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Have you seen this in-plug fuse arrangement before?

  1. Jun 18, 2017 #1

    berkeman

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    I bought a small floor fan yesterday, and in reading the instructions (yes, some of us do read the instructions), I was surprised to see this. Have any of you seen a fuse in the AC mains power plug like this before? If I hadn't read the instructions, it would have never occurred to me to look for a fuse there if the fan stopped working...

    Fan Power Plug Fuse.jpg
     
  2. jcsd
  3. Jun 18, 2017 #2

    anorlunda

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    No, I never saw that.

    One could save on copper in the lead wires if they were designed for max 5 amps. But savings would be offset by the complexity of the fused plug. I find it hard to imagine that copper savings is the motivation.

    Shock hazard also sounds dubious. The difference in hazard between 5A@120V and 15A@120V seems slight.

    Yes someone thought that the expense of that extra fuse was justified. Good mystery.
     
  4. Jun 18, 2017 #3

    Averagesupernova

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    I have seen electric fence chargers that have 2 fuses in the plug. One fuse for each wire, hot and neutral. I believe this is to help prevent lightning strikes from energizing the AC line. It really isn't to protect from overcurrent from a failure in the fence charger although obviously it would do that too.
    -
    So what say you @berkeman ? Any chances of a lightning strike on your fan? :DD
     
  5. Jun 18, 2017 #4

    berkeman

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    LOL, let's hope not! :smile:

    I'm guessing it's to protect against fire in the case of a stuck rotor, but who knows. I wonder if my other fans have fuses that are not accessible, and customers complained too much when the fans stopped working and they could not easily replace the fuse.
     
  6. Jun 18, 2017 #5
    Out of curiosity, do you know the cord length and AWG?
    Fuse specifics (is it a 5x20mm slo-blow glass fuse similar to a Bussmann GDC fuse)?

    I've seen fused plugs on Christmas tree lighting, but I'm puzzled for the same reasons as @anorlunda why one would be used for a fan motor and cordset. Mouser lists the 2000+ quantity cost of a GDC-5A fuse as $0.584 ($1.36 single unit), and I'm having a hard time understanding the manufacturer's cost justification if the idea was fusing allowed using thinner wire.

    It seems to me that plug fusing may have actually increased fire risk.
    • Fuse heating isn't much (fuse resistance is 0.00985 ohms; about 1/4 watt at 5 amps), but it won't dissipate very well surrounded by insulation.
    • 3 additional connection points - two at the fuse clip to fuse interface, and an additional connection from wire to clip - that are heated bu the fuse.
     
  7. Jun 18, 2017 #6

    Nidum

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    upload_2017-6-18_22-28-38.png

    This is the inside of the 3 pin plug used on most domestic and office equipment in the UK . Rated at 13 Amp but generally used on devices drawing up to about 10 Amp maximum . Various rated fuses can be fitted . Most common are 3 Amp and 13 Amp .
     
  8. Jun 18, 2017 #7

    anorlunda

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    So what risks do those fuses protect against?. Presumably, a short circuit will blow the fuse or breaker on the house's panel. Is it overload of the appliance, but not short circuit?

    An appliance with a motor can overload with a blocked rotor. But something like an oven either works at rated power or is short circuited, it is hard to think of an overload fault for a oven.


    I bet @sophiecentaur can help us.
     
  9. Jun 18, 2017 #8

    Averagesupernova

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    Most household fan motors will be what is called 'impedance protected'. Locked rotor current is limited by the nature of the design. It surprises me that a fan would have a fuse. Maybe it is simply standardization to be legal to use in various countries.
     
  10. Jun 18, 2017 #9
    I didn't find any smoking gun however this document gives some good background info... https://www.cpsc.gov/s3fs-public/portfan.pdf

    This write-up is a bit dated (2003) but what jumped out at me was the reference to other motor standards requiring a non-resettable overload device to protect the motor from overheating. Impedance protection is also discussed.

    I'm guessing there have been some changes in this standard, purchase required... https://standardscatalog.ul.com/standards/en/standard_507_9

    However there is also some allusion to the fused safety plug becoming standard equipment on a fan contained within the website of this manufacturer, who seems to fabricate appliance cords for the industry... http://www.yunhuanelectric.com/America-UL-fused-plug-power-cord-1-15P.html

    Maybe someone having hands-on experience with the standard can give us more? But to answer the OP, I recall seeing these plugs out there for some time, and I have also noticed them on some forms of portable lighting such as christmas decorations. I think it sums up as cheap insurance; in a world of uncertain applications at least the appliance can somewhat look out for it's own wiring from the point of supply.
     
    Last edited: Jun 18, 2017
  11. Jun 19, 2017 #10

    sophiecentaur

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    The standard 13A plug was designed in conjunction with the (240V) Ring Main system, which has a 32A fuse, feeding several outlets (at least five or six twin outlets). A fault in an appliance or even a partial short at the cable entry can produce a load which will not blow a 32A fuse but fry the lead and set fire to the carpet etc. There are many differing opinions about 120V vs 240V domestic systems but the higher voltage allows thinner conductors and more flexible leads for appliances. That's a real practical advantage. A plug fuse gives the appropriate protection for the appliance and its lead where the fuse at the panel has to be over-generous for low power appliances.
    People who aren't used to the Ring Main tend to treat the whole idea with suspicion and fear. Nonetheless, it works very well and there are very few accidents which can be attributed to that system. It's even better nowadays when all appliances are fitted with moulded plugs and the correct fuse. People do very few home repairs these days and appliances mostly get thrown out with the original fuse still in the plug.
    Also, Ring Main = Low Cost Installation.
     
  12. Jun 19, 2017 #11

    Nidum

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    @ Anorlunda : So what risks do those fuses protect against ?

    The primary purpose of a fuse is to protect the electricity supply system from any damage which could be caused by excess current flow due to a short .

    Basic theory is that fuses are always arranged so that in event of a short in one section of the supply system they prevent any excess current from being drawn from the next section along in the direction of the power station .

    In the standard UK electricity supply system there is a cascade of protection devices of increasing rating between your domestic appliance and the power station .

    Actual blow wire fuses are only used in the lower power end of the system . Solenoid and electronic breakers are almost universally used at higher powers .

    New build houses in UK now mostly have electronic combined RCD + overload protection systems .

    Nice little link with history though - most power supply companies still demand the fitting of a 100 Amp cartridge fuse on the ingoing feeder . They never seem to blow . You still see ones in use that are over 60 years old and with the trade name markings of long gone makers .
     
    Last edited: Jun 19, 2017
  13. Jun 19, 2017 #12

    sophiecentaur

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    You need to consider both upstream and downstream of the fuse. The safety aspect of fuses is primarily to protect downstream (assuming the cabling is correctly specified for any more upstream fuses).
    If that were the only function, you could have 32A fuses in each plug (which would make them superfluous, of course). The 'Primary' purpose of the Plug Fuse is to protect the appliance and its lead from all the. Frying the lead and burning the house down would involve a value of current that the Ring Main can easily supply. The 32A fuse is what 'protects the Ring. There are many faults, other than 'dead shorts' that need to be protected against.
     
  14. Jun 19, 2017 #13

    Nidum

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    The fuses in domestic appliances and plug connectors do provide some safety protection for the appliance and for the user but this is somewhat incidental to the primary purpose of the fuses .

    In practice this safety protection is often not very effective . They help by shutting off power to a shorting device and so help prevent possibility of fires but provide little protection against shocks . RCD's are much better at preventing shocks .
     
  15. Jun 19, 2017 #14

    sophiecentaur

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    There has always been a wire fuse in series with the company supply to all of my houses. Very few problems with a wire fuse failing to blow - although they can get tired and cut your supply when they shouldn't.
     
  16. Jun 19, 2017 #15

    anorlunda

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    Thanks @Nidum and @sophiecentaur . I always enjoy learning.


    Most means some. How do they decide which are and are not?
     
  17. Jun 19, 2017 #16

    sophiecentaur

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    Not in the case of a star or ring main. How would the ring need any protection from a faulty appliance that is drawing 25A, for instance? The house cabling is thick enough and there is a 32A fuse. The plug fuse is appropriate for the appliance and its lead.
    In fact, thinking more about it, any supply distribution system will have plenty more capacity than the individual legs will ever need. There is a 100A fuse at the input of UK houses. The 3 phase supply can handle many houses on the same phase with each house taking near its maximum. A serious fault in the house, involving, say 15kW of power would hardly be noticed by the supply.
    I have to agree, of course, that the company need to avoid an overload but that involves much more current than a single house wiring could handle. Bigger planned loads need to have a special supply, arranged with the company (3 phase, usually).
     
  18. Jun 19, 2017 #17

    sophiecentaur

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    For 'most' read 'all'. Fuseless square pin plugs do not exist in the UK (except, possibly as illegal imports but there would be no point). I think I can say that with certainty. (is that an oxymoron?). The last time that fuseless plugs were used for major appliances, it was 15A round pins, which were phased out in the '50s.
    Edit: All my remarks above, relate to Ring Mains. Any appliances taking more than 13A (and immersion heaters) are fed individually from the fuse box.
     
    Last edited: Jun 21, 2017
  19. Jun 19, 2017 #18

    Nidum

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    upload_2017-6-19_11-43-16.png

    This type of wall socket is the one used almost universally with the ring mains that @sophiecentaur described above . The plug goes with the socket . There are other types of plug and socket but they are rarely used now .
     
  20. Jun 19, 2017 #19

    anorlunda

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    What about minor appliances like a cell phone charger, or a LED night light?
    image.jpeg


    image.jpeg
     
  21. Jun 19, 2017 #20

    sophiecentaur

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    They are autonomous and, if they are type approved, then they will have their own internal protection. ("No user serviceable parts inside" - but I have tried on occasions) I think the lower picture is of a US socket?. The very low power devices catch fire relatively often, I believe. eBay -type imports could have absolutely anything inside them - even just a high value resistor and a diode and the authorities can't really do anything about that.
    There are no Outlets on a ring main that do not include a fuse - ' a fused spur' has the fuse integral and a fixed cable, feeding a permanent appliance or possible an alternative non-fused plug. The standard 13A plug is a big beast and it takes up a lot of wall space. That's the only downside of it I can think of. It is possible to feed a compact distribution panel from a fused spur on the ring. Nerdy hifi and computer types will use such a system in a 'home studio' environment where there are dozens of bits of low power equipment. But the easier alternative is to use a multiway extension.
     
    Last edited: Jun 19, 2017
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