Current limiting (was Sq D as combiner box) [RE-wrenches]

[Phil Undercuffler]

I've been away from my desk for a few days, so forgive me if someone has
already answered this.

All fuses and breakers will protect a circuit from excessive current draw
over time, such as plugging too many appliances into one outlet or circuit.
Some fuses are designed to protect quickly, to protect electronics, and
others are designed to allow higher current to flow for a short period of
time, for motor starting.

In a dead-short situation, however, an extremely high (on grid, a near
infinite amount of current) begins to flow.  In my Litttelfuse application
guide it mentions currents in the hundreds of thousands of amps.  A standard
fuse will allow this current to pass, even for a brief instant, which
understandably creates all sorts of havoc downstream.

A current limiting fuse, however, is designed to limit the current passed in
a severe short to much less than what would occur otherwise, and to open
within the first half cycle after the fault appears (wording from the
application guide, I'm not sure how that specifically applies to DC).

Now here's some more info from my application guide:
"NEC Section 110-22 states:  Where circuit breakers are applied in
compliance with the series combination ratings marked in the equipment by
the manufacturer, the equipment enclosure(s) shall be legibly marked in the
field to indicate the equipment has been applied with a series combination
rating.  The marking shall be readily visible and state the following:
CAUTION---SERIES COMBINATION SYSTEM RATED ___AMPERES.  IDENTIFIED
REPLACEMENT COMPONENTS REQUIRED.

So if we're using a current limiting fuse to protect downstream breakers
we're supposed to label the equipment to protect future generations.


Phil Undercuffler
Positive Energy

Injustice anywhere is a threat to justice everywhere--Martin Luther King
----- Original Message -----
From: <sunwise at cheqnet.net>
To: <RE-wrenches at topica.com>
Sent: Wednesday, April 04, 2001 7:54 PM
Subject: Re: Sq D as combiner box [RE-wrenches]



> is just some confusion regarding terminology -- Current Limiting vs.
> Current Interrupting.
>
> I thought all breakers were current limiting, that is their job, to
> limit the current flow on, for instance, a branch circuit.  They
> typically do not have the current interrupting ratings that fuses in the
> R and T class have.  A SD fuse will limit the DC current flow to/from a
> given device if the amperage begins to exceed the circuits design
> limits, like if someone plugged too many appliances ito a circuit with a
> conductor protected by a breaker.
>
> They perfom poorly at current interruption, like in a battery based
> system when someone sticks a screw driver into a DC branch circuit
> outlet box and welds then thing in there good.  In such a situation, the
> breaker would try to limit the current, but the DC rated/high current
> interrupting T class fuse that is protecting the breakers would save the
> day.


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