circuit breakers.. Re: [Beowulf] newbie's dilemma

Jim Lux James.P.Lux at
Thu Mar 2 07:51:20 PST 2006

At 06:33 AM 3/2/2006, Jim Lux wrote:

>Circuit breakers have a "time to trip" that varies inversely with the 
>amount of overload.  The actual "must trip" level is somewhat above the 
>rated current, and they're typically rated at 125%, 200% and sometimes 
>even higher.  They'll trip a lot faster at 200% than at 125% overload.

Found some typical times.. no trip at 100% load, 1 hour trip time at 135% 
of rated capacity, 5 seconds-40 seconds at 200%, down to tenths of a second 
at 1000%   (that's at a 40C breaker temperature, by the way)

If you're tripping breakers when you fire up all those computers, you've 
got a real problem.

So you can see why they want the actual wiring to be rated at substantially 
more than the breaker current rating. The heating in the wiring (and at 
connections,plugs, and receptacles, which is usually where the problems 
are) goes as the square of the current, so at that 135% overload, the 
heating it almost twice what it is at rated load.

here's a sample trip time vs overload chart:

but that's probably more information than you needed.
And what is this list for, if not a surfeit of information on obscure 
topics that are only tangentially related to computing.


James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

More information about the Beowulf mailing list