Just to clarify EN 3-7 allows Water-based extinguishers to be marked as being suitable for use on live electrical equipment up to 1 000 V a.c. at a distance of 1 m, but The British Standards Technical Committee FSH/2 have stated that the national practice is ”not marking the extinguisher if it passes the test
in BS EN 3-7:2004+A1:2007, Annex C, but marking a warning if the extinguisher failed the test or was not submitted”
It's therefore only UK traditions that mean water based extinguishers are not normally marked for electrical fires - a tradition that the new draft BS5306-8 for the selection & placing of extinguishers will continue (unless there is an overwhelming response to the draft to take up the mainland European approach)