On Sun, 2008-08-31 at 18:02 +0000, malc wrote:
Dave Liquorice wrote:

I should hope it places a resistor between L and N and across the
current transformer. So that it actually tests what it is supposed to
be doing rather than relying on another path that might not exist.

But a resistor between L & N only simulates a load so it won't trip the RCD.
A RCD relies on an imbalance between the current in the Live wire and the
current in the Neutral wire. So the test will put a resistor between L and
Earth or between N and Earth. If you put a resistor between L & N the same
current flows in both wires so won't trip the breaker.

The resistor goes from output L to input N (or vice-versa), bypassing
one side of the transformer.

Mike

On 31/08/2008 19:02, malc wrote:

But a resistor between L & N only simulates a load so it won't trip the RCD.
A RCD relies on an imbalance between the current in the Live wire and the
current in the Neutral wire. So the test will put a resistor between L and
Earth or between N and Earth. If you put a resistor between L & N the same
current flows in both wires so won't trip the breaker.

But the way Dave showed the resistor and "T" button diagonally across
the current transformer, it would register as an imbalance and cause it
to trip, besides RCDs that I've fitted don't *have* an earth connecton.

Trust me I work with electricity every day.

Hmmmm.

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Dave Plowman (News)"
saying something like:

The point is that other countries could have adopted this and did not

Can you name any developed country which has changed their plug and socket
design - apart from the UK?

cough
--
Dave
GS850x2 XS650 SE6a

"It's a moron working with power tools.
How much more suspenseful can you get?"
- House

Andy Burns wrote:

But the way Dave showed the resistor and "T" button diagonally across
the current transformer, it would register as an imbalance and cause
it to trip, besides RCDs that I've fitted don't *have* an earth
connecton.

Bloody ASCII art.

If there is no earth connection then it could still work as an RCD because
RCDs rely on the leakage created by a fault down to earth/through you or
whatever. So long as the current comes out of the live and doesn't return to
neutral there will be an imbalance and the device will trip [1]


Trust me I work with electricity every day.

Hmmmm.

Heh!

[1] OK I know we're talking about AC here but for simplicity's sake...

--
Malc
R1100RS old and tatty

You laugh at me because I am different
I laugh at you because you are all the same

On 31/08/2008 21:20, malc wrote:

Bloody ASCII art.

:-)

If there is no earth connection then it could still work as an RCD because
RCDs rely on the leakage created by a fault down to earth/through you or
whatever. So long as the current comes out of the live and doesn't return to
neutral there will be an imbalance and the device will trip [1]

Oh sure, in normal operation the current imbalance can arise because
some of the current returns via the earth conductor, or the actual muddy
earth, or the "wrong" neutral. I was just talking about how the test
button would cause an imbalance (without requiring an earth connection
to the RCD itself).

Anthony R. Gold wrote:

Codes live and evolve. It was suggested that just adding fuses to exiting
plugs would be "a good idea" and there is nothing in the design of the

I am not sure the suggestion (in itself) holds much merit. In many cases
you would be fusing the plug at or near that of the circuit. The result
would be an inability to discriminate in the event of a fault. This
would just increase service and rectification times.

It would only be a worthwhile exercise if you also increased the current
capacity of the standard circuits. However that would at a stroke
require all the plugs used on it to be fused. So it seems unlikely to
happen since the changeover would be too costly etc.


--
Cheers,

John.

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

Anthony R. Gold wrote:

Turn it round the other way, can you justify the need for a plug fuse
just on the basis of having parallel connection paths to the circuit
breaker? (or for that matter any other unique characteristic of a ring
circuit other than its high current delivery capacity)

I understand the sequence to be that post-war poverty and copper shortage
led to the power ring. The power ring resulted in bringing 3kW to each and

Yup, the original reason for the use of ring circuits was partly to save
copper. It was also IIUC, the intention to be able to supply enough
power for day to day needs and also heat a house if required. Hence =
7kW per circuit, and circuits being able to cover a wide floor area.

every power outlet, and having 3kW power available everywhere requires
additional protection for appliance cords and flexes. AFAIK no country
wires radially with the requirement that 3kW is available to each appliance
outlet.

We don't here either - with rings or radials.

The circuit designs assume diversity - i.e. the assumption that not
every outlet will require the full 13A. This matches real world usage
rather well.

IMO, in this chicken and egg argument, it is povertyring3kWfused plugs.
but you may see it differently.

I do, because a ring is not the only circuit that requires protection at
the plug. Also there is nothing intrinsic to the ring itself that
requires the fusing, only the magnitude of the current. Since radials of
equal power are also permitted in our wiring regs, it seems
inappropriate to attribute the requirement for fusing to a ring circuit.

--
Cheers,

John.

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

In article ,
John Rumm writes:
Anthony R. Gold wrote:

Codes live and evolve. It was suggested that just adding fuses to exiting
plugs would be "a good idea" and there is nothing in the design of the

I am not sure the suggestion (in itself) holds much merit. In many cases
you would be fusing the plug at or near that of the circuit. The result
would be an inability to discriminate in the event of a fault. This
would just increase service and rectification times.

It would only be a worthwhile exercise if you also increased the current
capacity of the standard circuits. However that would at a stroke
require all the plugs used on it to be fused. So it seems unlikely to
happen since the changeover would be too costly etc.

You can't just consider plugs in isolation. The 13A plug
was one part of a whole new wiring scheme, which included
the circuit layout, circuit protection, earthing, area
power requirements, etc.

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

Andrew Gabriel wrote:
In article ,
John Rumm writes:
Anthony R. Gold wrote:

Codes live and evolve. It was suggested that just adding fuses to exiting
plugs would be "a good idea" and there is nothing in the design of the
I am not sure the suggestion (in itself) holds much merit. In many cases
you would be fusing the plug at or near that of the circuit. The result
would be an inability to discriminate in the event of a fault. This
would just increase service and rectification times.

It would only be a worthwhile exercise if you also increased the current
capacity of the standard circuits. However that would at a stroke
require all the plugs used on it to be fused. So it seems unlikely to
happen since the changeover would be too costly etc.

You can't just consider plugs in isolation. The 13A plug
was one part of a whole new wiring scheme, which included
the circuit layout, circuit protection, earthing, area
power requirements, etc.

Indeed, so a non starter really...

--
Cheers,

John.

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

On Sun, 31 Aug 2008 18:02:28 GMT, malc wrote:

I should hope it places a resistor between L and N and across the
current transformer.

But a resistor between L & N only simulates a load so it won't trip the
RCD.

Correct.

A RCD relies on an imbalance between the current in the Live wire and
the current in the Neutral wire.

Correct.

So the test will put a resistor between L and Earth or between N and
Earth.

They might but then they are not producing a reliable test of the current
transformer. The above test relies on an external circuit (the Earth) that
an RCD does not require.

If you put a resistor between L & N the same current flows in both wires
so won't trip the breaker.

True enough but read what I wrote not what you think I wrote.

Trust me I work with electricity every day.

I'd rather not, your basic understanding seems to be some what lacking.

--
Cheers
Dave.

On Aug 31, 11:37*pm, "Dave Liquorice"
wrote:


Trust me I work with electricity every day.

I'd rather not, your basic understanding seems to be some what lacking.

Heh. So if you need a radiotherapy machine sorting don't come to me
then...

--
Malc

"Dave Liquorice" wrote in message
ll.net...
On Sun, 31 Aug 2008 18:02:28 GMT, malc wrote:

I should hope it places a resistor between L and N and across the
current transformer.

But a resistor between L & N only simulates a load so it won't trip the
RCD.

Correct.

A RCD relies on an imbalance between the current in the Live wire and
the current in the Neutral wire.

Correct.

So the test will put a resistor between L and Earth or between N and
Earth.

They might but then they are not producing a reliable test of the current
transformer. The above test relies on an external circuit (the Earth) that
an RCD does not require.

If you put a resistor between L & N the same current flows in both wires
so won't trip the breaker.

True enough but read what I wrote not what you think I wrote.

Trust me I work with electricity every day.

I'd rather not, your basic understanding seems to be some what lacking.

AIUI you either need a resistor to earth, which could potentially raise the
earth voltage to mains if its faulty, so would be unwise, or put a resistor
across one side of the current transformer, diverting some of the current
and tripping the device.
Maybe it would be easier to just short out a few turns to make it
unbalanced?

In article
,
Malc wrote:
On Aug 31, 11:37 pm, "Dave Liquorice"
wrote:


Trust me I work with electricity every day.

I'd rather not, your basic understanding seems to be some what lacking.

Heh. So if you need a radiotherapy machine sorting don't come to me
then...

No wonder the health service is in a state, then.

You stated you need to provide an earth path to test an RCD in response
to Mr Liquorice's post. You patently have no knowledge of a domestic CU
otherwise you'd know there is no earth connection to the RCD(s) - yet a
test button is provided.

--
*TEAMWORK...means never having to take all the blame yourself *

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

"Anthony R. Gold" wrote in message
...
On Mon, 1 Sep 2008 08:51:07 +0100, "dennis@home"
wrote:

AIUI you either need a resistor to earth, which could potentially raise
the
earth voltage to mains if its faulty, so would be unwise, or put a
resistor
across one side of the current transformer, diverting some of the current
and tripping the device.
Maybe it would be easier to just short out a few turns to make it
unbalanced?

Neither putting a resistor across one side of the current transformer nor
shorting part of the current transformer would allow the trip test button
to work without an operating load. Why not just listen to Dave?

Maybe, but I didn't say it didn't put a load across the coils too.
I pointed out how to get an imbalance not how to make one.

In uk.d-i-y, Andrew Gabriel wrote:
In article ,
Mike Barnes writes:
In uk.d-i-y, John Rumm wrote:
The crux of the issue is that if you have a circuit capable of
supplying a current significantly in excess of the rating of the
appliance flex, then you need a overcurrent protected plug. So its not
really anything to do with the circuit being a ring or not, even if
that is the most commonly encountered 32A circuit in the UK.

I agree, but most of the world doesn't have fused plugs. So is there a
major problem with, say, those 3A flexes running to laptop power bricks,
plugged into a radial circuit limited to (say) 16A??

They have to be short, so they can pass the fault current
required to trip a 16A breaker. In practice, this doesn't
seem to be a problem. The only effect in the UK when our
appliances became subject to EU rules in this area was that
the lengths of such flexs are now limited to 2 or 3 metres.
When using a 3A fused plug, the limit on length was much
longer than was ever likely to be encountered.

Most appliances don't have a failure mode whereby they can
overload their flex, so there isn't a risk of running 15A
continuously through a 3A flex. The flex does have to handle
a fault current though (dead short), and pass enough current
so that the 16A breaker trips before the flex overheats.
This requires the flex resistance to be low, and hence the
limit on flex lengths.

The one item which completely screws up such design
considerations is an extension cable. These are routinely
available in lengths and sizes which are not intrinsicly
safe. Even if they weren't, people would still plug them
together to contrive to make them so.

Thanks, that all makes perfect sense to me. Is that 2 or 3 metre flex
length limit for a 16A breaker or for a 32A breaker? And with such a
short flex would it be permissible to use a 13A fuse in the plug?

--
Mike Barnes

"dennis@home" wrote in message
...


"Anthony R. Gold" wrote in message
...
On Mon, 1 Sep 2008 08:51:07 +0100, "dennis@home"
wrote:

AIUI you either need a resistor to earth, which could potentially raise
the
earth voltage to mains if its faulty, so would be unwise, or put a
resistor
across one side of the current transformer, diverting some of the
current
and tripping the device.
Maybe it would be easier to just short out a few turns to make it
unbalanced?

Neither putting a resistor across one side of the current transformer nor
shorting part of the current transformer would allow the trip test button
to work without an operating load. Why not just listen to Dave?

Maybe, but I didn't say it didn't put a load across the coils too.
I pointed out how to get an imbalance not how to make one.




The trouble with this thread is that you're both right!
1. The 'domestic' style RCD does not have an earth connection.
2. The test resistor does go between the L & N poles

but

3. The resistor goes between the L load terminal and the N supply
terminal. This unbalances the RCD by taking the test current through the
RCD but not allowing it to pass back through the device. Cheap, simple and
mostly effective.

I hope that this clears up the working of the RCD which will therefore work
whichever polarity is connected to it. So now we're back to the original
problem of getting the polarity correct when connecting a continental supply
to a UK & Ireland caravan.

Pedro

In article ,
Mike Barnes writes:
In uk.d-i-y, Andrew Gabriel wrote:
In article ,
Mike Barnes writes:
I agree, but most of the world doesn't have fused plugs. So is there a
major problem with, say, those 3A flexes running to laptop power bricks,
plugged into a radial circuit limited to (say) 16A??

They have to be short, so they can pass the fault current
required to trip a 16A breaker. In practice, this doesn't
seem to be a problem. The only effect in the UK when our
appliances became subject to EU rules in this area was that
the lengths of such flexs are now limited to 2 or 3 metres.
When using a 3A fused plug, the limit on length was much
longer than was ever likely to be encountered.

Most appliances don't have a failure mode whereby they can
overload their flex, so there isn't a risk of running 15A
continuously through a 3A flex. The flex does have to handle
a fault current though (dead short), and pass enough current
so that the 16A breaker trips before the flex overheats.
This requires the flex resistance to be low, and hence the
limit on flex lengths.

The one item which completely screws up such design
considerations is an extension cable. These are routinely
available in lengths and sizes which are not intrinsicly
safe. Even if they weren't, people would still plug them
together to contrive to make them so.

Thanks, that all makes perfect sense to me. Is that 2 or 3 metre flex
length limit for a 16A breaker or for a 32A breaker? And with such a
short flex would it be permissible to use a 13A fuse in the plug?

It's for a 16A breaker. Any modern appliance with a 13A plug
also needs to remain safe when moved elsewhere in the EU and
connected to a 16A outlet without a fused plug.

You are correct that in theory you can use a 13A fuse in all
appliance plugs nowadays, except for old appliances with thin flexs
which are longer than EU regs now allow, and extension cords.

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

On Mon, 1 Sep 2008 00:26:26 -0700 (PDT), Malc wrote:

Trust me I work with electricity every day.

I'd rather not, your basic understanding seems to be some what lacking.

Heh. So if you need a radiotherapy machine sorting don't come to me
then...

TBH the thought of someone with your lack of basic knowledge, or perhaps
the inabilty to admit a mistake, being allowed to mess with radio therapy
machines worries me.

--
Cheers
Dave.

On Mon, 1 Sep 2008 15:35:37 +0100, Pedro wrote:

I hope that this clears up the working of the RCD which will therefore
work whichever polarity is connected to it. So now we're back to the
original problem of getting the polarity correct when connecting a
continental supply to a UK & Ireland caravan.

Which brings us back to RCDs not being polarity sensitive and if tripping
when connected with "reverse" polarity indicates something a miss with the
caravans wiring after the RCD.

I suspect that the incoming "neutral" is bonded to the caravan chassis and
the chassis is also connected to the incoming "earth". This is incorrect,
after the RCD the three supply wires "live", "neutral" and "earth" should
be kept and remain seperate. If a given the supply only provides "live"
and "neutral" then the caravan chassis must be connected to real ground
with an earth spike.

--
Cheers
Dave.

On 1 Sep, 09:25, "Anthony R. Gold" wrote:

Neither putting a resistor across one side of the current transformer nor
shorting part of the current transformer would allow the trip test button
to work without an operating load. *Why not just listen to Dave?

That's not what Dave wrote.

A resistor between L&N, but bypassing half of the current
transformer's bifilar winding (this is what Dave's diagram showed),
_will_ trigger the RCD without any load connected.

Shorting half of the transformer will need a load current too, agreed.

"Dave Liquorice" wrote in message
ll.net...
On Mon, 1 Sep 2008 15:35:37 +0100, Pedro wrote:

I hope that this clears up the working of the RCD which will therefore
work whichever polarity is connected to it. So now we're back to the
original problem of getting the polarity correct when connecting a
continental supply to a UK & Ireland caravan.

Which brings us back to RCDs not being polarity sensitive and if tripping
when connected with "reverse" polarity indicates something a miss with the
caravans wiring after the RCD.

I suspect that the incoming "neutral" is bonded to the caravan chassis and
the chassis is also connected to the incoming "earth". This is incorrect,
after the RCD the three supply wires "live", "neutral" and "earth" should
be kept and remain seperate. If a given the supply only provides "live"
and "neutral" then the caravan chassis must be connected to real ground
with an earth spike.

--
Cheers
Dave.



The UK & Ireland Wiring Regulations specifically forbid any connection other
than the PE (earth) to be made to any exposed metalwork in/on the caravan.
This means that it is illegal to connect the neutral to the chassis either
before or after the RCD. In the event of a polarity reversal someone could
be killed just stepping into or out of the van.

A caravan without an RCD (it is not legal to sell a van without an RCD in
the UK & Ireland and I think in western Europe) 'SHALL have' a protective
earthing spike which has been tested to ensure its effectiveness in meeting
the requirements of the Regulations. Clearly this is not practical with Joe
Public pitching his/her tourer. I can just see the site owner's face when
asked "What is the prospective short circuit capability of your supply and
what type and rating of protective device is immediately upstream of your
point of user supply?" On some sites I've been on this would then result in
me driving in at least 3 metres of earth rod alongside the van - probably
going through a cable or pipe at that depth!!

Before someone queries the first sentence, it is legal for the 12 volt
system to have one pole (usually the negative) connected to the
earth/chassis. Some manufacturers do this others don't. It can slightly
increase safety under certain fault conditions.

Pedro

"Anthony R. Gold" wrote in message
...
On Tue, 2 Sep 2008 03:13:51 -0700 (PDT), Andy Dingley
wrote:

On 1 Sep, 09:25, "Anthony R. Gold" wrote:

Neither putting a resistor across one side of the current transformer
nor
shorting part of the current transformer would allow the trip test
button
to work without an operating load. Why not just listen to Dave?

That's not what Dave wrote.

A resistor between L&N, but bypassing half of the current
transformer's bifilar winding (this is what Dave's diagram showed),
_will_ trigger the RCD without any load connected.

Shorting half of the transformer will need a load current too, agreed.

Maybe you are not using a threaded reader - I was replying to Dennis and
contradicting him and I was agreeing with Dave.

Yes he was contradicting something that was correct.
Best leave him alone and see if he learns anything. 8-)

Tony

"Lobster" wrote in message
...
Malcolm wrote:
Lobster wrote:
I'm in the middle of wiring up a cable for an electrical hook up to be
used while camping in France. I have a French 3-pin plug bought there
on a previous visit, but in opening it up I find there's absolutely no
indication which of the two power pins is live and neutral. Doesn't it
matter?

I will be using the cable with an RCD device; won't that object and trip
out if the wiring's the wrong way round? Or is it a case of "suck it
and see" when I get to France, and if it's wrong, reverse the wiring?

Thanks
David
A number of years ago when caravanning in France/Spain I was warned by a
friend about polarity problems when using an RCD. I fitted a switch
(DPDT) the the (old) van so that I could easily change polarity. Sure
enough on one site in France when a new arrival plugged my leadinto a
different socket on the same set of sockets I discovered that the system
had trippedbecause the polarity was reversed. On a site in Spain this
polarity reversal happened without the lead being unplugged!

You need three adaptors
16A blue to 16 A blue, polarity reversed
16A blue to continental
16A blue to continental, polarity reversed

If the RCD trips and won't reset, reverse the polarity. Our continental
cousins don't have the concept of live and neutral that we do

OK, thanks - think I've got it now! Will organise some more hardware
before my next trip.

I don't intend to use any power without an RCD, but just for my education
and information, if I *were* to do so, would all my kit
(laptop/fridge/chargers) all work OK without problems, then?

What do the French etc do about this polarity problem - do they not bother
with RCDs while camping or whatever?

David

Sorry 'Lobster' I didn't get back to you, I thinks there are all your
answers on here already!
Steve.

John Mann wrote:
In message , Lobster
writes
Malcolm wrote:
Lobster wrote:
I'm in the middle of wiring up a cable for an electrical hook up
to
be used while camping in France. I have a French 3-pin plug
bought
there on a previous visit, but in opening it up I find there's
absolutely no indication which of the two power pins is live and
neutral. Doesn't it matter?


However as much of Europe uses 2 pin reversible
plugs it can obviously be connected either way.

Which, I would have thought, is the obvious and simple answer :-)

--

Geoff

David Klyne wrote:

OK, thanks - think I've got it now! Will organise some more hardware
before my next trip.

I don't intend to use any power without an RCD, but just for my education
and information, if I *were* to do so, would all my kit
(laptop/fridge/chargers) all work OK without problems, then?

What do the French etc do about this polarity problem - do they not bother
with RCDs while camping or whatever?

Generally reverse polarity will make little difference to 230 volt
appliances in good working order. The problem with UK appliances occurs if
the appliance has a fault and remains on when you think it is off as we only
switched on the live side. Mind you newer caravans don't seem to have socket
switches now which makes it a bit more difficult. If its any help this is
how I have explained it on my website
http://www.davidklyne.plus.com/frenc...anelectric.htm The best option
is the have a short cable with CEE17 male and sockets at either end with one
end reversed. This can then be used with either you manin cable or with the
continental adaptor. At least that works for me. Have a good trip.

Thanks for all the responses. In the event the power supply was via
standard UK blue sockets, all the electrics worked OK, and nobody got
fried! [notwithstanding France being as wet as the UK apparently was
last week :-( ]

David

David