10 amp fuses - quick moan

[The Natural Philosopher]

15A round pin rather...

[Mark Carver]

You're right. I was only 0.56dB out though.

[Owain Lastname]

This is identical to the BS 546 15 ampere plug and socket, although in South Africa it is rated for 16 A for compatibility with the IEC 60898 standard current ratings.

Owain

[Animal]

Other foreign countries also rate our old round pin plugs at higher ampacity. I think the biggest diffrence is the US mains plug, here rated at 50v 2.5A

[John Rumm]

While I sympathise with the sentiment, you have to ask yourself what "more protection" actually means?

Will it make it respond to an overload more quickly? Yes. How likely is an overload? Pretty much nil. So what have you gained?

Will a 3A fuse clear a fault quickly? Yes. Will a 13A fuse clear a fault quickly? Yes. So what have you gained? In reality the ability to clear a "almost fault" condition more quickly - say a "nearly short". What have you lost? A bit of extra heat dissipated in the fuse, and a greater likelihood that the fuse will fail in time from thermal cycling on inrush current.

I fail to see the comparison...

Two 3kW heaters would be at most 26A (and in reality a bit less probably). The nominal design current (with diversity) for a double socket is 20A (although BS1363 requires testing at higher currents). Most of the better ones will handle 26A when clean and new. The load on one socket will generally be unable to exceed the circuit design current[1].

A ring circuit as a whole needs both overload and fault protection since the user can connect more load than the circuit is designed for. Hence the MCB has a thermal response that should mimic the diabatic heating of the circuit conductors, and cut the power should that exceed the maximum continuos maximum working temperature (typically 70 deg C).

When dealing with the spur, the MCB can no longer provide overload protection - since it will carry indefinitely a significantly higher current than the socket was designed to handle. So the responsibility for overload protection moves to the spur end of things, and with an unfused spur is achieved by design limitation (by requiring no more than one socket (single or double), or by the inclusion of a fuse (no limit on the number of sockets).

The circuit also needs fault protection, and this generally must be at the origin of the circuit. Here the MCB needs to respond to adiabatic heating quickly enough to limit the final circuit conductor temperature to around double the maximum safe continuos temperature (so 150 to 200 deg C typically depending on cable type). (The assumption made is that the circuit could already be operating at max temperature at the time the fault occurs). This is why MCBs have a magnetic response that kicks in at fault currents typically a multiple of the nominal rating of the device (so up to 160A for a B32 MCB, 320A for C32, and 640A for a D32)

[1] for the pedants, yes you could have an unbalanced ring with cables in insulation, and a couple of multi-way extension leads plugs into the two sides of a double socket.

[John Rumm]

Indeed, and that is why the spur does not need overload protection from the MCB, which is handy since at 32A it ain't going to get it!

[John Rumm]

There is, you just subtract 0.4 from the final reading :-)

[Animal]

What you gain is less chance of fire due to less energy & temp rise in the fault area. There's also a little bit of shock protection improvement.

[Mark Carver]

Never noticed that. I've just checked. I have a BS 546 plug bought here. Rated at 15A.  I have another bought in SA, rated at 16A.

I'd trust a BS546's pins to carry 16 amps a country mile more than any Schuko style's to carry even 10A !

[Scott]

As Animal pointed out, my arithmetic was at fault.

Thanks for the explanation. I did not fully understand it but I get the drift. More complicated than I thought.

[PeterC]

Inrush current is probably worst on fridge compressors. My fridge-freezer, single pot, is 125W but the current is given as 1.25A. I can't check, but it probably has a 13A fuse to deal with the starting and the flex loos like 1 mm, so a bit over-fused. I remember a fridge repairer asking if slow-blow BS 1362 fuses were available. I couldn't find anything in all my catalogues (mid-80s).

Now, coincidentally, getting a 13A fuse out for a new device I found one that had 'Type C' on it and wondered if it was somewhat delayed (brand on it is Pifco and it's certainly '80s at youngest).

[whisky-dave]

Yes we bought about 50 for the lab, very useful but we know what they will be used for.

Which they can be when used properly.

[Dave Plowman (News]

If a 13amp fuse doesn't blow at 26 amps continuous, if will have the same failing no matter what comes after it in the form of connectors.

[Dave Plowman (News]

By load I meant load, rather than fault condition.

[John Rumm]

You seem to have misunderstood the concept of fault protection. If the system has adequate fault protection, there is no risk of fire. That is the whole point of fault protection - to limit the let-through energy to something that will not cause a dangerous level of heating.

At fault current levels, any BS1362 fuse will have a fusing time well under 40 ms.

The biggest risk is from a intermediate level of over current - but not enough to be classed as a fault current, or from an arcing connection.

In that situation the lower rated fuse may perform better.

[John Rumm]

Yup, usually induction motors - and those bring two issues you need to deal with. The inrush is typically 5 to 9x the nominal run current. Also they may have a low power factor, which will make the apparent power higher, and hence the current draw higher as well (some of that current is in effect "given back" later in the cycle mains, but is still in the form of real amps that the circuit cables have to supply).

I have never seen anything described as such - although you could argue that the standard fuse is pretty slow blow - especially as it has a sand filling to air arc quenching.

It might have just been indicating that they have a fault current response more like that of a type C MCB than a type B.

[John Rumm]

Fairy snuff... it was the word "peak" in there that suggested something else to me.

[SteveW]

But the first IEC connector in the chain will be very overloaded and liable to overheat - which was the point raised earlier. A 10A fuse would likely have blown.

[Owain Lastname]

Or, if you buy your fuses from less-than-reputable suppliers, "if" it has a sand filling.

Owain

[PeterC]

That's what I wondered, but there were no other fuses like that. It came from Plessey Microsystems - lots of things were re-fused during checks as the electronic 'engineers' (oxymoron) had a habit of using 13A fuses even on

0.5 mm flex. The sparkies would do some and I checked anything that I hadn't seen before - in one case I just cut of the entire lead as the whole thing was dangerous!