Are SP or DP grid switches safest for 5A light sockets?

[BHI]

There seems to be a bit of debate as to whether SP or DP grid-switches are safest when wiring 5A lighting wall sockets (on a lighting circuit)... What are the latest thoughts if getting a neutral to, and switched-neutral from, the switch back box isn't an issue?

DP initally seems best to switch both live and neutral, but like the 13A mains socket discussions, are these actually (potentially) less safe as any failure in the live pole of the switch could still leave the sockets live even though they appear to be off at the lamp (since then only the neutral pole is open)?

I would personally tend towards the DP approach still, since this approach is also used in grid switch banks for 13A appliance isolation on mains ring spurs - but I'm open to persuasion otherwise in light of the above "risk"...

(I know there is also a separate debate about grid-switch point-loading in general...)

 

[333rocky333]

There seems to be a bit of debate as to whether SP or DP grid switches are safest

By who, do that many people use Grid stuff for lighting in domestic

Appliance isolation is a different matter as the appliances may reguire double pole and there often replacing the DP switched FCU

 

[BHI]

I think that the SP v DP debate tends to concern 13A switched sockets - ie, on a theorectial level, are the MK DP sockets acutally more or less "safe" than other brand SP sockets (if you discount variations in manufacturing quality)?

Just applying the same principals to remote 5A light socket switching... Or like the fixed ceiling lighting, would you simply stick with SP switches the for 5A sockets too if it's not an issue as you suggest?

 

[PrenticeBoyofDerry]

If you have an option to have the neutral switched, then I would have thought logically that would offer a more effective isolation to the load part of the circuit, this could help prevent complete power loss to other parts of the installation if a fault occurs at this section.
If that makes it a safer option then so be it!

 

[Iggifer]

When I put a 5a socket in my room, I deliberately went with MK grid so that I could use a DP switch, it meant using a 2G box for 3 switches but I'm happier with DP, it also removes the need for a connector block

Not sure I get the OP's point about a failure in the live terminal of the switch leaving the neutral closed but live open circuit - this is the exact situation you would have with a SP switch when it's open and operating normally?

 

[viewer]

Wasn't there a recent post with several replies referring , I think, to a pump, where a DP switch caused problems with tripping of the RCD due to slight differences in the L & N opening movements? Problem solved by moving to SP so maybe this should be a consideration?

 

[JohnW2]

Not sure I get the OP's point about a failure in the live terminal of the switch leaving the neutral closed but live open circuit - this is the exact situation you would have with a SP switch when it's open and operating normally?

I think the OP was talking about the opposite situation from that - i.e. one in which a switch fault (welded L contacts or broken L-side mechanism) meant that the L-side was always closed, even in the 'off' position. In that situation, the switch could be in the 'off' position and the load would appear to be 'switched off' (neutral of switch open), but L would still be getting to the load.

...theoretically possible, one can't deny, but I can't say I recall having ever previously heard that presented as an argument against using a DP switch/isolator!

Kind Regards, John

 

[flameport]

For items connected with plugs & sockets, there is no benefit to DP switching.

The switch is there to control the appliance only.
For isolation, remove the plug from the socket - a far safer method than any switch.

 

[JohnW2]

For items connected with plugs & sockets, there is no benefit to DP switching. The switch is there to control the appliance only. For isolation, remove the plug from the socket - a far safer method than any switch.

It's very difficult to argue with any of that.

In practical terms, if (as per the OP) the switch is remote from the socket, I suppose the wiring might be slightly simpler with a DP switch (removing the need to find some other way of dealing with the Ns). I also suppose that some people might use such a remote DP switch as a means of isolating the socket if one wanted to work on it, but I think I'd be keener on more upstream isolation!

Kind Regards, John

 

[BHI]

Thank you for all your replies - it seems like there are no overwhelming safety reasons to go for DP over SP, or vice-versa, if common-sense prevails and if just switching rather than full isolation of the sockets is the intention.

 

[ban-all-sheds]

In that situation, the switch could be in the 'off' position and the load would appear to be 'switched off' (neutral of switch open), but L would still be getting to the load.

Why would that rightfully be a problem?

 

[JohnW2]

In that situation, the switch could be in the 'off' position and the load would appear to be 'switched off' (neutral of switch open), but L would still be getting to the load.

Why would that rightfully be a problem?

Do I take it that your point is that, if one is going to work on the circuit, failure of a faulty switch to isolate it properly should be detected by 'testing for dead'?

If so, then that is true if one is going to work on the circuit (even though we know that, in reality, some people would not 'test for dead', and therefore would be at risk in this situation). However, in other situations, it could surely be a 'problem'? ... for example, if one used the DP switch to isolate an appliance because one thought that its casing had become live - failure to interrupt the L would then surely be 'a problem', wouldn't it?

However, as I've said, until recently I'd never heard this (fairly remote) possibility suggested as a downside of DP switching/isolation!

Kind Regards, John

 

[BHI]

Sorry, I think that the whole SP v DP debate stems from MK 13A sockets touting themselves as being DP-switched and therefore safer - yet some folk suggest this is not in fact the case. Perhaps just scaremongers / trolls?!

As you say though, no portable equipment should be worked on whilst plugged-in. Equally, no socket should be worked on unless the entire circuit is isolated at the CU. But I guess that a potential problem could theoretically arise when a socket is incorrect identified on the wrong circuit - so the wrong circuit is isolated, and since the faulty DP socket appears dead to any plugged-in equipment (if the socket is not directly tested properly), then the N-pole could be open yet the L-pole could acutally still be closed?

I have to agree that it's all a one-in-a-billion chance that would also require a string of "numpty" factors, so I apologise for the wild-goose chase...

 

[JohnW2]

Sorry, I think that the whole SP v DP debate stems from MK 13A sockets touting themselves as being DP-switched and therefore safer - yet some folk suggest this is not in fact the case. Perhaps just scaremongers / trolls?!

Indeed - but I don't think MK are unique in this respect. What is the point (i.e. what they believe makes DP switching in a socket 'safer') is somewhat less obvious. As you go on to say, if one wants isolation (e.g. to work on an appliance), one would pull the plug out! I suppose it could be more of a (theoretical) issue with a DP switch/isolator for a fixed-wired appliance, when the option to 'pull the plug out' does not exist.

...But I guess that a potential problem could theoretically arise when a socket is incorrect identified on the wrong circuit - so the wrong circuit is isolated, and since the faulty DP socket appears dead to any plugged-in equipment (if the socket is not directly tested properly), then the N-pole could be open yet the L-pole could actually still be closed?

That's true - but as I think BAS was implying in his recent post, even that should not be a problem if, as one should, one tests to confirm that the circuit/socket/whatever really is 'dead' before working on it, even after a supposed isolator has been operated.

Kind Regards, John

 

[ban-all-sheds]

Do I take it that your point is that, if one is going to work on the circuit, failure of a faulty switch to isolate it properly should be detected by 'testing for dead'?

I took that as read, hence why I couldn't see how it would rightfully be a problem.

If so, then that is true if one is going to work on the circuit (even though we know that, in reality, some people would not 'test for dead', and therefore would be at risk in this situation).

All bets are off in that case.

However, in other situations, it could surely be a 'problem'? ... for example, if one used the DP switch to isolate an appliance because one thought that its casing had become live - failure to interrupt the L would then surely be 'a problem', wouldn't it?

So the case has become live, but no RCD or overcurrent device has operated?