Re: GFI (again)

["L. R. Cartwright (Larry)" <physics@SCNC.CPS.K12.MI.US>]

> > On the 'up- side' a single GFI outlet will protect every other outlet on
> > THAT branch circuit.
> I find that idea hard to believe. Could someone please either refute it
> or explain it for me?

Absolutely true, that is the way Gfi's are usually installed.

> But remember (or realize) that all the outlets on a branch circuit
> are wired in a 'series' of outlets. All the juice that flows to ANY
> outlet must also flow thru the first (GFI) outlet.

Pretty close, but not quite right.  The outlets in a branch are wired in
*parallel* not series.  Normally the current going to the other outlets
passes by but not through the first outlet.  A gfi outlet unit consists of
two parts:  a standard 120v outlet and a ground fault breaker module in
series with that outlet.  The outlet closest to the service panel is
replaced with the gfi outlet unit.  The rest of the branch is connected in
parallel with the gfi-protected outlet in the unit.  Because the gf
breaker module is now wired in series with the entire parallel branch, it
disconnects the entire branch of outlets when it trips out.

           . . . . . . . .
           .  ground     .
           .  fault      .
           .  breaker    .
           .  module     .
           .  *****      .
     ----->---*   *--|------------|------------|
           .  *****  |   .        |            |
           .        ***  .       ***          ***
from       .        * *  .       * *          * *
service    .        * *  .       * *          * *
           .        ***  .       ***          ***
           .         |   .        |            |
     ----->----------|------------|------------|
           . . . . . . . .
              grounded         outlet2      outlet3
             gfi outlet1

As Steven Ratliff mentioned, it is possible to connect a gfi outlet so it
stands alone and does not protect any other outlets, but that is not a
typical installation.

Don't let just any yahoo with a toolbox install gfi equipment.  Proper
grounding of the gfi unit is critical to its correct functioning.
Installed by a competent electrician and tested on a regular basis, the
gfi is remarkably reliable applied physics.

Best wishes,

Larry

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Larry Cartwright
Physics, Physical Science, Internet Teacher
Charlotte High School, 378 State Street, Charlotte MI 48813
<physics@scnc.cps.k12.mi.us> or <science@scnc.cps.k12.mi.us>

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