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In Reply to: RE: Zerozone 100VA PSU Fuse Issue posted by jea48 on July 22, 2017 at 09:05:35
Jim, where did you get the factor of 2.5. I've never seen that high a factor before in my readings. Seems too high?? Thanks, T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
Follow Ups:
I can't remember if it originated with NEMA or UL electrical safety standards for transformers of 1000Vac and less. My above post was going from memory....
But the percentage rating for overcurrent protection can be found in 2014 NEC Article 450, 450.3, Table 450.3B
Duster's power supply uses a 100Va transformer. Max FLA rating is
100VA / 120Vac = 0.83 amps
Table 450.3B Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Less (as a percentage of transformer-rated current)
Primary Protection
Primary only protection
Currents Less Than 2 amps - 300%
Not sure I would go 300% on a 100VA power supply transformer though.
Note:
Currents of 9 amps or more, 125%.
Current less than 9 amps 167%.
Currents less than 2 amps, 300%
Throw in secondary protection and the percentages are 250% across the board. (see note 3)
Awsome info jea. Thanks! This seems to somewhat contradict some of the other info I've seen and heard that is quite a bit more conservative. Never saw this before. T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
This seems to somewhat contradict some of the other info I've seen and heard that is quite a bit more conservative. Never saw this before. T456
Tweaker456,
I should have qualified my response, given in my previous post, is for transformers that fall under the jurisdiction of the NEC. Mains power AC line overcurrent protection of power transformers used in cord and plug connected electronic equipment may be different. Though I believe the max 250% of the manufacture's FLA ampere rating of a power transformer is probably safe. But that may not be the case.
Bottom line though, the designer, manufacture, of the equipment has the final say on what amperage fuse size and type of protection fuse he requires to be installed in his equipment. If the equipment is under warranty and the user installs a bigger fuse and or different type fuse and through an equipment internal failure, or caused by a connected overload, damage to the equipment may/will void the manufacture warranty.
In this case the 1.6 amp fuse is 200% of the 100VA rating, 0.83 amp FLA max rating,(for 120Vac mains) of the transformer used in the power supply.
NOTE: It seems the seller that sold Duster the Power Supply said he could use a fuse higher than the 1.6 amp supplied fuse up to a 3 amp fuse.
I read the eBay seller's add and he states the AC input voltage being 115Vac not 120Vac. I can only assume 115V is from that of the manufacture's spec of the power transformer.
100VA / 115Vac = 0.87 amps (max rated primary continuous duty amps)
300% X 0.87 = 2.6 amps @ 115Vac
100VA / 120Vac = 0.83 amps
300% x 0.83 amps = 2.49 amps
350% x 0.83 amps = 2.9 amps
Not sure I would use a 3 amp fuse to protect a 100VA transformer. I don't think an AC Line 3 amp fuse will protect the full wave rectifier or voltage regulator from a connected overload.
Jim
.
In this case one sure can't rely on the manufacturer or seller. I'm working on getting fire extinguishers put in audio equipment so the standard can be raised to a factor of 600% (just kidding). A factor of 300% seems to me to imply that they would believe that most faults would be above that and that there is a very low likelihood of a constant 300% current draw in a fault situation. Otherwise 300% doesn't seem like a safe recommendation. T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
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