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Fuses? How Do They Differ?

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Old 04-28-2008, 08:16 AM   #1
paul 74
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Default Fuses? How Do They Differ?

This is not necessarily C2-related but it does have automotive relevance. In looking through the Owner's Manual for my latest old Corvette I had a peek at the fuse and breaker data. To my eye they all look very much the same: glass and cylindrical. But the descriptions differ. Here are examples:

-headlamp circuit, circuit breaker (seems straight-forward)
-backup lamps & turn signal, AGC fuse (20 amp)
-heater-A/C, 3AG fuse (25 amp)
-tail lamps, SFE fuse (20 amp)

What is the difference between AGC, 3AG, and SFE?

*Edit*-This pic shows the whole story. The year is 1974.


Last edited by paul 74; 04-28-2008 at 10:58 AM.
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Old 04-28-2008, 09:10 AM   #2
jim lockwood
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Quote:
Originally Posted by paul67 View Post
This is not necessarily C2-related but it does have automotive relevance. In looking through the Owner's Manual for my latest old Corvette I had a peek at the fuse and breaker data. To my eye they all look very much the same: glass and cylindrical. But the descriptions differ. Here are examples:

-headlamp circuit, circuit breaker (seems straight-forward)
-backup lamps & turn signal, AGC fuse (20 amp)
-heater-A/C, 3AG fuse (25 amp)
-tail lamps, SFE fuse (20 amp)

What is the difference between AGC, 3AG, and SFE?
The SFE fuse for the tail lights has a slower reaction time so the surge current of 4 brake light filaments coming on won't cause it to open up. One manufacturer of the era had trade name of "Slo-Blo" for this type of fuse. These were available by special order through Radio Shack a few years ago.

AGC vs 3AG, I couldn't say.

Jim
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Old 04-28-2008, 07:49 PM   #3
JohnZ
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AGC glass fuses are all the same length (1-1/4"), regardless of rating, from 1 amp to 30 amps. SFE (Society of Fuse Engineers) glass fuses are the same diameter as AGC fuses (1/4"), but are six different lengths depending on their amp rating (from 5/8" long for 1-amp to 1-7/16" long for 30-amp).
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Old 04-28-2008, 07:56 PM   #4
Bill Irwin
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Society of Fuse Engineers? Who would of thunk? Bill.
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Old 04-28-2008, 08:19 PM   #5
paul 74
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Thnks but why would an SFE 20-amp and an AGC 20-amp be included in the same fuse box?
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Old 04-29-2008, 09:08 AM   #6
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Brain spasm with my posting yesterday.

The SFE fuse designation is NOT a slow acting fuse. I believe the original automotive designation for a slow acting fuse, as is needed for brake lights, was SAE.

In any event slow acting fuses are available in the Bussman brand, the MDL series:

http://host1.publiquik.com/bussmann_...?familyUid=695

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Old 04-29-2008, 04:20 PM   #7
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Don't stress over this, just use the correct amp rated fuse!

The "slo blow" refers to the time it takes to blow when the fuse is in a severe overload condition.
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Old 04-29-2008, 05:13 PM   #8
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Quote:
Originally Posted by mikem350 View Post
Don't stress over this, just use the correct amp rated fuse!

The "slo blow" refers to the time it takes to blow when the fuse is in a severe overload condition.
I've come to that conclusion .
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Old 04-30-2008, 11:44 AM   #9
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Quote:
Originally Posted by paul67 View Post
Thnks but why would an SFE 20-amp and an AGC 20-amp be included in the same fuse box?
SFE fuses are 'automotive', rated only to interrupt 32 volts. The length depends on their current rating, ranging from 5/8" for a SFE-4, to 1-7/16" for a SFE-30. AGC fuses are 'general purpose', they are always 1/4" by 1-1/4", available in 250V ratings to 10 amps, & 32V ratings to 30 amps. An AGC-20 is the same length, & pretty much interchanges with an SFE-20. Both types are "fast acting", but according to Buss charts, the SFE is a little faster. An AGC-20 is rated to carry 27 amps for up to 60 minutes, while an SFE-20 will blow in under 2 minutes. However, the 60 minutes is given as a maximum, while the 2 minutes is apparently typical, so that may not be an accurate comparison.

The 3AG fuse is "fast" acting. Typical times at percent of rated current are: 4 hours at 110%, 1 hour at 135%, 5 seconds at 200% - all at 32 volts.

ACG fuses have about the same time@load ratings, but are rated at 110volts.

Which is specified should depend on several factors: whether ambient temperature of the device will effect the current draw, start up draw interaction if multiple devices are on the same fuse, and of course the preference of the designer.

Last edited by magicv8; 04-30-2008 at 12:01 PM.
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Old 04-06-2013, 10:03 PM   #10
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Old thread yep! But this is good reading ... especially for someone like me who just went through the gamut with the fuse panel.
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