In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).


I'm not worried. I just don't see the logic.


I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.
However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

?

Cheers,
David.

On 15/12/10 19:54, David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).


I'm not worried. I just don't see the logic.


I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.
However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

?

Cheers,
David.

Fuse in the plug at max of 13A.

Odd as it may seem, overload protection *can* exist downstream as well
as upstream.

In the case or a short circuit in the back of the socket, the 32A
breaker will still protect the cable (ie interrupt the current before
the cable overheats).

--
Tim Watts

David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).


I'm not worried. I just don't see the logic.


I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.
However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

The short version -

You have to consider the difference between overcurrent protection and fault
protection.

If an MCB is used for overcurrent protection then the circuit has to be
capable of running at the full current allowed by the MCB. For a B type MCB
this is 1.45 times the rating of the MCB.


If the MCB is to be used only for fault current protection (ie a short
circuit) then the cable only needs to be able to pass the short circuit
current long enough to allow the MCB to trip within the required times when
there is a fault.


A ring from 2.5 T&E is capable of meeting the 32 x 1.45 current overload
protection that a B type MCB provides.

Now a spur from a ring is "self" overcurrent protecting as is is only
allowed to feed 1 single socket or 1 double socket. The maximum normal
current that you get down this spur is 26A (assuming a double socket loaded
to the maximum) which is less than the 27A capacity of 2.5 T&E when clipped
direct (or buried in plaster) so there is no overcurrent problem and the MCB
is now only needed for short circuit protection to protect the spur.
However if there is a short on the spur (nail, angle grinder etc) then the
resistance reading at the end of the spur must be low enough to trip the MCB
in the times required for a short circuit. The resistance reading is made up
from the supply impedance plus the cable impedance, so the longer the length
of the spur the greater the chance that the spur is not compliant and may
not clear a short in the required time.

--

Cheers

Adam

On Dec 15, 7:57*pm, Tim Watts wrote:
On 15/12/10 19:54, David Robinson wrote:



In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.
However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

?

Cheers,
David.

Fuse in the plug at *max of 13A.

Odd as it may seem, overload protection *can* exist downstream as well
as upstream.

In the case or a short circuit in the back of the socket, the 32A
breaker will still protect the cable (ie interrupt the current before
the cable overheats).

Thanks Tim.

So on a 20A radial, the 20A is to protect against overloads (lots of
"up to 13A" loads, potentially causing damage over time without ever
tripping a 32A) whereas a short circuit would generate enough current
to trip a 32A before the cable suffered harm?

Where can I find that calculation? I saw those MCB and fuse trip-time
vs current graphs the posted the other week, but can't remember
reading a shot circuit disconnect time requirement.

Do I need to change my 40A to a 32A, or leave it as it is?

Cheers,
David.

On Dec 15, 8:25*pm, "ARWadsworth"
wrote:
David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.
However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

The short version -

You have to consider the difference between overcurrent protection and fault
protection.

If an MCB is used for overcurrent protection then the circuit has to be
capable of running at the full current allowed by the MCB. For a B type MCB
this is 1.45 times the rating of the MCB.

If the MCB is to be used only for fault current protection (ie a short
circuit) then the cable only needs to be able to pass the short circuit
current long enough to allow the MCB to trip within the required times when
there is a fault.

A ring from 2.5 T&E is capable of meeting the 32 x 1.45 current overload
protection that a B type MCB provides.

Now a spur from a ring is "self" overcurrent protecting as is is only
allowed to feed 1 single socket or 1 double socket. The maximum normal
current that you get down this spur is 26A (assuming a double socket loaded
to the maximum) which is less than the 27A capacity of *2.5 T&E when clipped
direct (or buried in plaster) so there is no overcurrent problem and the MCB
is now only needed for short circuit protection to protect the spur.
However if there is a short on the spur (nail, angle grinder etc) then the
resistance reading at the end of the spur must be low enough to trip the MCB
in the times required for a short circuit. The resistance reading is made up
from the supply impedance plus the cable impedance, so the longer the length
of the spur the greater the chance that the spur is not compliant and may
not clear a short in the required time.

brilliantly clear - thanks Adam.

Cheers,
David.

David Robinson wrote:
On Dec 15, 7:57 pm, Tim Watts wrote:
On 15/12/10 19:54, David Robinson wrote:



In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single
bit of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it.
(IIRC they even dropped the 2.5mm rating down to 18A, which is
"correct" in accordance with the calculations, but then increased
it back to 20A because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker
circuit (feeding a socket that can only supply the gas hob
ignition) and that seems very wrong too - but wiring a socket in
6mm is just too painful. However, the cooker circuit may yet drop
back to 32A (because I suspect that's all it needs) which makes it
no worse than the socket spur discussed above.

?

Cheers,
David.

Fuse in the plug at max of 13A.

Odd as it may seem, overload protection *can* exist downstream as
well as upstream.

In the case or a short circuit in the back of the socket, the 32A
breaker will still protect the cable (ie interrupt the current before
the cable overheats).

Thanks Tim.

So on a 20A radial, the 20A is to protect against overloads (lots of
"up to 13A" loads, potentially causing damage over time without ever
tripping a 32A) whereas a short circuit would generate enough current
to trip a 32A before the cable suffered harm?

NO. A 20 radial is a circuit with a 20A MCB. A spur from a 32A ring is a
different thing. ( my other post did not make that one clear)


Where can I find that calculation? I saw those MCB and fuse trip-time
vs current graphs the posted the other week, but can't remember
reading a shot circuit disconnect time requirement.


http://wiki.diyfaq.org.uk/index.php?title=Fuse

for the graphs.


Do I need to change my 40A to a 32A, or leave it as it is?

I would change it. Far easier than doing calculations. However I would not
use the 2.5 T&E for use with an electric cooker without making some very
accurate calculations.


--
Adam

"ARWadsworth" wrote in message
...


Do I need to change my 40A to a 32A, or leave it as it is?

I would change it. Far easier than doing calculations. However I would not
use the 2.5 T&E for use with an electric cooker without making some very
accurate calculations.

Its self protecting exactly as a spur on a 32A ring is (it only has a single
13 socket).
I would change it too, most electricians would reject it as they don't know
how to do the calculations and its just easier to use the regs than to
explain why its OK.

dennis@home wrote:
"ARWadsworth" wrote in message
...


Do I need to change my 40A to a 32A, or leave it as it is?

I would change it. Far easier than doing calculations. However I
would not use the 2.5 T&E for use with an electric cooker without
making some very accurate calculations.

Its self protecting exactly as a spur on a 32A ring is (it only has a
single 13 socket).

Yes it is at the moment, but it will not be if the OP installs an electric
cooker, and he has hinted that he may do so.

I would change it too, most electricians would reject it as they
don't know how to do the calculations and its just easier to use the
regs than to explain why its OK.

(the regs tell you what is OK and they give you the calculations, the regs
and what is OK are inclusive not exclusive)

I have to take the bigger view. I might be able to show that a certain
cooker is safe when installed with the 2.5 T&E, however the customer may
change the cooker for a higher powered one or sell the house and a new owner
may fit a higher powered cooker. I would rather see a cooker circuit
correctly fused to the actual cable rating and not to some diversity
calculations that mean nothing when a cooker is swapped.


--
Adam

"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...


Do I need to change my 40A to a 32A, or leave it as it is?

I would change it. Far easier than doing calculations. However I
would not use the 2.5 T&E for use with an electric cooker without
making some very accurate calculations.

Its self protecting exactly as a spur on a 32A ring is (it only has a
single 13 socket).

Yes it is at the moment, but it will not be if the OP installs an electric
cooker, and he has hinted that he may do so.

I would change it too, most electricians would reject it as they
don't know how to do the calculations and its just easier to use the
regs than to explain why its OK.

(the regs tell you what is OK and they give you the calculations, the regs
and what is OK are inclusive not exclusive)

You can do anything you like if you are competent to do the calcs, I believe
that's in the regs somewhere.
The majority of electricians mean the onsite guide when they refer to the
regs.


I have to take the bigger view. I might be able to show that a certain
cooker is safe when installed with the 2.5 T&E, however the customer may
change the cooker for a higher powered one or sell the house and a new
owner may fit a higher powered cooker. I would rather see a cooker circuit
correctly fused to the actual cable rating and not to some diversity
calculations that mean nothing when a cooker is swapped.

That I agree with.. diversity is a PITA.
It doesn't even work when someone swaps a few single plugs for doubles in a
ring and then plugs in a few fan heaters at one end.
That sort of things makes rings run out of spec.
I suppose its regarded as unlikely that some will use 9kW of heating in a
room, even one they are trying to dry out after the recent floods. ;-)

dennis@home wrote:
I would change it too, most electricians would reject it as they
don't know how to do the calculations and its just easier to use the
regs than to explain why its OK.

(the regs tell you what is OK and they give you the calculations,
the regs and what is OK are inclusive not exclusive)

You can do anything you like if you are competent to do the calcs, I
believe that's in the regs somewhere.
The majority of electricians mean the onsite guide when they refer to
the regs.

The same rules and regs apply to both the full BS7671 and the OSG.


I have to take the bigger view. I might be able to show that a
certain cooker is safe when installed with the 2.5 T&E, however the
customer may change the cooker for a higher powered one or sell the
house and a new owner may fit a higher powered cooker. I would
rather see a cooker circuit correctly fused to the actual cable
rating and not to some diversity calculations that mean nothing when
a cooker is swapped.

That I agree with.. diversity is a PITA.
It doesn't even work when someone swaps a few single plugs for
doubles in a ring and then plugs in a few fan heaters at one end.

That makes no difference in most cases. Most people do not have a massive
store of fan heaters ready to be used.

That sort of things makes rings run out of spec.

For a short time that will not matter.

I suppose its regarded as unlikely that some will use 9kW of heating
in a room, even one they are trying to dry out after the recent
floods. ;-)

But you know damn well the correct tool is a dehumidifer not a heater for
this job.

--
Adam

"ARWadsworth" wrote in message
...

I suppose its regarded as unlikely that some will use 9kW of heating
in a room, even one they are trying to dry out after the recent
floods. ;-)

But you know damn well the correct tool is a dehumidifer not a heater for
this job.

OK, 9 kW of heating + 1kW of dehumidifier.

dennis@home wrote:
"ARWadsworth" wrote in message
...

I suppose its regarded as unlikely that some will use 9kW of heating
in a room, even one they are trying to dry out after the recent
floods. ;-)

But you know damn well the correct tool is a dehumidifer not a
heater for this job.

OK, 9 kW of heating + 1kW of dehumidifier.

But that never happens. I doubt that three 3kW fan heaters will stay on for
long if they are in the same room. They have thermostats.

--
Adam

"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

I suppose its regarded as unlikely that some will use 9kW of heating
in a room, even one they are trying to dry out after the recent
floods. ;-)

But you know damn well the correct tool is a dehumidifer not a
heater for this job.

OK, 9 kW of heating + 1kW of dehumidifier.

But that never happens. I doubt that three 3kW fan heaters will stay on
for long if they are in the same room. They have thermostats.

The problem being that there is no way to know, it might be a big room with
the doors and windows open with a gale blowing through, they would stay on
then.
The reason why I brought it up is because I have seen it happen BTW.
Not everyone would think about running an extension lead from somewhere else
to make sure there wasn't a problem. They would leave it running until the
room was dry or it all failed.

dennis@home wrote:
"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

I suppose its regarded as unlikely that some will use 9kW of
heating in a room, even one they are trying to dry out after the
recent floods. ;-)

But you know damn well the correct tool is a dehumidifer not a
heater for this job.

OK, 9 kW of heating + 1kW of dehumidifier.

But that never happens. I doubt that three 3kW fan heaters will stay
on for long if they are in the same room. They have thermostats.

The problem being that there is no way to know, it might be a big
room with the doors and windows open with a gale blowing through,
they would stay on then.
The reason why I brought it up is because I have seen it happen BTW.
Not everyone would think about running an extension lead from
somewhere else to make sure there wasn't a problem. They would leave
it running until the room was dry or it all failed.


You are missing one important point.

Ring mains are designed for normal domestic use not for clearing floods.


--
Adam

"ARWadsworth" wrote in message
...

You are missing one important point.

Ring mains are designed for normal domestic use not for clearing floods.

Shame that very few know that it can be dangerous, something that could be
designed out.

dennis@home wrote:
"ARWadsworth" wrote in message
...

You are missing one important point.

Ring mains are designed for normal domestic use not for clearing
floods.

Shame that very few know that it can be dangerous, something that
could be designed out.


The IET must be stupid. Why did they not let you design the new regs for
them?

--
Adam

"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

You are missing one important point.

Ring mains are designed for normal domestic use not for clearing
floods.

Shame that very few know that it can be dangerous, something that
could be designed out.


The IET must be stupid. Why did they not let you design the new regs for
them?

So you don't think they got the compromise between cost and safety on ring
mains wrong then?
That would be why they added radials with breakers that actually match the
cables because they didn't need to.
Who knows, sometime in the future when all the old electricians have retired
rings will finally die.

On 16/12/10 08:58, dennis@home wrote:


"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

You are missing one important point.

Ring mains are designed for normal domestic use not for clearing
floods.

Shame that very few know that it can be dangerous, something that
could be designed out.


The IET must be stupid. Why did they not let you design the new regs
for them?

So you don't think they got the compromise between cost and safety on
ring mains wrong then?
That would be why they added radials with breakers that actually match
the cables because they didn't need to.
Who knows, sometime in the future when all the old electricians have
retired rings will finally die.

Rings have notable safety benefits over radials, namely that the CPC is
doubled up over two paths.

Also, show me a practical 32A radial with multiple socket drops noting
the BS standard restrictions on terminal capacities of socket and FCU
accessories.


--
Tim Watts

"Tim Watts" wrote in message
...

Rings have notable safety benefits over radials, namely that the CPC is
doubled up over two paths.

They also have several safety problems.
They can have latent faults that the householder will only find out about
when its too late.
Things like broken earths (which negates your above argument) can just be
there undetected for years unless you have regular inspections and real
continuity tests.
This isn't a solution though as the act of doing the test will make the
faults more likely to occur.


Also, show me a practical 32A radial with multiple socket drops noting the
BS standard restrictions on terminal capacities of socket and FCU
accessories.

Why do you need a 32A radial?
I don't think you will find them in the OSG.
4mm will do the job with many accessories and you could always crimp in a
joint if the accessory is to small. I wouldn't bother with a 32A radial to
replace a ring.

On 16/12/10 09:38, dennis@home wrote:


"Tim Watts" wrote in message
...

Rings have notable safety benefits over radials, namely that the CPC
is doubled up over two paths.

They also have several safety problems.
They can have latent faults that the householder will only find out
about when its too late.
Things like broken earths (which negates your above argument) can just
be there undetected for years unless you have regular inspections and
real continuity tests.
This isn't a solution though as the act of doing the test will make the
faults more likely to occur.


Also, show me a practical 32A radial with multiple socket drops noting
the BS standard restrictions on terminal capacities of socket and FCU
accessories.

Why do you need a 32A radial?

Because 20A is **** all use to me.

I do not want to be thinking - are these sockets on the same 20A circuit
- damn I cannot plug this 3kW appliance in because there are already two
high load appliances on that circuit.

I already also have enough RCBOs in my CU - I don't have space for even
more, let alone the expense.

I don't think you will find them in the OSG.

Bugger the OSG, it is a guide only. I think you WILL find them in
Appendix 15 of the 17th.

4mm will do the job with many accessories

4mm doesn't cut it for Reference Method B (a common installation method
in my house).

and you could always crimp in
a joint if the accessory is to small. I wouldn't bother with a 32A
radial to replace a ring.


--
Tim Watts

On 16/12/10 09:38, dennis@home wrote:

This isn't a solution though as the act of doing the test will make the
faults more likely to occur.

********. Have you ever done any testing and inspection work?

Inspections will disturb the accessory, so you do that first[1].

The testing requires disturbing the connections at the CU only and the
first test you do is an end-end test on all 3conductors which will pick
up this problem. In fact that is the one test you could validly do with
any old cheap multimeter or even a bulb and battery.

The next test is the 500V inter-conductor insulation test.

Once those have passed, you now know that the ring is continuous and has
no single wiring error (though it may have multiple erros that cancel.

The 3rd set of figure-8 tests prove the absence of any wiring error or
open circuit fault and demonstrate the soundness of the circuit both
under load and fault conditions for all bar the most obscure and
unlikely problem scenarios.

As long as you can reinsert the wires into the CU correctly you are
good. I add an extra test of my own which is to do a few live loop
impedance tests afterwards which would pick up any issues there.

[1] As we are talking about socket circuits. Lighting circuits are more
of a PITA as you need to bridge out any electronic devices which
unfortunately means fiddling with the accessories after the tests are done.

--
Tim Watts

dennis@home wrote:
"Tim Watts" wrote in message
...

Rings have notable safety benefits over radials, namely that the CPC
is doubled up over two paths.

They also have several safety problems.
They can have latent faults that the householder will only find out
about when its too late.
Things like broken earths (which negates your above argument) can
just be there undetected for years unless you have regular
inspections and real continuity tests.
This isn't a solution though as the act of doing the test will make
the faults more likely to occur.


Also, show me a practical 32A radial with multiple socket drops
noting the BS standard restrictions on terminal capacities of socket
and FCU accessories.

Why do you need a 32A radial?
I don't think you will find them in the OSG.

Pages 49 and 158 to name two references to 32A radials in the OSG.

--
Adam

dennis@home wrote:
"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

You are missing one important point.

Ring mains are designed for normal domestic use not for clearing
floods.

Shame that very few know that it can be dangerous, something that
could be designed out.


The IET must be stupid. Why did they not let you design the new regs
for them?

So you don't think they got the compromise between cost and safety on
ring mains wrong then?

No, I believe that the compromise is very good and we have a good safe
system.

That would be why they added radials with breakers that actually
match the cables because they didn't need to.

I am not sure what you mean.
All circuits have breakers designed to match the cable.

Who knows, sometime in the future when all the old electricians have
retired rings will finally die.

Maybe, it is suggested everytime there is an update to the regs.

--
Adam

On Dec 16, 12:44*am, John Rumm wrote:
On 15/12/2010 19:54, David Robinson wrote:

In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

This is one of those occasions where the responsibility for overload
protection and fault protection can be split. The fault protection (i.e.
very high short circuit currents) must always be at the origin of the
circuit, however the overload protection can be at the load end in some
cases. (others would include a 3A drop wire to a pendent fixing on a 6A
protected lighting circuit for example - the largest bulb you can get
won't come close to 3A so no chance of overload))

An unfused spur can power a total of one double or single socket. The
diverse load of a double socket is taken as being 20A. (That is lower
than the current carrying capacity of 2.5mm^2 T&E in all but the most
adverse installation methods). Even the theoretically possible 26A is
just under the maximum rating when clipped direct or buried in masonry.
* So the conditions at the load end (i.e. only one double socket)
enforce the overload protection.

The next question concerns the fault protection. i.e. what happens when
you nail through the cable or some other drastic fault occurs. Here you
will get a fault current that limited only by the the resistance of the
wires themselves and that of the supply and earth connections. This is
one of the reasons for their being a maximum cable length specified for
most circuits - to ensure the so called "Earth Loop Impedance" (i.e.
round trip resistance from supply through circuit wires, and to earth)
can't get too high.

A 32A breaker will typically need as much as 160A to open "instantly"
i.e. on the magnetic part of its trip response. Instant in this case
means 0.1 secs or less.

See charts he

http://wiki.diyfaq.org.uk/index.php?...ypes_B.2CC.2CD

Lets say you work out that your prospective fault current is going to be
200A, you now need to assess[1] what happens when you try and stick that
down the skimpy 1.5mm^2 earth wire. Needless to say that will get hot,
and quickly. Its also going to be heating so fast that the natural heat
losses to its surroundings are going to be negligible during the time
scale we are interested in. So we have what can be though of as
adiabatic heating.

What you need to check is that it won't melt before its done its job and
tripped the MCB. For this you use what is called the adiabatic equation:
s = sqrt( I^2 x t ) / k, where s = the minimum cross sectional area of
copper required in the conductor, and k is a factor specific to the the
type of cable (115 in the case of PVC T&E).

So 200A, a check of the MCB charts says this is plenty to open the
breaker in 0.1 secs. So our sum becomes s = ( 200 x 200 x 0.1 ) / 115 =
0.56mm^2, which is significantly less than the 1.5mm^2 we actually have
an hence is ok.

[1] In reality you can skip this stage since these are "standard"
circuit designs where as long as you obay the length limits, you know
the design "works".

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).

Current carrying capacities are listed he

http://wiki.diyfaq.org.uk/index.php?title=Cables#T.26E

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.

Well the overload protection is achieved by the 13A maximum load on the
socket. The only other question is will it be adequately fault
protected. Assuming the rest of the circuit is a more substantial cable,
the the loop impedance ought to be fairly low to that point. Hence it
seems reasonable to assume you will get *fault current in excess of
200A. That will open the 40A circuit breaker in 0.1 secs, and we know
from the sum above that the wire will handle it.

Yes, the rest of the circuit is 6mm T+E, from CU to cooker switch (inc
socket) to cooker outlet. It's only the last jump from cooker outlet
to adjacent single socket that's 2.5mm.

However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

Indeed. In fact due to the nature of the load presented by cookers, even
a very powerful cooker with a theoretical peak load over 60A will
usually be fine on a 32A circuit. (cooker diversity is calculated as 10A
plus 30% of the remainder). So a 60A cooker would need a circuit
provisioned for 10A + 0.3 x 50 = 25A, or 30A if there is also a socket
on the cooker point.

IIRC the cooker claims to be 13A but instructs use of heat resistant
2.5mm cable into a cooker outlet, not a 13A plug. So I've followed
those instructions.

Cheers,
David.

On Dec 15, 9:48*pm, "ARWadsworth"
wrote:
dennis@home wrote:
"ARWadsworth" wrote in message
...

Do I need to change my 40A to a 32A, or leave it as it is?

I would change it. Far easier than doing calculations. However I
would not use the 2.5 T&E for use with an electric cooker without
making some very accurate calculations.

Its self protecting exactly as a spur on a 32A ring is (it only has a
single 13 socket).

Yes it is at the moment, but it will not be if the OP installs an electric
cooker, and he has hinted that he may do so.

No, see my reply to John: the rest of the circuit is 6mm T+E, and the
cooker will be run from a cooker outlet, not the socket.

Cheers,
David.

On 16/12/10 10:31, David Robinson wrote:

IIRC the cooker claims to be 13A but instructs use of heat resistant
2.5mm cable into a cooker outlet, not a 13A plug. So I've followed
those instructions.


13A plugtops do have quite low specified limits on operating
temperature. Whilst it won't be a problem for a gas cooker that only
draws power for a light, spark and maybe a low powered warming cupboard,
it might be anticipated to be an issue for a cooker that actually draws
3kW (eg dual fuel with a lumpy electric oven).
--
Tim Watts

"Tim Watts" wrote in message
...
On 16/12/10 09:38, dennis@home wrote:


"Tim Watts" wrote in message
...

Rings have notable safety benefits over radials, namely that the CPC
is doubled up over two paths.

They also have several safety problems.
They can have latent faults that the householder will only find out
about when its too late.
Things like broken earths (which negates your above argument) can just
be there undetected for years unless you have regular inspections and
real continuity tests.
This isn't a solution though as the act of doing the test will make the
faults more likely to occur.


Also, show me a practical 32A radial with multiple socket drops noting
the BS standard restrictions on terminal capacities of socket and FCU
accessories.

Why do you need a 32A radial?

Because 20A is **** all use to me.

I do not want to be thinking - are these sockets on the same 20A circuit -
damn I cannot plug this 3kW appliance in because there are already two
high load appliances on that circuit.

But you need to do that if you want to plug them into one end of a 2.5 mm
ring, even more so with three appliances.
It causes imbalance currents in the ring that can take them out of spec.
Its why fixed heating shouldn't go on a ring, they don't like high load
stuff at one end.

"Tim Watts" wrote in message
...
On 16/12/10 09:38, dennis@home wrote:

This isn't a solution though as the act of doing the test will make the
faults more likely to occur.

********. Have you ever done any testing and inspection work?

Inspections will disturb the accessory, so you do that first[1].

The testing requires disturbing the connections at the CU only and the
first test you do is an end-end test on all 3conductors which will pick up
this problem. In fact that is the one test you could validly do with any
old cheap multimeter or even a bulb and battery.

The next test is the 500V inter-conductor insulation test.

Once those have passed, you now know that the ring is continuous and has
no single wiring error (though it may have multiple erros that cancel.

The 3rd set of figure-8 tests prove the absence of any wiring error or
open circuit fault and demonstrate the soundness of the circuit both under
load and fault conditions for all bar the most obscure and unlikely
problem scenarios.

As long as you can reinsert the wires into the CU correctly you are good.
I add an extra test of my own which is to do a few live loop impedance
tests afterwards which would pick up any issues there.

[1] As we are talking about socket circuits. Lighting circuits are more of
a PITA as you need to bridge out any electronic devices which
unfortunately means fiddling with the accessories after the tests are
done.

every time you disconnect the wire and reconnect it you work harden the
copper and make it more likely to fail due to passing traffic, etc. If the
consumer unit was correctly designed it would have test links so you could
do the tests without disconnecting the wires. But why do things right when
you can do them cheap?

"ARWadsworth" wrote in message
...

Who knows, sometime in the future when all the old electricians have
retired rings will finally die.

Maybe, it is suggested everytime there is an update to the regs.

That would be because a lot of engineers don't think rings are safe.

On Dec 15, 8:25*pm, "ARWadsworth"
wrote:
David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single bit
of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC
they even dropped the 2.5mm rating down to 18A, which is "correct" in
accordance with the calculations, but then increased it back to 20A
because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit
(feeding a socket that can only supply the gas hob ignition) and that
seems very wrong too - but wiring a socket in 6mm is just too painful.
However, the cooker circuit may yet drop back to 32A (because I
suspect that's all it needs) which makes it no worse than the socket
spur discussed above.

The short version -

You have to consider the difference between overcurrent protection and fault
protection.

Queue :::Jerry:::

MBQ

Man at B&Q wrote:
On Dec 15, 8:25 pm, "ARWadsworth"
wrote:
David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single
bit of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it.
(IIRC they even dropped the 2.5mm rating down to 18A, which is
"correct" in accordance with the calculations, but then increased
it back to 20A because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker
circuit (feeding a socket that can only supply the gas hob
ignition) and that seems very wrong too - but wiring a socket in
6mm is just too painful. However, the cooker circuit may yet drop
back to 32A (because I suspect that's all it needs) which makes it
no worse than the socket spur discussed above.

The short version -

You have to consider the difference between overcurrent protection
and fault protection.

Queue :::Jerry:::

And Dennis.

--
Adam

On 16/12/10 11:53, dennis@home wrote:

every time you disconnect the wire and reconnect it you work harden the
copper and make it more likely to fail due to passing traffic, etc. If
the consumer unit was correctly designed it would have test links so you
could do the tests without disconnecting the wires. But why do things
right when you can do them cheap?

Whilst I don't disagree with the sentiment, a decently laid out CU isn't
too bad - and your idea would require at least 3 terminals per current
terminal (L,N and PE) all with per terminal links and test points - so
that's 9 links and test points per device.

I wish that all dimmers came with a small shorting plug though, that
could be inserted from the back of the device - that would save some
faffing.

--
Tim Watts

dennis@home wrote:
"ARWadsworth" wrote in message
...

Who knows, sometime in the future when all the old electricians have
retired rings will finally die.

Maybe, it is suggested everytime there is an update to the regs.

That would be because a lot of engineers don't think rings are safe.

But 60 years of history prove them wrong.

Are you going to clarify what you meant by

"That would be why they added radials with breakers that actually match the
cables because they didn't need to."

--
Adam

"David Robinson" wrote in message
...
On Dec 15, 9:48 pm, "ARWadsworth"
wrote:
dennis@home wrote:
"ARWadsworth" wrote in message
...

Do I need to change my 40A to a 32A, or leave it as it is?

I would change it. Far easier than doing calculations. However I
would not use the 2.5 T&E for use with an electric cooker without
making some very accurate calculations.

Its self protecting exactly as a spur on a 32A ring is (it only has a
single 13 socket).

Yes it is at the moment, but it will not be if the OP installs an
electric
cooker, and he has hinted that he may do so.

No, see my reply to John: the rest of the circuit is 6mm T+E, and the
cooker will be run from a cooker outlet, not the socket.

It doesn't actually significantly change the level of protection if you fit
a full power cooker.
It does change the possibility of tripping the circuit as it allows more
current to be drawn.
However it is still safe (well as safe as a spur on a ring) as there is no
way to overload it that won't trip the circuit.

You will trip the plug fuse if you overload the socket whatever cooker you
have and trip the MCB if you overload the total circuit.
The same is true for if you fit a double socket but I wouldn't put a double
on a 2.5 mm spur in a kitchen, someone is bound to plug in a big tea urn and
a combo microwave and overload it for a couple of hours. That's what Murphy
told me.

There is quite a lot of difference between it being safe and it being user
friendly, you don't want circuits to trip frequently to maintain safety.

On Dec 16, 12:07*pm, "ARWadsworth"
wrote:
Man at B&Q wrote:

On Dec 15, 8:25 pm, "ARWadsworth"
wrote:
David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is supposed
to protect the cable to the fitting, and the fuse in the plug is
supposed to protect the cable to the appliance, how is the single
bit of 2.5mm T+E run to a single socket spur adequately protected by
anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it.
(IIRC they even dropped the 2.5mm rating down to 18A, which is
"correct" in accordance with the calculations, but then increased
it back to 20A because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker
circuit (feeding a socket that can only supply the gas hob
ignition) and that seems very wrong too - but wiring a socket in
6mm is just too painful. However, the cooker circuit may yet drop
back to 32A (because I suspect that's all it needs) which makes it
no worse than the socket spur discussed above.

The short version -

You have to consider the difference between overcurrent protection
and fault protection.

Queue :::Jerry:::

And Dennis.

You don't think...

MBQ

Man at B&Q wrote:
On Dec 16, 12:07 pm, "ARWadsworth"
wrote:
Man at B&Q wrote:

On Dec 15, 8:25 pm, "ARWadsworth"
wrote:
David Robinson wrote:
In a bout of man flu, I lay in bed wondering: If the MCB is
supposed to protect the cable to the fitting, and the fuse in the
plug is supposed to protect the cable to the appliance, how is
the single bit of 2.5mm T+E run to a single socket spur
adequately protected by anything?

You've basically got 2 bits of 2.5mm in parallel (usually unequal
lengths), feeding a single bit of 2.5mm. 32A MCB.

Whereas if it was a standard radial circuit, you'd have just the
single bit of 2.5mm, and you'd only be allowed a 20A MCB on it.
(IIRC they even dropped the 2.5mm rating down to 18A, which is
"correct" in accordance with the calculations, but then increased
it back to 20A because that's what everyone does anyway).

I'm not worried. I just don't see the logic.

I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker
circuit (feeding a socket that can only supply the gas hob
ignition) and that seems very wrong too - but wiring a socket in
6mm is just too painful. However, the cooker circuit may yet drop
back to 32A (because I suspect that's all it needs) which makes it
no worse than the socket spur discussed above.

The short version -

You have to consider the difference between overcurrent protection
and fault protection.

Queue :::Jerry:::

And Dennis.

You don't think...

Well they certainly don't......

--
Adam

"Tim Watts" wrote in message
...
On 16/12/10 11:53, dennis@home wrote:

every time you disconnect the wire and reconnect it you work harden the
copper and make it more likely to fail due to passing traffic, etc. If
the consumer unit was correctly designed it would have test links so you
could do the tests without disconnecting the wires. But why do things
right when you can do them cheap?

Whilst I don't disagree with the sentiment, a decently laid out CU isn't
too bad - and your idea would require at least 3 terminals per current
terminal (L,N and PE) all with per terminal links and test points - so
that's 9 links and test points per device.

Less if its not a ring though.


I wish that all dimmers came with a small shorting plug though, that could
be inserted from the back of the device - that would save some faffing.

You should invent one.. a crimp on termination pin for each wire with a
suitable socket should do it.

--
Tim Watts

"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

Who knows, sometime in the future when all the old electricians have
retired rings will finally die.

Maybe, it is suggested everytime there is an update to the regs.

That would be because a lot of engineers don't think rings are safe.

But 60 years of history prove them wrong.

How does it?
Nobody knows how many faulty rings there are as the householder doesn't know
and very few are ever checked.
There could be 10 there could be 100,000, until people get hurt no one
cares.


Are you going to clarify what you meant by

"That would be why they added radials with breakers that actually match
the cables because they didn't need to."

It is illogical to rely on downstream breakers to protect up stream cables.
It makes modification to the circuit potentially dangerous.

People do modify the circuits by putting the wrong fuse in plugs (including
nails) rendering them useless for protecting the upstream circuit against
overload.
There was even a large batch of "genuine" fuses for plugs that didn't
protect the circuit a few decades back.
These led to certification being required to show batch testing had been
done.
God knows how many of these faulty fuses are still out there.
Do you check the plug fuse for certification when you come across one?

dennis@home wrote:
"ARWadsworth" wrote in message
...
dennis@home wrote:
"ARWadsworth" wrote in message
...

Who knows, sometime in the future when all the old electricians
have retired rings will finally die.

Maybe, it is suggested everytime there is an update to the regs.

That would be because a lot of engineers don't think rings are safe.

But 60 years of history prove them wrong.

How does it?
Nobody knows how many faulty rings there are as the householder
doesn't know and very few are ever checked.
There could be 10 there could be 100,000, until people get hurt no one
cares.

Ditto for faulty radials.

Are you going to clarify what you meant by

"That would be why they added radials with breakers that actually
match the cables because they didn't need to."

It is illogical to rely on downstream breakers to protect up stream
cables. It makes modification to the circuit potentially dangerous.

Only if you do not know what you are doing.

People do modify the circuits by putting the wrong fuse in plugs
(including nails) rendering them useless for protecting the upstream
circuit against overload.

Non of which alters the load on the spur or makes any difference at all to a
fault current on the spur.

There was even a large batch of "genuine" fuses for plugs that didn't
protect the circuit a few decades back.
These led to certification being required to show batch testing had
been done.
God knows how many of these faulty fuses are still out there.
Do you check the plug fuse for certification when you come across one?

And the relevance to spurs and radials is?

--
Adam

Tim Watts wrote:
On 16/12/10 09:38, dennis@home wrote:


"Tim Watts" wrote in message
...

Rings have notable safety benefits over radials, namely that the CPC
is doubled up over two paths.

They also have several safety problems.
They can have latent faults that the householder will only find out
about when its too late.
Things like broken earths (which negates your above argument) can
just be there undetected for years unless you have regular
inspections and real continuity tests.
This isn't a solution though as the act of doing the test will make
the faults more likely to occur.


Also, show me a practical 32A radial with multiple socket drops
noting the BS standard restrictions on terminal capacities of
socket and FCU accessories.

Why do you need a 32A radial?

Because 20A is **** all use to me.

20A radials often use more cable when installing than a ring, offer very
little in the way of diversity and require just as much (if not more) work
to install than a ring.

There are times when they are useful but not very often.

--
Adam