I've heard lots of people stating that houses need re-wiring after 25 years
or so, but can't for the life of me figure out why this should be the case.
Copper tends not to corrode, and PVC stays flexible for many many years so
is this 25 year thing designed to give electricians a good earner or is
there some other reason? I'm curious as I am about to buy a 26 year old
house which has not been re-wired and am wondering what to look out for.

Cheers

Hellraiser..........

On Sun, 5 Feb 2006, Hellraiser wrote:

I've heard lots of people stating that houses need re-wiring after 25 years
or so, but can't for the life of me figure out why this should be the case.
Copper tends not to corrode, and PVC stays flexible for many many years so
is this 25 year thing designed to give electricians a good earner or is
there some other reason? I'm curious as I am about to buy a 26 year old
house which has not been re-wired and am wondering what to look out for.

A 26-year old installation is unlikely to require a complete rewire but a
full inspection and test is probably in order. It might be worth
considering replacing the consumer unit with a modern split-load one. It
is also quite likely that the number of socket outlets will be
insufficient for the number of electronic devices in most homes these
days.

HTH

--
Alistair Riddell - BOFH
Microsoft - because god hates us

In article ,
"Hellraiser" writes:
I've heard lots of people stating that houses need re-wiring after 25 years
or so, but can't for the life of me figure out why this should be the case.
Copper tends not to corrode, and PVC stays flexible for many many years so
is this 25 year thing designed to give electricians a good earner or is
there some other reason? I'm curious as I am about to buy a 26 year old
house which has not been re-wired and am wondering what to look out for.

No PVC wiring will have yet worn out, providing it's not
been subject to excessive heat or other damaging factors.
The accessories (switches, sockets, consumer unit, etc)
should all be regarded as potentially at end of life and
in need of inspection and possibly replacement. All the
earthing should be checked and ideally brought up to
current standards. A 1980 installation might be slightly
low on numbers of sockets, although not as bad as a 1960's
installation. A check for extensions to the original system
which have not been correctly done would be good too.

--
Andrew Gabriel

"Andrew Gabriel" wrote in message
...
In article ,
"Hellraiser" writes:
I've heard lots of people stating that houses need re-wiring after 25
years
or so, but can't for the life of me figure out why this should be the
case.
Copper tends not to corrode, and PVC stays flexible for many many years
so
is this 25 year thing designed to give electricians a good earner or is
there some other reason? I'm curious as I am about to buy a 26 year old
house which has not been re-wired and am wondering what to look out for.

No PVC wiring will have yet worn out, providing it's not
been subject to excessive heat or other damaging factors.
The accessories (switches, sockets, consumer unit, etc)
should all be regarded as potentially at end of life and
in need of inspection and possibly replacement. All the
earthing should be checked and ideally brought up to
current standards. A 1980 installation might be slightly
low on numbers of sockets, although not as bad as a 1960's
installation. A check for extensions to the original system
which have not been correctly done would be good too.

Excellent, I can replace switches/sockets easily enough, I had to do that in
the house I am in now. I don't believe the property has a consumer unit, how
much would it cost (approx) to get one fitted? I notice that CUs themselves
are quite cheap, but I am unsure of the amount of work needed to fit one,
and obviously there would be the cost of MCBs etc on top, but just a very
rough ballpark would do me for now. I am looking at getting the kitchen
refitted so would most likely need a new ring main for that, would it be
best to have a CU fitted at the same time and perhaps reduce the cost?

Sorry the questions are a little vague, but till I move in I can't provide
much more info

Cheers

Hellraiser...........

In article ,
"Hellraiser" writes:

Excellent, I can replace switches/sockets easily enough, I had to do that in
the house I am in now. I don't believe the property has a consumer unit, how
much would it cost (approx) to get one fitted? I notice that CUs themselves

It must have one somewhere, unless it's been stolen and all the
electricity is off.

are quite cheap, but I am unsure of the amount of work needed to fit one,
and obviously there would be the cost of MCBs etc on top, but just a very
rough ballpark would do me for now. I am looking at getting the kitchen
refitted so would most likely need a new ring main for that, would it be
best to have a CU fitted at the same time and perhaps reduce the cost?

Yes. Alternatively, engage an electrician to test the installation
and replace CU if necessary when you move in, and to leave spare
ways to connect up new kitchen circuits when you have that done.

I should have mentioned getting the installation tested in previous
posting. It might flag up something more serious if you want to be
sure of no surprises later, although personally I wouldn't bother
for a 1980's house.

--
Andrew Gabriel

Excellent, I can replace switches/sockets easily enough, I had to do that
in
the house I am in now. I don't believe the property has a consumer unit,
how
much would it cost (approx) to get one fitted? I notice that CUs
themselves

It must have one somewhere, unless it's been stolen and all the
electricity is off.

ahem yes, I meant it didn't have a modern CU, just a standard fusebox

are quite cheap, but I am unsure of the amount of work needed to fit one,
and obviously there would be the cost of MCBs etc on top, but just a very
rough ballpark would do me for now. I am looking at getting the kitchen
refitted so would most likely need a new ring main for that, would it be
best to have a CU fitted at the same time and perhaps reduce the cost?

Yes. Alternatively, engage an electrician to test the installation
and replace CU if necessary when you move in, and to leave spare
ways to connect up new kitchen circuits when you have that done.

I should have mentioned getting the installation tested in previous
posting. It might flag up something more serious if you want to be
sure of no surprises later, although personally I wouldn't bother
for a 1980's house.

Great, that sets my mind at rest - many thanks for your advice.

Hellraiser...........

Hi;

I have changed several consumer unit to a modern day split un trip to
replace those old fuse wire jobbies. Usually though one is stressed in
sorting out the trip problems that occur after this, but after that no
probs.

I also add loads of sockets as Alister says, as I reckon kit these days
consume so little power that its even worth doubling tose old single
sockets. Rather dates the present day Reg's that seems to be lagging
behind reality.

One thing I do do now though, is always get rid of old Flouescent
fitting as they will alway give you trip problems.

If you do have the floors etc up then its always worth running power
for an Electric shower for when the gas runs out.

Regards
Ian

"Alistair Riddell" wrote in message
rg.uk...
On Sun, 5 Feb 2006, Hellraiser wrote:
SNIP

A 26-year old installation is unlikely to require a complete rewire but a
full inspection and test is probably in order. It might be worth
considering replacing the consumer unit with a modern split-load one. It
is also quite likely that the number of socket outlets will be
insufficient for the number of electronic devices in most homes these
days.
What does a split load CU (as opposed to single) do, please ?

And why would you replace a single (maybe fused) with a split load one ?

Hellraiser wrote:
"Andrew Gabriel" wrote in message
...
In article ,
"Hellraiser" writes:
I've heard lots of people stating that houses need re-wiring after 25
years
or so, but can't for the life of me figure out why this should be the
case.
Copper tends not to corrode, and PVC stays flexible for many many years
so
is this 25 year thing designed to give electricians a good earner or is
there some other reason? I'm curious as I am about to buy a 26 year old
house which has not been re-wired and am wondering what to look out for.

No PVC wiring will have yet worn out, providing it's not
been subject to excessive heat or other damaging factors.
The accessories (switches, sockets, consumer unit, etc)
should all be regarded as potentially at end of life and
in need of inspection and possibly replacement. All the
earthing should be checked and ideally brought up to
current standards. A 1980 installation might be slightly
low on numbers of sockets, although not as bad as a 1960's
installation. A check for extensions to the original system
which have not been correctly done would be good too.

Excellent, I can replace switches/sockets easily enough, I had to do that in
the house I am in now. I don't believe the property has a consumer unit, how
much would it cost (approx) to get one fitted? I notice that CUs themselves
are quite cheap, but I am unsure of the amount of work needed to fit one,
and obviously there would be the cost of MCBs etc on top, but just a very
rough ballpark would do me for now.

CUs tend to come with mcbs and RCD fitted. But I cant see any reason to
replace your existing one. Its only worth replacing if its damaged,
inadequate, or there is some specific problem with it. Wire fuses are
still compliant for new installs.


I am looking at getting the kitchen
refitted so would most likely need a new ring main for that,

what for? Ie I doubt it.

would it be
best to have a CU fitted at the same time and perhaps reduce the cost?

Sorry the questions are a little vague, but till I move in I can't provide
much more info

Cheers

Hellraiser...........

What problem are you trying to solve? I get the feeling you dont have
one.

I would check switches, sockets, lights etc, and maybe add more
sockets. I'd check it doesnt have a shower on the ring or something
like that. Beyond that its upto you how much you want to spend, but
really you're likely to see more return from spending on other areas.

Replacing a CU is far more work than replacing a fuse wire maybe twice.


NT

On Mon, 6 Feb 2006 16:58:53 -0000 someone who may be "Ian_m"
wrote this:-

What does a split load CU (as opposed to single) do, please ?

Generally, some circuits are protected by an RCD, like socket
outlets, while other circuits are not, like lights. This has the
advantage that a fault on an appliance will probably not plunge the
whole house into darkness.

And why would you replace a single (maybe fused) with a split load one ?

If one is replacing a consumer unit anyway, perhaps to get more
ways, then there are advantages in going for a split one at the same
time.

Personally I prefer cartridge fuses in a consumer unit to MCBs,
because of the high breaking capacity, but most people are better
off with MCBs.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

What problem are you trying to solve? I get the feeling you dont have
one.

I would check switches, sockets, lights etc, and maybe add more
sockets. I'd check it doesnt have a shower on the ring or something
like that. Beyond that its upto you how much you want to spend, but
really you're likely to see more return from spending on other areas.

Replacing a CU is far more work than replacing a fuse wire maybe twice.

You could be right, I was just getting edgy as the surveyor mentioned that
there was no modern CU there. If it works though....

Hellraiser...........

Hellraiser wrote:

What problem are you trying to solve? I get the feeling you dont have
one.

I would check switches, sockets, lights etc, and maybe add more
sockets. I'd check it doesnt have a shower on the ring or something
like that. Beyond that its upto you how much you want to spend, but
really you're likely to see more return from spending on other areas.

Replacing a CU is far more work than replacing a fuse wire maybe twice.

You could be right, I was just getting edgy as the surveyor mentioned that
there was no modern CU there. If it works though....

Hellraiser...........

He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for concern.

NT

wrote in message
oups.com...
Hellraiser wrote:

What problem are you trying to solve? I get the feeling you dont have
one.

I would check switches, sockets, lights etc, and maybe add more
sockets. I'd check it doesnt have a shower on the ring or something
like that. Beyond that its upto you how much you want to spend, but
really you're likely to see more return from spending on other areas.

Replacing a CU is far more work than replacing a fuse wire maybe twice.

You could be right, I was just getting edgy as the surveyor mentioned
that
there was no modern CU there. If it works though....

Hellraiser...........

He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for concern.

NT


That's a very sweeping statement. How can you say it is not a cause for
concern?? Have you inspected every eighties electrical installation in the
country

Stephen Dawson wrote:
wrote in message

He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for concern.

NT


That's a very sweeping statement. How can you say it is not a cause for
concern?? Have you inspected every eighties electrical installation in the
country

The fact that its 1980s is not a cause for concern.

Obviously that doesnt preclude problem for other reasons.

Why ask the d--- obvious? If the OP were retarded I dont think theyd be
sat here asking these questions.


NT

He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for concern.

NT


That's a very sweeping statement. How can you say it is not a cause for
concern?? Have you inspected every eighties electrical installation in
the
country

The fact that its 1980s is not a cause for concern.

Obviously that doesnt preclude problem for other reasons.

Why ask the d--- obvious? If the OP were retarded I dont think theyd be
sat here asking these questions.

Indeed. The initial question was one of deterioration of wiring over a
period of time, I think the response was more along the lines of 1980's
electrics being fairly durable and as such should not have deteriorated
unduly unless they have been exposed to exceptional conditions (i.e.
connections arcing due to them being too loose, etc).

Hellraiser...........

"Hellraiser" wrote in message
...
He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for
concern.

NT


That's a very sweeping statement. How can you say it is not a cause for
concern?? Have you inspected every eighties electrical installation in
the
country

The fact that its 1980s is not a cause for concern.

Obviously that doesnt preclude problem for other reasons.

Why ask the d--- obvious? If the OP were retarded I dont think theyd be
sat here asking these questions.

Indeed. The initial question was one of deterioration of wiring over a
period of time, I think the response was more along the lines of 1980's
electrics being fairly durable and as such should not have deteriorated
unduly unless they have been exposed to exceptional conditions (i.e.
connections arcing due to them being too loose, etc).

Hellraiser...........


Granted, the wiring my not have deteriorated since the 1980's but the
terminations may have done, and in my experience PVC cale down break down,
the termination points do. So to go back the the OP, have a periodic done by
a competent sparks and ct on the information given.

Steve Dawson

"Stephen Dawson" wrote in message
...

"Hellraiser" wrote in message
...
He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for
concern.

NT


That's a very sweeping statement. How can you say it is not a cause for
concern?? Have you inspected every eighties electrical installation in
the
country

The fact that its 1980s is not a cause for concern.

Obviously that doesnt preclude problem for other reasons.

Why ask the d--- obvious? If the OP were retarded I dont think theyd be
sat here asking these questions.

Indeed. The initial question was one of deterioration of wiring over a
period of time, I think the response was more along the lines of 1980's
electrics being fairly durable and as such should not have deteriorated
unduly unless they have been exposed to exceptional conditions (i.e.
connections arcing due to them being too loose, etc).

Hellraiser...........


Granted, the wiring my not have deteriorated since the 1980's but the
terminations may have done, and in my experience PVC cale down break down,
the termination points do. So to go back the the OP, have a periodic done
by a competent sparks and ct on the information given.

Steve Dawson

I have got to get a new keyboard!!!

Stephen Dawson wrote:

Granted, the wiring my not have deteriorated since the 1980's but the
terminations may have done, and in my experience PVC cale down break down,
the termination points do. So to go back the the OP, have a periodic done by
a competent sparks and ct on the information given.

Steve Dawson

I fairly much concur. Just for clarity, what I meant was a 1980s
install would have been upto a good safety standard when new, so the
fact that it is 1980s is not in itself a cause for concern. As Steve
says, some fittings may have deteriorated, particularly IME light
switches, which tend to become stiff enough in action that they can
stay in the halfway position, which isnt a good idea.

It is also possible for pvc cable to deteriorate, but this does not
normally happen. If outdoor pvc cable is cracking up, replace. If its
unpainted, paint with gloss paint to protect it from uv.

A periodic inspection can be done, but I'm not sure the return, ie
safety improvement per money spent, is of as good a value as various
other things you could do. Also you might get a list of recommendations
that amount fo bringing it upto new standard, which in most cases I'm
doubtful would gain any significant safety improvement.

Despite the popular perception of the dangers of electrickery, the
reality is the death rate is orders of magnitude lower than deaths
caused by stairs, and injuries are orders fo magnitude lower than those
caused by beds, DIY accidents, and so on.

Perhaps at some point it could even be that improving your wiring could
be more dangerous than any risk it removed


NT

Hellraiser wrote:
He has to cover his arse. They often tell people to have drain
inspections when nothings wrong. 80s wiring is not a cause for concern.

NT


That's a very sweeping statement. How can you say it is not a cause for
concern?? Have you inspected every eighties electrical installation in
the
country

The fact that its 1980s is not a cause for concern.

Obviously that doesnt preclude problem for other reasons.

Why ask the d--- obvious? If the OP were retarded I dont think theyd be
sat here asking these questions.


Indeed. The initial question was one of deterioration of wiring over a
period of time, I think the response was more along the lines of 1980's
electrics being fairly durable and as such should not have deteriorated
unduly unless they have been exposed to exceptional conditions (i.e.
connections arcing due to them being too loose, etc).

Having recently changed my CU, on a 1980 house, I found the wiring to be
in excellent condition. No insulation resistance problems, visually the
wiring looks fine. The only point to note is that the cpc on the 2.5mm
T+E is 1mm, not 1.5mm. MCBs disconnect in the event of a fault current
much faster than fuses - if the cable is protected with a fuse, then the
disconnect time and resistance is such that the cable will get too hot
in the event of a fault, possibly causing damage (as I understand it, it
*isn't* a fire risk). Since I now have a shiny new CU with MCBs, no problem.

Since I'm gutting the kitchem, and moving a few sockets around, I'm
going to rewire it anyway, but I'm in no rush to do the rest of the house.

Ben

Indeed. The initial question was one of deterioration of wiring over a
period of time, I think the response was more along the lines of 1980's
electrics being fairly durable and as such should not have deteriorated
unduly unless they have been exposed to exceptional conditions (i.e.
connections arcing due to them being too loose, etc).

Having recently changed my CU, on a 1980 house, I found the wiring to be
in excellent condition. No insulation resistance problems, visually the
wiring looks fine. The only point to note is that the cpc on the 2.5mm
T+E is 1mm, not 1.5mm. MCBs disconnect in the event of a fault current
much faster than fuses - if the cable is protected with a fuse, then the
disconnect time and resistance is such that the cable will get too hot
in the event of a fault, possibly causing damage (as I understand it, it
*isn't* a fire risk). Since I now have a shiny new CU with MCBs, no
problem.

Since I'm gutting the kitchem, and moving a few sockets around, I'm
going to rewire it anyway, but I'm in no rush to do the rest of the house.

Well, that's nice to know - I tend to be immensely unlucky with most things
house related (my house sale has fallen thru twice already and it looks like
it might again unless my buyer gets his arse into gear) so it's good that
there should be no problems

Cheers

Hellraiser..........

In article ,
says...

Having recently changed my CU, on a 1980 house, I found the wiring to be
in excellent condition. No insulation resistance problems, visually the
wiring looks fine. The only point to note is that the cpc on the 2.5mm
T+E is 1mm, not 1.5mm. MCBs disconnect in the event of a fault current
much faster than fuses - if the cable is protected with a fuse, then the
disconnect time and resistance is such that the cable will get too hot
in the event of a fault, possibly causing damage (as I understand it, it
*isn't* a fire risk). Since I now have a shiny new CU with MCBs, no problem.


If the total Zs of the circuit is within the specified limits, the fuse
/ MCB will go within the time required (0.4s or 5s generally). For a 30A
BS3036 fuse, the max Zs is 1.14 ohms, for an equivalent BSEN60898-B MCB
rated at 32A, it is 1.50 ohms. If your circuit is within the BS3036
figure, changing to MCBs is going to really achieve anything, certainly
not on safety grounds. It just makes it more convenient to reset.

If you have a TN-C-S supply, with theoretically maximum Ze of 0.35 ohms,
this allows you to have R1+R2 of 0.79 ohms. Working backwards:

P+CPC = 3.16 ohms (assuming a proper ring circuit)

CPC = 2.5P

Gives P resistance of about 0.90 ohms, or about 120m of cable. Should be
possible to wire the average sized house on one ring and still fall
within the limits for a fuse, let alone an MCB, to meet the 0.4s
disconection time.

wrote:
Despite the popular perception of the dangers of electrickery, the
reality is the death rate is orders of magnitude lower than deaths
caused by stairs, and injuries are orders fo magnitude lower than those
caused by beds, DIY accidents, and so on.

There was thread the other day, kicked off by somebody concerned about
the presence of old rubber wiring in his property; the general consensus
of the replies was that he was virtually advised not to enter the
death-trap of a house until it was sorted out.

Now, I'm thinking there must still be thousands of rubber-wired
properties around the country - I've certainly come across plenty in my
time - and while I'm not for a minute arguing against the desirability
of rewiring ASAP under such circumstances, I'm just wondering how many
fatalities/injuries/house fires can be attributed to this ancient
wiring? Are there stats available? How dangerous *really* is it?

David

Lobster wrote:
wrote:
Despite the popular perception of the dangers of electrickery, the
reality is the death rate is orders of magnitude lower than deaths
caused by stairs, and injuries are orders fo magnitude lower than those
caused by beds, DIY accidents, and so on.

There was thread the other day, kicked off by somebody concerned about
the presence of old rubber wiring in his property; the general consensus
of the replies was that he was virtually advised not to enter the
death-trap of a house until it was sorted out.

Now, I'm thinking there must still be thousands of rubber-wired
properties around the country - I've certainly come across plenty in my
time - and while I'm not for a minute arguing against the desirability
of rewiring ASAP under such circumstances, I'm just wondering how many
fatalities/injuries/house fires can be attributed to this ancient
wiring? Are there stats available? How dangerous *really* is it?

David


This is a very good question, and it would be nice to have some data.
It could tell us more about the safety of both ancient rubber installs
and more modern installs, when all we have is combined data - which I
gather isnt that accurate anyway.

I suppose also the risks would vary by person too. I suspect some would
be more vulnerable to shock than others.

NT

On Wed, 08 Feb 2006 10:18:36 +0000 someone who may be Ben Blaukopf
wrote this:-

MCBs disconnect in the event of a fault current
much faster than fuses

Far too sweeping a generalisation to be correct, though it is a
common misconception often expressed with absolute certainty. There
are fuses and fuses, there are MCBs and MCBs and there are faults
and faults.

Never forget that an MCB takes an irreducible minimum time to
operate. The magnetic coil has to attract a bit of iron, this has to
release a mechanism, springs have to go boing, things have to pivot
and contacts have to move some distance apart before the circuit is
opened. No matter how high the current this minimum time is
irreducible for a particular design. In contrast with all this
mechanical stuff, all a fuse wire has to do is melt. Rapid melting
is something HBC fuse links do very rapidly at high currents, faster
than the fastest MCBs. In fact even rewirable fuse wires melt
rapidly at high currents, though (like many ranges of MCB) they are
unable to break the fault current.

With a small overload of long duration the position is reversed. The
irreducible minimum operating time of the MCB is now irrelevant, as
the controlling factor is how quickly the thermal part of the
mechanism activates the contacts. HBC fuses will take longer to
operate (though one can argue this is an advantage in a properly
designed circuit). Rewirable fuses are rather uncertain in such
circumstances.

MCBs do have advantages of ease of operation and resetting, though
if one operates more than once in a blue moon then there is probably
something wrong with the circuit or something connected to it. They
are thus particularly suitable for those who don't know much about
electricity, provided that the correct range is selected to break
the prospective fault current (or adequate backup provided).

On the other hand some people sleep a bit better at night knowing
that they live near a substation, but if a serious fault was to
develop in their wiring a HBC fuse will react incredibly quickly to
disconnect the particular circuit concerned in far less than a
quarter of a cycle and limit the fault current. With typical MCBs
the same fault would probably have caused the main "electricity
board" fuse to operate, cutting off all electricity, hopefully
before the MCBs had caught fire because they were unable to break
the fault. Some MCBs are a lot better than typical MCBs, but there
are still limits on how fast a series of mechanical components can
move.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:

MCBs do have advantages of ease of operation and resetting, though
if one operates more than once in a blue moon then there is probably
something wrong with the circuit or something connected to it.

This is true, though not the whole story. The time it takes to replace
a fuse wire at a guess 2 or 3 times in the life of an installation is
less than the time taken to go get some mcbs and retrofit them. Oft
overloooked in the rush to '''upgrade.'''

And the cost is 50p.


NT

Gary Cavie wrote:
In article ,
says...

Having recently changed my CU, on a 1980 house, I found the wiring to be
in excellent condition. No insulation resistance problems, visually the
wiring looks fine. The only point to note is that the cpc on the 2.5mm
T+E is 1mm, not 1.5mm. MCBs disconnect in the event of a fault current
much faster than fuses - if the cable is protected with a fuse, then the
disconnect time and resistance is such that the cable will get too hot
in the event of a fault, possibly causing damage (as I understand it, it
*isn't* a fire risk). Since I now have a shiny new CU with MCBs, no problem.



If the total Zs of the circuit is within the specified limits, the fuse
/ MCB will go within the time required (0.4s or 5s generally). For a 30A
BS3036 fuse, the max Zs is 1.14 ohms, for an equivalent BSEN60898-B MCB
rated at 32A, it is 1.50 ohms. If your circuit is within the BS3036
figure, changing to MCBs is going to really achieve anything, certainly
not on safety grounds. It just makes it more convenient to reset.

Those are the maximum impedances to achieve the disconnect time, but not
to avoid thermal damage to the cable.

t = k^2S^2/I^2

Zs is 1.14ohms, so I is 240V/1.14ohms == 210A
t is therefore 0.299s, and if the disconnect time is any longer, the
cable may be damaged by heat. A type B MCB will disconnect much faster
than that because I/In 5, but looking at
http://www.tlc-direct.co.uk/Figures/3.13b.gif
a BS3036 30A fuse will just meet the 400ms disconnect requirement, but
certainly not 299ms.

If we required that Zs was no more than 0.69ohms (plucking a figure from
the OSG), then t becomes 100ms, and S is 350A. At that point, a BS3036
30A fuse still doesn't disconnect fast enough - but a BS88 does - just.
Which is, as far as I can see, why BS88 is okay with 1mm cpc, but BS3036
isn't.

On 9 Feb 2006 02:02:26 -0800 someone who may be
wrote this:-

The time it takes to replace
a fuse wire at a guess 2 or 3 times in the life of an installation is
less than the time taken to go get some mcbs and retrofit them. Oft
overloooked in the rush to '''upgrade.'''

And the cost is 50p.

Agreed. It is even easier to take the spare HBC fuse out of the
carrier and put this in the correct part of the carrier, after one
has located the fault. If one has a "wylex style" board then one may
have to locate the spare fuses, if they are not in a handy place.

Rewirable fuses suffer from the person who thinks they know what
they are doing. Such people can fail to put the wire in the right
places, even if they get the right sized wire.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:

Rewirable fuses suffer from the person who thinks they know what
they are doing. Such people can fail to put the wire in the right
places, even if they get the right sized wire.

hard to imagine... What have you seen done?

NT

On 9 Feb 2006 07:59:51 -0800 someone who may be
wrote this:-

Rewirable fuses suffer from the person who thinks they know what
they are doing. Such people can fail to put the wire in the right
places, even if they get the right sized wire.

hard to imagine

Different carriers have different routes for the wire. If one
doesn't follow this route then the melting wire may melt parts of
the carrier and/or consumer unit.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54