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Home > Archive > Electrical code Compliance > August 2005 > GFI breakers
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| Robert Bodling 2005-08-19, 3:21 pm |
| Can some one explain how a single pole GFI breaker works?
| |
| phil-news-nospam@ipal.net 2005-08-19, 7:21 pm |
| On Fri, 19 Aug 2005 18:14:52 GMT Robert Bodling <robertbodling@yahoo.com> wrote:
| Can some one explain how a single pole GFI breaker works?
The line/hot conductor that is subject to overcurrent protection in the
breaker is run in parallel with the neutral conductor through a current
sensor to detect how much residual current is not matched in the two
conductors. The white insulated pigtail attached to GFI breakers is
connected to the neutral bus bar. The breaker has 2 terminals, one for
the circuit line/hot wire, and one for the circuit neutral. Thus the
circuit neutral path runs through the breaker (and the sensor inside)
to allow this current residual detection function to be done.
Since current flows out one wire, and back in the other wire, having
both of these wires running through a single current transformer, the
currents will cancel each other out as far as the CT sees it. The CT
winding is then fed to a detection circuit (separately powered) that
constantly measures the current (as a voltage drop across a burden
resistor) to determine if the current exceeds the design threshold.
That threshold may be around 30 milliamps for equipment protection
purposes, or between 2 and 6 milliamps (5 milliamps nominal) for people
protection (such as the GFI protection required for certain circuits in
homes and other places). When the current exceeds the threshold limit
for a specific time duration, the circuit inside will cause the breaker
to open the line/hot conductor.
A double pole GFI is not much different. Both line/hot wires, plus the
neutral wire (only a single pigtail is needed), run through the current
sensor. The current threshold levels are the same. When any leakage
current is detected, the breaker is tripped to open both line/hot wires.
Breakers at or above a certain current rating will not have room for
three terminals on a double pole space, since the terminals have to be
larger. Square-D double pole 60 amp breakers are an example. These
cannot be used on split voltage circuits because no facility is there
to measure the neutral current. These can only work on 2-wire 240 volt
circuits.
Additional reading:
http://electronics.howstuffworks.co...it-breaker4.htm
http://electronics.howstuffworks.com/question117.htm
http://en.wikipedia.org/wiki/Residual-current_device
http://homerepair.about.com/cs/elec...FCI_what_is.htm
http://www.codecheck.com/gfci_principal.htm
http://www.elec-toolbox.com/Safety/safety.htm#GFCIs
http://www.electrical-contractor.ne..._Fact_Sheet.htm
http://www.osha.gov/SLTC/etools/con...dents/gfci.html
--
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
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| Rich Webb 2005-08-19, 8:21 pm |
| On Fri, 19 Aug 2005 18:14:52 GMT, "Robert Bodling"
<robertbodling@yahoo.com> wrote:
>Can some one explain how a single pole GFI breaker works?
If (current in hot wire) is not equal to (current in neutral wire)
then (trip breaker).
--
Rich Webb Norfolk, VA
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