Just curious.

Saw an installation (wash hand basin) where the copper pipes came out of the 
wall into plastic push fit elbows, then a short run of copper, then plastic 
push fit tap connectors.

The earth bonding cables were connected to the copper pipes between the 
plastic elbow & plastic tap connector.

Looking at it, the cables seemed 'isolated' from the taps & pipes by the 
plastic fittings, then I realised that the pipes are full of water.

Is that how it works?  Would it still protect if the system were drained 
down?


-- 
Dave - The Medway Handyman
www.medwayhandyman.co.uk

On Sat, 28 Jun 2008 09:24:55 +0000, The Medway Handyman wrote:

> Just curious.
> 
> Saw an installation (wash hand basin) where the copper pipes came out of
> the wall into plastic push fit elbows, then a short run of copper, then
> plastic push fit tap connectors.
> 
> The earth bonding cables were connected to the copper pipes between the
> plastic elbow & plastic tap connector.
> 
> Looking at it, the cables seemed 'isolated' from the taps & pipes by the
> plastic fittings, then I realised that the pipes are full of water.
> 
> Is that how it works?  Would it still protect if the system were drained
> down?

This is an example of gold plated compliance with the regs. No 
supplementary bonding is needed if there's less than 0.5m of Copper pipes 
in the supply.


-- 
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk 
Gas fitting FAQ  http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On 2008-06-28, The Medway Handyman  wrote:
> Just curious.
>
> Saw an installation (wash hand basin) where the copper pipes came out of the 
> wall into plastic push fit elbows, then a short run of copper, then plastic 
> push fit tap connectors.
>
> The earth bonding cables were connected to the copper pipes between the 
> plastic elbow & plastic tap connector.
>
> Looking at it, the cables seemed 'isolated' from the taps & pipes by the 
> plastic fittings, then I realised that the pipes are full of water.
>
> Is that how it works? 

No. Tap water doesn't conduct electricity that well.


-- 
          "Be thankful that you have a life, and forsake your vain
                 and presumptuous desire for a second one."
               [email me at huge {at} huge (dot) org <dot> uk]

The Medway Handyman wrote:

Spose I better say it before someone else does...

No such thing as "earth bonding". There is earthing, and there is 
equipotential bonding (main and supplementary), but they serve different 
purposes.

> Saw an installation (wash hand basin) where the copper pipes came out of the 
> wall into plastic push fit elbows, then a short run of copper, then plastic 
> push fit tap connectors.

Kind of pointless. Note that if you are complying with the 17th edition 
there is no longer a requirement for supplementary bonding in rooms 
containing a bath or shower. A room with just a hand basin and a loo 
say, did not require it under the 16th edition either.

> The earth bonding cables were connected to the copper pipes between the 
> plastic elbow & plastic tap connector.
> 
> Looking at it, the cables seemed 'isolated' from the taps & pipes by the 
> plastic fittings, then I realised that the pipes are full of water.
> 
> Is that how it works?  Would it still protect if the system were drained 
> down?

No, just someone getting a bit overenthusiastic.

-- 
Cheers,

John.

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

"John Rumm"  wrote in message 
news:b4udndsQ_Kpi3PvVRVnyhAA@posted.plusnet...
> The Medway Handyman wrote:
>
> Spose I better say it before someone else does...
>
> No such thing as "earth bonding". There is earthing, and there is 
> equipotential bonding (main and supplementary), but they serve different 
> purposes.
>
>> Saw an installation (wash hand basin) where the copper pipes came out of 
>> the wall into plastic push fit elbows, then a short run of copper, then 
>> plastic push fit tap connectors.
>
> Kind of pointless. Note that if you are complying with the 17th edition 
> there is no longer a requirement for supplementary bonding in rooms 
> containing a bath or shower. A room with just a hand basin and a loo say, 
> did not require it under the 16th edition either.
>

The best I have seen is a full plastic pipework job for a loo and sink job 
and someone had put clamps around the tap tails and bonded the taps 
together!

Dave, The electrical resistance of water is here

http://www.hep2o.co.uk/bititesguideearth.htm

Although this will not fully answer your question.

Adam

John Rumm wrote:
> The Medway Handyman wrote:
>
> Spose I better say it before someone else does...
>
> No such thing as "earth bonding". There is earthing, and there is
> equipotential bonding (main and supplementary), but they serve
> different purposes.
>

Could you expand on that John - or point me to somewhere I could read up on 
it?

>> Saw an installation (wash hand basin) where the copper pipes came
>> out of the wall into plastic push fit elbows, then a short run of
>> copper, then plastic push fit tap connectors.
>
> Kind of pointless. Note that if you are complying with the 17th
> edition there is no longer a requirement for supplementary bonding in
> rooms containing a bath or shower. A room with just a hand basin and
> a loo say, did not require it under the 16th edition either.
>
>> The earth bonding cables were connected to the copper pipes between
>> the plastic elbow & plastic tap connector.
>>
>> Looking at it, the cables seemed 'isolated' from the taps & pipes by
>> the plastic fittings, then I realised that the pipes are full of
>> water. Is that how it works?  Would it still protect if the system were
>> drained down?
>
> No, just someone getting a bit overenthusiastic.

So, would these bonding wires be better if they were connected to the copper 
pipes coming out of the wall?


-- 
Dave - The Medway Handyman
www.medwayhandyman.co.uk

The Medway Handyman wrote:
> John Rumm wrote:
>> The Medway Handyman wrote:
>>
>> Spose I better say it before someone else does...
>>
>> No such thing as "earth bonding". There is earthing, and there is
>> equipotential bonding (main and supplementary), but they serve
>> different purposes.
>>
> 
> Could you expand on that John - or point me to somewhere I could read up on 
> it?

Good earthing will ensure that in the event of a fault the supply can be 
disconnected quickly by providing a path for a large fault current to 
flow - hence causing rapid operation of a Circuit Protective Device. It 
will also limit the voltage rise of anything earthed that you can touch 
to a safe level (under 50V typically) during the fault. So if a live 
wire falls off the inside of your washing machine and makes contact with 
the case, the big fault current will open the fuse in the plug or 
circuit breaker on the circuit quickly, and should you also happen to be 
touching the machine at the time, the voltage you are exposed to should 
be low enough to not place you at significant shock risk.

Supplementary equipotential bonding is not designed to clear a fault 
condition, or limit the absolute touch voltage. What it is supposed to 
do is electrically tie together any pipe or service or other part that 
could under fault conditions introduce a dangerous potential into the 
room. In doing this it attempts to create a Faraday cage - where 
anything and everything that you can touch will be at the same 
electrical potential[1] as everything else - even if it is elevated way 
above true ground. So for example a fault that could leave a hot tap at 
230V due to it being connected to a faulty inline water heater, and a 
well earthed cold tap connected directly to a rising main, would pose a 
serious shock hazard - touching both taps would expose you to a 230V 
potential difference and a "stiff" supply. If the taps are bonded 
together (and anything else that may take on an elevated voltage under 
fault conditions like the earth wires of any circuits feeding power into 
the room), there is the possibility that both taps will rise to 230V 
under fault conditions - however touching both exposes you to zero volts 
of potential difference. (equipotential zones only work well where you 
are insulated from any true earth reference - its no good making sure 
that all the taps float up to the same voltage if you are standing on 
something that is conductive and is going to stay put at 0V).

[1] in reality the bonding may fail to tie all elements together at 
exactly the same potential, however it must limit any potential 
difference to 50V or under.


>>> Saw an installation (wash hand basin) where the copper pipes came
>>> out of the wall into plastic push fit elbows, then a short run of
>>> copper, then plastic push fit tap connectors.
>> Kind of pointless. Note that if you are complying with the 17th
>> edition there is no longer a requirement for supplementary bonding in
>> rooms containing a bath or shower. A room with just a hand basin and
>> a loo say, did not require it under the 16th edition either.
>>
>>> The earth bonding cables were connected to the copper pipes between
>>> the plastic elbow & plastic tap connector.
>>>
>>> Looking at it, the cables seemed 'isolated' from the taps & pipes by
>>> the plastic fittings, then I realised that the pipes are full of
>>> water. Is that how it works?  Would it still protect if the system were
>>> drained down?
>> No, just someone getting a bit overenthusiastic.
> 
> So, would these bonding wires be better if they were connected to the copper 
> pipes coming out of the wall?

There are a range of possible answers to this depending on circumstance.

Generally with plastic pipe installations, supplementary bonding of the 
pipes is not required (although it may be required between the earths of 
  say lighting and power circuits if they are both accessible in the 
room plus any other extraneous metallic parts (CH Pipes for example)).

In rooms which are not "special locations" (i.e. places likely to put 
you at increased danger from electric shock (typically because you might 
be wet)), there is also no requirement. So for example a cloakroom with 
loo and sink would not require it )although it is commonly seen in such 
circumstances).

Under the 17th edition it is also sometimes permitted to have no EQ 
bonding in a room with a bath or shower providing that the main EQ 
bonding is in place, and all the circuits feeding the room have 
additional protection from a RCD with 30mA (or lower) trip threshold.

The one you would need to take care with is where you have say a 
bathroom where there is some quantity of exposed copper that is in turn 
connected to plastic pipe and then the taps. If the exposed pipe is 
likely to be accessible then it could pose a risk an should be bonded, 
even if the taps don't since they are isolated  via plastic pipe.

-- 
Cheers,

John.

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

>
> Good earthing will ensure that in the event of a fault the supply can be
> disconnected quickly by providing a path for a large fault current to
> flow - hence causing rapid operation of a Circuit Protective Device. It
> will also limit the voltage rise of anything earthed that you can touch
> to a safe level (under 50V typically) during the fault. So if a live
> wire falls off the inside of your washing machine and makes contact with
> the case, the big fault current will open the fuse in the plug or
> circuit breaker on the circuit quickly, and should you also happen to be
> touching the machine at the time, the voltage you are exposed to should
> be low enough to not place you at significant shock risk.
>
> Supplementary equipotential bonding is not designed to clear a fault
> condition, or limit the absolute touch voltage. What it is supposed to
> do is electrically tie together any pipe or service or other part that
> could under fault conditions introduce a dangerous potential into the
> room. In doing this it attempts to create a Faraday cage - where
> anything and everything that you can touch will be at the same
> electrical potential[1] as everything else - even if it is elevated way
> above true ground. So for example a fault that could leave a hot tap at
> 230V due to it being connected to a faulty inline water heater, and a
> well earthed cold tap connected directly to a rising main, would pose a
> serious shock hazard - touching both taps would expose you to a 230V
> potential difference and a "stiff" supply. If the taps are bonded
> together (and anything else that may take on an elevated voltage under
> fault conditions like the earth wires of any circuits feeding power into
> the room), there is the possibility that both taps will rise to 230V
> under fault conditions - however touching both exposes you to zero volts
> of potential difference. (equipotential zones only work well where you
> are insulated from any true earth reference - its no good making sure
> that all the taps float up to the same voltage if you are standing on
> something that is conductive and is going to stay put at 0V).
>
> [1] in reality the bonding may fail to tie all elements together at
> exactly the same potential, however it must limit any potential
> difference to 50V or under.
>
>
>
> There are a range of possible answers to this depending on circumstance.
>
> Generally with plastic pipe installations, supplementary bonding of the
> pipes is not required (although it may be required between the earths of
>   say lighting and power circuits if they are both accessible in the
> room plus any other extraneous metallic parts (CH Pipes for example)).
>
> In rooms which are not "special locations" (i.e. places likely to put
> you at increased danger from electric shock (typically because you might
> be wet)), there is also no requirement. So for example a cloakroom with
> loo and sink would not require it )although it is commonly seen in such
> circumstances).
>
> Under the 17th edition it is also sometimes permitted to have no EQ
> bonding in a room with a bath or shower providing that the main EQ
> bonding is in place, and all the circuits feeding the room have
> additional protection from a RCD with 30mA (or lower) trip threshold.
>
> The one you would need to take care with is where you have say a
> bathroom where there is some quantity of exposed copper that is in turn
> connected to plastic pipe and then the taps. If the exposed pipe is
> likely to be accessible then it could pose a risk an should be bonded,
> even if the taps don't since they are isolated  via plastic pipe.
>
> --
> Cheers,
>
> John.
>
Now this answer is a example of why this group is so good. I knew quite a
lot of what is in it, but I don't think I have ever seen such a concise and
comprehensive summary of the subject, and moreover written in plain English.
Thanks John!

Charles F

CJF wrote:
>> Good earthing will ensure that in the event of a fault the supply
>> can be disconnected quickly by providing a path for a large fault
>> current to flow - hence causing rapid operation of a Circuit
>> Protective Device. It will also limit the voltage rise of anything
>> earthed that you can touch to a safe level (under 50V typically)
>> during the fault. So if a live wire falls off the inside of your
>> washing machine and makes contact with the case, the big fault
>> current will open the fuse in the plug or circuit breaker on the
>> circuit quickly, and should you also happen to be touching the
>> machine at the time, the voltage you are exposed to should be low
>> enough to not place you at significant shock risk.
>>
>> Supplementary equipotential bonding is not designed to clear a fault
>> condition, or limit the absolute touch voltage. What it is supposed
>> to do is electrically tie together any pipe or service or other part
>> that could under fault conditions introduce a dangerous potential
>> into the room. In doing this it attempts to create a Faraday cage -
>> where anything and everything that you can touch will be at the same
>> electrical potential[1] as everything else - even if it is elevated
>> way above true ground. So for example a fault that could leave a hot
>> tap at 230V due to it being connected to a faulty inline water
>> heater, and a well earthed cold tap connected directly to a rising
>> main, would pose a serious shock hazard - touching both taps would
>> expose you to a 230V potential difference and a "stiff" supply. If
>> the taps are bonded together (and anything else that may take on an
>> elevated voltage under fault conditions like the earth wires of any
>> circuits feeding power into the room), there is the possibility that
>> both taps will rise to 230V under fault conditions - however
>> touching both exposes you to zero volts of potential difference.
>> (equipotential zones only work well where you are insulated from any
>> true earth reference - its no good making sure that all the taps
>> float up to the same voltage if you are standing on something that
>> is conductive and is going to stay put at 0V).
>>
>> [1] in reality the bonding may fail to tie all elements together at
>> exactly the same potential, however it must limit any potential
>> difference to 50V or under.
>>
>>
>>
>> There are a range of possible answers to this depending on
>> circumstance.
>>
>> Generally with plastic pipe installations, supplementary bonding of
>> the pipes is not required (although it may be required between the
>>   earths of say lighting and power circuits if they are both
>> accessible in the
>> room plus any other extraneous metallic parts (CH Pipes for
>> example)).
>>
>> In rooms which are not "special locations" (i.e. places likely to put
>> you at increased danger from electric shock (typically because you
>> might be wet)), there is also no requirement. So for example a
>> cloakroom with loo and sink would not require it )although it is
>> commonly seen in such circumstances).
>>
>> Under the 17th edition it is also sometimes permitted to have no EQ
>> bonding in a room with a bath or shower providing that the main EQ
>> bonding is in place, and all the circuits feeding the room have
>> additional protection from a RCD with 30mA (or lower) trip threshold.
>>
>> The one you would need to take care with is where you have say a
>> bathroom where there is some quantity of exposed copper that is in
>> turn connected to plastic pipe and then the taps. If the exposed
>> pipe is likely to be accessible then it could pose a risk an should
>> be bonded, even if the taps don't since they are isolated  via
>> plastic pipe.
>>
>> --
>> Cheers,
>>
>> John.
>>
> Now this answer is a example of why this group is so good. I knew
> quite a lot of what is in it, but I don't think I have ever seen such
> a concise and comprehensive summary of the subject, and moreover
> written in plain English. Thanks John!

I'll second that.  Thanks John, I am a wiser man.


-- 
Dave - The Medway Handyman
www.medwayhandyman.co.uk

On Sun, 29 Jun 2008 04:50:38 +0100, John Rumm wrote:

Great explanation John - it would make a good wiki article, say
"Earthing and Bonding". 

Couple of points/clarifications. Faraday Cages are about shileding
the enclosed space from electrical fields rather than about electrical
safety:
	http://en.wikipedia.org/wiki/Faraday_cage
Anyway it's not an everyday concept so doesn't help explain something to
the Ordinary Joe in the street.

> ... So for example a fault that could leave a hot tap at 
> 230V due to it being connected to a faulty inline water heater, and a 
> well earthed cold tap connected directly to a rising main, would pose a 
> serious shock hazard - touching both taps would expose you to a 230V 
> potential difference and a "stiff" supply.

"stiff" supply? You mean
a) a low impedance supply, or
b) a supply that turns you into a stiff? :-)

-- 
John Stumbles

This message has been rot13 encrypted twice for added security

On 2008-06-29, John Rumm  wrote:

> In doing this it attempts to create a Faraday cage 

Um, no it doesn't.


-- 
          "Be thankful that you have a life, and forsake your vain
                 and presumptuous desire for a second one."
               [email me at huge {at} huge (dot) org <dot> uk]

Huge wrote:
> On 2008-06-29, John Rumm  wrote:
> 
>> In doing this it attempts to create a Faraday cage 
> 
> Um, no it doesn't.
> 
> 
I know what he meant.  It isn't a Faraday cage (RF field densities at 
mains type voltages?  OUCH!) but generally, great post, thanks John.

Andy

"Huge" <Huge@nowhere.much.invalid> wrote in message 
news:g47s9m$3rn$3@anubis.demon.co.uk...
> On 2008-06-29, John Rumm  wrote:
>
>> In doing this it attempts to create a Faraday cage
>
> Um, no it doesn't.
>

There is no such thing as a perfect faraday cage.  Most let some EM 
radiation through of some sort.  At 50Hz the one described will do nicely.

John Stumbles wrote:
> On Sun, 29 Jun 2008 04:50:38 +0100, John Rumm wrote:
> 
> Great explanation John - it would make a good wiki article, say
> "Earthing and Bonding". 

Prolly not a bad idea...

> Couple of points/clarifications. Faraday Cages are about shileding
> the enclosed space from electrical fields rather than about electrical
> safety:
> 	http://en.wikipedia.org/wiki/Faraday_cage
> Anyway it's not an everyday concept so doesn't help explain something to
> the Ordinary Joe in the street.

yup there are better explanations with more general appeal...


>> ... So for example a fault that could leave a hot tap at 
>> 230V due to it being connected to a faulty inline water heater, and a 
>> well earthed cold tap connected directly to a rising main, would pose a 
>> serious shock hazard - touching both taps would expose you to a 230V 
>> potential difference and a "stiff" supply.
> 
> "stiff" supply? You mean
> a) a low impedance supply, or
> b) a supply that turns you into a stiff? :-)

Yes, and quite possibly yes! ;-)


-- 
Cheers,

John.

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

"The Medway Handyman"  wrote in message 
news:7ZG9k.17592$E41.784@text.news.virginmedia.com...
> I'll second that.  Thanks John, I am a wiser man.
>

None the wiser, I suspect, but hopefully better informed  :o)


-- 
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

On 2008-06-29, Andy Champ  wrote:
> Huge wrote:
>> On 2008-06-29, John Rumm  wrote:
>> 
>>> In doing this it attempts to create a Faraday cage 
>> 
>> Um, no it doesn't.
>> 
>> 
> I know what he meant. 

Oh, me too ...


-- 
          "Be thankful that you have a life, and forsake your vain
                 and presumptuous desire for a second one."
               [email me at huge {at} huge (dot) org <dot> uk]

On 29 Jun, 14:42, "Tim"  wrote:

> There is no such thing as a perfect faraday cage.  Most let some EM
> radiation through of some sort.  At 50Hz the one described will do nicely.

A Faraday cages is a closed conducting surface enclosing a space from
which it excludes static electric fields. Its effect on EM radiation
is (a) secondary and (b) partial.

Ian

Bob Mannix wrote:
> "The Medway Handyman"  wrote in
> message news:7ZG9k.17592$E41.784@text.news.virginmedia.com...
>> I'll second that.  Thanks John, I am a wiser man.
>>
>
> None the wiser, I suspect, but hopefully better informed  :o)

:-)



-- 
Dave - The Medway Handyman
www.medwayhandyman.co.uk

On 30 Jun, 10:37, The Real Doctor  wrote:
> On 29 Jun, 14:42, "Tim"  wrote:
>
> > There is no such thing as a perfect faraday cage.  Most let some EM
> > radiation through of some sort.  At 50Hz the one described will do nicely.

?? what has EM radiation got to do with it?


the Faraday Cage efffect is an observation about electric fields (with
or without an associated magnetic field) and conductors.

the wikipedia article [http://en.wikipedia.org/wiki/Faraday_cage } is
decidedly unsatisfactory.  It goes overboard with examples from HF -
off beam if not OTT


>
> A Faraday cages is a closed conducting surface enclosing a space from
> which it excludes static electric fields.

that's very close to it.  However mentioning "static" in the
definition misleads.  A better definition is:

Faraday Cage: a space enclosed by an envelope which is a perfect
conductor. (A perfect conductor being one with zero conductance
between any 2 points.).

By definition the electric field gradient across such a space, between
any 2 points on the envelope, must be zero. Shielding from external
electric fields becomes a consequence of the definition, not some
magic property.

[Nor is there strictly any requirement that the "space" should be a
simple 3D volume nor that the envelope is a closed surface - but that
is an excursion into topology.]

A practical attempt to build a Faraday Cage is never going to be
100%.  What we have to do, as in any engineering project, is come as
near as possible without over-engineering.

In the case of a bathroom, field frequencies are zero (ie DC) or low
frequency (ie 50Hz or possibly higher if transformers or electronic
voltage converters are present).  Bonding every conductor in sight is
sufficient to achieve a satisfactory approximation to a Faraday Cage.
The electric field gradient between any 2 possible contact points is
zero (or near enough) & the wet bather is protected from shock.



 Its effect on EM radiation
> is (a) secondary and (b) partial.

 & irrelevant

A bonded bathroom is, within its normal working conditions, a
sufficient approximation to a Faraday Cage. Full stop.

jim wrote:

> A bonded bathroom is, within its normal working conditions, a
> sufficient approximation to a Faraday Cage. Full stop.

Sorry it isn't; it's an equipotential zone in which there won't be 
dangerous potential differences (at power frequency) between certain 
items of metalwork.  That's all.  A static or slowly changing charge 
placed outside the zone would still result in a corresponding electric 
field inside the zone, unless there's an awful lot or pipework, or the 
walls, floor and ceiling happen to be conductive.  Any Faraday cage 
effect is purely incidental, and largely irrelevant to the purpose of 
the bonding.

-- 
Andy

"Andy Wade"  wrote in message 
news:6cvvdhF508fU1@mid.individual.net...
> jim wrote:
>
>> A bonded bathroom is, within its normal working conditions, a
>> sufficient approximation to a Faraday Cage. Full stop.
>
> Sorry it isn't; it's an equipotential zone in which there won't be 
> dangerous potential differences (at power frequency) between certain items 
> of metalwork.  That's all.  A static or slowly changing charge placed 
> outside the zone would still result in a corresponding electric field 
> inside the zone, unless there's an awful lot or pipework, or the walls, 
> floor and ceiling happen to be conductive.  Any Faraday cage effect is 
> purely incidental, and largely irrelevant to the purpose of the bonding.
>

I think you're splitting hairs.  I can see your point, but it does 
approximate to a farday cage.  Even in a Faraday cage you can have a 
conductor at main potential, and often used to power equipment.  However, in 
practice, equipotential bonding has the same effect of ensuring the electric 
field in the vicinity is negligible.

On 1 Jul, 23:53, jim  wrote:
> On 30 Jun, 10:37, TheRealDoctor wrote:


> > A Faraday cages is a closed conducting surface enclosing a space from
> > which it excludes static electric fields.
>
> that's very close to it.  However mentioning "static" in the
> definition misleads.  A better definition is:
>
> Faraday Cage: a space enclosed by an envelope which is a perfect
> conductor. (A perfect conductor being one with zero conductance
> between any 2 points.).

I spent quite a lot of my life working with perfect conductors, and I
am pretty sure they didn;t have zero conductance ...

> By definition the electric field gradient across such a space, between
> any 2 points on the envelope, must be zero.

That's an interesting idea on two counts. Firstly, I think you mean
"electric field" and not "electric field gradient". Secondly, are you
claiming that there can be no electric field between two points at the
same potential?

> Shielding from external
> electric fields becomes a consequence of the definition, not some
> magic property.

No, it's a property of a closed conducting surface.

> [Nor is there strictly any requirement that the "space" should be a
> simple 3D volume nor that the envelope is a closed surface - but that
> is an excursion into topology.]

Do give an example of a non-closed surface forming a Faraday cage,
then.

> In the case of a bathroom, field frequencies are zero (ie DC) or low
> frequency (ie 50Hz or possibly higher if transformers or electronic
> voltage converters are present).  Bonding every conductor in sight is
> sufficient to achieve a satisfactory approximation to a Faraday Cage.
> The electric field gradient between any 2 possible contact points is
> zero (or near enough) & the wet bather is protected from shock.

Keeping everything at the same potential certainly prevents shock. I
am little confused about why you think that has anything to do with
Faraday cages.

Ian

"The Real Doctor"  wrote in message 
news:477c6784-9fd9-463c-9dec-12e17f3666c2@y38g2000hsy.googlegroups.com...
> On 1 Jul, 23:53, jim  wrote:
>> On 30 Jun, 10:37, TheRealDoctor wrote:
>
>
>> > A Faraday cages is a closed conducting surface enclosing a space from
>> > which it excludes static electric fields.
>>
>> that's very close to it.  However mentioning "static" in the
>> definition misleads.  A better definition is:
>>
>> Faraday Cage: a space enclosed by an envelope which is a perfect
>> conductor. (A perfect conductor being one with zero conductance
>> between any 2 points.).
>
> I spent quite a lot of my life working with perfect conductors, and I
> am pretty sure they didn;t have zero conductance ...
>

I've never seen a perfect conductor, nor anything with zero conductance!  If 
I did stumble across a perfect conductor, I'm sure it would have an infinite 
conductance.

>> By definition the electric field gradient across such a space, between
>> any 2 points on the envelope, must be zero.
>
> That's an interesting idea on two counts. Firstly, I think you mean
> "electric field" and not "electric field gradient". Secondly, are you
> claiming that there can be no electric field between two points at the
> same potential?
>
>> Shielding from external
>> electric fields becomes a consequence of the definition, not some
>> magic property.
>
> No, it's a property of a closed conducting surface.
>
>> [Nor is there strictly any requirement that the "space" should be a
>> simple 3D volume nor that the envelope is a closed surface - but that
>> is an excursion into topology.]
>
> Do give an example of a non-closed surface forming a Faraday cage,
> then.

Simply when you have an opening far smaller than the wavelength of interest. 
Depends on what you mean by closed?

>
>> In the case of a bathroom, field frequencies are zero (ie DC) or low
>> frequency (ie 50Hz or possibly higher if transformers or electronic
>> voltage converters are present).  Bonding every conductor in sight is
>> sufficient to achieve a satisfactory approximation to a Faraday Cage.
>> The electric field gradient between any 2 possible contact points is
>> zero (or near enough) & the wet bather is protected from shock.
>
> Keeping everything at the same potential certainly prevents shock. I
> am little confused about why you think that has anything to do with
> Faraday cages.
>

An equipotential zone in a bathroom at 50 Hz, where the wavelength is 6 km, 
is going to look much like a faraday cage at 50Hz, akin to a real Faraday 
cage having some holes at the higher end of the rf spectrum.