SP or DP?

[amfisted]

Just wondering when they introduced a choice between single pole and double pole wall sockets.

I've just replaced a double socket converter with a 1 gang socket. The switch on the converter was arcing when operated, even though the socket is only a couple of years old and rarely used.

I went to purchase a single 13amp switched wall socket from Scrooficks, and was confronted with the choice between 1 gang SP and 1 gang DP. In the past I've simply asked for a 1 gang socket at our local electrical factors, and nobody has ever requested that I specify SP or DP.

Looking at the instructions, the difference appears to be that with the DP, there's an earth connection to the back box, whereas the SP instructions simply show the three conductors connected to the socket terminals.

So is a DP intended for use with metal back boxes only?

 

[Risteard]

Nothing to do with it whatsoever. A single pole breaks only one pole with the switch (i.e. the phase (line) conductor) whereas a double pole breaks both live conductors.

 

[amfisted]

Thanks for that. In what circumstances would you choose an SP socket over DP, or vice versa?

 

[bernardgreen]

The advantage of double pole is that it prevents a Neutral to Earth fault in the appliance tripping the RCD,

With Single pole even though the switch is OFF a Neutral to Earth fault in the appliance can still cause the RCD to trip.

 

[amfisted]

I see, thanks.

So that event I had the other day-- when I was working on the lighting circuit, the power was off and the RCD tripped due to Neutral and Earth touching, could not have happened if I'd been working with the power off on a socket which is DP.

Makes sense now.

 

[winston1]

Nor would it have happened if you had removed the plug, which is a much better method of isolation than any switch.

There is no requirement for a socket to have a switch anyway.

 

[amfisted]

My daughter used to tell me off for turning off switches on appliances which had just been used: kettles, toasters etc. She thought it was unnecessary, but now she does it herself.
I've always done it. Without switches I would be a broken man.

 

[ericmark]

Switching the neutral can only be done if the line is switched at the same time, so with German sockets where line and neutral can be swapped, you need to switch both to ensure the line is not left on and neutral off. However switches do fail, so where the socket is polarised as all British sockets are, it is considered safer to only switch line so we can't get a situation where the line part of switch is on and neutral off, which could make one think the items is not live when it is.

Similar to the silly plastic lumps which plug into sockets, in Germany there are no shutters on most of their sockets, so there is a good reason for these

however in the UK these

in the UK actually make the socket more dangerous See this web site for more so copying EU items and adapting for use in UK does not always work.

Having a MCB double pole would help isolating faulty circuits to stop a RCD from tripping, but can't get single width ones for UK consumer units, you can get double pole RBCO's but what is the point as then you have not got multi circuits on same RCD.

Sockets over a set size do need switches to ensure power is broken quick enough however 13A is not enough to cause a problem arcing when the plug is withdrawn, and normally we don't use items which run as soon as plugged in, normally there is a switch on the item being used.

 

[FrodoOne]

For comparison, Australia/NZ regulations require a socket-outlet to be “individually controlled by a separate switch that .... operates in all active (Line) conductors,” subject to three “exceptions”:

• “A single switch may be used for the control of two socket-outlets located immediately adjacent to each other.” (subject to the current rating of the switch to be at least equal to the (a) total current rating of the sockets concerned; or (b) the current rating of the overcurrent protection device concerned, whichever is the lesser value.) (AS/NZS 3000:2007 (Wiring Rules) - 4.4.4.1)
• “A socket-outlet that is switched by the insertion and withdrawal of the plug shall be deemed to meet the requirements ....”
• ”A socket-outlet that is rated at not more than 10 A, installed for the connection of a fixed or stationary appliance or a luminaire and that is not readily accessible for other purposes, need not be controlled by a switch.”

However, "stationary appliances" (such as fans) and most “luminaires” may be controlled by a remote switch, which would switch the supply via the socket-outlet concerned. Exceptions could be devices such as illuminated “Exit” signs, which require connection to the power supply at all times.

Each switch or means of operating a switch, for a socket-outlet shall be (a) as close as practicable to the socket-outlet, and (b) marked to indicate the socket-outlet(s) or the connected electrical equipment that it controls, with the exception that marking is not required where the socket-outlet controlled is obvious because of the location of the switch.

Double pole switches are required in caravans and mobile homes:

• All switches installed in transportable structures and intended to be connected to the site supply shall operate in all live (active and neutral) conductors." (AS/NZS3001:2008 (Electrical Installations – Transportable structures and vehicles) - 3.6.2

(https://en.wikipedia.org/wiki/AS/NZS_3112)

 

[AndyPRK]

you may want to use dual pole where wet goods a concerned.
e.g
washing machine, dishwasher etc. but if these are under the counter sockets, little point

 

[JohnW2]

For comparison, Australia/NZ regulations require a socket-outlet to be “individually controlled by a separate switch that .... operates in all active (Line) conductors,” subject to three “exceptions”: ....
“A socket-outlet that is switched by the insertion and withdrawal of the plug shall be deemed to meet the requirements ....”

That seems daft - since that one 'exception', alone, surely removes any point in the regulation, doesn't it? Any socket-outlet can surely be 'switched' by withdrawal of the plug, can't it?

Kind Regards, John

 

[AndyPRK]

sounds like the arch is on the switch contacts, rather than the plug pins

 

[Taylortwocities]

You shouldn't be unpluging, or switching off, something that is still energised, If you turn the (whateveritis) off, then you will not get any arching.

 

[ericmark]

I do remember a socket where after plugging in you turned the plug which made the switch, and to remove you pressed a red button

however other than GEC Large Steam Turbines I have not seen them used. I am sure however if you move around the world there are other examples of where the user is forced to plug in then switch on, and switch off then unplug which is really a good idea, however unless forcing the user to switch off first, then no real point having the switch.

And with a 13A plug and socket there is nothing to stop the user removing while still energised.

 

[JohnW2]

You shouldn't be unpluging, or switching off, something that is still energised ...

Whilst what you say is clearly true in terms of unplugging, if it were not acceptable to 'switch off' something that was already energised, it would have to remain energised for ever after it had first been energised!

I would suspect that most wall sockets (e.g. BS1363) are probably more capable of breaking a load current than are many of the switches within appliances/equipment.

Kind Regards, John