Just moved into a house and directly under the fuse board (almost attached to it) is a switch (looks like a light switch). No idea what it does but flicking it on causes a huge load on fuse board. Anyone have any ideas what it could be?
Could it be an immersion heater switch? Do you know what current it’s drawing?
No unfortunately not. There’s a separate immersion switch beside the hot press
Wait till it’s dark
Flip the switch
Go outside, it could be this deadhomersociety.com/2009/12/25/quote-of-the-day-333/
Have you looked in the attic and are you also using unaccountably large amounts of water? 
Ah the joys of random wiring!
How huge is the load? Are the trip switches on the fuseboard labelled? Isolate the ring to which the switch is wired (only one tripswitch up). See if you can have the meter not turning except when the offending switch is on. Time how long one rotation of the disc on the meter takes in seconds. Multiply by the “number of turns per unit” given on the meter faceplate (e.g. mine says 187.5 rev/kWh). Divide the answer into 3,600,000. That gives the power being drawn in Watts. (e.g. 3,600,000 / (20 seconds per turn x 187.5 turns per unit) = 960 W). Now you know the load and which ring it’s on. What was the question again? 
Is it for the UV lamps in the attic?
I remember a story I heard once about someone who had a bit of damp in the attic and it was recommended that they put a three bar electric fire up there to dry it out, which they did.
Unfortunately they forgot about it!
25 years and a couple of changes of owner later, the new owners couldn’t understand why the electric bills were so high and eventually found this heater!
I can’t wait to find out what this is it’s most thrilling… and boy what a story! 25 years!!! 
Any news? 
Was there a new elec shower installed?
my money’s on elec shower - I have one [switch] but it’s a back up; odd that it’s drawing current though
Electric underfloor heating?
Does your neighbour have one of these
Thanks all for replies - only just bought the house - there are 2 electric showers upstairs allright alas no batcave. Was out playing soccer last night so didn’t get time to investigate further - will do at weekend
I could be wrong but I thought that one could only have one 30amp cable in a housing unit and that would only be enough for one electric shower, and possible one electric oven. I am not an electrician so am not sure, but seem to remember being told something like this years ago, so maybe someone has done a little bit of illegal wiring?? Also I would want to get to the bottom of this if it is as you say drawing power continuously??
In a previous house I rented there was such a switch and you would change it depending on which electric shower you wished to operate.
Ah I see, so presumably it would be ok to have two electric showers as long as they are not being operated at the same time. Did not know this, thanks.