If the exact part numbers of the fuses in question are known that would allow analysis of their characteristics. Without additional information I would agree with Jean-Paul that the delay characteristic of the Schurter fuse was inadequate, or that the fuse type itself was not suitable for the application, rather than conclude that the Schurter fuses are of poor quality. A glance at the Schurter catalog of a type SPT 5x20 shows it has the approval of six Safety Agencies from around the world. Not really likely all these agencies would put their stamp of approval on a piece of doo-doo.
Bullseye. In my case, I think this was "user caused" but now have distrust of these "3 in 1" units. Now ordering the hi temp holders another member pointed to. I also threw in a couple holders I saw at Newark just to have a look at them.
Russellc
I just ordered littlefuse versions, I saw two different ones. Spec looked identical, and they may be but got some of both just in case. They will be on here in a couple days.
Russellc
Especially when it's welded shut! One came out with a little persuasion. The other I posted a pic of, has to be REALLY persuaded!
Normally, just a push frees them, then pull them out without damage.
Took top off the AlephJ, looking at the business side with small flashlight, I can't see obvious damage YET, lower part is fairly
Covered up by row of power supply caps.
Wire isn't burnt on top, wires are all shrink wrapped and that dust rubbed off making it appear blacker. Better pics, and of all connectors once removed.
Normally, just a push frees them, then pull them out without damage.
Took top off the AlephJ, looking at the business side with small flashlight, I can't see obvious damage YET, lower part is fairly
Covered up by row of power supply caps.
Wire isn't burnt on top, wires are all shrink wrapped and that dust rubbed off making it appear blacker. Better pics, and of all connectors once removed.
Attachments
This is exactly what I thought 20 years ago!
My friend sent me an ARC D90 amplifier and a bag of 5A slow blow fuses from RadioShack and complained that it kept blowing fuses. I put a Dim bulb tester in series to power the amp and it worked fine, but as soon as I plugged the amp directly from the wall outlet, the fuse blew on power. After a few tries I decided to look in the owner's manual and found out it required a Bussmann 5A DML fuse! Once I installed the DML fuse the amp started up fine and he didn't have to replace the fuse for the next 10 years until he sold the amp.
Currently, I only use Bussmann DML glass fuses or Littlefuse 326 series ceramic fuses in high power amplifiers to take advantage of their higher interrupting ratings and protect against high inrush currents.
Slower fuses do not protect against high inrush currents*, slow start circuits do. Normally from 500VA and up such circuits are used certainly with toroid transformers. Circuit breakers and RCDs as used in recent installations are more picky and usually switch off immediately when switching on such loads when connecting them directly. Some manufacturers and DIYers use the simplest method like using a CL-60 but more complex circuits with relays exist.
https://www.mouser.de/ProductDetail/Amphenol-Advanced-Sensors/CL-60?qs=w3gjcs3NwciEQjXGFRAAAA==&srsltid=AfmBOoprM1_O4-g7DNTUik8b9TLK7jvFob6QSl7deIXYxn3yQjpkibO7
*Using extra slow fuses is allowing the grid and wiring to the amplifier (home installation, power cord) to have above normal inrush currents.
Another possibility that Russellc experiences molten fuse holders can be that the used faston connectors are not branded stuff and do not have the required clamping force for having a low enough ohmic connection with the fuse holders contacts. This would also mean heat and possible damage.
https://www.mouser.de/ProductDetail/Amphenol-Advanced-Sensors/CL-60?qs=w3gjcs3NwciEQjXGFRAAAA==&srsltid=AfmBOoprM1_O4-g7DNTUik8b9TLK7jvFob6QSl7deIXYxn3yQjpkibO7
*Using extra slow fuses is allowing the grid and wiring to the amplifier (home installation, power cord) to have above normal inrush currents.
Another possibility that Russellc experiences molten fuse holders can be that the used faston connectors are not branded stuff and do not have the required clamping force for having a low enough ohmic connection with the fuse holders contacts. This would also mean heat and possible damage.
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Thanks, I ordered 2 different types of 2.5 amp 250 volt slow blows, will check the numbers. Should be here today or tomorrow. Little fuse.
If not, I will pick some up. Ordered over 200 bucks of fuses and holders, I never thought I would go down the fuse rabbit hole, but needs to be done. My problem a bit different, mine didn't blow....
Russellc
If you think about it, there is a limitation on the wattage dissipation for any fuse holder. It would follow that the limitation for the fuse element in a combination power inlet, fuse holder and switch unit would be the lower than that of a fuse holder only. Its not like these things have fins for heat dissipation on them. Looking at the data sheet for fuses you will find a dissipation value. It is entirely possible that a given fuse choice exceeds the allowable dissipation for the power inlet unit.
This issues I know and had several times to find out a solution for more reliability.
Are there large toroidal transformers in use, only high quality single parts for power switch, fuse holder and IEC male connector so as RF mains filter are much more better.
Combi power entry mains modules (incl. main switch, fuse holder and sometimes integrated RF line filter like those under
https://www.mouser.de/ProductDetail/Schurter/4301.3203
https://www.mouser.de/ProductDetail/Schurter/43046081
https://www.mouser.de/datasheet/2/358/typ_6765-1275662.pdf)
are only useful, if no big transformer like in power amplifiers is in use (e. g. RIAA stages, cd player, pre-amplifier and so on).
I think, the main reason for the observed damages as to find in the images from #1+2 are the inrush current after switch on.
Over the time the contact resistances will be higher and higher and thus also the thermal stress - especially on the main switch and fuse holder contacts - with the showed consequences anytime.
The Schurter power inlet is rated at 10A.
The CL-60 is 10 ohms cold, max 5A hot. 120V AC implies a maximum average current of 12A.
Mark Johnson measured 15A peak inrush current on a 400VA Antek transformer with the Amphenol CL-60, and switched to an Ametherm SL15-60002 which peaked at 4A.
Mark's thread, with calculations for limiters, is here: https://www.diyaudio.com/community/...n-usa-160w-class-a-first-watt-designs.329520/
The CL-60 is 10 ohms cold, max 5A hot. 120V AC implies a maximum average current of 12A.
Mark Johnson measured 15A peak inrush current on a 400VA Antek transformer with the Amphenol CL-60, and switched to an Ametherm SL15-60002 which peaked at 4A.
Mark's thread, with calculations for limiters, is here: https://www.diyaudio.com/community/...n-usa-160w-class-a-first-watt-designs.329520/
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Just suppose the way it was till they went kaput. No CL-60 was hurt as there was none (AFAIK). If there was a CL-60 then it is even more strange as inrush current was limited in that case.
Also the loads impedance should be taken into account. The load is not a short circuit so to speak. Just as surge currents that always occur but are so short they usually don’t cause damage but it is a good thing to limit them. In the EU this is mandatory above 500VA.
The very best soft start is to limit inrush current and then bridge the thermistor. Just a thermistor is a tad too simple.
IMHO inrush currents are not the cause as the heat was there for a longer time. The inrush current lasts a few periods so hundred milliseconds tops. This is so short that no significant heat will occur.
Also the loads impedance should be taken into account. The load is not a short circuit so to speak. Just as surge currents that always occur but are so short they usually don’t cause damage but it is a good thing to limit them. In the EU this is mandatory above 500VA.
The very best soft start is to limit inrush current and then bridge the thermistor. Just a thermistor is a tad too simple.
IMHO inrush currents are not the cause as the heat was there for a longer time. The inrush current lasts a few periods so hundred milliseconds tops. This is so short that no significant heat will occur.
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Taking it apart this PM. These power supplies will be totally rebuilt and these ideas for heavier duty parts and how to implement is much appreciated.
Mouser and Digikey to be delivered today.
In the mean time, Ive been keeping tabs on how hot fuse drawers are getting. BA3, which has a very large 1000VA transformer with 31-32 volt rails would tend to make you think it hot, but not so much. drawer feels just warm. the PoP amp of Zenmod, that drawer feels pretty darn warm. Not alarming warm, but much warmer than the BA3. PoP is using a temporary power supply until I get the dual mono PS installed, single 300VA transformer, 24 ish volt rails. Wiring to Schurter should be larger diameter, and it is connected with unsoldered fastons, but soon will be soldered, if that style unit is still going to be used. That doesnt help.
The two amps in question here are the M2 and AlephJ clones."Store kit" AlephJ, back when first a group buy with 6l6. The M2 was the Teabag board many here used. Power supply on both units are identical, 500 VA Antek 18+18 volt. Both have 8 33K capacitor teabag power supply boards, standard Clone CL-60 inrush setup.
These are the only two of my clones this happened to. They were also both part of a temporary experiment a couple years ago, and I am suspicious damage may have occurred then. Have run regularly since that time.
Removing cooked input now.
Russellc
Mouser and Digikey to be delivered today.
In the mean time, Ive been keeping tabs on how hot fuse drawers are getting. BA3, which has a very large 1000VA transformer with 31-32 volt rails would tend to make you think it hot, but not so much. drawer feels just warm. the PoP amp of Zenmod, that drawer feels pretty darn warm. Not alarming warm, but much warmer than the BA3. PoP is using a temporary power supply until I get the dual mono PS installed, single 300VA transformer, 24 ish volt rails. Wiring to Schurter should be larger diameter, and it is connected with unsoldered fastons, but soon will be soldered, if that style unit is still going to be used. That doesnt help.
The two amps in question here are the M2 and AlephJ clones."Store kit" AlephJ, back when first a group buy with 6l6. The M2 was the Teabag board many here used. Power supply on both units are identical, 500 VA Antek 18+18 volt. Both have 8 33K capacitor teabag power supply boards, standard Clone CL-60 inrush setup.
These are the only two of my clones this happened to. They were also both part of a temporary experiment a couple years ago, and I am suspicious damage may have occurred then. Have run regularly since that time.
Removing cooked input now.
Russellc
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One other feature of these two affected amps, is that they are both in the older stye Deluxe 4U which is only 300mm deep.
Neither of them seemed excessively hot, may have to rethink that obviously.
Russellc
Neither of them seemed excessively hot, may have to rethink that obviously.
Russellc
