I have a number of electric storage heaters on a single ring covred by a 30A
MCB. The heaters are 3 x 1.7Kw and a single 3.3Kw. These are all on 24hr
fused spur timers and set to come on at times that offset the load, i.e. the
3.3 and a single 1.7, then two 1.7's etc etc. So in theory the max load is a
3.3 and 1.7 or 5Kw which according to my understanding is 5000/240 = 20.8333
or 21amps? So... why does it trip out occasionally (once every 24 horus) ?
No other loading on the circuit but the MCB buzzes and the buzzing gets
louder the higher the load, ie. put another .17kw heater on.

So, is there something fundamentally wrong with my understanding or what?

Thanks