On 08/11/2012 07:52, harry wrote:
On Nov 7, 10:35 pm, John Rumm wrote:
On 07/11/2012 20:20, Graham. wrote:









On Wed, 7 Nov 2012 10:40:34 -0800 (PST), David Robinson
wrote:

Not my question, or belief, but found here...

http://forums.digitalspy.co.uk/showthread.php?t=1755197

Is there a counter-argument?

Thanks,
David.

Here's a counter argument I made a couple of years ago in response to
those that say it's only there to protect the cable.

[It] May not be mandated for appliance protection, but isn't it true
to say that choosing
a plug fuse value sensibly having regard for the maximum current
demand of
the appliance rather than than just the capability of the cable has
always been good practice?

Indeed - but its not a counter argument. The logic Humax (or whoever)
are applying is that the 13A fuse is adequate to protect the flex. That
is demonstrably true.

However what you say is also true, and in no way conflicts with that.

More closely matching the actual size required will mean that when
things do go pear shaped, the actual energy let through may be a little
lower. Less spark, less heating stress on the cables etc.

The appliance manufactures seem to think so.

Take the case of three items I regularly install, all with supplied
0.75mm2 IDE cables

Strange place to be using a disk interface cable ;-)

(IEC perhaps?)

A Dell PC with a 5A fuse in the plug
A Cisco router with a 10A fuse
A Brother laser printer with 13A fuse.

Of course this might now create issues with interchangability.
I would be interested to see your comments about the above.

For stuff running with "normal" IEC leads its does not make any real
difference. The appliance will have an internal fuse if it needs it, and
the cables are quite sturdy (assuming they are not Chinese knock offs!)
The kit will have been designed to run safely on 16A circuits with no
plug fuse (as it will need to in most countries) so the flex will be
specced to suit.

IME leads for IT kit doe get swapped about without any regard for their
rating (most people have not even spotted there are two different specs
of lead and fuse). So 3A fusing may be pushing your luck depending on
what it gets connected to.


Not all faults are dead shorts. Transformer overheat/burnout for
example.

For the purposes of this discussion they are (its in the definition of
"fault current")

Transformers are unlikely to be able to present any sustained load above
their design maximum for long - the wire size on their primary winding
will typically be *significantly* thinner than that of the supply flex.

Not all equipment has internal fuses.

If it needs overload protection then it is required to have its own
protection. Remember in most markets, protection via a plug fuse is not
an option.

3Kw/13a of sustained energy in an electronic device/small electric
motor could start a nice little fire.

Which would be an overload and not a fault. If there is a plausible
overload scenario, then the device in question must include its own
protection.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/