Most sunny days my system is running the out buildings (non-essential) using nice free sun power. If we manage loads carefully we can run the whole property off of solar when the sun is shining. The known condition that we can’t handle is the lawn mower - the start up current is too high and the system will trip.
But sometimes we have an outage when the sun is up. Then we lose the out buildings. Then the poolpump doesn’t run and we have to run an extension lead if we want to do any washing.
So I thought about having a change over switch so that I can move the sub-db for the out buildings onto the essential side of the main DB when I need to. Potential problem: I switch over to essential. I forget about it. Things run fine until the gardener starts up the lawn mower.
So I thought of using a timer switch in the DB. Most of the ones you can fit into a DB are double throw. So you can switch between two circuits. But they’re usually single pole, allowing you to switch the live connection between two circuits.
I like a timer because I can always set it to switch back to non-essential if I forget. Say at 1 in the morning. Then I have to explicitly switch back to essential if I want to do that, knowing that if I forget to manually switch back then the timer will take care of that.
But to use the SPDT switch I think I have to tie the two neutrals together - the neutral from the grid and the neutral from the inverter. This seems like not a very good idea to me - but I can’t say exactly why. I’d rather be cautious about it than just go ahead and blow something up.
I have a suitable manual cutover switch already.
Does this timer idea have any merit, or there are other red flags that I, in my benighted state, am not seeing?
You need a DPDT timer, or you need to make up your own.
You do get DPDT contactors, and you can switch one of those with a timer.
You can also use two contactors, physically and electrically interlocked, switched with a timer. That would be the safest by far, but a bit costly: Two contactors plus the interlock kit, plus the timer.
Thank you. Are these electro-mechanical or solid state? The sub-db used to have a timer switch that couldn’t handle much current, but it could switch a contactor that could carry enough current for the geyser element. This worked, but the contactor was noisy when engaged. In the out buildings this was not a problem, but in the main DB in the house it would increase my grumpiness.
There is no affordable way to do this safely & legally. The only acceptable way would be with a changeover switch (it must be break-before-make like all the approved changeover switches). A DPDT contactor cannot guarantee that you won’t connect your backup to grid under all circumstances.
What you want is an ATS and one that certified will be >R10k.
Of course you are perfectly correct, I may have been entirely unclear that I don’t support such a thing. That would be the minimum you need to avoid a shared neutral, which is a bit of a no-no.
I think it can be done safely with two contactors that are physically and electrically interlocked. Basically the same hardware you’d use for a delta-star starter on a three phase motor. Transfer speed is not a big deal here.
Of course you cannot rule out those corner cases, a short in one contactor that allows it to open (so the mechanical interlock allows the other one to close), but still results in a connection to both grid and inverter. For that, you indeed need something very expensive.
Left field idea.
I think the simplest and cheapest solution will be something like a CBI Astute setup with some rules, on the plug socket/s which the garden engineer use for the lawn mower. If there’s no grid then switch them off, or only allow 500W or something.
The whole house is on essentials. The geyser never used to be, but the actual heating is by a heatpump, and the draw is low enough that I get away with it on essentials.
There is a possibility of a trip if we turn on enough kitchen appliances at the same time.
I know you only used it as an example, but just saying, there’s no way I’ll trust a ATS costing only R500.
From my days in the AV industry I actually detest them, in 99% of buildings where we operated we just switched them to manual. Fun times when Eskom and a generator start chasing each other via the ATS, the smoke will come out guaranteed.
Smart sockets in the kitchen can solve it by limiting or switching off when there is no grid.
It will make for interesting times with the wife, but it will work.
