Hi to all this is my first post on this group but would like to say
how helpful this group has been to read in the past.
Right then, i have bought an house which needs plenty of work but at
the moment i am replacing a few plug sockets in my living room. Upon
looking to buy the sockets (2 gang & 1 gang) i have noticed the option
of 'dp and sp' I understand that theabbreviation is single pole or
double pole but am unsure as to which is needed. Any advice on which i
would need or an expanded explanation would be great, thanks again.

Probably going to get shot down in flames here but I'll have a stab at
answering..

As I understand it a double pole switch will break the circuit/isolate both
the live and neutral circuits, a single pole will just do the live circuit.
No idea what the regs etc say but the dp switches provide an extra level of
safety and guess they should be used in kitchens etc

HTH

Jim

Fishmilk wrote:
Hi to all this is my first post on this group but would like to say
how helpful this group has been to read in the past.
Right then, i have bought an house which needs plenty of work but at
the moment i am replacing a few plug sockets in my living room. Upon
looking to buy the sockets (2 gang & 1 gang) i have noticed the option
of 'dp and sp' I understand that theabbreviation is single pole or
double pole but am unsure as to which is needed. Any advice on which i
would need or an expanded explanation would be great, thanks again.

Your understanding is correct. Double pole are the better ones to get
since they will completely isolate something when switched off. The
single pole ones will leave the neutral connected which is in reality
still a "live" connection.


--
Cheers,

John.

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

On Mon, 26 Feb 2007 13:45:30 -0800, Fishmilk wrote:

Hi to all this is my first post on this group but would like to say
how helpful this group has been to read in the past.
Right then, i have bought an house which needs plenty of work but at
the moment i am replacing a few plug sockets in my living room. Upon
looking to buy the sockets (2 gang & 1 gang) i have noticed the option
of 'dp and sp' I understand that theabbreviation is single pole or
double pole but am unsure as to which is needed. Any advice on which i
would need or an expanded explanation would be great, thanks again.

There was a thread on this a few weeks or months back. IIRC although DP
sound like a better idea, unless they're designed to guarantee breaking
the live first when switching off and making it last when closing they're
liable to trip RCDs, because there can be an instant in which neutral is
disconnected and live still connected, allowing transient current to flow
through any capacitance between L and E.

On Mon, 26 Feb 2007 23:09:36 +0000, John Rumm
wrote:

|!Fishmilk wrote:
|! Hi to all this is my first post on this group but would like to say
|! how helpful this group has been to read in the past.
|! Right then, i have bought an house which needs plenty of work but at
|! the moment i am replacing a few plug sockets in my living room. Upon
|! looking to buy the sockets (2 gang & 1 gang) i have noticed the option
|! of 'dp and sp' I understand that theabbreviation is single pole or
|! double pole but am unsure as to which is needed. Any advice on which i
|! would need or an expanded explanation would be great, thanks again.
|!
|!Your understanding is correct. Double pole are the better ones to get
|!since they will completely isolate something when switched off. The
|!single pole ones will leave the neutral connected which is in reality
|!still a "live" connection.

But if everything is installed and working correctly there will at worst
there will only be very few volts between neutral and earth which you will
hardly notice if you touch it and will not kill you.
--
Dave Fawthrop dave hyphenologist co uk Compare and contrast
Sharia Law http://en.wikipedia.org/wiki/Sharia
European Convention on Human Rights http://www.hri.org/docs/ECHR50.html
Then sign this petition http://petitions.pm.gov.uk/Ban-Sharia

In article ,
John Rumm wrote:
Your understanding is correct. Double pole are the better ones to get
since they will completely isolate something when switched off. The
single pole ones will leave the neutral connected which is in reality
still a "live" connection.

I'm not convinced. With house wiring - unless there is a serious fault -
the neutral will always be at a safe voltage. And a more likely fault is
failure of the socket switch itself. I suppose I can understand the need
for DP switching for an isolator - but with a plug and socket you isolate
by removing the plug.

--
*Why do psychics have to ask you for your name? *

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

"Dave Fawthrop" wrote in message
...
On Mon, 26 Feb 2007 23:09:36 +0000, John Rumm
wrote:

|!Fishmilk wrote:
|! Hi to all this is my first post on this group but would like to say
|! how helpful this group has been to read in the past.
|! Right then, i have bought an house which needs plenty of work but at
|! the moment i am replacing a few plug sockets in my living room. Upon
|! looking to buy the sockets (2 gang & 1 gang) i have noticed the option
|! of 'dp and sp' I understand that theabbreviation is single pole or
|! double pole but am unsure as to which is needed. Any advice on which i
|! would need or an expanded explanation would be great, thanks again.
|!
|!Your understanding is correct. Double pole are the better ones to get
|!since they will completely isolate something when switched off. The
|!single pole ones will leave the neutral connected which is in reality
|!still a "live" connection.

But if everything is installed and working correctly there will at worst
there will only be very few volts between neutral and earth which you
will
hardly notice if you touch it and will not kill you.
--
Dave Fawthrop dave hyphenologist co uk

The major problem in Dave's contribution stem from the words 'IF' .
Accidents are usaully a combination of more than one event - [if the cables
are installed correctly ; if the Earthing is correct; if the neutral is
correctly balanced; if ....; if .... .]
IMHO, the advantges of DP versus SP outwieigh any cost impact - particularly
as you're d-i-y.
I was taught (brainwashed) to;- stand on a duck board; keep one hand in a
pocket; have the fuse feeding the circuit in the top-pocket of one's
overalls; have an safety observer; ensure there was a 'Brinkly stick' (aka
de-bollocking stick) nearby ..... and to check, recheck and check again to
ensure that equipment was isolated .... but that perhaps was a counsel of
perfection. In your case; a table-lamp would still have continuity, via
it's neutral wire, to the mains circuit if it's only isolated by a SP
switch. BTW; if'm you're changing an switched outlet , I wouldn't bother
fitting a single gang ... make it a double!

--

Brian

Brian Sharrock wrote:
snip


BTW; if'm you're changing an switched outlet , I wouldn't bother
fitting a single gang ... make it a double!


I agree entirely and will join you. Make mine a double too.
;-)

Dave Plowman (News) wrote:

I'm not convinced. With house wiring - unless there is a serious fault -
the neutral will always be at a safe voltage.

Agreed, although it is perhaps worth keeping in mind that while the
voltage difference is likely to be low, the effect of neighbouring
properties can mean that the neutral looks like a fairly low impedance
current source wrt earth - certainly on TN-S installs.

And a more likely fault is
failure of the socket switch itself. I suppose I can understand the need
for DP switching for an isolator - but with a plug and socket you isolate
by removing the plug.

Again true, but I still can't see any reason for bothering with single
pole switched accessories when good quality DP ones can be had for
little more.

--
Cheers,

John.

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

"Dave Plowman (News)" wrote in message
...
In article ,
John Rumm wrote:
Your understanding is correct. Double pole are the better ones to get
since they will completely isolate something when switched off. The
single pole ones will leave the neutral connected which is in reality
still a "live" connection.

I'm not convinced. With house wiring - unless there is a serious fault -
the neutral will always be at a safe voltage. And a more likely fault is
failure of the socket switch itself. I suppose I can understand the need
for DP switching for an isolator - but with a plug and socket you isolate
by removing the plug.

The big problem with that is there is no easy way for the user to detect
such a fault.
So you have the lifetime of the installation for the fault to occur and then
nothing to stop you being zapped unless you fit the secondary safety devices
(the DP isolators in this case).

Its the reason why I don't like ring mains.. they have decades to develop an
almost undetectable (to the user) fault which can overload the cables.
Not a good design as far as safety goes, it was just cheaper at a time when
copper was expensive.
A trade off of cost vs. safety but costs are different now and we still use
the same old system.
Radials suffer from fewer non detectable (for the user) faults so are better
for most users.

The only "real" alternative is to have frequent safety checks and hope the
fault is detected before someone is zapped or the house burned down.
The recent train crash shows that this is not a good strategy if you are
designing systems to be safe.
(The safe way to deal with the points issue is to bolt them shut. They are
used so infrequently that they don't need to be operable by someone in a
signal box all the time and a point is always going to be a potential
fault.)

On 27 Feb, 10:17, "Brian Sharrock" wrote:
"Dave Fawthrop" wrote in message
...
On Mon, 26 Feb 2007 23:09:36 +0000, John Rumm
wrote:
|!Fishmilk wrote:

|! the moment i am replacing a few plug sockets in my living room. Upon
|! looking to buy the sockets (2 gang & 1 gang) i have noticed the option
|! of 'dp and sp' I understand that theabbreviation is single pole or
|! double pole but am unsure as to which is needed. Any advice on which i

standard single pole is fine.


|!Your understanding is correct. Double pole are the better ones to get
|!since they will completely isolate something when switched off. The
|!single pole ones will leave the neutral connected which is in reality
|!still a "live" connection.

But if everything is installed and working correctly there will at worst
there will only be very few volts between neutral and earth which you
will
hardly notice if you touch it and will not kill you.

The major problem in Dave's contribution stem from the words 'IF' .
Accidents are usaully a combination of more than one event - [if the cables
are installed correctly ; if the Earthing is correct; if the neutral is
correctly balanced; if ....; if .... .]
IMHO, the advantges of DP versus SP outwieigh any cost impact - particularly


Facts so far true, but its only half the story, and conclusion is thus
not really so.

The downside with dp is that earth leakage that was a tiny risk with
on an sp socket can become dangerous when on a dp socket. This occurs
when an appliance leaks to case at the neutral end of the element, a
condition that is not rare in reality. Harmless on sp, though not
totally risk free, but it becomes an immediate danger on a dp switched
socket.

The only way to know which is safer is to look at the numbers of
electrocutiuons and fatal fires on sp versus dp socket systems, and we
simply dont have the data.

Some kind of interim best assesment would thus require further
thinking.


NT

On 27 Feb, 13:44, "dennis@home" wrote:
"Dave Plowman (News)" wrote in ...
In article ,
John Rumm wrote:

Its the reason why I don't like ring mains.. they have decades to develop an
almost undetectable (to the user) fault which can overload the cables.
Not a good design as far as safety goes, it was just cheaper at a time when
copper was expensive.
A trade off of cost vs. safety but costs are different now and we still use
the same old system.
Radials suffer from fewer non detectable (for the user) faults so are better
for most users.

Another case of awareness of only one part of the issue.

If a bad connection occurs:

in a ring circuit it results in one cable carrying twice the current.
It does not get twice as hot because only one of the 2 conductors is
running at 2x current. It does not melt or catch fire because of the
safety margins designed into cable choices. It can and does continue
to work for decades.

in a radial circuit the bad connection carries the current, intense
heat is produced and it eitrher burns o/c or catches fire.

Neither is perfect, but the radial scenario is much much worse. Ring
circuits are a clever safety feature, its just a shame most dont grasp
the concept properly.


NT

In article ,
John Stumbles writes:
On Mon, 26 Feb 2007 13:45:30 -0800, Fishmilk wrote:
There was a thread on this a few weeks or months back. IIRC although DP
sound like a better idea, unless they're designed to guarantee breaking
the live first when switching off

They aren't, and this can cause plug-in RCD's to trip, which in some
cases also monitor for broken neutral, which they see when the live
makes first or breaks last. I've seen this a number of times.
Plug in a 3-neon socket tester and switch a double pole socket on
and off slowly, and you'll find many such sockets go through fault
conditions transitly.

and making it last when closing they're
liable to trip RCDs, because there can be an instant in which neutral is
disconnected and live still connected, allowing transient current to flow
through any capacitance between L and E.

The only argument I have thought of for them is where you have an
RCD protected circuit. People quite reasonably think that if an
appliance is faulty and tripping the RCD, switching off the socket
will stop this, which it won't in the case of a neutral-earth short
and a single pole switch. This seems to me to be about the only
argument for double-pole switching of socket outlets, albeit a very
weak argument. As a counter argument, the extra switch contact and
mechanical parts would seem to add unnecessary potential points of
failure, although socket outlets are generally extremely reliable,
so that's weak too. Frankly, I wouldn't spend to much time worrying
about it one way or the other.

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

wrote:

The downside with dp is that earth leakage that was a tiny risk with
on an sp socket can become dangerous when on a dp socket. This occurs
when an appliance leaks to case at the neutral end of the element, a
condition that is not rare in reality. Harmless on sp, though not
totally risk free, but it becomes an immediate danger on a dp switched
socket.

I don't follow your train of thought there... could you expand?


--
Cheers,

John.

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

dennis@home wrote:

Its the reason why I don't like ring mains.. they have decades to develop an
almost undetectable (to the user) fault which can overload the cables.
Not a good design as far as safety goes, it was just cheaper at a time when
copper was expensive.
A trade off of cost vs. safety but costs are different now and we still use
the same old system.

We use the system because in almost every case it out performs a radial
circuit. Think through each of the fault scenarios, and rings win more
often.

Radials suffer from fewer non detectable (for the user) faults so are better
for most users.

Like a break in earth continuity?

The only "real" alternative is to have frequent safety checks and hope the
fault is detected before someone is zapped or the house burned down.

In real terms though people don't get zapped or houses burnt down in any
statistically significant numbers as a result of fixed house wiring, and
that is without frequent safety checks.



--
Cheers,

John.

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

In article ,
dennis@home wrote:
I'm not convinced. With house wiring - unless there is a serious
fault - the neutral will always be at a safe voltage. And a more
likely fault is failure of the socket switch itself. I suppose I can
understand the need for DP switching for an isolator - but with a plug
and socket you isolate by removing the plug.

The big problem with that is there is no easy way for the user to detect
such a fault. So you have the lifetime of the installation for the
fault to occur and then nothing to stop you being zapped unless you fit
the secondary safety devices (the DP isolators in this case).

I'm not quite sure how a DP isolator *on a socket* can improve safety in
event of any fault. And given the majority of faults are likely to be the
fitting itself or the connection to it increasing the complexity might
well increase the fault rate.

Single pole socket outlets have been around for a *very* long time in the
UK. Where's the evidence they are not satisfactory? Indeed my worries are
that with a more likely fault condition they *reduce* safety in the same
way as fusing both line and neutral can.

--
*Middle age is when it takes longer to rest than to get tired.

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

On 28 Feb, 01:30, John Rumm wrote:
wrote:

The downside with dp is that earth leakage that was a tiny risk with
on an sp socket can become dangerous when on a dp socket. This occurs
when an appliance leaks to case at the neutral end of the element, a
condition that is not rare in reality. Harmless on sp, though not
totally risk free, but it becomes an immediate danger on a dp switched
socket.

I don't follow your train of thought there... could you expand?

I'm not clear which bit isnt clear, but will try.

Each method has consequent risks, dp and sp switching.

Suppose you have an appliance with leakage from the neutral end of a
heating element (or just a N wire if you wish) to the case. With sp
switching the user will not receive any shock, as the leaky current is
at or close to neutral voltage. The sp switching renders it a minorish
safety issue.

Now, put that appliance on a dp socket and it now has the capacity to
kill every time you switch the socket off or on. Why? If at any moment
L is connected and N open, the formerly neutral leak becomes a live
leak. Instead of leaking at 10v from earth its now leaking 240v.

IRL mechanical switches open and close messily. Look at the waveforms
on a scope and you'll see that any toggle movement at all causes
intermittent contact on both poles. Thus the above will occur.


NT

wrote:

On 28 Feb, 01:30, John Rumm wrote:
wrote:

The downside with dp is that earth leakage that was a tiny risk with
on an sp socket can become dangerous when on a dp socket. This occurs
when an appliance leaks to case at the neutral end of the element, a
condition that is not rare in reality. Harmless on sp, though not
totally risk free, but it becomes an immediate danger on a dp switched
socket.

I don't follow your train of thought there... could you expand?

I'm not clear which bit isnt clear, but will try.

Yup, you go the bit ;-)

Each method has consequent risks, dp and sp switching.

Suppose you have an appliance with leakage from the neutral end of a
heating element (or just a N wire if you wish) to the case. With sp
switching the user will not receive any shock, as the leaky current is
at or close to neutral voltage. The sp switching renders it a minorish
safety issue.

Now, put that appliance on a dp socket and it now has the capacity to
kill every time you switch the socket off or on. Why? If at any moment
L is connected and N open, the formerly neutral leak becomes a live
leak. Instead of leaking at 10v from earth its now leaking 240v.

While I accept that switch bounce is a real and demonstrable phenomenon,
I am not convinced that it would pose much if any risk in this case
without the presence of other faults and the absence of other
protections methods.

Firstly a faulty appliance like this would trip a RCD if so protected.
Secondly, even without a RCD, the earthing of the casework of the
appliance should limit the touch voltage to below 50V (as is one of its
primary design requirements)

IRL mechanical switches open and close messily. Look at the waveforms
on a scope and you'll see that any toggle movement at all causes
intermittent contact on both poles. Thus the above will occur.

The bounce duration of most mechanical switches is typically well under
40ms, often under 20ms. That is the time from starting the switch
operation until stabilisation of the output. During the bounce period,
typically half that time (on average) will be spent in the "made"
condition. Of that time we are only interested in times where the made
condition for both poles does *not* coincide. Thus reducing the window
of exposure still further. So the total risk window is going to be a
couple of ms at worst, or significantly less than one mains cycle. When
you look at this in the context of a RCD that is considered to be
working correctly and providing adequate protection from serious shock
when it is able to respond within 40ms, it suggests a minimal risk from
this scenario since the total "let through" energy is limited by the
duration of the event.


--
Cheers,

John.

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

In article ,
John Rumm wrote:
Firstly a faulty appliance like this would trip a RCD if so protected.
Secondly, even without a RCD, the earthing of the casework of the
appliance should limit the touch voltage to below 50V (as is one of its
primary design requirements)

Both of which if working correctly negate the need for DP switched sockets.

--
*Very funny Scotty, now beam down my clothes.

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

"Dave Plowman (News)" wrote in message
...
In article ,
John Rumm wrote:
Firstly a faulty appliance like this would trip a RCD if so protected.
Secondly, even without a RCD, the earthing of the casework of the
appliance should limit the touch voltage to below 50V (as is one of its
primary design requirements)

Both of which if working correctly negate the need for DP switched
sockets.

DP switching is there so that architects can specify them so they look like
they know everything. They then go on to specify SELV fans in utility rooms
and kitchen sinks "earth bonding"
Is there one case of a person being killed by the use of a SP switched
socket as opposed to a DP switched socket? I doubt it

Adam

On 28 Feb, 12:14, John Rumm wrote:
wrote:
On 28 Feb, 01:30, John Rumm wrote:
wrote:

The downside with dp is that earth leakage that was a tiny risk with
on an sp socket can become dangerous when on a dp socket. This occurs
when an appliance leaks to case at the neutral end of the element, a
condition that is not rare in reality. Harmless on sp, though not
totally risk free, but it becomes an immediate danger on a dp switched
socket.

I don't follow your train of thought there... could you expand?

I'm not clear which bit isnt clear, but will try.

Yup, you go the bit ;-)

Each method has consequent risks, dp and sp switching.

Suppose you have an appliance with leakage from the neutral end of a
heating element (or just a N wire if you wish) to the case. With sp
switching the user will not receive any shock, as the leaky current is
at or close to neutral voltage. The sp switching renders it a minorish
safety issue.

Now, put that appliance on a dp socket and it now has the capacity to
kill every time you switch the socket off or on. Why? If at any moment
L is connected and N open, the formerly neutral leak becomes a live
leak. Instead of leaking at 10v from earth its now leaking 240v.

While I accept that switch bounce is a real and demonstrable phenomenon,
I am not convinced that it would pose much if any risk in this case
without the presence of other faults

switch bounce is not the prime issue. Or even the issue at all in
practice.

But neither type of socket, sp or dp, poses any risk when there ar eno
other faults. IRL there sometimes are other faults, and thats where
the 2 behave differently, both with minor pros and cons.

and the absence of other
protections methods.

indeed, switching is just one more layer of safety, rather than the
only one.


Firstly a faulty appliance like this would trip a RCD if so protected.

If. If an RCD is present and working, there really is no benefit to dp
switching.


Secondly, even without a RCD, the earthing of the casework of the
appliance should limit the touch voltage to below 50V (as is one of its
primary design requirements)

Should on new installs, sure. Does on the millions of older TT
installs? Almost no chance of that. And many appliances arent earthed.

IRL mechanical switches open and close messily.

The bounce duration of most mechanical switches is typically well under
40ms, often under 20ms.

For a switch in peak condition, sure. Over half a second of crap is
not unusual for a switch in not such good condition.


Of that time we are only interested in times where the made
condition for both poles does *not* coincide. Thus reducing the window
of exposure still further. So the total risk window is going to be a
couple of ms at worst, or significantly less than one mains cycle.

No, but this is not the issue. I was just pointing out there that even
with switches designed to close in a certain order, that sequence is
not guaranteed.


NT

Dave Plowman (News) wrote:
In article ,
John Rumm wrote:
Firstly a faulty appliance like this would trip a RCD if so protected.
Secondly, even without a RCD, the earthing of the casework of the
appliance should limit the touch voltage to below 50V (as is one of its
primary design requirements)

Both of which if working correctly negate the need for DP switched sockets.

You don't *need* a switch at all... and I certainly would not reject a
DP switched one in favour of a SP one unless there were some other
reason to do so.

(This is mostly academic anyway since most switched sockets available
these days are DP switched)

--
Cheers,

John.

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

wrote:

The bounce duration of most mechanical switches is typically well under
40ms, often under 20ms.

For a switch in peak condition, sure. Over half a second of crap is
not unusual for a switch in not such good condition.

We are talking socket switches here! Just how much mass do you suppose
the contacts would need to have to sustain a under damped oscillation
for that duration?

(unless you are talking about very dirty contacts that are arcing - then
the number of poles is the least of your worries!)

Of that time we are only interested in times where the made
condition for both poles does *not* coincide. Thus reducing the window
of exposure still further. So the total risk window is going to be a
couple of ms at worst, or significantly less than one mains cycle.

No, but this is not the issue. I was just pointing out there that even
with switches designed to close in a certain order, that sequence is
not guaranteed.

AFAIAA most DP switches are not usually designed to close in any
particular order... if they were then chances are they would.

--
Cheers,

John.

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