Nigel Goodwin wrote:

> in the UK.  Generally light switches only switch the live conductor,
> and switched mains socket only switch the live.  But most seperate
> switches, for such things as immersion heaters, electric showers,
> storage heaters switch both live and neutral.

  The philosophy behind this hinges on the likelihood of wiring
modifications being made and service being performed using only the
local switch for isolation.  "Fixed" wiring is expected to have active
(live if you like) and neutral irrevocably defined, and the neutral is
known to be bonded to ground, so only the active need be isolated.

  Once the appliance has a flexible cord connecting it to an outlet (or
even a wiring box), there«s a chance that the cord will be damaged and
rewired, possibly by an unqualified person.  Without the guarantee of
which wire is which, you must isolate both.  The same applies to circuit
breakers and it is very reasonable of them to isolate both lines if
tripped.

  Neutral fuses are in much the same situation.  If there is any chance
the supply polarity be reversed, then the neutral fuse becomes the
active fuse and will be blown if a short to ground occurs.  Because this
is at that time effectively the active fuse, the situation is safe.  It
may become unsafe when moved out of the fault location for service but
well, if you are testing something with a blown fuse you don«t just plug
it in and turn it on.  *Do* you?

  By the way, a little commonsense here suggests the active fuse be
rated between 70% and 50% of the neutral, so it blows preferentially for
internal overload while the neutral *only* blows for a ground short.  (I
say *commonsense*; the standards may say different for all I know!)
--
  Cheers,
        Paul B.