Re: Why does laptop PSU have an earth wire?

Chris Whelan <cawhelan@xxxxxxxxxxxxxxxxxxxxx> wrote in
news:KPuUf.107$NN4.65@xxxxxxxxxxxxxxxxxxxx:

Frazer Jolly Goodfellow wrote:

Chris Whelan <cawhelan@xxxxxxxxxxxxxxxxxxxxx> wrote in
news:TzmUf.36$NN4.8@xxxxxxxxxxxxxxxxxxxx:

Frazer Jolly Goodfellow wrote:


When a filament lamp fails it strikes an arc, causing
current to increase rapidly.

If the filament has a resistance R ohms and breaks into two
parts of resistance R1 and R2, where R1 + R2 = R. Given the
voltage is fixed, for the current to increase to a level
sufficient to blow a 5 amp supply fuse the arc would need to
have negative resistance.

I'm sorry, I don't understand what a "negative resistance" is.
If such a thing existed, we would have had perpetual motion
long ago!

Precisely.

I think you missed the point I was responding to:

">>> When a filament lamp fails it strikes an arc, causing
current
to increase rapidly."

I didn't say that.

I know. If you backtrack, you'll see that I was replying to a
poster called Thoss and you hijacked that response.

You failed to respond to the rest of my post, where I explained
in clear and simple terms how a failing filament lamp can can
cause the protective device to operate. Not that I particularly
care, but do you now understand how this may happen?

Yes, but although it may happen, I think that:

"... often a failing filament shorts the lamp out ..."

only applies for very small values of 'often' :-)
.

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