine so that it does not stall in heavy smoke conditions - when the ambient air's oxygen levels may be too low to support combustion...
Added by Rob Johnson on 26 July 2022.