In article ,
John Rumm writes:
It tells you there is no associated overcurrent trip. It probably
indicates that you have high leakage that is "sensitising" the RCD (i.e.
using most of its trip current budget, and leaving it close to the point
of tripping), and hence it does not take much to put it over the edge.
Even transients on the mains that could originate outside of the house
may trigger the trip.

My parents had a couple of trips recently, but knew it was a desk fan.
Fan is double insulated and all plastic case, so it can't generate
earth leaks. Eventually I opened the plug, and found a dead spider
nesting bwteen the live and (unused) earth connection.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]