Fuses - again

[Scott]

Fuses are available as 1A, 2A, 3A, 5A, 7A, 10A and 13A yet only 3A and

10A seem to be in common use.

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

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.

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

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?

[Mike Clarke]

In the event of an internal fault in an appliance containing electronics then the chances are that one of the components will release its smoke and permanently disable the appliance much faster than the 'correctly' rated fuse would blow. So not much point in trying to protect anything except the lead.

[Caecilius]

BS 1362 plug fuses already have a margin greater than 50% built in. A

13 amp fuse can take 20 amps for a long time before it fails.

See:

[John Rumm]

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)

You know how to have fun!

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.

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)

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

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.

[Mike Clarke]

I expect a cable rated for 13A would also survive for longer than it would take for the 13A fuse to blow.

[Scott]

Not as bad as a former colleague who complained when the managing agents replaced the manufacturer's software in the lift with generic software, which was detrimental to the user experience of the lift :-)

[Andy Burns]

Apparently showers (Riba seem to be either 8.5, 9.5 or 10.5kW) can run from a 13A FCU with some 1mm flex

[John]

or perhaps a better sense of satisfaction of having done it "right)

Good enough for me.

[The Natural Philosopher]

In general the better the motor efficiency the worse is the surge.

10x is easily achievable wioth 'good' motors.

cheap chinese motors used in e.g. car window winders are deliberately pants to that they can be stalled without burning out wires or fuses.

[Fredxx]

Doesn't his fix make a mockery of safe-zones?

[Fredxx]

Car window winders are an especially bad example. They use load sensors to ensure nothing is trapped, such as a child's arm, in the window.

What we did before regulations! Apart from a few children losing arms and suffocated children of course.

[ARW]

yes.

[John Rumm]

Looks like they pinched our scan :-)

[John Rumm]

Like true for a cable rated at 3A as well...

[Fredxx]

It would have been better to move his switch down to be horizontally in line with the cable.

Happy to show my ignorance, while 50mm from a wall corner is a safe-zone, I didn't think the same was true behind an architrave for a door way.

[ARW]

I totally agree with you.

That was a bit of messing about with the camera looking for the "joist".

Nothing that a connector rod would not have found in seconds.

Although IMHO he did well fitting a dry liner box in that wall.

[Steve Walker]

IIRC the IEC plugs commonly used on computer, hi-fi, entertainment systems, even (hot version) kettles these days, are only rated for 10A. As they may be swapped between devices, it'd make sense to make them all rated 10A and use a 10A fuse.

SteveW

[charles]

I'd be more concerned about the lead having an earth connection. One I bought off ebay recently had none.

[Roger Hayter]

I would have thought it was exactly where a nail might go, I agree.

[Andy Burns]

Didn't he saw he didn't plaster over it? So if it was left visible, no problem :-P