# fuse blown up

Discussion in 'Electronic Design' started by [email protected], Nov 25, 2007.

1. ### Guest

I connected a piece of nichrome wire( heating element) the one u'll
find in old electric stoves ~4cms (don't know its resistance exactly)
to Ac current. when I switched on, fuse was blown up in my home.
Does the fuse blown due to

1. short circuit or

If what I did (connecting ni-cr wire directly) was wrong then are
there any circuits for connecting it AC mains. Any suggestions

2. ### Fred BloggsGuest

don't go near anything electrical, you're not there yet...

3. ### D from BCGuest

I'm no expert on fuses but here's what I know (or think I know : ) )

A fuse is like a light bulb. If it's overdriven with current...it
heats up, melts and/or oxidizes? . That's the burn out.
P=I^2R where R is the fuse element. R increases with temperature like
most conductors.
When P is sufficient for making the fuse element reach the melting
temperature ..it blows.

Your fuse blows because it can't sustain the current for your circuit.
D from BC

4. ### Jon SlaughterGuest

Um, its important to know the resistance. The resistance tells you how much
current is going to be pulled and hence you then can determine if the
breaker will blow/fuse will go.

Since the voltage in AC mains is ~120Vrms AC, by ohms law

120Vrms / R = I

So if R = 0.001ohm then your pulling 120k Amps. The average outlet in a
house only lets about 15A through.

If R = 1 ohm, then I = 120 Amps and this is still to large.

I believe most electrical stoves run in houses run on a seperate branch that
allows more current(which means they must have larger wires or better
insulated wires) to pass.

In any case you need to measure the resistance.

If the resistance is large enough(say > 10 ohms) then it should be ok. Make
sure you don't have any other current hungry devices connected on the same
branch though. If you have a two friges, a stove, a TV, 2 lamps, a computer,
etc... all connected to the same socket then your going to trip the breaker

5. ### Tam/WB2TTGuest

Look at it this way. Suppose the full length of the nichrome wire was 40 cm.
You cut off 4 cm of that. If the whole 40 cm draws 5 amps, then your 4 cm
was trying to draw 50 amps.

Tam

6. ### Tom Del RossoGuest

How can you know what the resistance is going to be after it gets hot until
you've tried it? Maybe a variac would help, but measuring the cold
resistance doesn't reveal much.

7. ### Robert BaerGuest

Are you sure that it was not blown *down* ??
Methinks that wire was rather short, and most definitely drew at
least 3 times the fuse rating.

8. ### Robert BaerGuest

Stoves in the US do take a lot of current; they run off of 220V to

9. ### Robert BaerGuest

With a short piece, that would be an excellent way to ruin the
variac, if it is not fuse protected.

10. ### Don KlipsteinGuest

And if the fuse did not blow, that short piece of nichrome would have
gone blooey, spitting a few drops of molten nichrome in the process. Or
the wires to that piece of nichrome might have done that instead. This
experiment sure sounds like a fire hazard to me!

If you use 1/10 the length of a heating element, you will be dissipating
into it 10 times the amount of power that the full length takes (give or
take due to variation of resistance with temperature). 100 times rated
power per unit length (or whatever in this ballpark it turns out to be) is
going to blow *something*!

- Don Klipstein ()

11. ### JamieGuest

And if the fuse did not blow, that short piece of nichrome would have
gone blooey, spitting a few drops of molten nichrome in the process. Or
the wires to that piece of nichrome might have done that instead. This
experiment sure sounds like a fire hazard to me!

If you use 1/10 the length of a heating element, you will be dissipating
into it 10 times the amount of power that the full length takes (give or
take due to variation of resistance with temperature). 100 times rated
power per unit length (or whatever in this ballpark it turns out to be) is
going to blow *something*!

- Don Klipstein ()[/QUOTE]
Something I found on the NET for all to look at on this subject..

http://www.chemical-ecology.net/papers/heater-p.htm

12. ### Tom Del RossoGuest

The OP never exactly said if he used the whole 120V length or not.

13. ### Jon SlaughterGuest

It doesn't matter. Resistance doesn't decrease with temperature so it will
pull max current when cold. Also, Nichrome has a very small coefficient so
there is very little change.

14. ### ian fieldGuest

Yes he did - he stated a 4cm length.

15. ### ian fieldGuest

Yes - if you do any more experiments, video the event and post on youtube

16. ### Frithiof Andreas JensenGuest

Replace wimpy Fuse with Nail
Try Again
Perhaps try bigger nail

17. ### EeyoreGuest

LOL !

I saw a remarkable electrical display once in Bombay that could have been caused
by such an experiment.

Graham