Posted by: @jancold

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My question is can I common the same 'phase' of the CT's and use a single pole switch to select either CT.

It depends

An unloaded current transformer can output a dangerously high voltage which may cause your switch to arc over, or it can cause arcing inside the CT, damaging it. In your project, one CT is always unloaded.

But some current transformers have a built in load resistor, so they are never unloaded.

Thus, which one do you have? You can post a picture of the CTs, it's written on the label. If it has volts output, then it has an internal load resistor and it is safe to switch it. If it has a ratio like "1000:1" then it is current output and is not safe to use unloaded.

@bobflux thanks, this is the CT I got recommended for my inverter.

It's a current output CT (33mA output for 100A input or 1:3000) so it should never be used unloaded.

It's the opposite of a battery: a battery is a voltage source, so leaving it unloaded is fine, it just outputs its nominal voltage, but a battery shouldn't be shorted because then current can be very high.

A CT is a current source: it's fine to short the output, it'll just output its tiny measurement current into the short. But leaving it open means it'll still try to output current into an open circuit, which makes a very high voltage. It's a 3000:1 transformer so whatever voltage on the input will be multiplied by 3000 on the output if there is no load.

You could hack something but you'd need make-before-break switches, so each CT is either shorted or connected to the meter, and during the transition both are shorted, but never open/unloaded.

What inverter is it? I get the feeling you're trying to find a hardware solution to an uncooperative software problem... many such cases...

Can it work with a smartmeter instead of CT?

@bobflux It is the solis s6 EH1P8K-L plus. In the summer I'm OK with the water heating coming from the grid but not in the winter with the pump running continuously   If there is another way to achieve what I want ie to make the EV charging invisible to the inverter that would be great. My battery installer was not as helpful as I'd hoped leaving me to do all the setting up and I have only just got them to change the RCBO to a bi directional. Octopus put the ASHP and EV on one CU which I agreed to as at the time I had not thought about battery drain. However if there is another way to do it, I am very interested.

If the Solis inverter is the same version as mine, (shipping with Meter Model SDM120CTM with its own CT), it can use a meter input on Pins 9/10 using RS485 or a CT directly onto pins 1/2. The inverter can be set to use one or the other via the onboard menu (and perhaps via the app). The inverter may throw a warning if the config is not fully adjusted in one step, but it sorts itself out on its own..

Of course if the CT not in use needs shorting, it will make sense to set that up on the wiring (I would just have two identical CTs) and just have a switch that also shorts the unused circuit.. 

With my IOG tariff and requirements (our EV is on a small CU and the heatpump on a massive wasteful Hager CU), I was able to just move the CT once and avoid discharging the home battery when we rather use the extra discounted period(s). 

If our EV was plugged in that often, I would have looked into how to make the most of the cheap leccy. Something for the HA lab to handle..

In your case, it may be best to have the EV RCBO moved to a separate dedicated CU or to one shared with low energy-use circuits (but wired directly from the house supply). 

If the goal is to avoid discharging the battery during cheap electricity times, you could use the Solis scheduler (I think it's called "timed charge")

Posted by: @bobflux

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If the goal is to avoid discharging the battery during cheap electricity times, you could use the Solis scheduler (I think it's called "timed charge")

If I understood right, the challenge is that IOG offers ad-hoc additional periods where the EV is charged (and Octopus drives the charging). When the EV charger starts, the inverter starts draining the home battery to avoid importing, as it assumes it would be an expensive import... (you are probably aware that Intelligent Octopus Go offers a base of 6 hours of low cost electricity at a fixed time, making that part very simple to program the inverter for)

@bobflux I wiĺ have a look at the inverter manual.

I had Solis check the config of my inverter to allow charging batteries overnight and a period of exporting roughly from 4 to 7... Made a few tweaks when I tested the smart port for a while but am just using grid and backup ports now. 

Happy to check the settings, timed periods, etc in case it helps. 

Last winter my  battery ,32KWh, could run the house completely, with a some leeway, on the cheap rate with the battery recharging  during the 2330 to 0530 period. However if IOG decided to charge my car outside of those hours it not only discharged from the battery but was of no use to Octopus and I would end up using higher rate grid import. Thanks for the explanation and ideas and they have stopped me from doing the wrong thing with the CT's.