Electrical codes and plugs/sockets was Re: [Beowulf] Opteroncooling specifications?

Jim Lux James.P.Lux at jpl.nasa.gov
Mon May 1 13:20:19 PDT 2006

At 12:16 PM 5/1/2006, Michael Will wrote:
>And this is generally the message - short term it will work
>but long term continuous load should stay at 80%. There are different
>timeouts for breakers to be triggered at different over-80% levels.

The overcurrent protection device should be able to carry 100% forever 
without tripping. (at least, that's the specification)..

Once you go over the rated trip current, then there's usually a decreasing 
trip time for increasing current characteristic.  The time/overcurrent 
curve is different for different applications (e.g. for fuses, you have 
fast blow and slow blow, etc.)

Generally speaking, the specs call for "must trip" at 120% load eventually.
A typical spec might be to trip within 100 seconds at 200% load
and within 1 second at 10x load.
Typically, at 100x rated current, it should trip within 1 cycle (16 ms)

On a large installation, there's a fair amount of work in what's called 
"coordination", which means making sure that the downstream breaker trips 
before the upstream breaker, so you don't interrupt power to more loads 
than you need to.  This is nontrivial, because consider a typical residence 
might have 20 or more  branch circuits, each at 15-20 Amps, but have only a 
150-200 Amp service.  Start looking at even a small building with offices, 
and you've got literally hundreds of branch circuits, with a fairly complex 
tree of panels and subpanels.


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