On 29/12/2019 13:01, Scott wrote:
Fuses are available as 1A, 2A, 3A, 5A, 7A, 10A and 13A yet only 3A and
10A seem to be in common use.

3A, and 13A were typically the most common. (and originally the only
other commonly available value was 5A).

There seems have been a growth in use of 10A however - probably down to
the flood of dubious "13A" accessories and extension leads landing here
from the middle kingdom.

(Multiway extension leads being one of the few cases where the fuse
might have to offer overload protection)

I've been checking some fuses (when I got bored over Christmas!) and

You know how to have fun!

many seem to be far higher than needed, most notably a printer fitted
with a 10A fuse when the rated current is 2.5A. I changed this to 5A
(to leave some headroom). The toaster does not need 13A; 7A is fine.
LED lamps do not require 3A. 1A seems fine.

Most appliances are designed with leads that will still have adequate
protection from a 13A fuse, since the designer must assume that is what
the less well informed may well fit.

Note that in most cases the protection being discussed is "fault" (i.e.
short circuit) protection and not overload. In those cases, so long as
the fuse blows quickly enough, the actual rating is not really that
important. However not "false" blowing on inrush etc, is more important.

I appreciate that the fuse is intended to protect the lead not the
appliance, but surely there is a side-effect of protecting the
appliance? Is there any benefit in fitting a fuse significantly
larger than needed? Common sense suggests go for maximum protection.

Keeps the production line easier, and smaller inventory of parts etc.
However your basic point is correct, you *might* get a "better" result
with a more closely matched fuse. (where better is hard to assess -
perhaps a lower fire risk in some cases, or perhaps a better sense of
satisfaction of having done it "right)

I appreciate that motors have a surge at the start. I believe a 50%
margin should be added generally (more for a motor).

Depends on the motor, inrush on some induction motors can be 9x nominal.

When the ring main system was introduced (in 1947, I believe), the
correct fuse was used for each appliance. Why have we moved away from
this commonsense arrangement and apparently degraded a safety feature?

In those days appliances may not have had adequate internal protection,
and so relied on the plug fuse. Also many of the flexes would not have
had adequate fault protection with a 13A fuse.



--
Cheers,

John.

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