DP switch for LV and ELV

JohnW2 · 7 Feb 2019

I suspect I can predict some of the replies, but I wondered what people's thoughts are about using one pole of a DP switch to switch LV ('230V') and the other pole to switch ELV (~12V).

The insulation within the switch would obviously be adequate, given that it would be designed to have an LV potential between its poles.

I could avoid doing this by using relay(s), but in some senses I would then merely have moved the issue to somewhere else, since there would still, unavoidably, be LV and ELV connected to one 'something' (a relay, rather than a switch).

I've certainly done it in the past, but would be interested to hear thoughts before I 'do it again'!

Kind Regards, John

ericmark · 7 Feb 2019

I see nothing wrong with the idea, it is often unavoidable. Clearly all cable for LV but you know that.

bernardgreen · 7 Feb 2019

The only comment I would make is that the ELV circuit should be Earthed to ensure any fault in the switch (*) creates a current to Earth va the ELV and thus trips the RCD on the LV supply.

(*) Any moisture or other leakage path from LV to un-earthed ELV in or around the switch could raise the wiring of the ELV circuit to hazardous potentials without any indication of the hazard. The ELV circuit would function normally.

JohnW2 · 7 Feb 2019

ericmark said:
I see nothing wrong with the idea, it is often unavoidable. Clearly all cable for LV but you know that.

Yes, as I said, it is essentially unavoidable since, even if one used relay(s), there would still be a need for some component (even if a relay, rather than a switch) to have both LV and ELV connections.

Kind Regards, John

JohnW2 · 7 Feb 2019

bernardgreen said:
The only comment I would make is that the ELV circuit should be Earthed to ensure any fault in the switch (*) creates a current to Earth va the ELV and thus trips the RCD on the LV supply. .... (*) Any moisture or other leakage path from LV to un-earthed ELV in or around the switch could raise the wiring of the ELV circuit to hazardous potentials without any indication of the hazard. The ELV circuit would function normally.

Yes, I thought about that, but it's not totally straightforward ...

... if one side of the ELV were earthed, I wouldn't normally think of switching the earthed side, so if there were a fault from the ELV to the non-earthed side of the ELV, whether or not that would result in enough current flowing to trip an RCD would depend upon what was going on in the ELV circuitry. Also, of course, if there were a requirement for the ELV to be SELV, one could not earth it.

What would you do if you wanted to switch a floating (e.g. SELV) ELV supply with an LV-powered relay?

Kind Regards, John

bernardgreen · 7 Feb 2019

Some relays are designed with enhanced isolation between coil and contact with breakdown voltage of 5 kV.

JohnW2 · 7 Feb 2019

bernardgreen said:
Some relays are designed with enhanced isolation between coil and contact with breakdown voltage of 5 kV.

Indeed - but (depending on the nature of the 'enhanced isolation') that doesn't necessarily help in relation to your "moisture or other leakage path", does it?

Kind Regards, John

bernardgreen · 7 Feb 2019

JohnW2 said:
ELV supply with an LV-powered relay?

LV to the primary of a transformer, voltage from the transformer secondary to operate the relay, one side of (or the centre tap of) the secondary to be Earthed,

Or do what the super carefull GPO telephone engineers did for switching a mains lamp from a low voltage ( 48 v ) control circuit.

bernardgreen · 7 Feb 2019

JohnW2 said:
that doesn't necessarily help in relation to your "moisture

Anti creep profiles in the insulation reduce the risk of leakage in moist atmospheres

Harry Bloomfield · 7 Feb 2019

bernardgreen said:

I remember installing one of those for a mill owner in the early 1960's. The noise in their weaving shed was deafening, so they wanted a system which would make it obvious the phone was ringing or had rung, on the night shift, with no one in the office. I was asked if I could think of a solution. I devised a system which flashed a couple of 240v 100w coloured spot lamps, alternately once triggered and kept flashing until a reset button was pressed.

JohnW2 · 7 Feb 2019

bernardgreen said:
LV to the primary of a transformer, voltage from the transformer secondary to operate the relay, one side of (or the centre tap of) the secondary to be Earthed,...

Do I take it that you're talking about an ELV secondary voltage? If so, then fair enough, but that is presumably only an approach for the 'very cautious'.

Or do what the super carefull GPO telephone engineers did for switching a mains lamp from a low voltage ( 48 v ) control circuit.

I can't fully work out what is going on there - can you perhaps give a brief description?

Kind Regards, John

Harry Bloomfield · 7 Feb 2019

JohnW2 said:
I can't fully work out what is going on there - can you perhaps give a brief description?

Kind Regards, John

The 48v solenoid is switched on by the ring current, that then operates the mercury switch and the mercury switch switches the mains circuit.

JohnW2 · 7 Feb 2019

Harry Bloomfield said:
The 48v solenoid is switched on by the ring current, that then operates the mercury switch and the mercury switch switches the mains circuit.

Thanks - what's going on to the left of the relay armature?

Kind Regards, John

Harry Bloomfield · 7 Feb 2019

JohnW2 said:
Thanks - what's going on to the left of the relay armature?

Kind Regards, John

It just looks like a bit of gash wiring, it probably went to a pair of 50v terminals originally for the phone line connection.

PhillCO2 · 7 Feb 2019

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.

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.

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DP switch for LV and ELV

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