Date   : Wed, 19 Aug 2009 20:39:46 +0100
From   : afra@... (Phill Harvey-Smith)
Subject: Retro Reunited: Econet Cabling, Demos, etc.

Sprow wrote:
> In article <20090819162105.11227h369vq8ltgc@...>,
>    <afra@...> wrote:
>> Quoting Darren Grant <darren.grant@...>:
> 
>>> On 19/08/2009 13:59, "Ian Stocks" <bbcmicro@...> wrote:
>>>
>>>> Whilst we are talking about cabling. Any one know what happens about
>>>> mains distribution? Here at work, our H&S guy has a fit if you
>>>> daisy-chain multi-way extensions. I pressume they dont care at the show.
>>> It doesn't really matter as long as they are rated to carry 13A then the
>>> fuse will blow before you can overload anything. You could have 100 
>>> sockets chained together and it wouldn't be a problem as they all go 
>>> through a single 13A fuse.
> 
> No you can't - as the earth return will exceed permitted resistance.

I asked my Wife to briefly look at this as she is deputy saftey officer 
for the university department that I work in, acording to her research 
daisy chaining wasn't banned, but was frowned upon, and it was 
considdered better to say have 4x 4 blocks connected to a 5th and then 
to the wall in a tree formation rather than one into another in a long 
chain. Which would I guess reduce the earth return resistance from each 
socket. Also it was sugested that if you do daisy chain that you fit an 
RCD where it goes into the wall.

>> Whilst in principle I agree with you that's not the whole story, each  
>> thing plugged into the chain also has to be correctly fused too, so  
>> that if it develops a fault which causes it to draw too much current,  
>> it will blow it's fuse, ifen if it's not enough to blow the 13A one in  
>> the main plug. This is why I tend to fit 1A fuses to most of my retro  
>> gear as it's normally all they need.
> 
> The fuse exists to protect the flex to the appliance, the appliance itself
> should have a fuse appropriate to its use (for example, if the appliance has
> a 500mA PSU it should have a 500mA fuse in it, but might be wired up with 5A
> cable so could reasonably have a 5A fuse in the plug).

Though in reality the fuse should be slightly higher than the PSU rating 
to allow for inrush current etc. A common mis-conception is that the 
fuse is there to protect you, it's not, it's there to stop the equipment 
  drawing too much current and catching fire.

> Fitting a lower fuse rating than the flex rating is OK, particularly if the
> internal fuse is hard to get at!

Indeed, one of the things that really annoys me is that new pluhs are 
generally supplied with a 13A fuse so people who don't know any better 
make the asumption that this is what is needed and fit it, even if the 
equipment only needs 500mA. Take into account also that for most mains 
fuses there is typically a time/current curent curve that determines 
when they fail, so draw say 14A from a 13A circuit and it may take a 
while to fail, the more current over the rated value the quicker it fails.

Cheers,

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.