On 01/01/2020 13:07, Scott wrote:

Okay, but any sensible person would replace like for like. You could
say the same about 3A fuses.

That works on the assumption that most people are sensible.

There are many who would happily keep trying the next larger size fuse
until it 'worked'.

On Wed, 1 Jan 2020 14:39:45 +0000, Mike Clarke
wrote:

On 01/01/2020 13:07, Scott wrote:

Okay, but any sensible person would replace like for like. You could
say the same about 3A fuses.

That works on the assumption that most people are sensible.

There are many who would happily keep trying the next larger size fuse
until it 'worked'.

So all appliances are designed to be safe with a 13 amp fuse? The
flex on the lamp I bought from John Lewis does not look to me that it
could carry 13 amps but I could be wrong. I won't be trying it out.

On 01/01/2020 14:54, Scott wrote:
So all appliances are designed to be safe with a 13 amp fuse?

Generally yes - for some value of 'safe' in that many appliances will
self destruct and go open circuit before the fuse goes.

On Wednesday, 1 January 2020 14:54:33 UTC, Scott wrote:
On Wed, 1 Jan 2020 14:39:45 +0000, Mike Clarke
wrote:
On 01/01/2020 13:07, Scott wrote:

Okay, but any sensible person would replace like for like. You could
say the same about 3A fuses.

That works on the assumption that most people are sensible.

There are many who would happily keep trying the next larger size fuse
until it 'worked'.

So all appliances are designed to be safe with a 13 amp fuse? The

no, though it's often said. Most is though.

flex on the lamp I bought from John Lewis does not look to me that it
could carry 13 amps but I could be wrong. I won't be trying it out.

It can, as I found out, but not really safely.


NT

On 01/01/2020 14:54, Scott wrote:
On Wed, 1 Jan 2020 14:39:45 +0000, Mike Clarke
wrote:

On 01/01/2020 13:07, Scott wrote:

Okay, but any sensible person would replace like for like. You could
say the same about 3A fuses.

That works on the assumption that most people are sensible.

There are many who would happily keep trying the next larger size fuse
until it 'worked'.

So all appliances are designed to be safe with a 13 amp fuse?

Yes

The flex on the lamp I bought from John Lewis does not look to me that it
could carry 13 amps but I could be wrong.

See the example I posted higher up the thread.

Contrary to common expectations, that flex is likely to be
conservatively oversized when protected by a 13A fuse.

Fuses can be called upon to provide fault current protection and/or
overload protection.

In the case of a lamp there is no likely or plausible situation that
would result in a significant and prolonged overload, since the largest
lamp it is designed to take would probably be a 100W incandescent bulb,
and that has a maximum draw of under half an amp.

So the fuse only has to worry about fault protection.

(also remember, if you plug that lamp in Germany, it will have only a
16A MCB in the consumer unit protecting it)

I won't be trying it out.

If you read up on the concepts of fault currents as opposed to overload
current, you might be reassured.

Fuses can be used to provide protection for either, but in most cases[1]
for a modern appliance they are only required to provide protection
against fault currents. (The appliance itself should include any
protection required to mitigate against for overload current, should
that be necessary)

http://wiki.diyfaq.org.uk/index.php/...oad_curre nts

[1] Once exception being multi socket extension leads.

--
Cheers,

John.

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On Wednesday, 1 January 2020 14:39:48 UTC, Mike Clarke wrote:
On 01/01/2020 13:07, Scott wrote:

Okay, but any sensible person would replace like for like. You could
say the same about 3A fuses.

That works on the assumption that most people are sensible.

There are many who would happily keep trying the next larger size fuse
until it 'worked'.

and sometimes that is a sensible option. If your appliance has had its 13A fuse replaced with a 2A and it blows on occasion, trying a 3A is a fair thing to do by someone that doesn't understand what fuse to choose.


NT

On 01/01/2020 13:07, Scott wrote:
On Wed, 01 Jan 2020 12:37:20 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Scott wrote:
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?

Because to know what the correct size fuse is, you'd need to know the
cable size. Somewhat easier with removable plugs, but guesswork with
moulded types.

How is it 'guesswork'? The manufacturer must know when they select
the fuse.

And perhaps most only want to get the thing working again, so will fit any
fuse they have to hand, or can buy easily. Most likely 13 amp.

Okay, but any sensible person would replace like for like. You could
say the same about 3A fuses. By your logic all electrical appliances
should be fitted with a 13A cable just in case someone decides to fit
a 13A fuse at a later stage.

Which is exactly what they do...

Note "13A cable" has no meaning in this sense. Since we are talking
about fault current situations and not continuous current handling
capacity. i.e. you will need a flex with at least 1.5mm^2 conductors to
carry 13A continuously. However a 0.5mm^2 flex may still be adequately
protected by a 13A fuse while carrying 150A of fault current.

What matters, is will the cable survive long enough without damage to
open the protective device. To know that you can do some arithmetic to
prove it.

For example, say you have a device with 1.8m of lead, and the flex has
0.5mm^2 CSA conductors. Lets assume its protected by a 13A fuse, and the
plug is in the furthest socket from the consumer unit on the circuit. To
make matters worse, let's assume the circuit itself only just meats the
maximum allowable earth loop impedance at that socket for a circuit
protected by a B32 MCB.

So let's take the circuit earth loop impedance at the socket as 1.37
Ohms[1].

We can take the loop impedance of the flex at around 87 mOhms/m, or 0.16
Ohms total. So if we add those, we get a total of 1.53 Ohms at the
appliance end of the "3A" flex.

So if we have a fault right at the appliance end, that will give a
prospective fault current of around 230 / 1.53 = 150A

Now we look at the fusing time for a 13A BS1362 fuse[2]. At 150A that's
nicely down in the 0.01 second range or less. So the last step is to
work out what the minimum cross sectional area of cable is needed to
survive that 150A of fault current for the duration. We can get this
using the adiabatic equation [3]:

s = sqrt( I^2 x t ) / k

where k is a constant dictated by the flex construction - we will take
it as 115 for a PVC flex. Plug the numbers in and we get:

s = sqrt( 150^2 x 0.01 ) / 115 = 0.13 mm^2

Hence we can conclude that our 0.5mm^2 CSA conductor flex is plenty
large enough to withstand the fault when protected by a 13A fuse.







[1]
http://wiki.diyfaq.org.uk/index.php/...Loop_Impedance


[2] http://wiki.diyfaq.org.uk/index.php/...FusingTime.png

[3]
http://wiki.diyfaq.org.uk/index.php/...diabatic_Check


--
Cheers,

John.

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| Internode Ltd - http://www.internode.co.uk |
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\================================================= ================/

On Wednesday, 1 January 2020 12:40:59 UTC, Dave Plowman (News) wrote:
In article ,
Scott wrote:

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?

Because to know what the correct size fuse is, you'd need to know the
cable size. Somewhat easier with removable plugs, but guesswork with
moulded types.

And perhaps most only want to get the thing working again, so will fit any
fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the load ie the appliance.


NT

In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

--
*Why do the two "sanction"s (noun and verb) mean opposites?*

Dave Plowman London SW
To e-mail, change noise into sound.

On Fri, 03 Jan 2020 14:26:51 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

What appliances do not need a fuse?

On Friday, 3 January 2020 16:15:15 UTC, Scott wrote:
On Fri, 03 Jan 2020 14:26:51 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

What appliances do not need a fuse?

Aga cooker. Gas ascots. My electric toothbrush doesn;t have a fuse, and niether does the charging station (that I know of) the 2 pin plug goes into an atapter
which does have a 3 amp fuse in it IIRC, same as with my shaver.

But I guess it does depend on what you mean by an appliance, and whether that
includes what powers the appliance(s)

"Scott" wrote in message
...
On Fri, 03 Jan 2020 14:26:51 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

What appliances do not need a fuse?

Kettles, toasters, convection ovens, irons etc etc etc.

On 03/01/2020 16:15, Scott wrote:
On Fri, 03 Jan 2020 14:26:51 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

What appliances do not need a fuse?

A table lamp would be a resonable example. No typical use case where it
could cause an overload. So all you need worry about is fault protection
for the cable.

(There are times you can even quite legitimately wire a fixed circuit
without including overload protection[1])

[1]
http://wiki.diyfaq.org.uk/index.php/...istics_of_load



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
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On Friday, 3 January 2020 14:36:02 UTC, Dave Plowman (News) wrote:
In article ,
tabbypurr wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

Start by doing some reading. You'll soon find yourself in posession of a clue.

On 03/01/2020 14:26, Dave Plowman (News) wrote:
In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on the
load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If the
appliance needs a fuse it will be in or on it.

Although NT could have worded that better, what he is saying is a
reasonable assessment if you start from the principle that any modern
appliance will have a flex fitted that will still fail safe when plugged
into a 16A protected circuit with no additional fusing, as would be the
case in the rest of Europe.

Then the fuse selected (if one is not just defaulting to 13A) will then
depend on the appliance load. E.g it would not be appropriate to fit a
3A fuse to the plug of your Washing Machine or Kettle.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In article ,
John Rumm wrote:
On 03/01/2020 14:26, Dave Plowman (News) wrote:
In article ,
wrote:
And perhaps most only want to get the thing working again, so will fit
any fuse they have to hand, or can buy easily. Most likely 13 amp.

The correct fuse doesn't depend on cable size at all, it depends on
the load ie the appliance.

********. The fuse in a plug is only there to protect the cable. If
the appliance needs a fuse it will be in or on it.

Although NT could have worded that better, what he is saying is a
reasonable assessment if you start from the principle that any modern
appliance will have a flex fitted that will still fail safe when plugged
into a 16A protected circuit with no additional fusing, as would be the
case in the rest of Europe.

Correct.

Then the fuse selected (if one is not just defaulting to 13A) will then
depend on the appliance load. E.g it would not be appropriate to fit a
3A fuse to the plug of your Washing Machine or Kettle.

But perfectly safe.

If the internals of a device need protection by a fuse - like many
electronic devices, it will be done so at the device itself - and not rely
on the plug fuse. Becuase virtually nothing is made for the UK only, and
very few countries have plug top fuses.

--
*Yes, I am an agent of Satan, but my duties are largely ceremonial

Dave Plowman London SW
To e-mail, change noise into sound.