I've just be taking a look inside one of them low power night lights. The kind
that has a 13 amp plug as part of its construction so just plugs into a 13 A
socket. It consists of a small pcb that does all the switching logic and drives
a high-brightness led. It has a CE mark and is double insulated. Thing is, there
is no sign of a fuse inside the unit. (Not that I can see anyway). So afaik we
have a 0.6W device protected by the ring main rated mcb (15A/30A?) + RCD (30mA).
So, 1) the device in normal operation is taking less than the trip current of
the RCD (~2.5mA : 30mA ie 12 times less) 2) a fault in the device would easily
melt the wires etc within in before getting anywhere near 15 or 30A.
Just some thoughts on safety anyway. Make any sense?

"mike" <mike@mail.nomail> wrote in message
news:eud0i193ugosoh17n086qds09qlucdsivg@4ax.com...

> Thing is, there
> is no sign of a fuse inside the unit. (Not that I can see anyway).
....
> 2) a fault in the device would easily
> melt the wires etc within in before getting anywhere near 15 or 30A.


Sounds like a fuse to me...

On Thu, 8 Sep 2005 14:43:30 +0100, "Andrew" <-> wrote:


>
>"mike" <mike@mail.nomail> wrote in message
>news:eud0i193ugosoh17n086qds09qlucdsivg@4ax.com...
>> Thing is, there
>> is no sign of a fuse inside the unit. (Not that I can see anyway).
>...
>> 2) a fault in the device would easily
>> melt the wires etc within in before getting anywhere near 15 or 30A.
>
>Sounds like a fuse to me...


You mean the wiring within the unit is designed to melt like a fuse!!!! I don't
think I can belive that one.

In article , 
mike@mail.nomail says...

> I've just be taking a look inside one of them low power night lights. The kind
> that has a 13 amp plug as part of its construction so just plugs into a 13 A
> socket. It consists of a small pcb that does all the switching logic and drives
> a high-brightness led. It has a CE mark and is double insulated. Thing is, there
> is no sign of a fuse inside the unit. (Not that I can see anyway). So afaik we
> have a 0.6W device protected by the ring main rated mcb (15A/30A?) + RCD (30mA).
> So, 1) the device in normal operation is taking less than the trip current of
> the RCD (~2.5mA : 30mA ie 12 times less) 2) a fault in the device would easily
> melt the wires etc within in before getting anywhere near 15 or 30A.
> Just some thoughts on safety anyway. Make any sense?
> 
> 

Fuses only really protect power leads, and as the device doesn't have a 
power lead (and it's going to fail open circuit long before the mains 
wiring gets warm) I don't think you need to worry  :-)

Sometimes a weakness in a PCB link would act as a low current fuse...

On Thu, 8 Sep 2005 15:33:38 +0100, Rob Morley  wrote:


>In article , 
>mike@mail.nomail says...
>> I've just be taking a look inside one of them low power night lights. The kind
>> that has a 13 amp plug as part of its construction so just plugs into a 13 A
>> socket. It consists of a small pcb that does all the switching logic and drives
>> a high-brightness led. It has a CE mark and is double insulated. Thing is, there
>> is no sign of a fuse inside the unit. (Not that I can see anyway). So afaik we
>> have a 0.6W device protected by the ring main rated mcb (15A/30A?) + RCD (30mA).
>> So, 1) the device in normal operation is taking less than the trip current of
>> the RCD (~2.5mA : 30mA ie 12 times less) 2) a fault in the device would easily
>> melt the wires etc within in before getting anywhere near 15 or 30A.
>> Just some thoughts on safety anyway. Make any sense?
>> 
>> 
>Fuses only really protect power leads, and as the device doesn't have a 
>power lead (and it's going to fail open circuit long before the mains 
>wiring gets warm) I don't think you need to worry  :-)


I'm not worried - just interested - it seemed a little incongruous that's all.

mike wrote:


> >> I've just be taking a look inside one of them low power night lights. The kind
> >> that has a 13 amp plug as part of its construction so just plugs into a 13 A
> >> socket. It consists of a small pcb that does all the switching logic and drives
> >> a high-brightness led. It has a CE mark and is double insulated. Thing is, there
> >> is no sign of a fuse inside the unit. (Not that I can see anyway). So afaik we
> >> have a 0.6W device protected by the ring main rated mcb (15A/30A?) + RCD (30mA).
> >> So, 1) the device in normal operation is taking less than the trip current of
> >> the RCD (~2.5mA : 30mA ie 12 times less) 2) a fault in the device would easily
> >> melt the wires etc within in before getting anywhere near 15 or 30A.
> >> Just some thoughts on safety anyway. Make any sense?



Fuses come in a few different forms, some of which look nothing like
traditional glass or ceramic fuses. But these types of products usually
use a safety resistor instead. This is a series resistor that does 2
jobs.
1. it reduces inrush current
2. if it gets too hot it fails safely open circuit, thus acts like a
slow thermal fuse.

These effectively give very low current fusing.


Thin PCB tracks will fuse too, but they would be limited to LV use, you
wouldnt want to do that for mains fusing. A fuse wire link soldered
into the pcb will have the same problem, burnt pcb would tend to bridge
the fuse, so its not good enough on mains.


NT

In article ,
   mike <mike@mail.nomail> wrote:

> I've just be taking a look inside one of them low power night lights.
> The kind that has a 13 amp plug as part of its construction so just
> plugs into a 13 A socket. It consists of a small pcb that does all the
> switching logic and drives a high-brightness led. It has a CE mark and
> is double insulated. Thing is, there is no sign of a fuse inside the
> unit.


Likely, the PCB itself would fail close to the pin connections if there
was some form of overload.

-- 
*I almost had a psychic girlfriend but she left me before we met *

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.