Date: Wed, 2 Mar 2005 14:01:40 -0500

Author: "=?iso-8859-1?b?UmFt824=?= O. Torres-Isea"

Subject: RE: Circuit Breaker Question

Post:

Hello,

You could make it "pop" in a few minutes with enough current...but the circuit
should use rated wire. If the wiring is able to dissipate the heat easily the
thermal element is not going to actuate. The bimetallic element is "sized"
based on the thermal characteristics of rated wires.

Incidently, for short-circuit conditions the thermal element is too slow (too
much damage happens before it heats up) and thus a magnetic tripping element is
used for actuation (residential breakers are Thermal-Magnetic Circuit Breakers).
Industrial breakers may also incorporate electronic-trip units.

As far as the use of AC or DC...you are right...under overload condition it does
not matter (is just the heating effetc of the current that is used)...BUT...the
short-circuit capacity of a breaker is lower for DC than for AC (breaker
typically need to be "de-rated" for DC applications!)...with AC there are lots
of zero-current conditions during which the arc that develops may be
quenched...but this does not happen with DC and the breaker must force arc
quenching by stretching the arc.

Just a little bit of extra info. My apologies for going on and on.

Ramon Torres-Isea
--
Adjunct Lecturer and Supervisor,
Advanced Physics Teaching Laboratories,
Department of Physics,
The University of Michigan,
Ann Arbor, Michigan

Phone:(734)764-3443
Fax: (734)764-5153




Quoting David Kardelis :

> Thanks, I thought it would take a few minutes not hours. In my old house, I
> had two 1500 W heaters on a 20 A circuit. If I ran both on high (25a)it would
> pop in less than a minute. I figured I would get a similar response on this
> little test setup, I figured I was drawing as much as 35A one time, Circular
> saw 10A, Router 15A, 900 light(7.5a) and a 1500W Heater(12.5A). I figured
> the tools weren't at full current since they weren't stalled but I did cut
> wood with them. Didn't blow the main breaker for the building outlet either
> but I figured those were at least 20A circuits.
>
> dave
>
> David Kardelis Ph.D.
> Chairman; Dept of Chemistry and Physics
> College of Eastern Utah
> 451 E 400 N
> Price, UT 84501
>
> david.kardelis@ceu.edu
> 435-613-5258
> 435-613-5125 (lab phone)
> 435-613-5996 Fax
>
> > -----Original Message-----
> > From: owner-tap-l@listproc.appstate.edu [mailto:owner-tap-
> > l@listproc.appstate.edu] On Behalf Of Ramón O. Torres-Isea
> > Sent: Wednesday, March 02, 2005 9:55 AM
> > To: tap-l@listproc.appstate.edu
> > Subject: Re: Circuit Breaker Question
> >
> > Hello,
> >
> > A breaker "pops" for one of two reasons: an overload condition (125% -
> > 600% of
> > nominal current) or a short-circuit condition (above 600%). How fast it
> > "pops"
> > depends on the reason. If under short-circuit condition it should do so
> > instantaneously (in milliseconds). If under overload, it depends on the
> > level
> > of current and it may take as long as 2 hours!
> >
> > These values are all in accordance with the UL 489 Standard tests...which,
> > for
> > the overload conditions, are conducted with the breaker connected with 4'
> > of
> > rated wire.
> >
> > So...in your case...you should be using rated wire (12 or 14 gauge)
> > through out
> > your circuit...but since you only have an overload condition...it won't
> > "pop"
> > immediately. However, with enough overload current you should be able to
> > trip
> > it in minutes. To find out exactly how fast, you need to consult the
> > breaker's
> > tripping curves or charts (available from the manufacturer).
> >
> > Good luck!
> >
> > Ramon Torres-Isea
> > --
> > Adjunct Lecturer and Supervisor,
> > Advanced Physics Teaching Laboratories,
> > Department of Physics,
> > The University of Michigan,
> > Ann Arbor, Michigan
> >
> > Phone:(734)764-3443
> > Fax: (734)764-5153
> >
> >
> >
> >
> > Quoting David Kardelis :
> >
> > > I have a little demo to show how a house is wired. It has a Breaker box
> > > hooked to a 120V Power cord, Hooked to the breaker are a couple sockets
> > and
> > > outlets. I have not ever been able to pop the breaker. I am using a 15A
> > > breaker. I hook a 1500W heater and 3- 300W bulbs, a 7 amp Circular saw,
> > and
> > > a ˝" drill all running at the same time and nothing happens.
> > >
> > >
> > >
> > > So I figured the breaker was bad rare but possible, so I hook it to a DC
> > > power supply and put 35A through it, doesn't pop. Must be bad.
> > >
> > >
> > >
> > > Go to the hardware store buy 2 new ones for $3 each, hook them up to the
> > DC
> > > power Supply put 35A through them for a couple minutes and neither pops.
> > >
> > >
> > >
> > > Is there a reason a Breaker will not work with DC current. I haven't
> > every
> > > ripped one open, but I was under the impression it worked on a
> > bimetallic
> > > strip so all that matters is amperage for heating
> > >
> > >
> > >
> > > Thanks dave
> > >
> > >
> > >
> > > David Kardelis Ph.D.
> > >
> > > Chairman; Dept of Chemistry and Physics
> > >
> > > College of Eastern Utah
> > >
> > > 451 E 400 N
> > >
> > > Price, UT 84501
> > >
> > >
> > >
> > > david.kardelis@ceu.edu
> > >
> > > 435-613-5258
> > >
> > > 435-613-5125 (lab phone)
> > >
> > > 435-613-5996 Fax
> > >
> > >
> > >
> > >
>
>
>
>
From rueckner@fas.harvard.edu Wed Mar 2 14:09:09 2005

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