replacing a high voltage fuse.

LOCUS,

Sorry, a had a problem of my opinion.

steps:
1.First measure power supply, let it is (E) volt
and current rating is (I) ampere > 0.75amp.

2. Let take power supply source resistance =Rs ohm.
Fuse impedence =Z1 ohm.
Load impedence =Z2 ohm.

You choose 600v-0.75amp fuse.ok.

Significance of 0.75amp of fuse:

1.let, 0.75amp =E/(Rs+Z1+Z2).............(A)
Rs=0 ohm.(assume)
Z1=constant(by ohm meter)
z2 = (E/0.75)-Z1 ohm.

So this Z2 ohm is minimum value for maximum of 0.75ampere.
Under calculation, if you insert Z2 ohm then I1=0.75 ampere
will pass through the circuits.


conclusion: So under calculation of E,Z1
you can insert Z2 ohm of device.(for max of 0.75amp)
After knowing Z2 value you can insert higher
than Z2 value then current will less than 0.75amp.



Significance of 600v of fuse:

1. But if you insert a device whose impedence is less than
Z2 ohm then by equation (A), (greater than 0.75 ampere)=I2 will
circulate through the circuit.
2. then I2 squared Z1 is relatively larger than I1 squared Z1.
3. So in this case fuse will be heated. and tempereture
will rise.

4.Let we assume fuse element is positive tempereture coefficient.
so if temp rises then resistance rises by Rt=R0(1+alpha*t).
where,
Rt = final resistance.
R0 = initial resistance.
alpha = tempereture coefficient of fuse element.
t= tempereture difference.

since, rising current is constant i.e.,greater than 0.75amp
voltage accross the fuse will be V=I2*Z1.
5. If voltage accross the fuse is (V=I2*z1) greater than 600V
then fuse melts.

conclusion:
So you must insert of Z2 impedence for 0.75ampere.(under
calculation)



[ nb. It is also true that if fuse current rating is greater than
power supply rating then powersupply may dammage.
But no effect on fuse.]

Please Stop

In my opinion, it is suggestions like this that get people hurt or killed. I understand your attempt at calculations, however there is a lot more to fuse sizing/construction then you are taking into account. The safest bet is always to replace with the exact same fuse or better (interrupting rating). End of story. This is only my opinion, take it for what it is worth (12 years as an electrician and seeing one of these suckers blowing)

YES - PLEASE STOP

saifullah99319,

You are trying to offer advice on fuses by approaching them as if they were a resistor.

FUSES ARE NOT RESISTORS

A fuse is a different beast altogether.

As Screamin said,

ADVICE LIKE YOURS CAN GET PEOPLE KILLED!

Steve

letting the smoke out

I agree with the more conservative ratings approach to fuse replacement. If you can't get the exact fuse in the original design in most cases it is better to get one with a higher voltage rating, lower current rating, or shorter time delay.
But as with everything real world considerations (actual load, "normal" spikes...) should be observed.

Need I say anything about keeping all legs of a circuit (sucha as a three pase motor) fused equally?

$0.02

Like what is it with fuses lately? Is it that difficult to replace a bad one with a good one of the same type. The only math needed in replacing one is the ability to read the numbers and ratings on the old one. I think I'll go divide by zero for a little while. At least no one gets hurt that way.

Cheers
SBC (B is for B**** today) sorry

Give up already-

saifullah99319-
Have you ever replaced a high voltage fuse in real life?

saifullah99319 said:

LOCUS,

Sorry, a had a problem of my opinion.

conclusion: So under calculation of E,Z1
you can insert Z2 ohm of device.(for max of 0.75amp)
After knowing Z2 value you can insert higher
than Z2 value then current will less than 0.75amp.
conclusion:
So you must insert of Z2 impedence for 0.75ampere.(under
calculation)

nb. It is also true that if fuse current rating is greater than
power supply rating then powersupply may dammage.
But no effect on fuse.]

Click to expand...

Saifullah you are totally and completely wrong.

Read this thread: http://www.plctalk.net/qanda/showthread.php?s=&threadid=4155

You have the mathematics right, but you are using it to model a situation where the equations are not applicable in the form you are trying to apply them.

Ohm's law applies through the entire circuit. You are doing your analysis as though the entire circuit voltage drop takes place across the fuse. THIS IS NONSENSE!!!!!! The voltage drop across a good fuse is very close to zero, and the impedence is very close to zero. In point of fact that is how those of us that know what we are doing check a fuse - if we don't have continuity (essentially 0 Ohms resistance) the fuse is blown. If you don't believe me go to any electronics store and measure the resistance across a few fuses of several ratings!

By Ohms law at 0 resistance the voltage drop is 0 at any current. If you put more current through the fuse than its rating, I-squared-R heating melts the fusible link, opening the circuit.

On a blown fuse, the resistance is essentially infinite, making the current 0 at any voltage.

BEWARE OF APPLYING MATHEMATICAL ANALYSIS TO MODEL INAPPROPRIATE PHYSICAL PHENOMENON!!!!

For example, it takes one woman nine months to have a baby. If I use four women, mathematically it would only take 2.25 months! This is numerically correct but physiologically stupid.

If I build a steel column of 1 sq. in cross sectional area and load it with 12,000 pounds, the stress is 1/3 of yield and the column will not be permanently deformed. This is based on the fundamental equation of stress = load divided by area. However, if I add the information that the column is 120 feet tall, the mathematcial result is the same but the column WILL FAIL because of buckling. The moral: when you use mathematical analysis for modelling physical phenomena make sure you understand the origins, limitations, and underlying assumptions for the formula you are using! Otherwise you can make a deadly mistake.

Analog fuse

You know, I had an experience with a fuse that did not completely fail in a car stereo application. A 35A fuse inline to my amplifier actualy started to melt one time. It was enough to drop the voltage to the amp to about 2 volts if I recall. Had >12 on the hot side of the fuse. Hmmm? Pulled the fuse, looked good, tested on a NON AUTO-RANGING DVM (on too high of a range) and it showed good. Correcting the range showed that the fuse had about 1300 Ohms! Never saw that before. It happened on my boat a year later and that was an instant fix that time.

By the way, Tom, I like the idea of the four women. There's just no pleasing me!

You'll always remember the weird ones!

Scott

The Fuse Saga

Damn, you know, I'd hate to be a teacher nowadays...the students just don't listen.

Ya know gentlemen,

This is beggining to make more of a case for properly sized circuit breakers!

They are more expensive in the short term but I have had customers in the past replace the wrong size fuses in some of my equipment. We all know the results.

Regards,
Chris

Fuse selection should be taken very seriously. During a fault, there can be huge fault currents. The fault currents available depend on the impedence of the transformer supplying the circuit. Some of the things to keep in mind when selecting a fuse are the fuses "interupting capacity" and how quickly a fault can be cleared (usually in cycles). Here are a couple of good links to check out.

The Basics of Selecting Industrial Fuses — Part 1

The basics of selecting industrial fuses — part 2

For example, it takes one woman nine months to have a baby. If I use four women, mathematically it would only take 2.25 months! This is numerically correct but physiologically stupid.

Click to expand...

That is hysterical!

This is beggining to make more of a case for properly sized circuit breakers!

Click to expand...

I disagree, there are applications for both. Circuit breakers can not totally replace fuses. Basically fuses provide better (more safe) protection than circuit breakers.

saifullah99319
What is your explanation for the various lengths of fuses, ranging from 1/4in. to approx. 2ft. in length.

PROBLEM;
If I have two fuses with the exact voltage and current rating on them except one of them is a glass fuse 1/4" in total length and the other is a ceramic fuse 2" in length. The 25A ceramic fuse blows, on my 250vdc air compressor. Are you saying I can, safely, replace the ceramic fuse with the glass one?

Roger

I have to take it that the math(s) is correct because you learned gentlemen say its right - I wouldnt know.
I would just take the numbers from it an order another one.

But saifullah99319 reminds me of an old saying

He could mathematically calculate how many pickled onions were in the jar but he couldnt get the lid off.