Re: CBus wiring

["David Buckley" <db@xxxxxxxxxxx>]

--- In ukha_d@xxxxxxx, "Paul Robinson" <ukcueman@y...>
wrote:
> Anybody know if there's a significant difference in using
> 6A MCBs instead of 1A/2A MCBs on the output legs of the dimmers?
> I got a load of 6A MCBs cheaply, and I've installed those,
> along with one 10A MCB per dimmer on the input side. Was it
> a mistake to use the 6A MCBs?

One would immediately think it was, but the probable answer is no,
they are fine.

First the question -  why on earth would you bother to install MCBs
on the output side of the CBus dimmers?

I think there are a few reasons.

a) To protect downstream wiring and equipment from overcurrent.
1.0mm wiring can safely be breakered at 6A, so there isnt a wiring
overcurrent problem.  Even if the 6A breker wasn't there, under most
conditions 1.0mm cable can be breakered at 10A and be safe.

b) to defend the CBus dimmer against overload - this you wont do, as
the max load is 1A or 2A per channel.  But on a fixed installation,
this element is in your hands as to how you allocate lights to
simmers, and how much power each lamp uses.

c) To defend the CBus triacs from damage caused by short at full
power.  Although a 6A breaker is much bigger than a 6A breaker when
you are talking about an operating current of one or two amps, under
a (say) 300A short circuit current, both breakers will beahve pretty
much the same.  Maybe they'll save the traic, maybe they wont.

The interesting thing here is that for exactly the same reasoning,
the 10A feed breaker (assuming its the same "type" as the 6A
breakers) is equally likely to trip as the 6A breaker.  You have no
effective discrimination.  To make this work correctly, you need
a "slower" breaker upstream, so under short conditions, the
faster
breaker will let go first.