On Sun, 28 Nov 2004 20:23:18 -0000, "TonyK"
strung together this:

I have a number of electric storage heaters on a single ring covred by a 30A
MCB.

Oh dear, a bad start.

The heaters are 3 x 1.7Kw and a single 3.3Kw. These are all on 24hr
fused spur timers

Well, 3.3kw is over the maximum size of fixed appliance that should be
connected to a ring final circuit. The spur will no doubt be rated at
3kw.

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?

Hmm, not neccesarily. Depends on what your actual supply voltage is,
it could be more than 20.8333A. There is also the probability of a
start up current that is greater than the running load.

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.

Probably because you're a bit close to the limit of the MCB with it
all on. Assuming the voltage is closer 230 the total load with it all
on is 29.summat amps, not a good idea.
Only option in my opinion is rewire it with radials to each heater and
have a central CU\contactor\timeclock controlling them.

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

I would say so, yes.
--

SJW
A.C.S. Ltd
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