Do I need to use a Voltage sensor?
Yes if you need to do accurate readings. Yes if you are measuring for current direction (e.g. import/export in solar panel).
For any other readings using an estimated voltage to obtain is usually sufficient.
Why is the CT not showing zero's when not connected?
Reading values with the CT unconnected will output a small value. This is normal.
The CT sensor is an analog sensor and has noise. i.e. It will always measure an approximate value. Hence zero + noise when not connected.
Even if nothing is connected there will be always a tiny current running in the burden resistor inducing a small voltage. In turn this will convert into a small power. For a board setup for the SCT-013-000 with 100A fitted burden resistor one might find a background noise under 100W.
If the admitted noise value appears too high then one might have to opt for lower range burden resistor.
Why is voltage reading zero then? Voltage will show zero when unplugged because the sketch is programmed to give zero if it times out finding enough sign changes. No sensor -> no ac signal -> no sign changes. Note that voltage sensors are also analog and same applies concerning noise. The zero in that case would rather mean "timeout" rather than "value is 0".
CT is clamped but reading does not move
1. Make sure the SCT is clamped around A SINGLE PHASE.
Clamping the CT on the whole mains cable will never read anything as it measures two currents. One positive plus one negative which sums to 0 (+noise).
The easiest quick test is to connect the CT at the meter level on a single phase. Otherwise if willing to measure the power used by a single application then one should make a special cable that split phases.
2. If using RPICT7V1 or RPICT4V3 or RPICT3V1 make sure the voltage sensor is plugged in and voltage reading is correct.
3. Make sure the load applied is sufficient and adapted. If using a 100A range board then a load of minimum 1kw should be measure to be clearly visible. Anything in the vicinity of 100 watt would be hidden in noise.
Which pin are used on RPICT series?
1 - 3.3V (RPICT3T1, RPICT3V1, RPICT4T4)
2 - 5V (RPICT7V1, RPICT8, RPICT4V3)
6 - GND
8 - TXD
10 - RXD