Sending 240VAC + 24VDC on the same 3 conductors.

[paulca]

It's just an idea I have been rolling around in my head.  Questions:

1.  Is it possible?
2.  Is it wise?
3.  Is it safe?
4.  Is it worth the effort versus adding another conductor?

On 1, I figure I could use a circuit capable of biasing the 50Hz 240V by the 24V DC as offset.  Current capacity might be a concern.  Q2 sounds unlikely in this case and Q3 is probably a NO not DIY!

4.  is a more tricky one.  It cost me £120 to install the existing 3 core heavy 6mm^2 cable.  Adding another won't be much cheaper.

On other options, given that the 3rd conductor is presently un-used as I do not earth reference the DC..... yet.

There is therefore a 4th conductor, in the form of the local earth wiring.  If I was to send the DC+ up the 3rd conductor and earth reference it at either end using a "local PE" connection.  Similarly I could Semi-TT the earth of the VAC and only send the Live/Hot and do my own N->E bonding at either end.  <-- all of these sound like they will score a flat 0 on the "Is wise, is safe" questions.

We are talking about 240VAC @ a max of 5A and 24VDC at a maximum of 5A.

[KaneTW]

Possible? Sure.
Wise/Safe? I wouldn't do it. Due to lack of double or reinforced insulation the 24VDC would be never be SELV or PELV and would have to be treated similarly carefully as low voltage.
Worth the effort? Almost certainly no.

[Siwastaja]

Mains-powered transformers and motors will saturate easily due to DC offset, causing heating and possibly destruction. A few volts is normal; 24V is a lot.

But let's pretend you don't have any such DC offset sensitive load. But the question is, why? What do you think you would gain with that? What are you actually building?

[paulca]

I'm converting the 12VDC solar system to 24VDC + 240V invertor.

While sending the 240V up the 6mm conductors and creating my own 24V on the other end sounds like a lot less hassle, the power drop in the invertor, even at idle, has me thinking it's a part time item.  Turn it on, run something off it for an hour whatever, switch it off.

However I like having my always on low voltage DC from the system I can run and charge lots of low power devices with it and don't need to worry leaving it on all night.

On offset.  I was obviously thinking of removing the offset to get my 24V back again.

[jonpaul]

would use,a CT trsf, see phantom power.

Will violate electric regulations and local code.

Any shock, injury, fire, loss will be your liability and not covered by any insurance of your property, real estate or personal liability.

Suggest you research electrical safety isolation in your area and hire a top lawyer.

Jon

[Siwastaja]

On offset.  I was obviously thinking of removing the offset to get my 24V back again.

Wat?

I have read your posts like 3-4 times and still don't get what you are suggesting, at all. You want 240VAC with 24VDC offset (i.e., a sinusoidal waveform between -315 and +363 V)? Then you would sometimes run this without the offset (240VAC, sinusoidal between -339V and +339V)?

And somehow you would power 24V loads from this -315V to +363V supply, and it would be somehow easier than using off-the-shelf power supply our of standard 240VAC (without offset)?

From guessing about the "power drop of invertor" thing, maybe you want sometimes 240VAC, sometimes 24VDC, on the same conductors? Is this what you mean, nothing about offsetting the AC?

[Terry Bites]

Do you value you life and the live's of others? Do you own anything of value? Go ahead!
Phantom power only works for very low currents. Nahhh.
Just buy that cable, live long and prosper.

[paulca]

I figured it was  "No".  Just rolling ideas.

From guessing about the "power drop of invertor" thing, maybe you want sometimes 240VAC, sometimes 24VDC, on the same conductors? Is this what you mean, nothing about offsetting the AC?

That is an option.  Has it's own safety issues.

In fairness it's probably not worth bothering.  For 6Amp 240VAC I can use much, much cheaper cable.  I can simply add another 3 core (2mm^2) 13amp outdoor cable up the same route and cable tie it to the original.  It's black cable, black ties, back of the house.

All this is intended to be off grid.  I just want to power a few things off it.  Ideally an office set up of circa 200-500W for a few hours a day maybe.

In another thread I have a schematic of the circuit.  The inverter is earthed, fused and the output passes via an RCD etc.

In the UK house insurance covers you for accidental damage.  Unless you delibrately set your house on fire (arson), it would be difficult to argue negligence on the consumer side.  Tampering with the mains installation wiring (anything that needs unscrewed!), might find you partly liable.  This will not touch the mains wiring, save for a  protective earth tap.  It will power only one extension.