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
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.
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.
|!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.
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 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!
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.
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.
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.)
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.
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.
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.
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.
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.
Like a break in earth continuity?
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.
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.
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.
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)
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.
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
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