On 08/11/2012 12:51, wrote:
On Thursday, November 8, 2012 10:46:10 AM UTC, John Rumm wrote:
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 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")

Welcome to the strange world of Aurthur Brown. The wiring regs use
'fault current' to specifically mean a dead short, but obviously in
the real world appliance faults occur that produce current (fault
current by definition) that covers the full range from a smallish
increase than only overheats a motor to a dead short.

Indeed. However the wiring regs separate these out into overload and
fault since they need to be treated separately.

Introducing a
narrow perspective definition that applies specifically to wiring and
using it outside of where it makes good sense is not productive, and

Except in this case we are not using it outside. The link the OP
referred to was a very specific question about whether a small
electronic device can be adequately "protected" by a 13A fuse. The
answer to which is yes. The protection that the fuse is there to offer
is to limit the effects of fault current - i.e. stopping the mains lead
bursting into flames, vaporising and spewing hot metal and plastic about
the place etc when someone sticks a metal chair leg through it. Once you
can demonstrate that you have achieved that level of protection, then
your job (and that of the fuse in the plug) is done.

This is a completely separate issue from minimising damage to the device
itself because some internal motor got latched on when it should not
have, and its now sucking more power than it ought. Yes that is a
"fault" in the device, and that may result in overcurrent (wiring regs
use of the term). If its a problem, then the designer can do something
about it.

insisting it must mean that elsewhere is illogical. In the world of
electronics (which many appliances are) fault current does not
usually mean a dead short across the mains.

True, but not relevant to the point under discussion.

If you want to protect electronics you are designing now from damage
resulting from overload, then you need a fuse (or thermal cutout etc).
However you can't rely on the one in the plug, since outside the UK it
won't be there.

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.

In practice not all has, particularly old appliances

Remember in most markets, protection via a plug fuse is not an
option.

Old UK appliances were typically designed for only the UK market

Indeed. And for those, a lower rating fuse may be required in the plug.

Don't confuse the situation as it applies to fault current protection
for the flex on a new device one is putting onto the market in the EU
now, with old kit built to different requirements and standards.

Also don't assume that just because a 13A fuse will meet the fundamental
requirement for fault protection on new kit that there is no benefit you
using a lower value fuse in some cases. There might be.

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.

But in practice a fair few don't. There are still significant numbers
of old electrical goods in use spanning the range from pre-95 to
1920s. And not all new goods meet the standards legally required of
them.

As I said, in these cases if the maker said the appliance must be
protected by a 3A fuse then so be it. If they designed it to rely on a
3A fuse in a plug, that is fine, use one and not a 13A. They would not
be able to do it now (for a product to be marketed internationally) but
it does not mean it did not happen in the past.

Fault current protection is a minimum requirement - something that must
be taken into account and protection put in place for. None of that
precludes doing more than the minimum.




--
Cheers,

John.

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