This inverter can be selected to be a CT inverter, and you get a Current Transformer which you can use to prevent pushing power out to the grid.
Type: 120A/40mA, supplied by solis.
In my case, the current measurement point is 60m from the inverter. can we extend the measurement cable that long?
A current transformer’s signal is a current, not a voltage. The CT will try to supply the current that represents the current in the measured cable. So it’s unlikely to vary with the length of the signal cable. (similar to a 4-20mA signal)
It’s easy to test however: Insert a multi meter in series with the CT signal cable and compare the readings (AC current mA range)with and without the cable.(i.e. disconnect the signal and measure at the CT and then with the inverter + 60m cable connected)
(From memory you can run a 4-20mA cable 600m)
Right. But a very long cable increases the total resistance. At the end of the cable, there is almost certainly a small load resistor to turn it into a voltage signal that can be read by a microcontroller.
The current is usually a very small amount. For example, a pretty typical CT (the SCT-013-000) shows a 50mA output when clamped around a 100A wire, so for your average single phase house you’re dealing with max 25mA or thereabouts. That means you don’t have to be too concerned about a voltage drop… but… I would at least use shielded wire to avoid turning it into a massive radio antenna 
I did recently have a case where someone extended the wire going to such a CT up to 300 meters. And it even worked for a while… until it didn’t.
Yes, because its a current signal and not a voltage signal. On the mine we have send mA signals up to 250 meters on many of our instrumentation equipment.
Yes indeed!
If one were to do a professional installation you need to convert the CT signal to an analog signal (typically 4-20mA) and then convert it back to what the inverter input is configured for.
If the inverter input isn’t able to be configured for different signals then it sends a message that they don’t encourage this method.
I have used a self-twisted pair without shielding over the ground for an even smaller CT input ( micro-amps) over a further distance very close to a sub-station. This was a temporary test set-up for a few hours. It was remarkably accurate, I didn’t think it stood a hope.
I would use a proper shielded pair for anything permanent, accuracy won’t be an issue.
What will be an issue is insulation.
I would run a decent earth wire between the termination fixtures, don’t rely on the shielding to be the earth connection.
Otherwise that twisted pair will succumb to lightning.
Bingo.
This is an unfamiliar approach to me. In instrumentation we normally prefer to only earth on side of the shielded cable to prevent creating additional ground loops. We would run a few hundred control loops in each section and only earth the one side, normally in the marshalling rack at the distribution side and never at the field devices.
Do you recommend running a separate earth cable next to the signal cable and earthing both sides?
The shield or earth is only to get rid of EMF and to keep the signal “Clean”, Its not suppose to act as an earth conductor, the instrument on the one side and the Control system whether a PLC or DCS on the other side, should have their own proper earthing/earth conductor.
EDIT: Wait a minute, I only understood what you said after answering, I gues we are saying the same thing here. I do apologise…
@JacoDeJongh, thanks for clarifying the role of the shield. You are correct of course.
I was trying to get across that lightning will use that shield and indeed the twisted pair as well, (regardless of any earthing), if it is not provided with an alternative path.
