Specifications for switch?

Is the inverter derived mains still on in a power cut ? If so then there are minimum isolation requirements, to meet (to make incredible,  the risk of feeding inverter power into an isolated street main and hurting the chaps working to repair it ). 

If not then any double pole (I assume single phase, 4 pole if 3 phase) change over with adequate contacts and a rating that treats the 2 supplies at least as well as separate phases should be enough.
There are some nice rotary ones out there.
Mike.

Some thoughts ...

Is the solar kit connected to the mains? If so, why change between the two?

Any break before make switch should do, but then you have to consider the earthing arrangement.

I have thought about this for when I get a PV array installed. If, e.g. in the height of the summer, I am generating 4 kW and using very little, it would be better to heat my water cylinder (what else, if not a large battery?) than pass the surplus into the mains. Clearly with gas heating, there must be a break even point when the cost of leccy (PV + mains) and gas are the same. It may in fact be better to have a low-powered immersion heater (do 1 kW examples exist?) to use up a smaller surplus.

I do not think that this is as easy as it seems. I can also see a situation where the PV battery is charged up and the surplus is going into the Grid. Potentially, it could supply the immersion heater for, say, an hour and then charge up again.

Am I thinking along similar lines to your client?

To be honest there appear to be things to check further before committing.

All I know is that there is a 3phase TT supply. One phase feeds the main house and another feeds a heat pump. The heat pump cannot keep up with the demand for hot water, ergo the requirment for a facility to turn on an immersion heater on one of the 2 tanks associated with the heat pump when demand is too high.

When the heat pump was fitted, a tank was supplied which I am guessing is generally heated by a hot water coil from the heat pump unit but there is also an immersion heater present.

Now, what gets complicated is the way the RISC rebate is paid, with a disqualification if the aux immersion is connected to the 'dirty fossil fueled mains side. Since all of this was done, solar panels have been added but I don't know how the solar generated supply is divided up, but the solar folks removed the prewired dirty mains connection to the immersion heater from the heat pump CU and wired it directly to their new solar supply. Problem is, the heat pump cannot keep up with HW demand so am thinking of fitting a change over s/w so that the heater can be fired by the dirty mains for short periods when extra HW is required in a hurry.

So, the idea is one T&E to the immersion heater FCU. One supply in T&E from the mains, and another from the solar, with a change over switch to swap between the two.

I asked about the type of switch because of my ignorance of the regs surrounding the nature and rating of switch contacts related to renewebles and all this nonsense about suff having to be bidirectional. But I suppose that it doesn't matter because the solar supply is I think - must check - coming directly from the inverter already oven baked for 230VAC.

I had a quick search and found that immersion control products exist that could divert energy to the heater when there is sufficient solar being generated. However, I feel like this problem requires more complex controls than what has been described. i.e. if the heat pump requires the additional hot water but there is insufficient solar to operate the immersion; what needs to happen?

What needs to happen is that someone can turn the immersion heater from solar to mains electricty when required, and then back again when the demand for hot water has fallen back to within th ability of the heat pump to provide it.The instances when demand is exceeded are few in number but do happen at peak times during the spring/summer months.

There are 6 mixer showers coming off the system at present! And that is before we factior in DHW for the kitchen sink and other applications.

whjohnson said:

Now, what gets complicated is the way the RISC rebate is paid, with a disqualification if the aux immersion is connected to the 'dirty fossil fueled mains side.

I see, but strictly-speaking, the presence of a switch should change nothing.

I do not see any problems doing this with a single appliance, provided that (as I mentioned earlier) the switch is break-before-make so that the two sources can never be connected together.

Does the solar have a separate output for feeding the immersion?

Normally, the output of the inverter is just wired into the mains.  In which case, creating two separate circuits for feeding the immersion, and adding a switch to choose one or other of them, is pretty pointless.  You have no control over whether the immersion is getting solar or mains electricity.

Some inverters have a UPS-like output for running important appliances, such as freezers.  But even they will be running off the mains if it's night time and the battery (if fitted) is flat.

The off-the-shelf solar diverters monitor the mains supply using a current clamp, and turn on the immersion if they see you exporting spare electricity.  I'm not sure if they have an emergency bypass.

I'm a bit confused so far. If it's a conventional grid-tied PV system there won't be separate solar and mains electricity system to switch between - they're both connected together already. It might therefore be just a matter of turning on the immersion at an appropriate time - by the normal laws of physics it'll then draw what it needs, the solar system will provide what it can and if insufficient the rest will come from the grid (or the excess exported to the grid if the PV is generating more than the whole installation including the immersion needs). Otherwise you're into all sort of problems matching demand with variable PV output. Battery PV systems tend to operate on similar principle just that some of the PV output can be time delayed to try and better match the installation's demand.

If there's no choice about when the HP need backup from the immersion, I might be as simple as letting the HP turn it on as it wishes - if there's spare solar capacity that'll be used otherwise the grid will make up the difference as normal. 

  - Andy.

AJJewsbury said:

If it's a conventional grid-tied PV system there won't be separate solar and mains electricity system to switch between

Precisely. So, whjohnson, which is it please?

This is one of the questions I have to find the answer to. At first when the heat pump went in,  and there were no panels, a cable was run to a FCU next to the tank from the aux consumer unit which also feeds some sockets and lights in the plant room. This was connected just to a 16A mcb and left switched off

The solar guys came along, pulled out the immersion heater cable from the cu and ran it into an adaptbale box. Into this box they ran another twin and earth from upstairs and I think it comers from the solar switchgear next to the house CU which is on a different phase.

It may well be that.the solar feeds into the house cu to the grid.

The heat pump has its own independent consumer unit. Both this and the aux CU are fed from a different phase to that of the house. Another questions which phase is the solar feeding back to?

Hopefully find the answers this afternoon.

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UPDATE: Solar feedsinto main house cu and a solar-to-hot water device has already been fitted but it just cannot keep up. Short version is, no point in fitting a change over switch after all. We're keeping it simple. I have returned the installtion to it's original config, with a direct connection to the mains supply. It seems the customer gets paid more by selling juice back to the grid rather than to attempt a saving by diverting a proprtion of  it to run the immersion.

This is the mad UK energy market at its best!