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In Reply to: RE: Fuse question posted by henryrancourt@att.net on September 22, 2016 at 08:41:52
Using the simple equation, fuse = 3 X VA/VmainsWhere did you come up with the 3 X? There in 300%.
Maybe electronics is different than electrical wiring for sizing the primary winding fuse size of a power transformer.
For the primary fuse for a power transformer, the minimum fuse size is 125% of the FLA of the transformer. 250% being the maximum fuse size of FLA.
FLA = transformer nameplate VA rating / nameplate voltage rating.
.
Edits: 09/22/16Follow Ups:
Jea, If you search "Transformer Protection-Merson" he has a chart that indicates a factor of 3 for a primary fuse if the current is under 2A. There is a differential factor depending on current according to Merson. That should answer the question of where 300% does or might come from. Probable not appropriate for a 300va situation though. Tweaker
If you search "Transformer Protection-Merson" he has a chart that indicates a factor of 3 for a primary fuse if the current is under 2A. There is a differential factor depending on current according to Merson.
Merson uses NEC, (National Electrical Code), Table 450.3(B), Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Less ( as a Percentage of Transformer Rated Current) .
Not sure NEC should be used for sizing the fuse for a power transformer in a piece of audio equipment, that has a cord and plug that plugs into an electrical wall outlet. That technically does not fall under the domain of NEC.
As for NEC Table 450.3(B) the less than 2 amps 300% figure is more for a control transformer in the eyes of NEC, not a power transformer. An example would be a control transformer used for the control voltage of a motor starter circuit.
I think you need to look elsewhere for the proper sizing of the AC Line safety fuse for a piece of audio equipment. Maybe UL or NEMA.
//
NEC 90.1
National Electrical Code 90.1 Purpose:(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.
B) Adequacy. This Code contains provisions considered necessary for safety. Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.
FPN: Hazards often occur because of overloading of wiring systems by methods or usage not in conformity with this Code. This occurs because initial wiring did not provide for increases in the use of electricity. An initial adequate installation and reasonable provisions for system changes provide for increase in the use of electricity.
(C) Intention. This Code is not intended as a design specification or instruction manual for untrained persons.
(D) Relation to Other International Standards. The requirements in this Code address the fundamental principles of protection for safety contained in Section 131 of International Electrotechnical Commission Standard 60364-1, Electrical Installation of Buildings.
FPN: IEC 60364-, Section 131, contains fundamental principles of protection for safety that encompass protection against electric shock, protection against thermal effects, protection against overcurrent, protection against fault currents, and protection against overvoltage. All of these potential hazards are addressed by the requirements in this Code.
Look Closely at Section (B) Adequacy it reads as follows: "Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electric use."
Edits: 10/08/16
Thanks for the info. I got the fuse = 3 X VA/Vmains from a different site where there was info on building amps with the ICs I listed.
If min is 125% and max is 250% basic would be 187.5%. So using your calculation, with the basic % value, the fuse amperage would = (300/115)X 1.1875 = 3.0978. Is that correct?
Disregard anything you see on that other website if that's how they pick their fuses.
I usual start with the min, and if that keeps blowing then there's room to increase it if necessary. 3A should be fine.
Dan Santoni
nt.
What does +1 nt mean?
thanks
+1 means someone agrees with a post. You might consider reading "Transformer Protection-Mersen". It has calculations that differ somewhat in how conservative they are based on the current of the circuit. I myself don't see why one would require both primary and secondary fuses but I could be wrong. Maybe an EE can weigh in on this. T456
The primary fuse essentially protects the power transformer and everything downstream, as a last resort. Rail fuses protect, potentially, the output stage devices and the speakers. (Note that the OP states these are "fast-blow" types, which makes sense.) I have seen many commercially marketed amplifiers, both tube and solid state types, that are fused in this manner. (But I am definitely NOT an EE.)
Both my Atma-sphere amplifier (tube, OTL) and my Threshold amplifier (solid state) are doubly fused in this manner. On one occasion, a rail fuse blew in the Threshold because of a short at the output of that one channel, thus saving the entire amplifier from damage, as well as my delicate woofers.
Seems like a good answer. I guess you can't be too careful although it's hardly the case that all amps are fused in this way. It is still possible IME that the transformer primary fuse would or could protect the amp but of course not a likely as FB's on the rails. How did the short at the output come about?? (Not that I have never done it). The designers have their reasons but protection has it's price in sound quality. This does not mean that I am recommending something dangerous. T456
I think the negative effect on sound quality of using a fuse is way over-stated, most often on this website. I am not saying there is NO effect; I am saying that the effect is small and worth tolerating for the peace of mind that comes with using a proper fuse when and where necessary. But that's just my 2 cents.
On the Threshold, it was no big deal. The speaker wire ends very briefly touched each other, shorting hot to ground. That can have disastrous consequences if there is no fuse on the rail. In this case, no problem, except I needed to replace that one fuse.
I'm wondering why some have rail fuses and some don't. May have something to do with the cost of repair?? I'm glad my amp only has one, at least until if and when it blows for some reason. T456
and have explained why there are slow blow and fast blow fuses? Tweak, We all know you have your way of thinking on power amps, AC mains, and how everything bigger is better and the Engineers have another.If something ever does go wrong with your amp's output stage or power supply, you will wish you had all those fuses with proper values in place that you think are not necessary.
Edits: 10/04/16
There is only one fuse in my power amp. The value is the max value the designer "allows". Anyone "should" be able to hear that a bigger fuse sounds better and no fuse at all, a short, will sound the best ,IMHO. This is not to say anyone should do this but it does lay the ground work for the rational for bigger than necessary diodes to be used. Bigger solid state outputs will also sound better. A bigger power supply, bigger transformers, bigger cap banks, bigger wire will also sound better, not only in my opinion but the opinion of Nelson Pass. You might want to read what the engineer Nelson Pass has to says about power supplies and bigger. Also a bigger power supply will often if not always have a bigger , safely and appropriately installed, bigger fuse. A big problem companies have with bigger is the bigger cost . Big Tweaker
> > > Bigger solid state outputs will also sound better. A bigger power supply, bigger transformers, bigger cap banks, bigger wire will also sound better,
This is just wrong. No surprises here I guess.
I don't know what amplifier you own, but if you were to look inside, it's possible that the rails are fused internally (either with a conventional fuse that is not mounted on the exterior of the chassis or using just a fine wire) or that there is some sort of current-sensitive device in the circuit that is designed to blow before damage is done to the output devices.
99.9% certain that there are no internal fuses. Looked inside once over 3 tears ago. It sounds a bit odd that some other device other than a fuse would be used for the purpose of protection??? Fuses, like prophylactics have pleasure reduction built in to them. No doubt about it, necessary or not. Of course, always use the correct fuse. T456
There it is.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
"The designers have their reasons but protection has it's price in sound quality. "
In the end there are trade-offs and everything is a compromise. One of my DIY amps had an output tube (KT-120,one of four) short plate to grid. Fortunately there was a 1K grid stopper that limited current so the secondary of the expensive interstage transformer feeding it wasn't damaged. Wasn't enough current draw to blow main fuse. The grid stopper did send up a smoke signal clueing me to the problem.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I myself don't see why one would require both primary and secondary fuses but I could be wrong.
I agree. In fact if the primary winding overcurrent protection is fused correctly overloading the secondary will cause the primary fuse protection to blow.
Edits: 09/23/16
nt.
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