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Improper protection of "wall wart" supply (U.K.) #165195 06/22/07 11:56 AM
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pauluk Offline OP
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This seems to be getting all too common.

Typical "wall wart" power supply, the LV cord had become damaged where it exits the grommet, resulting in a short. There is no fusing on either side of the transformer, not even an overtemperature cut-out on the primary.

In this particular case the primary burned open circuit before any more damage could be done, but the xfmr was very hot.

This particular unit was plugged into a power strip and thus had 13A protection at least, but this type of poorly designed unit could just as easily be plugged directly into an outlet on a ring circuit protected at 32 amps.

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Re: Improper protection of "wall wart" supply (U.K.) [Re: pauluk] #165197 06/22/07 12:37 PM
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Posts: 165
Retired_Helper Offline
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confused Paul, what is meant by a "ring circuit?" I've seen the term in your postings before. Does it have a US equivalent that you know of? Thanks! smile

Re: Improper protection of "wall wart" supply (U.K.) [Re: Retired_Helper] #165202 06/22/07 01:44 PM
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electech Offline
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This wall-wart may be made with fuseable magnet wire (dictionary spelling would be fusible, but I see fuseable more often). In the state the supply is in right now, following the "output loading" test that was accidently performed, it should still pass a 3000 VAC hipot test from primary to secondary (2000 VAC for 120V equipment). If not, then someone messed up.
Are you sure there isn't a thermal fuse up against the primary winding under the first two layers of tape?


Re: Improper protection of "wall wart" supply (U.K.) [Re: Retired_Helper] #165207 06/22/07 07:27 PM
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32VAC Offline
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Originally Posted by Retired_Helper
confused Paul, what is meant by a "ring circuit?" I've seen the term in your postings before. Does it have a US equivalent that you know of? Thanks! smile


http://en.wikipedia.org/wiki/Ring_circuit

Re: Improper protection of "wall wart" supply (U.K.) [Re: 32VAC] #165218 06/23/07 05:23 AM
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Retired_Helper Offline
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smile Thank you, 32VAC. That's a big help!

Re: Improper protection of "wall wart" supply (U.K [Re: electech] #165222 06/23/07 09:36 AM
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pauluk Offline OP
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See also these threads:

Ring circuits UK style.

Ring circuits revisited

Ring circuits? Spur unit? RCD Device?

This article (PDF) from the IEE journal:

Origin of the BS1363 plug & socket

The key point is that every BS1363 or fused spur connection to a ring should be protected by a fuse of no more than 13 amps.

The imported "wall warts" are just using molded BS1363 plugs in the cases and not providing the proper fusing which is part of the specification.

Quote
Are you sure there isn't a thermal fuse up against the primary winding under the first two layers of tape?


Yep. I stripped the outer layers back and the wires just go straight on to the primary winding.




Re: Improper protection of "wall wart" supply (U.K [Re: pauluk] #165227 06/23/07 01:07 PM
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Retired_Helper Offline
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smile Paul: thanks for the additional citations. I think I'll just stick with good old American radials (sounds like a tyre commercial!) I have no idea if my sparky's Navy education included UK wiring; I would guess not! crazy

Re: Improper protection of "wall wart" supply (U.K [Re: pauluk] #165232 06/23/07 02:51 PM
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wa2ise Offline
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Originally Posted by pauluk


In this particular case the primary burned open circuit before any more damage could be done, but the xfmr was very hot.




UL would consider that sufficient protection to keep it from starting a fire that burns the house down. The fact that it becomes trash is not important to them.

Re: Improper protection of "wall wart" supply (U.K [Re: wa2ise] #165233 06/23/07 03:41 PM
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Posts: 8,379
Trumpy Offline
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Paul,
I note on the case, the brand of US Robotics, is this a US made product designed for the UK market?
Second question,
How hard was it to open the case?, I remember posting a thread about these devices some time ago and the problems I had with actually getting the plastic case open to investigate what had happened to the power supply unit on my cordless drill charger.
I note that yours actually has screws in it, not the common plastic welding.

Re: Improper protection of "wall wart" supply (U.K [Re: Trumpy] #165296 06/25/07 09:31 AM
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pauluk Offline OP
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Quote
is this a US made product designed for the UK market?


It's the P.S.U. for a 3Com/U.S. Robotics external modem, about 8 or 9 years old. This appears to be a specific international version of the modem different from the North American version -- Only has CE etc. approval marks not UL/CSA/FCC, documentation and onboard help systems don't include American/Canadian options, and so on. Says Made in Hungary on the sticker.

The P.S.U. is the one specifically for the U.K., Irish Republic, Malta, etc. No clue as to its country of manufacture.

Quote
How hard was it to open the case?


Not hard at all after I put a large screwdriver between the sections and just snapped the threads out! wink

Yes, it was screwed together, but with screws having a peculiar head for which I had no tools. Why can't they just use regularly slotted or Philips on something like this? frown

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