Re: CBus wiring

["David Buckley" <db@xxxxxxxxxxx>]

--- 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.