Join Two Separate Lighting Circuits. Hmm?

[bankruatsteve]

Here's one for the EE's and/or sparky's who know. For myself, I keep coming across brain cells that I've killed, so looking for explanations...

The situation is: I have up/down CU's separate but both fed by a master cutoff. I have two strings of garden lights. One is fed from the up CU, the other is fed from the down CU. Neither are protected by RCD - one string (200W total) is connected to a 10a light circuit, the other (150W total) is connected to a 24a (mostly) receptacle circuit.

I have a timer thingy that now is no longer needed for its original purpose. I was thinking of connecting that (the timer) to one of the garden light strings - which I can easily do and a no brainer.

So here is the QUESTION: What if I connect the two light strings together? ie: so that both are controlled by the timer. If that is all I do, the two different circuits would be connected. I suppose if I left the switch of the non-timer circuit OFF, there would be no LIVE competition, but the neutral would be joined. (?) And, if I did switch the non-timer circuit switch ON, it would (I assume) power the lights and backfeed into the timer. And, vice-versa from the timer circuit into the power circuit.

Is this enough description? The help I would ask is: If the QUESTION is a bad thing to do - then why? If it doesn't really matter - then why? I don't think I'm looking for alternatives unless you have a good one.

Rgds.

Steve

[Forkinhades]

Quick answer would be do not join neutrals from 2 different CU's period. Although they go back to one source.

If your timer is 4 pole, could be a workaround though, but still dont like the idea of one piece of kit being fed from 2 different sources.

You dont seem to have much of a load, so why dont you just join together at the timer, but disconnect one circuit from one CU and load side, both neutral and live, and clearly mark them up once disconnected.

[doglover]

You should purchase an additional timer and have one installed on each circuit (check the wattage rating of the timer). Or disconnect one of the feeds and feed both light strings from one circuit and timer.

[doglover]

I think you are more curious if your proposition would work or not rather than just being provided a solution. Assuming the garden light strings are wired in parallel what you suggest would work except for the timer. It needs a power source on it's line side. If it was backfed the circuit would be open at the timer unless it was already in a closed position from prior operation. Yet with the garden lights wired in parallel if you closed the switch at the opposite end from the timer the lights will still light up. And if you closed the switch at the timer end the lights would operate as per the timer settings (keeping in mind if one switch is closed the other must be opened prior to any switching being performed)... ... I have a headache already...

So, as you can see it just is not the way to do things. Not to mention the electrical codes it goes against.

[Crossy]

You dont seem to have much of a load, so why dont you just join together at the timer, but disconnect one circuit from one CU and load side, both neutral and live, and clearly mark them up once disconnected.

That's the way to do it

[bankruatsteve]

After some reflection, I've decided this was a stupid post and, while no one actually answered my QUESTION, I have waken the dead and realize that by combining the two circuits, one would (if both switch ON) energize the other circuit and which would make the relative MCB ineffective as a cutoff - so that's bad.

As for joining the circuits at the timer, yah - if I was in the planning stages - at this point that is not practical. This is retro-fit so the only way I would join the circuits will be with 4-5m of cable/conduit and some trenching across the driveway and then disconnect at the non-timer circuit switch end.

So, that's it - not worth the discussion I thought it might have. Aagh.

[Forkinhades]

More to the point you can google borrowed neutrals

ATB

[bankruatsteve]

More to the point you can google borrowed neutrals

ATB

I understand borrowed neutrals so why would I "google". In the scenario I posted, what neutral is borrowed - if that is your point? My point was the QUESTION in the OP.

But again... I don't think this needs more discussion. Thanks anyway.

[Forkinhades]

^ ok steve, so you are happry with the answers?