RE: Re: CBus wiring

["Dean Barrett" <dean@xxxxxxxxxxxxxxxx>]

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.