Are 3A plug fuses really necessary? Why not always 13A?

[David Robinson]

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

there a counter-argument?

Thanks, David.

[John Williamson]

It depends on the cross sectional area of the cable cores. As is correctly pointed out in the thread, the fuse is to protect against cable faults, not internal faults in the box. If the cores are too small, a 13 A fuse may not blow or carry enough current to rapidly trip the MCB, leading to a cable that at the best is going to melt the insulation, and at worst may start a fire.

[GB]

Just anecdotally, I had a decorator in my house using a steam stripper. He asked to borrow an extension lead, and I conscientiously checked the extension lead was man enough for the job, then left him to it. Half an hour later, the power went off.

Investigating, I found that he hadn't bothered to unreel more of the extension cable than he needed to reach the job - despite clear instructions on the reel to that effect. The entire lot inside the reel had melted, and the power went off when the cores touched. This was a man who had been doing this sort of work for over 10 years, mind.

[Hugh - in either England or Sp]

Have to admit I have done the same. Stripper well within reels capability but it cut out after a while. My reel has an overheat cut-out fortunately. Lesson learned for all types of wiring.

If I take anything bought in Spain back to the UK I will have to change the plug or use it through a correctly fused adaptor. Using them unfused on a 30A ring main could be a recipe for disaster.

Worries me sightly that lamps and other itmes out here with thin flex plug into fairly highly rated power sockets.

[Graham.]

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?

The appliance manufactures seem to think so.

Take the case of three items I regularly install, all with supplied

0.75mm2 IDE cables

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.

[meow2222]

All modern appliances and leads are required to be safe with a 13A fuse. However the reality is that fires do happen, and there are at least some faults where a

3A would blow before a 13A, so maybe preventing a small percentage of fires.

Old appliances are another matter, and many are not properly protected by a 13A fuse.

So its better to keep using 3A where suitable.

NT

[ARW]

I am sure that some boiler installation manuals state the use of a 3A fuse - and there is no legal reason why a boiler cannot be powered using a plug and unswitched socket......

[Bill Wright]

13A fuses can go with a right bang, frightening old ladies. 3A fuses are nicer.

Bill

[polygonum]

Please can you provide a reference to when that changed? And, ideally, what the requirements actually are - in an accessible form.

[Dave Liquorice]

That would be interesting.

The big bad assumption made in the digital spy thread was to assume only short circuit faults and that massive currents will flow. In that particular case there is little difference between a 3A or 13A fuse.

However if the short circuit current is only high, maybe limited by weak spring tension in the fuse holder, corosion of the holdler/fuse cap, dodgy wire termination some where, and say only 20A is flowing. Thene there is a significant difference between a 3A and 13A fuse.

[Dave Liquorice]

And was his comment "That's odd. The last time I used a cable reel like that one, the same thing happened".

I hope he paid for a replacement.

[The Natural Philosopher]

Or more to the point, under overcurrent, which would you rather have go? a thin piece of flex with plastic insulation lying over a newspaper, or a fuse in a plug.

[John Rumm]

Counter to what?

Frank1 is obviously missing a clue.

grahamthompson and Nigel Goodwin seem to have explained more than adequately.

Its easy enough to check. From the BS7671:2008 table 41.3[1], the maximum earth loop impedance for a circuit protected by a B32 MCB is

1.44 ohms. That is what gives a fault current on the longest path of the circuit of around 160A - or enough to open a B32 MCB on the "instant" part of its trip response.

So we could take that as an example of the minimum fault current the flex will need to withstand when someone shorts it.

Lets say the appliance has a flex with 0.5mm^2 CSA conductors. For a short flex (i.e. typically 1.8m) you can ignore the additional resistance of the flex, so can just assume the prospective fault current of 160A. Now that should open a 13A fuse in less than 0.1 secs (probably significantly less). So all you need to know is, will the flex will carry that fault current for long enough to allow the fuse to do its stuff without either vaporising or bursting into flames.

So we can work out the minimum size (s) of conductor required for that with the adiabatic equation[2] and the PVC cable type k value of 115.

s = sqrt( 160^2 x 0.1 ) / 115

s = 0.44mm^2

Which is less that the actual flex CSA of 0.5mm^2, so in this particular example the flex would actually be adequately protected by the 32A MCB at the origin of the circuit (although the fuse will probably open before the MCB gets a chance to operate)

If you try that sum on a shorter circuit - or one with a lower Ze, then you may get an ELI of say 0.5 ohms at the end of the flex. That gives a fault current of 460A which would need a conductor size of aver 1mm^2 to be protected by the main MCB, so here the fuse would be essential.

(if the circuit were protected by a BS3036 rewireable fuse, or the circuit a tad out of spec, the flex longer or thinner etc then the fuse in the plug would be essential).

[1]

[Peter Crosland]

Only the terminally stupid would use a 13A fuse when a 3A is appropriate.

[John Rumm]

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.

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

(IEC perhaps?)

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.

[John Rumm]

If there is only 20A of fault current, then there is 11.5 ohms of resistance somewhere! The > 4kW you are dissipating in the fuse holder might be more of a worry than whatever is going on with the flex ;-)

[John Rumm]

You want to see the bang with the counterfeit ones arriving with your latest mains lead from China via ebay:

[Graham.]

Damn. that will teach me to proof read my own regurgitated words. In the original thread you quipped: "You can see why SATA became popular if the IDE leads were that size"

[meow2222]

Unfortuately no, I've not kept any notes of those points. One thing it has = changed is the minimum permissible conductor cross section in appliance lea= ds, if you look at appliances from the 50s and 60s, some shavers & clocks e= tc have very thin conductors. Now all new flexes need to be able to survive= the i squared t of a 13A fuse blow.

NT

[Frank Erskine]

In the early days of colour TV I was told by a TV engineer that the correct plug fuse for a CTV was 7A to allow for the switch-on current surge of the degaussing circuitry.

I don't know whether or not "intermediate" BS1362 fuses such as 5A or

7A are still available.

In fact, was there not also a 1A BS1362 fuse, used for clock plugs, but not shaver adaptors (or was it the other way round?)?