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In Reply to: RE: Zerozone 100VA PSU Fuse Issue posted by jea48 on July 25, 2017 at 16:55:05
jea48 wrote:
"NOTE: The 1.6 amp fuse in the 100VA power supply is supposed to be a slow blow fuse, right?"
That is correct. The stock fuse for the 50VA version is 5A fast blow.
Follow Ups:
The stock fuse for the 50VA version is 5A fast blow.
I forgot to mention. A 5 amp AC line fuse increases the secondary output to 40 to 50 amps.
***** 0.42 amps X 1200% = 5.04 amps YIKES! *****
120Vac X 5 amps = 600VA600 watts / 12 volts DC = 50 amps.
Jim
Edits: 07/25/17
Based on your information, I assume the Zerozone 50VA 3A PSU should have an approximate 0.8A 250V slow blow ceramic fuse. I plan to order an AMR Audio 1A 250V silver alloy slow blow ceramic fuse, which should certainly be better than the stock 5A 250V fast blow glass fuse. I simply don't know what they were thinking.
How about the slow blow vs. fast blow question, Jim?
Usually a designer of a piece of equipment will use a slow blow AC line fuse if the equipment draws high inrush current when the equipment is first powered up. Also if the equipment, under normal operation, draws short above normal durations of current. Example would be a decent size power amplifier.
In this case the 100VA/watt power supply unit. When the piece of equipment is first turned on, from a cold start, the caps in the DC power supply draw larger amount of current at first until they charge.Also, I imagine, in the case of a stand alone DC power supply unit that is built to power a remote piece of equipment inrush start up current may also be a factor for using the slow blow fuse. The lag time of a slow blow fuse passes the high inrush current for a time until the current drops, falls off, to normal current level which is within the normal part of the fuse's ampere rating.I question the designer's reasoning for using an AC line in the 100VA/watt power supply with, a 12Vdc 6 amp max output rating, choice of ampere rating of the fuse used. For the 100VA power supply unit I can understand using a slow blow fuse. What I don't understand is the designer's reasoning for using a fuse rated at almost 200% of rated 100VA/watt rating of the unit. I mean as stated above the slow blow section of the fuse allows for cold start up inrush current, as well, it will allow for short duration draws of current placed on the power supply from the connected DC load. Jmho, the slow blow AC line fuse should have been sized, for the 100VA/watt power supply unit closer to 110% to 125% of its' max output connected load rating than at 200%. A 1 amp slow blow fuse by my calculation.
Manufacture rating of the 100VA power supply in the eBay ad.
115Vac input
12Vdc output, max 6 amp connected load rating.100VA / 115Vac = 0.87 amps. (safe FLA continuous duty)
0.87 X 110% = 0.96 amps.0.87 X 125% = 1.09 amps.
Edits: 07/27/17
Protection - Table 450.3(B)
PRIMARY 1000 VOLTS & LESS
PRIMARY PROTECTION ONLY
Currents of 9 Amps or More 125%
Currents Less Than 9 Amps 167%
Currents Less Than 2 Amps 300%
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
https://www.audioasylum.com/audio/tweaks/messages/20/204476.html
But that is for transformers that fall under the jurisdiction of NEC. Not necessarily for cord and plug connected power supplies for audio and or video equipment. I think for overcuurent protection in this case it would be more under UL, NEMA, or some other safety equipment standards.
Where you see the 2 amps or less being 300% somewhere deeper in the code, that 300% figure is meant more for the overcurrent protection of control transformers not a small transformer used for power.
Control transformers are used in appliances and equipment to operate control devices that switch on and off larger power devices. Example a gas fired furnace found in a home. It will use a step down 120V to 24V control transformer to supply the power for the various control circuits. One being to start and stop the blower fan. (Note: Normally small VA rated transformers, say 25VA or so, will have built in fuse protection)
In commercial buildings and industrial facilities control transformers are found in all kinds of equipment. Example, motor control centers, Automated lighting switching equipment. Simple thing as an automatic door opener when you walk into a store.
This code also uses the no more than 300% factor on trannies under 2A. It also states that in control circuits the factor should be no more than 500%. Primary fuse. So it looks like some control circuits can use a higher amp fuse but the 300% figure for Duster's situation is correct. T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
UL and CSA (North American) Standards
North American standards, including UL 508, National Electric Code 450 , and the Canadian Electrical Code, Part 1, require
overcurrent protection on all control circuit transformers. There are two options for overcurrent protection.
.
Again relates to NEC. You need to find something that pertains to cord and plug connected equipment/appliances. Specifically consumer electronics.
I don't see any exclusion for our type uses anywhere I look. Here is what Cooper Bussman has to say. They state the standards discussed (300%) and then say it may not be adequate protection. I think I'm done. I getting a headache. While 300% looks to be the standard it looks like you, I and Bussmann think 300% is pushing it. They like the 125% thing. T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
Edits: 07/27/17
Article 450 Transformers
Step up your knowledge of NEC transformer requirements
Extracted from Mike Holt's Understanding the National Electrical Code textbook for EC&M Magazine
Article 450 begins with the terse statement:"This article covers the installation of all transformers." Then lists eight exceptions. In essence, Article 450 covers power transformers, transformer vaults, and most kinds of lighting transformers (Figure 450-1).
Note: Graphic images are not contained in this newsletter; they will be in the magazine.
Overcurrent Protection
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
Article 450 begins with the terse statement:"This article covers the installation of all transformers." Then lists eight exceptions. In essence, Article 450 covers power transformers, transformer vaults, and most kinds of lighting transformers (Figure 450-1).
Read NEC Article 90.1(A)(B)(C). Then 90.2(A)
I don't see where the NEC guidelines state that it is for control transformers only. Not saying it isn't there, I just don't see it, yet? So this looks like a control transformer would only be operating for a short time?? A transformer under 2A is by definition a fairly small transformer don't you think? I'd like to see the UL and or NEMA guidelines. I'll look for them. Maybe you have their publication. T456
"The Borg is the ultimate user. They're unlike any threat your Federation has ever faced."
- Q, 2365
So this looks like a control transformer would only be operating for a short time??
No not at all. A control transformer may be energized 24/7, 365 days a year.
A transformer under 2A is by definition a fairly small transformer don't you think?
Yes. Especially to the building electrical industry.
Something to remember about NEC. It is bare minimum electrical safety standards. It is not intended to be used as a design manual or a how to manual. See NEC Article 90.
If you wired a building, and say there was a fire and it turned out it was determined it was caused by faulty electrical wiring. If the customer sued you and for your defense in court you said to the judge, "judge I wired the building per City electrical code as well as the National Electrical Code". You will lose the case.
Read NEC Article 90.1 Purpose. (A) (B) & (C).
Then read 90.2 Scope. (A) Covered.
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