Fuse selection

Thread Starter

#12

Joined Nov 30, 2010
18,217
I haven't been able to figure out how to use the published specs on fuses so I am posting this opportunity for education.
For instance, an antique class A vacuum tube amplifier idles at 0.425 amps. The usual start surge is 1.3 amps and the meter occasionally reads as high as 1.95 amps. (Fluke 87V with a 250 usec peak current detect capability.)
0.6A slow works nicely and has been working for years. (I am not stuck with an inoperable amplifier.)
I would like some education about how to use published information to declare a proper fuse.
 

DickCappels

Joined Aug 21, 2008
7,424
I have used the I2T ratings to help find the ("I squared T")

From Google:
I2t is an expression of the available thermal energy resulting from current flow. With regard to fuses, the term is usually expressed as melting, arcing, and total clearing I2t. The units for I2t are expressed in ampere-squared-seconds [A2s]. Melting I2t: the thermal energy required to melt a specific fuse element.

These are usually listed in fuse catalogs, often with blowing charts.
 

Thread Starter

#12

Joined Nov 30, 2010
18,217
I have used the I2T ratings to help find the ("I squared T")

From Google:
I2t is an expression of the available thermal energy resulting from current flow. With regard to fuses, the term is usually expressed as melting, arcing, and total clearing I2t. The units for I2t are expressed in ampere-squared-seconds [A2s]. Melting I2t: the thermal energy required to melt a specific fuse element.

These are usually listed in fuse catalogs, often with blowing charts.
That is exactly what I don't understand! Being able to measure a 250 usec peak doesn't mean the time the peak is happening is 250 usec. I gave a good example. Is I squared 0.425 amps squared or is it 1.3 amps squared or 1.95 amps squared? This amplifier has been in use since 2009. Is time equal to 11 years?
 

Thread Starter

#12

Joined Nov 30, 2010
18,217
Happy New Year!

It depends on where the fuse is at.
The fuse is:
1) In a fuse holder, panel mount, glass with metal ends, about 1/4" diameter and 1.25" long.
2) In series with the primary of the power transformer.
 

Thread Starter

#12

Joined Nov 30, 2010
18,217
"Read the free literature." Got it. All I need now is, "How much time does the start pulse last?"
How to measure it? An oscilloscope that can store a measurement of the current in the first one second?
That explains why amateurs guess a lot. Not many have that kind of equipment.
 

nsaspook

Joined Aug 27, 2009
8,381
"Read the free literature." Got it. All I need now is, "How much time does the start pulse last?"
How to measure it? An oscilloscope that can store a measurement of the current in the first one second?
That explains why amateurs guess a lot. Not many have that kind of equipment.
Rules of thumb.

Rule #1: Informed guessing is OK.
Rule #2: The Fluke 87V is your friend.

Fusing for X component or board level protection can be tricky but most of the time fusing is there to prevent heat related overload effects so it's sized to pop when things go bad or short inside the equipment resulting it a much higher than normal current flow so precise value selection is a lot less critical. Things with motors or hefty solid-state DC power supplies can have very high start pulses while old tube amplifiers with tube rectifiers are soft starting devices because it takes time for the filaments to warm to full operational temperature..
 

Thread Starter

#12

Joined Nov 30, 2010
18,217
Rules of thumb.

Rule #1: Informed guessing is OK.
Rule #2: The Fluke 87V is your friend.

Fusing for X component or board level protection can be tricky but most of the time fusing is there to prevent heat related overload effects so it's sized to pop when things go bad or short inside the equipment resulting it a much higher than normal current flow so precise value selection is a lot less critical. Things with motors or hefty solid-state DC power supplies can have very high start pulses while old tube amplifiers with tube rectifiers are soft starting devices because it takes time for the filaments to warm to full operational temperature..
That was helpful! Not exactly what I need to declare a fuse size, but I'm still making friends with my new meter. (I didn't know a record function was happening in the background.)
 

Thread Starter

#12

Joined Nov 30, 2010
18,217
@#12 He's Ba-a---ck !
Happy New Year!
Max.
Just passing through as I learn about my new meter. It has so many unlabeled functions that I made a cheat card and put it in my wallet. It has several, "Hold this button during start up to cause that effect." and a couple of, "You would never guess which button toggles that function because it isn't labeled."
 

nsaspook

Joined Aug 27, 2009
8,381
That was helpful! Not exactly what I need to declare a fuse size, but I'm still making friends with my new meter. (I didn't know a record function was happening in the background.)
It tells you what the peak and avg currents are for the duration you record the Max/Min/Peak function. This gives you the information needed for fuse sizing.
 
Not certain on the regulations and industry standards governing internal fusing for supplementary overload protection within appliances, but at the branch circuit level the manufacturer's time-current curves are often used in selecting suitable cartridge fuses. Smaller glass fuses might have the same information published somewhere.
 

DickCappels

Joined Aug 21, 2008
7,424
The fuse manufacturers have their specifications. The design engineer and safety engineer are caught between them and the industry standards to chose the components that keep their design within boundaries of applicable specifications.

If you are working at home as a hobbyist your main concern is protecting your project from being a shock or fire hazard while not having to replace the fuse.
 
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