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Old 26th April 2011, 08:49 PM   #631
infinia is offline infinia  United States
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Even if you have tested the transformer for UL listing, you are not exempt from installing adequate protection measures which include secondary fuses most of the time.
that's news to me I have never seen that in consumer or industrial gear. Sounds like some weird requirement for instrumentation not UL. cite please. I stopped working in power in 1986, About a year prior I had developed a custom SMPS for Tektronics. I had to work with UL to get it listed, the UL engineer was greener than me, so he had us going off on tangents from time to time, but luckily had help from a Tek senior fellow who pretty much wrote the book on safety, step in when needed. I don't think much has changed since peeking in from time to time.


Your answer to AT sounds like your are making a case against primary protection more than a case for secondary.
but I'll see or add to his answer later.
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Old 27th April 2011, 03:38 PM   #632
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post630.
What is "selective fuse coordination"? Did you mean discrimination?

What? I did not ignore it. I couldn't have, if I don't know what you are referring to.

The rest of your post is logically flawed. I cannot draw any sensible conclusions/rules from the contradictory statements in there.
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Old 27th April 2011, 06:08 PM   #633
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Originally Posted by infinia View Post
that's news to me I have never seen that in consumer or industrial gear.
I have been working with industrial gear for several decades now and I always see it done that way. Circuit-breaker before the transformer for lead protection (short-circuit protection only) and another circuit-breaker or a fuse right behind the transformer for its own protection against shorts and overload.

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Your answer to AT sounds like your are making a case against primary protection more than a case for secondary.
Both are required.

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post630.
What is "selective fuse coordination"? Did you mean discrimination?
Yes, seems that I found the American translation instead of the British one.
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Old 27th April 2011, 06:16 PM   #634
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breaker ? What products are you talking about? KVA sized.
I don't think chip amp PS's are in that category. cite the safety regulation and post the applicable paragraph otherwise your UL guy made you go above and beyond your competitors, potentially affecting your reliability and costs.
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Old 28th April 2011, 08:01 PM   #635
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breaker ? What products are you talking about? KVA sized.
Circuit-breakers are available with nominal current ratings from 0,1 A upward, maybe even smaller. No kVA there. Of course it makes sense to replace the circuit-breaker with a fuse for your amp power supply for reasons of physical size.
You can see the products I am talking about in the link in post #630. They range from 20 VA upward.

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your UL guy made you go above and beyond your competitors, potentially affecting your reliability and costs.
Have a look beside my post. That is the German flag you see there. I am not working according to UL, but according to IEC, EN and DIN regulations.

And your assessment is wrong. The way I describe transformer protection is the quasi standard in Germany. So no advantage for competitors.

There is no regulation that obliges you to work that way. If you deal with this stuff on a regular basis you know that regulations tell you what you need to achieve, not how to achieve it. So if you can proove that a different arrangement is equally safe, nobody will forbid you to use it. You will however have to expend more effort to achieve that, which makes it unattractive for most companies.

Take another look into the link in post #630. On page 6 there is an unregulated power supply (GNC). The datasheet recommends a circuit-breaker for the primary which you can of course again replace with a fuse. In the right two columns it lists the fuses/circuit-breakers that are installed inside the power-supply. Lamentably they don't provide a schematic, but they do install the fuse/circuit-breakers between transformer and rectifier. That company has a far too big output to be wasting money on unnecessary fuses. Which leads me to the conclusion that they also found this arrangement to be the most economical way to comply with all applicable safety regulations.
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Old 29th April 2011, 12:19 AM   #636
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Have a look beside my post. That is the German flag you see there. I am not working according to UL, but according to IEC, EN and DIN regulations.

And your assessment is wrong. The way I describe transformer protection is the quasi standard in Germany. So no advantage for competitors.
OK didn't mean to offend, it was sexist too. lol replace your "UL guy" with "your representative for the intended markets safety organization". or VDE gal



So I still think after reviewing the sales PDF that additional secondary over load protection goes above and beyond what is sensible and practical for consumer and light industrial equipment and you don't cause it's in "the code". BTW what does this do to wall warts in Germany?
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Old 29th April 2011, 04:41 PM   #637
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Most wall warts use transformers with such a high stray inductance that they are virtually short-circuit-proof. You wouldn't want such a transformer in your amp, though, because high stray inductance means high regulation, too.
Most wall-warts are cheap, so you cannot rely on them being sufficiently well designed. Therefore a built-in temperature fuse is compulsory in them, at least in IEC member countries. If you are lucky it is of the self-resetting type, otherwise each overload or short renders the wall-wart useless.

So, how do you go about transformer protection when the inrush current makes it impossible to right-size the primary fuse for overload protection?
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Old 29th April 2011, 05:45 PM   #638
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In-rush is a transient condition, so most SB fuses, breakers, or XFMR heating doesnt respond until an average level over their time constant is exceeded. if the surge levels under the time curve exceed some primary protection or for that matter any other component in the path , like switches , rectifiers, etc. THEN Adding slow start circuitry is generally a good design practice this is rarely a safety concern but more of a reliability issue.
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Old 29th April 2011, 06:44 PM   #639
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Originally Posted by infinia View Post
In-rush is a transient condition, so most SB fuses, breakers, or XFMR heating doesnt respond until an average level over their time constant is exceeded. if the surge levels under the time curve exceed some primary protection or for that matter any other component in the path , like switches , rectifiers, etc. THEN Adding slow start circuitry is generally a good design practice this is rarely a safety concern but more of a reliability issue.

In otherwords design the primary current overload to the XFMR max ratings, not to a transient event. Turn on surges are mainly because of low Req not Leq. in larger > 500-600 VA EI cores and less 30% larger toroids.
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Old 29th April 2011, 07:27 PM   #640
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I have a 140r soft start feeding the 240Vac primary of a 160VA toroid transformer.
An F1A fuse feeding both the soft start and a direct feed to a 3VA EI has not failed yet due to nuisance blowing (fatigue).

If I had some T500mA and T800mA fuses, I would be reporting on those as well.

The secondary fuses (4off), after the smoothing caps (+-20mF), feeding two dual polarity amplifiers are F1.6A, and they too have not blown.
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