In an earlier contribution to this discussion,
TonyK wrote:

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

Are you sure that the on periods never overlap, so as to have them all on at
the same time. If you do this, you have 8.4kW = 35 amps.

If that's not the case, it might be a transient current problem. Heaters
which draw 20 amps in the steady state probably draw considerably more
current than that for the first few seconds when you turn them on - because
the element resistance will be quite low when cold. Maybe you need a trip
which can stand a short-term overload without tripping.
--
Cheers,
Set Square
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