At 03:05 PM 7/16/01 -0400, you wrote:
>I am embarrassed to say that I do not get it.  What's the 500w load in your
>diagram? In your scenario, is the faulty neutral at the breaker box or at
>the lamp?

Where I drew the // lines in it (below). The 500W load might be other lights,
whatever. The important thing is that it is higher current draw (lower
impedance) than the lamp.

If the neutral is broken entirely, then the voltage will divide across
the two loads. Assuming (for simplicity) the two loads are resistive, and
that the 100W lamp is R = 144 ohms, the 500W load is 28.8 ohms (fixed
for simplicity), then the 100W bulb would see 240 * (144/(144+28.8) = 200V.
Obviously it won't last very long at all!

If the loads on either side of the neutral are exactly balanced, then there
will be no obvious ill effects from breaking the neutral.

>Every outlet/lamp fixture in my house has 3 wires - what I typically think
>of as 'line', 'ground', and 'neutral'.  Ground and neutral are at the same
>potential and 'line' is +-120vac.  I think of ground as an alternate return
>path to ground in case the line is accidentally exposed. I have never quite
>understood why ground and neutral should be electrically isolated.

It's a safety issue. The safety ground is not supposed to carry current
except under fault conditions. Typically something like the housing of a
clothes washer is connected to ground. Imagine if it was connected to
neutral and the neutral went open (you don't have to think about the other
line to see this..) all of a sudden the metal housing is at 120VAC (in series
with the internal motor or whatever), which will happily fry you without
getting much voltage across it.

>So you are saying that with a faulty neutral, a 120vac appliance with see
>240vac?

It can see anything between close to zero to close to 240, depending on
how imbalanced the loads are. But a flickering (to dim) could just be a
bad connection in series with the load. Flickering (to more brightly)
pretty much nails it down to being a bad neutral.

Best regards,

>
>120VAC
>o-------x    <----------x
>        |               |
>   [100W bulb]          |
>N       |               |
>o--//---x              240VAC
>        |               |
>    [500W load]         |
>        |               |
>o-------x    <----------x
>120VAC

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