The SQUARE D Back-Fed Main Circuit Breaker Retaining Kits provide a cost-effective way to back feed a two-pole 10–125 amp circuit breaker on 12–42 circuit QO® convertible main load centers, or 8–40 circuit HOMELINE®convertible main load centers.
http://static.schneider-electric.us/docs/Electrical Distribu...

Can circuit breakers be reverse-fed or back-fed?
Any circuit breaker that does NOT have Line and Load identifying markings may be reverse fed. This includes all Square D industrial thermal-magnetic and electronic trip circuit breakers with the exception of Powerpact H and J frame breakers that have field interchangeable trip units. It also includes miniature breakers with the exception of all GFI, EPD, AFI, and CAFI types.
http://www.schneider-electric.us/sites/us/en/support/faq/faq...

In the 2011 code: 690.10 (E)

(E) Back-fed Circuit Breakers. Plug-in type back-fed circuit breakers connected to a stand-alone inverter output in either stand-alone or utility-interactive systems shall be secured in accordance with 408.36(D). Circuit breakers that are marked “line” and “load” shall not be backfed.
http://www.wind-sun.com/ForumVB/showthread.php?13581-Back-fe...

There are some situations where it's necessary to use a "back-fed" circuit breaker. Some panels utilize "back-fed" mains. Sometimes a secondary power source feeds into a "back-fed" circuit breaker in the panel. (They're usually required to have a clip or screw to hold them in place.) (If it's a secondary source, it may require an interlock with the primary source as well.)

A requirement I've seen for these breakers is that they have to be listed for use as a back-fed breaker.
:
Some breakers can be used in back-feed installations. One exception is Ground-Fault breakers. Do not install one of those as a back-fed device. It will burn out in seconds when attempting to trip.
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To put it simply, if the power is coming from the panel bus bars, through the breaker, then out to a load, it's NOT back feed.

If the power is coming from a source outside the panel, like a generator or inverter, through the breaker, then to the panel bus bars, it's back feed.

There are also "back feed mains" in some panels. Basically, they use a back feed breaker to act as the main breaker.
http://electricalknowledge.com/forum/topic.asp?ARCHIVE=true&...

If the steam-distribution pipe is fitted above the humidifier, the condensate can be fed back to the unit by placing the end of the hose in the filling cup.
nordmann-engineering.com
Ist das Dampfverteiler-Rohr oberhalb des Befeuchters montiert, kann das Kondensat zum Gerät zurückgeführt werden, indem der Schlauch in den Füllbecher gehängt wird.
nordmann-engineering.com

Surplus kinetic braking energy is converted into electric power and fed back into the supply system in sinusoidal form, thereby helping to preserve the environment [...]
drives.lt-i.com
Überschüssige kinetische Bremsenergie wird in elektrische Energie umgewandelt und sinusförmig in das Versorgungsnetz zurückgespeist, was die Umwelt schont und den Geldbeutel entlastet.
drives.lt-i.com

During the ramping-up, the actuated contactors are checked to confirm that they have actually switched, each having a potential-free contact which is fed back to the central unit for monitoring.
boening.com
Während des Hochlaufens werden die angesteuerten Schütze dahingehend geprüft, ob sie auch wirklich geschaltet haben, indem je ein potentialfreier Kontakt in die Zentraleinheit zurückgeführt wird.
boening.com

For controlling the function of these contactors, each output block must have one normally closed contact per contactor fed back in series to the corresponding terminal of the SafeC 400 controller (Figure 9).
csa.cedes.com
Zur Überprüfung der Funktion dieser Schütze muss für jeden Abschaltblock je ein Öffnerkontakt pro Schütz in Reihe zu der entsprechenden Klemme der SafeC 400-Einheit rückgeführt werden (Abbildung 9).
csa.cedes.com

http://www.linguee.de/englisch-deutsch/uebersetzung/fed back...

to feed back
rückkoppeln

to feed back
rückmelden

to feed back
zurückleiten

electr. tech. to feed sth. back to sth. [e.g. signals, data]
etw. zurückführen zu etw. [Dat.]
http://www.dict.cc/?s=feed back