Fuses and induction,

> One can indeed blow fuses by connecting an 
> inductor (e.g. variac) to the mains alone. The mechanism has to do 
> with residual magnetism in the core. If switch-on coincides with a
> mains peak of the "wrong" polarity, the core temporarily saturates
> losing most of its inductance and presenting a near zero resistance 
> to the mains. This is actually a real headache for designers of large 
> power amps who particularly choose to use toroidal transformers where
> leakage inductance is rather low. I have popped circuit breakers by
> simply switching on a variac turned to zero. A matter of luck in 
> timing basically.
Hi Malcom and all,
This is what Ive experienced with 0-280V units above the 10A mark,
even on a breaker triple that capacity! You need some 'motorstart'
fuses to handle the initial BLAST !!
On my new controller unit (40A variac + plus fuses + meteres)  Ive
made provision for a 'standby' state switch. This way the variac
can be left in an active state to avoid many inconvenient visits
to the (breaker box):( Ive got a green light switch for 'active
state' and a red one for 'active output state'.

This seems to add a little saftey, as well as the "Doing rounds
to the fuse box" syndrome.   

Well it works ok for me!........................