Next message: Jim Fidler: "TWL: Re: Another dumb electrical question"
No, Marin, your assumption is wrong.
The GFI has nothing to do with the ground circuit so you do get protection
on a two prong appliance.
The way a GFI works is it looks for an imbalance in the current going to the
device and the current returning.
If there is no path to ground, earth or anything else, at the appliance,
every bit of current going in the hot wire must come back from the neutral.
By putting opposing windings, that cancel each other out, on a transformer,
any imbalance shows up as a voltage on the secondary winding of the
transformer and that is used to trip the GFI. It can be made very sensitive
and detect thousandths of an amp difference in a circuit drawing tens of
amps.
So it doesn't care where it went, (or for that matter where it came from) if
they are not in balance it trips because some external circuit is involved
that should not be there.
This is a somewhat simplified description - there are other ways of doing it
than using a transformer.
Andina Foster,
> If I correctly understand the function of a GFI (Ground Fault
> Interrupter), it will trip if there is an anomaly in the ground circuit.
> So you becoming the ground for a faulty hair dryer will be sensed by the
> GFI as an anomaly or fault, and it will almost instantly shut off the
> current to the socket.
>
> Now if I have a cabin heater on my boat connected to a GFI using a
> two-prong plug, and that heater shorts out and starts burning, it's my
> understanding that the GFI will not see a problem in this because the
> ground path is not being changed, so it will continue to allow current
> to flow to the burning heater. Of course there will probably be a
> circuit breaker on the AC circuit the heater is plugged into, and this
> breaker should trip, cutting off the current. But while I'm not sure
> exactly what causes a breaker to trip- heat, maybe?- there seems to be
> plenty of evidence that a circuit breaker might not act in time to shut
> off the power to prevent the heater from starting to burn. And we all
> know where that usually leads.
>
> So my question is...... is there any type of (affordable) electrical
> device that can sense a short circuit the instant it happens and react
> to shut off the current as fast as a GFI does in the event of a ground
> fault? Or is a circuit breaker the only (or best) device available to
> do this?
>
> ______________________________
> C. Marin Faure
> GB36-403 "La Perouse"
> Bellingham, Washington
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