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In Reply to: Re: MA-2: Crucial fact to know posted by Springbok10 on September 24, 2004 at 17:10:14:
"FNM" is merely the Bussman designation type for a 13/32 X 1-1/2" midget dual element time delay fuse. The dual element construction provides a relatively long time delay during start-up to allow for the high inrush currents that exist when energizing reactive components and loads such as bus capacitors, motors, transformers and some heating elements. "TNM" is the equivilent Ferraz-Shawmut fuse type designation. Of course the MA-2 amplifier includes both bus capacitance and transformers which will dominate the inrush current profile.These are both good fuses (FNM and TNM). Slight differences will exist in their time-current clearing profiles, especially during the first few milliseconds when the higher inrush current exists. It doesn't necessarily mean one fuse is better than the other. I tried to compare the short time clearing characteristics of each fuse however, Bussman does not provide the 8 ampere fuse curve on their site so I was unable to do so. Neither fuse is UL class "CC" rated and I suspect this is due to the longer time or, higher inrush current tolerance required for this particular fuse's target application.
A class "CC" designated fuse (also 13/32 X 1-1/2")in the MA-2 amp would have probably assured a bit more consistency from one manufacture to another but, it likely would have not provided the overall protective coordination needed(short term and long term clearing action). I don't think the class "CC" time delay fuse provides the initial delay that exists in the non-class "CC" type "FNM" (or "TNM") type configuration. And still, even the class "CC" designation does not ensure two fuses will have identical clearing characteristics - just that they will be a bit closer.
It is always recommended to replace a fuse with the identical type, and from the same manufacturer if possible. The Bussman brand should be widely available as this is an old company that has been around for many years and has excellent distribution channels. If you are in a location where the Bussman brand is simply not available then I am sure Atmasphere can pick a substitute from what is available in your area (Ferraz, Littlefuse, etc). And, in this case one size larger ampere rating in the "TRM" line may be acceptable however, this guidance needs to come from Atmasphere. Never change fuse ratings without consulting the manufacturer of the equipment.
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Follow Ups
- Re: MA-2: Crucial fact to know - Neal Hood 18:38:09 09/24/04 (0)
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