[Gllug] OT: Merging UPS outputs

[John Winters]

On Wed, 2005-08-31 at 23:50 +0100, Chris Hunter wrote:
> John Winters wrote:
> 
> > 15A plugs are used in theatres for a very simple reason - they don't
> > have fuses in them.  Every time a bulb blows it will take the fuse out
> > and a single lantern might be connected through several pieces of tail,
> > each with a plug and socket.  Working through all of those to replace
> > the fuses would be impossible.  Instead each circuit is individually
> > fused back at the dimmer rack (with special fast-blow fuses to protect
> > the dimmers) so you know exactly where to go to replace the fuse.
> 
> There are NO fuses that fail fast enough to protect triacs!  The only 
> way is to use grossly over-rated triacs, or to use a different approach 
> - back-to-back thyristors.

I presume you mean there are no fuses which will *guarantee* to protect
triacs.  I've certainly used kit where the triacs blew well under half
the time, so the fuses succeeded at least a bit.

Having said that, all the serious kit uses (or I should say used,
because my experience is more than 20 years ago) back to back
thyristors.  Even those aren't immune to blowing though.  A regular
maintenance operation on Mini II racks was to replace one from a pair of
thyristors.  The symptom was that a channel would dim from full to half,
but no further because one of the thyristors was permanently on.
Thyristors too need fast-blow fuses to be protected, and even those
don't always manage it.

I'm not sure what the current racks use, but they're mightily
impressive.  The 36 ways of patchable Tempus racks in our local theatre
were recently replaced with 96 ways of hard wired racks, all in a rather
smaller space than the Tempus racks had.  When I saw where they were
proposing to put them I said, "What about the heat?" to which the answer
was, "There isn't any".  And amazingly, there isn't.  I don't know how
the new versions work, but where the old racks could be used to make
toast the new ones seem to stay stone cold all the time.

John

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