On Aug 2, 2:55*pm, "Dave Plowman (News)" wrote:
In article
,
* wrote:

If it can't achieve the required disconnect time, then it is at best
marginal. (and that's just the old 0.4 sec requirement).
There is no required disconnect time for existing installations.
Required times apply to new wiring. Whether a domestic socket circuit
takes 0.45s or 0.35s to fuse makes absolutely blank all difference to
real world death rates. And /that/ is whats relevant here, nowt else.

One thing with fuses is it's all too easy to fit a larger one to an
overloaded circuit - I've seen this many times.

Yes FWIW In some cases changing from 5A to 10A can remain withint
the cable ratings and be a deliberate upgrade.


NT

In article ,
John Rumm writes:
John Rumm wrote:
Andy Hall wrote:

What are the small grey wires at the bottom? Part of the automation
system?

MK RCBOs have a dedicated earth connection on each device. Not so
commonly found on other brands...

Sorry, brain fart, make that Mem! (MK and Hager are similar though IIRC)

Original MK RCBO's (which I think were made for them by
Merlin Gerin) have an earth connection. They use it for
detecting L-N swapped, and disconnected N, both of which
cause it to trip (and I presume the earth connection
must carry the trip solenoid current in the case of
disconnected N).

Later MK ones also seem to have an earth connection, but
don't include any of the extra supervisory features AFAIK.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Aug 2, 6:28*pm, John Rumm wrote:
wrote:
On Aug 2, 4:00 am, John Rumm wrote:
wrote:
On Aug 1, 1:13 am, John Rumm wrote:
wrote:

Rewirable fuses are still fully compliant for new installation. You'd
derive very little safety benefit from adding MCBs. The reason the
fuses didnt pop is that any trip device takes a cvertain amount of i
times t to blow, and the fault, while it may have looked impressive,
didnt draw enough i x t, and the fault blew itself open circuit first.
You may have been scared by this, but safetywise its pretty much a non-
issue.
While this is true, there are two caveats worth mentioning: If the
cables are the older style PVC with only a 1mm^2 CPC (modern cable has
1.5mm^2) then it is worth moving away from rewireable fuses since the
protection will be marginal or insufficient in many cases.
1mm2 T&E CPC will not have any difficulty at all blowing rewirable
fuses. Its a long way from marginal. What can be marginal is its
compliance with disconnect times for new wiring regulations - but
thats nowhere near enough of an issue to prompt a CU replacment for
99% of the population.
If it can't achieve the required disconnect time, then it is at best
marginal. (and that's just the old 0.4 sec requirement).

There is no required disconnect time for existing installations.
Required times apply to new wiring. Whether a domestic socket circuit
takes 0.45s or 0.35s to fuse makes absolutely blank all difference to
real world death rates. And /that/ is whats relevant here, nowt else.

Disconnect times have been specified in the regs for many years.

Yes, but there is no requirement for existing installs to meet them.
And whats relevant is the real world safety implications.


The
difference of .1 sec etc may not have a huge impact, however when
disconnect times start creeping up into the multiple of seconds then the
risks do become more significant (not only due to prolonged exposure to
high voltage earthed metalwork, but also that the CPC in the cable will
fail, or the cable sustain damage before the fuse operates).

As we have both said, the question of whether a BS 3036 semi-enclosed
fuse CU should be replaced is a broader question that needs to take
account of a number of factors. Compatibility with the installed wiring
is one factor.

I think the biggest factor is the ratio of safety benefit to expense.
Since the safety factor is about zero, and there are mass killers in
the average house, twiddling with the fusebox is both pointless and
counterproductive.


The improving stats for safety of electrical installations in general
would seem to disprove that the safety factor is zero,

those are due to many factors, so cant prove or disprove anything. The
slow disappearance of 1950s and earlier installations is doubtless one
factor.


also blanket
statements about the pros and cons of changing a CU are not really much
use since they concentrate on too small a part of the picture. There
will be times when swapping out a working BS3036 CU for a modern one
will bring no improvement in safety, and there are times when it will
bring about a dramatic and significant improvement.

yes.


If the power
(i.e. socket) circuits currently have no RCD protection then again it is
worth introducing this, which realistically will also mean a CU swap..
Again, the benefit exists but is so small that only a miniscule
percentage of the population would consider it worthwhile to fit the
new CU. Almost no-one rewires their house every time a minor safety
advance comes along.

No, I disagree strongly with this. There is no way you can call a RCD a
"minor safety advance". RCD protection makes a significant impact on the
survivability of electric shocks, and unlike nonsense like part P, the
the use of them has a big impact preventing death and serious injury as
a result of (ab)use of hand held appliances (which as we know is a
significant number, unlike those attributable to fixed wiring faults).

I agree theyre much more significant than fuses/mcbs, but. Twenty
something deaths a year, of which some are down to appliance abuse.

IIUC the = 20 per year are those attributable to fixed wiring I
understand, not the those from appliance abuse.

I thought it was total electrocutions, but I dont have a reference to
hand.


RCDs would indeed stop some of those deaths. But they also encourage
people to do things they never would have before, causing more of the

This is probably true - but only in a very small subset of cases where
the user of the appliance has a good technical appreciation for what the
RCD is and does. IME the vast majority of people have little grasp of
what is in the CU or what it does, other than there being lots of
"little switches" that they need to reset from time to time, and never
necessarily even associate the need to reset them with any other event.

So I would be very surprised if having a CU with RCDs prefitted is going
to influence behaviour for many people.

True enough, but I think there's more to it. An illustration we can
hopefully all agree on...

Decades ago lots of schookids got a shock at some point or another. We
all learnt from that how nasty and scary it was, either directly or by
seeing how shaken our friend was. That taught people about electricity
and changed peoples' behaviour, everyone's.

As safety practices advanced, most kids grew up without learning this.

Now today its the other way round. Kids muck about, short things out
and get a shock, but the RCD trips, and the whole experience is a lot
less nasty. The lesson the rest of the kids learn from this is
different: if you **** about with it, it'll bite very briefly, and the
thingy will trip. That's it.

This influence's almost everyone's behaviour, not just those with some
understanding of the technicalities. Its one of several reasons why
safety innovations so often fail to deliver the benefit that early
promoters claim.

Then here are the many that put themselvs at risk using RCDs. Typical
case: RCD on drill plug. It does nothing to protec the user from
drilling into a live wire. Nothing at all, but they think it does so
are a lot less careful.


(you only need look at the group
of accidents that occur with extension leads in the garden - its not
usually mowing over it that causes the shock, its the dopey pillock
picking up the severed ends to look at them!)



situations, and they cause loss of sair lighting, loss of lighting in
fires etc, which we know kills people. So the real world safety cost
or benefit is anything but clear. We simply don't have the figures
yet.

Well this has been known about for some time (most of the reign of the
16th edition) and remedies put in place to address the problem.

What remedies have you in mind? AIUI the 17th's requirement for RCDs
on all circuits has merely ensured these safety problems will apply to
every house newly wired.


But once again... if you want to save lives in the home, there are
much greater results to be had by spending much less on the real risk
issues.

Again, it depends on circumstances. In situations where students are
likely to overload circuits in their lodging house, and replace fuses
with tinfoil etc, or where there are lots of ankle biters roaming about
trying to push bits of metal into sockets to see what happens, you have
a real risk that needs addressing as a mater of urgency.

In both cases there are much bigger risks. If you look up the death
rates you'll immediately see that. Kids have been putting things in
sockets for over half a decade, but the resultant deaths are few. The
big risks to them lie elsewhere. The deaths from heart disease and
cancer are several orders of magnitude larger, and we know that what
kids do does affect their later life outcomes - just for one example.


In other cases
where a couple of adults share a flat, never need to mow a lawn, and do
nothing more exotic than plug in the TV and the phone charger there is
relatively little risk with 3036 fuses and no RCD.

Its not that bad. There are many millions of houses with rewirables
and no RCD, and the electrocution death rate is tiny.

For various reasons a sense of proportion is too often missing from
safety discussions.


NT

On Aug 2, 9:03*am, Terry Fields wrote:

Are any figures available that show injury and/or deaths rates are
less under the new regime (in this case an RCD CU) than the old (a
fused CU)?

A casual reading of the group suggests that there are a number
potential benefits from the technical advances in going from the
former to the other latter - but how does it work out in practice?


AFAIK the data isnt collected.


NT

wrote:

Disconnect times have been specified in the regs for many years.

Yes, but there is no requirement for existing installs to meet them.
And whats relevant is the real world safety implications.

"Existing installs" must include the large number done during which time
these requirements have been in force. Note that disconnection times
also have a big impact on a secondary risk with electrical installations
- i.e. fire. (as I recall someone was often keen to point out, there are
lots of these each year, and some are electrical in origin ;-)

The
difference of .1 sec etc may not have a huge impact, however when
disconnect times start creeping up into the multiple of seconds then the
risks do become more significant (not only due to prolonged exposure to
high voltage earthed metalwork, but also that the CPC in the cable will
fail, or the cable sustain damage before the fuse operates).

As we have both said, the question of whether a BS 3036 semi-enclosed
fuse CU should be replaced is a broader question that needs to take
account of a number of factors. Compatibility with the installed wiring
is one factor.
I think the biggest factor is the ratio of safety benefit to expense.
Since the safety factor is about zero, and there are mass killers in
the average house, twiddling with the fusebox is both pointless and
counterproductive.


The improving stats for safety of electrical installations in general
would seem to disprove that the safety factor is zero,

those are due to many factors, so cant prove or disprove anything. The
slow disappearance of 1950s and earlier installations is doubtless one
factor.

I think that is partly the point I was making - you can't look at just
one part of the installation and say safe/not safe based on that.

IIUC the = 20 per year are those attributable to fixed wiring I
understand, not the those from appliance abuse.

I thought it was total electrocutions, but I dont have a reference to
hand.

Yup in fact, I am not sure anymore either... looking at some docs that
figure does include both sources, but then others (like those originally
cited in in the Part P RIA spoke of several K deaths/year (before later
acknowledging they had misunderstood their own stats)).

RCDs would indeed stop some of those deaths. But they also encourage
people to do things they never would have before, causing more of the
This is probably true - but only in a very small subset of cases where
the user of the appliance has a good technical appreciation for what the
RCD is and does. IME the vast majority of people have little grasp of
what is in the CU or what it does, other than there being lots of
"little switches" that they need to reset from time to time, and never
necessarily even associate the need to reset them with any other event.

So I would be very surprised if having a CU with RCDs prefitted is going
to influence behaviour for many people.

True enough, but I think there's more to it. An illustration we can
hopefully all agree on...

Decades ago lots of schookids got a shock at some point or another. We
all learnt from that how nasty and scary it was, either directly or by
seeing how shaken our friend was. That taught people about electricity
and changed peoples' behaviour, everyone's.

As safety practices advanced, most kids grew up without learning this.

Now today its the other way round. Kids muck about, short things out
and get a shock, but the RCD trips, and the whole experience is a lot
less nasty. The lesson the rest of the kids learn from this is
different: if you **** about with it, it'll bite very briefly, and the
thingy will trip. That's it.

This influence's almost everyone's behaviour, not just those with some
understanding of the technicalities. Its one of several reasons why
safety innovations so often fail to deliver the benefit that early
promoters claim.

Yup, I understand how many safety improvements can become counter
productive and alluded to that in my comments above.

I don't really have any data to support a general reducing attitude to
shock risk though (or any to the contrary either)

Then here are the many that put themselvs at risk using RCDs. Typical
case: RCD on drill plug. It does nothing to protec the user from
drilling into a live wire. Nothing at all, but they think it does so
are a lot less careful.

To be honest I have never seen anyone use an RCD[1] on a drill. Mowers
and hedge trimmers yes, but never a drill.

[1] as in explicitly make a decision to use one, rather than just
plugging it into a RCD protected circuit since that is what was close.

With modern (class II drills) its unlikely to be an issue either way.

situations, and they cause loss of sair lighting, loss of lighting in
fires etc, which we know kills people. So the real world safety cost
or benefit is anything but clear. We simply don't have the figures
yet.

Well this has been known about for some time (most of the reign of the
16th edition) and remedies put in place to address the problem.

What remedies have you in mind? AIUI the 17th's requirement for RCDs
on all circuits has merely ensured these safety problems will apply to
every house newly wired.

The same remedies apply in the 17th as earlier editions - namely the
requirement for discrimination between faults and circuit protective
devices. Split load CUs, multiple RCDs and RCBOs should ensure that you
don't lose lighting as the result of a single fault elsewhere. The only
time loss of lights as a result of a fault is a notable problem is with
"whole house" RCDs.

But once again... if you want to save lives in the home, there are
much greater results to be had by spending much less on the real risk
issues.

Again, it depends on circumstances. In situations where students are
likely to overload circuits in their lodging house, and replace fuses
with tinfoil etc, or where there are lots of ankle biters roaming about
trying to push bits of metal into sockets to see what happens, you have
a real risk that needs addressing as a mater of urgency.

In both cases there are much bigger risks. If you look up the death
rates you'll immediately see that. Kids have been putting things in
sockets for over half a decade, but the resultant deaths are few. The
big risks to them lie elsewhere. The deaths from heart disease and
cancer are several orders of magnitude larger, and we know that what
kids do does affect their later life outcomes - just for one example.

I was not suggesting that one should focus all efforts on reduction of
risk with electrical installations, there are of course plenty of others
that would benefit from attention. That is not a reason to ignore
potentially dangerous electrics either.

In other cases
where a couple of adults share a flat, never need to mow a lawn, and do
nothing more exotic than plug in the TV and the phone charger there is
relatively little risk with 3036 fuses and no RCD.

Its not that bad. There are many millions of houses with rewirables
and no RCD, and the electrocution death rate is tiny.

Note we are not only talking about electrocution risk here, but also
fire risk, and accident risk directly and indirectly associated with
shocks in general - which is a much bigger number than actual deaths
from electrocution.

For various reasons a sense of proportion is too often missing from
safety discussions.

Quite agree - the big killers tend to be things that are no longer even
news.

--
Cheers,

John.

/================================================== ===============\
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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Andrew Gabriel wrote:

Original MK RCBO's (which I think were made for them by
Merlin Gerin) have an earth connection. They use it for
detecting L-N swapped, and disconnected N, both of which

Interesting point, but I would expect you need quite an inventive
solution to swap L&N on most CUs given the nature of the live connection
to the device... (still I suppose it is easy enough to swap them at
the tails on the way in)


--
Cheers,

John.

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

John Rumm wrote:

Terry Fields wrote:

Are any figures available that show injury and/or deaths rates are
less under the new regime (in this case an RCD CU) than the old (a
fused CU)?

Its a good question, and I don't know how easy it would be to get
figures in the exact form required... Until recently[1] deaths from
electrocution have fallen year on year in absolute terms for quite a
number of years, even though usage of appliances has risen. This won't
be attributable to just one improvement in safety, but a collective
effect of which RCDs have been a part quite a number of years now.

[1] In the last couple of years this trend seems to be showing signs of
reversing - strangely coincident with the introduction of Part P

That was my impression too, based on comments on the ng; but I was
wondering if any officcial figures were available.

It seems to be common for 'the authorities' to set a policy (i.e Part
P) but to fail to put in to place any monitoring of the effects of
that policy. Perhaps it's a question of 'if you don't want to know the
answer, then don't ask the question'.

A casual reading of the group suggests that there are a number
potential benefits from the technical advances in going from the
former to the other latter - but how does it work out in practice?

Not sure I follow the question - do you mean what will be the change in
user experience? or the actual process of making the change?

It was just a convoluted way of trying to link the technicalities such
as the release time for an RCD, or better earthing arrangements, with
reduction or otherwise in casualties, but if there's no figures
collected, then there's no way to link the cause with the effect. It
all seems to run on a 'feel-good' factor.

wrote:

On Aug 2, 9:03*am, Terry Fields wrote:

Are any figures available that show injury and/or deaths rates are
less under the new regime (in this case an RCD CU) than the old (a
fused CU)?

A casual reading of the group suggests that there are a number
potential benefits from the technical advances in going from the
former to the other latter - but how does it work out in practice?

AFAIK the data isnt collected.

NT

One can imagine the conversation:

"How effective has Part P been?"

"Oh, I wouldn't know, we only set policy, we don't monitor it!"

"So you can't say whether it has done any good?"

"Of course it's done good, we wouldn't have brought it in,
otherwise!".

groan

Terry Fields wrote:
John Rumm wrote:

Terry Fields wrote:

Are any figures available that show injury and/or deaths rates are
less under the new regime (in this case an RCD CU) than the old (a
fused CU)?
Its a good question, and I don't know how easy it would be to get
figures in the exact form required... Until recently[1] deaths from
electrocution have fallen year on year in absolute terms for quite a
number of years, even though usage of appliances has risen. This won't
be attributable to just one improvement in safety, but a collective
effect of which RCDs have been a part quite a number of years now.

[1] In the last couple of years this trend seems to be showing signs of
reversing - strangely coincident with the introduction of Part P

That was my impression too, based on comments on the ng; but I was
wondering if any officcial figures were available.

It seems to be common for 'the authorities' to set a policy (i.e Part
P) but to fail to put in to place any monitoring of the effects of
that policy. Perhaps it's a question of 'if you don't want to know the
answer, then don't ask the question'.

IIRC there was a question asked in the house recently on the apparent
rise coincident with part P. A search of Hansard will probably turn up
something.

A casual reading of the group suggests that there are a number
potential benefits from the technical advances in going from the
former to the other latter - but how does it work out in practice?
Not sure I follow the question - do you mean what will be the change in
user experience? or the actual process of making the change?

It was just a convoluted way of trying to link the technicalities such
as the release time for an RCD, or better earthing arrangements, with
reduction or otherwise in casualties, but if there's no figures
collected, then there's no way to link the cause with the effect. It
all seems to run on a 'feel-good' factor.

There are figures available and well documented for the effects of shock
and the implications of shock current and duration of exposu

http://www.memonline.com/rcd3.html

So RCD protection offers very much more than a "feel good factor" -
unless you include the ability to breathe and absence of ventricular
fibrillation as "feel good" ;-)

Note also RCDs are used not only to lessen shock risks, but also as a
earth fault protection mechanism. A TT install with a 6 ohms fault loop
impedance could end up sinking 40A for an extended period of time into a
protective conductor without tripping a typical 32A MCB on a power
circuit. This would (in the absence of the RCD) result in at best cable
damage, and possibly fire.



--
Cheers,

John.

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