DP switch for LV and ELV

[JohnW2]

I don't know your purpose but this kind of thing is common in industrial systems, including 24V logic being influenced by 110, 230, or 400/440V contactors or other circuit components. In these cases one normally sees an insulated auxiliary contact block(s) mechanically linked with the higher potential contactor, often for manual controls modular switches with multiple separate switch contact blocks.

Yes, I'm sure it must be common and, as I said, I've certainly done it a good few times in the past, in one way or another (for 'domestic' purposes).

It still doesn't remove the consideration of multiple different potentials with different sources within the same enclosure, you have to rely on the fact that only someone competent should be opening it up and poking around at the end of the day, and mark everything up physically and in drawings as best as possible.

All true. However, merely having LV and ELV within the same enclosure is very common (and 'accepted'). Furthermore, I imagine that one of the common reasons for using relays is (as in most of my applications) the situation in which a low voltage is being used to switch a higher one, or vice versa, in which case the 'low' and 'higher' voltages are necessarily connected to terminals on the same component.

In the context of 'fixed electrical installations/wiring', the expectation will be that any enclosure contains LV (230V, 400V or whatever), so the additional presence of ELV within the same enclosure would not really introduce any 'safety' issues (in the sense of an awareness of what voltages were present). There would be a concern (warranting labelling etc.) if ('the other way around') something which 'appeared to be' a piece of entirely ELV equipment also contained LV potentials, but that would be a pretty rare situation.

So, I don't really have concerns about having substantially different voltages within the same enclosure, but I asked the question in my OP specifically in relation to a DP switch - since I suspected (seemingly wrongly - at least so far!) that I might get more 'negative' comments about that specific issue than have.

Kind Regards, John

 

[bernardgreen]

So, I don't really have concerns about having substantially different voltages within the same enclosure,

The most common hazard is the ELV remote from that enclosure that has become Live due a fault in that enclosure.

 

[Harry Bloomfield]

A novel approach I took to isolation many years ago, was in a pumping station in south Wales - a prototype pumped storage system. The control panel monitored water pressure on the input and output of a pump. Simple upper and lower limits triggered by a needle on the large glass faced gauges. I guess it must have had a delay, ignoring the pressures on start up, until things settled. The panel had been designed so the entire control system ran at 240v. So live 240v contacts just behind the gauge glasses in a sometimes very wet environment.

I added some small, earthed core transformers, with well separated 240v and 12v windings. The 240v it series with the 240v relay, the 12v sides connecting to the pressure switches. Short the 12v winding out, with the pressure switch - the relays pulled in.

Water level detection back then, used a similar idea. A tiny transformer, mains side operating a very sensitive relay, low voltage side had two metal rods (galv. steel pipe) - long one deep into the water then a shorter one. The bottom tip of the short one set the trigger level. When water made contact with both probes, relay made and operated a contactor, which in turn switched pumps on or off.

 

[JohnW2]

So, I don't really have concerns about having substantially different voltages within the same enclosure

The most common hazard is the ELV remote from that enclosure that has become Live due a fault in that enclosure.

Indeed, but that doesn't alter, or detract from, the statement of mine that you quoted.

In fact, although this would obviously not always be the case, in the situation I am currently dealing with, there will be no "ELV remote from the enclosure". All the ELV will be within the enclosure, it's sole purpose being to power some electronics which control relays which switch 230V. The enclosure therefore has 230V entering it and various switched 230V lines leaving it, and no other connections to the outside world.

Ironically, the issue you raise is relevant in relation to the other issue I recently discussed, about remote LV switching of 230V. In that case, it is only the isolation provided by the 230V->12V PSU which prevents the 'remote ELV' becoming live - so there is then an argument for earthing one side of the ELV.

Kind Regards, John

 

[JohnW2]

The most common hazard is the ELV remote from that enclosure that has become Live due a fault in that enclosure.

It's just occurred to me that that risk inevitably exists with any SELV situation - since the isolation within whatever is converting LV to ELV is the only thing preventing the SELV wiring from 'becoming live' and, if it's SELV, one is not allowed to earth the ELV. That means that one cannot arrange things so that an RCD would hopefully operate if that isolation were to become compromised.

Kind Regards, John

 

[mattylad]

You could always simply use the switch to changover contacts on 2 relays.
One for LV and the other for ELV.

That way you have the complete isolation at the contacts that you may require.

 

[JohnW2]

You could always simply use the switch to changover contacts on 2 relays.
One for LV and the other for ELV.

Yes - but (in addition to being a hassle and probably unnecessary 'overkill'), as I've said, that would merely move the issue (of 'one component' having terminals, and internal parts, connected to both LV and ELV) from the switch to one of the relays.

Kind Regards, John

 

[ban-all-sheds]

the other pole to switch ELV (~12V).

AC or DC?

 

[JohnW2]

AC or DC?

DC (very low current)

 

[JohnW2]

AC or DC?

Given my uncharacteristically brief response to this question, I thought that I might as well explain what the immediate application (of a DP switch switching both LV and ELV) is all about ....

I have two bits of (related but totally separate) electronic circuitry, both requiring a 12V DC supply (voltage not critical). Both will only be 'switched on' (with a manual switch) very occasionally, so that it would make no sense to leave a 12V PSU permanently powered. The 'simplest' solution would be to just have two 12V PSUs, and for the switches to switch the 230V inputs to them. However, being mean, I wanted to do it with just one PSU - so, per diagram below, I plan to use two DP switches, one pole of each of them in parallel switching the mains input to the PSU and the other pole switching the output of the PSU to the appropriate load ...

Given that I am perfectly happy with doing that, there really isn't any justification in trying to make things any more complicated. However, more as an academic exercise than anything else, I toyed with a way which would require just a SP switch for each load (switching the 12V output of the PSU) with a battery, some diodes and a (12V coil) relay switching the 230V input to the PSU. The below assumes a non-rechargeable battery (which could go flat!). If I wanted to use a rechargeable one, I would presumably either have to make the circuitry a bit more complicated or else use a battery which didn't mind being permanently 'floated' across the output of the PSU whenever it was on. I would obviously have to remember to press the push switch (as well as switching off the main switch) when I wanted to 'switch off'. However, as I said, I don't think that this degree of 'complexity' would be justified (if I were that concerned, I'd just use two PSUS!) ...

Kind Regards, John

 

[ban-all-sheds]

Or you could use a normal 3-gang 1-way switch - one gang to turn on the PSU, the other two to switch the 12V.

 

[JohnW2]

Or you could use a normal 3-gang 1-way switch - one gang to turn on the PSU, the other two to switch the 12V.

Ah yes, I forgot to mention that 'even simpler' (even simpler than having two PSUs) approach.
If it were 'just me', I might well have done that. However, having to operate two switches to turn one thing on (and, even more so, having to operate two switches when one turned the thing off, even though it 'went off' after only one of the switches was operated) would, I fear (even with appropriate labelling), potentially result in confusion (and mistakes) for some potential users.
It's also not a very 'nice/neat' solution - a bit like having to operate two switches to get a room light to come on (and having to remember to operate both when one wanted to turn the light off - even though it would 'actually go off' if only one of the switches was operated.)