Re: Why does laptop PSU have an earth wire?

Frazer Jolly Goodfellow wrote:

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.

Hijacked? In my newsreader my post shows as 1:10am, yours as 2:19.

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' :-)

Not true in the particular circumstances I have described. IOW, where the
lamp is not cap up, the fuse is 2A and/or the protective device is an MCB
with other lamps already switched on in that circuit.

If I had to "guestimate" a frequency, I would say around one time in five if
the above conditions apply.

As an example, my MIL had to move to a smaller property a year ago. I have
twice had a panic-stricken call from her (she is elderly) to say "All my
lights have gone again". On both occasions, the 6A lighting circuit MCB has
tripped. On resetting it, one of the lamps in her wall lights had blown.
The MCB has never tripped at any other time.

She now won't use the wall lights. I repeat, this has happened twice in a
year in one house. I don't consider that a very small value of often in
this context.

Chris

--
Remove prejudice to reply.
.

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