Re: Re: CBus wiring

["Paul Robinson" <ukcueman@xxxxxxxxxxx>]

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?

----- Original Message -----
From: "Dean Barrett" <dean@xxxxxxx>
To: <ukha_d@xxxxxxx>
Sent: Wednesday, August 04, 2004 10:10 PM
Subject: RE: [ukha_d] Re: CBus wiring


> We're about to commence a Cbus job with 70+ 1 amp MCB's, so if anyone
want
> to join me on our bulk buy I'll be getting good prices for them !!
>
> We protect all incoming supplies to Cbus dimmers with 10amp MCB, and
all
> outgoing with 1 amp or 2's on the 2 amp/channel dimmers.
>
>
>
> Dean.
>
>
> -----Original Message-----
> From: David Buckley [mailto:db@xxxxxxx]
> Sent: 04 August 2004 21:37
> To: ukha_d@xxxxxxx
> Subject: [ukha_d] Re: CBus wiring
>
> --- In ukha_d@xxxxxxx, Robert Jennings <robert@j...> wrote:
> > Frank,
> >
> > I just came back from the CBus training course. The guy giving
the
> > course recommended fusing or circuit breaking all connections
into the
> > CBus modules as a short circuit could blow it. An expensive
short.
>
> Its really difficult to protect a triac from damage following a short
> circuit.
>
> The problem is that the short circuit currrent can be (actually,
should be)
> really large.  Hundred or even thousands of amps.  The fuse or circuit
> breaker will keep this current flowing for milliseconds until it blows
or
> trips.  During these milliseconds, the semiconductor device will be
> dissipating heat many times its normal capability, and will fail to
> dissapate this heat.  Thus the device junction over-temperatures and
dies.
> All this happens in one half-cycle of the mains waveform.
>
> I've said jokingly in the past that it is amazing that triacs are so
willing
> to lay down thier lives to protect a fuse, but, as the saying goes,
"oft
> truth said in jest", and thats certainly the case here.
>
> The best option is to have a triac that adequately rated for the short
> circuit current for the duration it is likely to flow, which means a
vastly
> overspecced device, along with a "normal" MCB or fuse.  This
device will,
> however, be rather larger and more expensive than the normal 12A tab
devices
> commonly used.  Look inside any stage lighting dimmer, the triacs are
far
> too big.  But they do survive shorts quite well.
>
> Next best option is a semiconductor fuse.  These do not fit normal
> fuseholders, and are quite pricy, and are not 100% successful, but
they do
> work quite well.  Perhaps a 5A fuse on each of the power inputs of a
CBus 8
> channel dimmer would work quite well, but its a bit of a flyer.  A 1A
fuse
> per output channel would be much better.
>
> Cheapskate version of this is a small HRC fuse.  A well known stage
dimmer
> rated at 10A/channel used a 7A HRC fuse as protection.  But with
tiddly
> dimmers rated at 1A, its hard to get a HRC fuse small enough...
>
> Ordinary MCBs are the last option, but the protection rate wont be
good with
> these under short conditions.
>
> The real fit-and-forget answer is to use IGBT dimmers, not triac
dimmers.
> These can run into shorts all day, as they notice the overcurrent
event as
> it is happening in microseconds, and shut down the output.  Every now
and
> again the dimmer will try the output again, in case its been fixed (or
fixed
> itself), and when the coast is is clear, deliver power again. So no
fuses,
> and the dimmer looks after itself.
>
> [OT]One of my favorite devices in the whole world is the Cyberex
static
> transfer switches (DSTS), used to maintain power to high criticality
loads
> when power (or UPS, or generators) fail.  Properly used in a correct
system
> configuration, these things are key to high availability power.
>
> The switching elements in these boxes are thyristors (SCRs), which are
> similiar devices to triacs only the only condut one way, so a pair are
> needed in parallel in opposite directions for AC.  You just cant get
big
> enough triacs for this kind of job.
>
> Anyway, the big DSTS are available in three types, i,ii,iii.  The
difference
> is how much the SCRs are over-rated to withstand overcurrent events. 
Type i
> has fues protected SCRs, and these will pop under short conditions. 
Result
> is loss of load power.  Types ii (most commonly used) and type iii
will hold
> the short circuit current until such time as a downstream breaker
outside
> the DSTS opens, clearing the fault.  Thus the rest of the loads
(assuming
> the source power system has sufficient capacity) will not lose power.