Frequently Asked

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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 around 0.2A (50W for 240V).

If the admitted noise value appears too high then one might have to opt for lower range burden resistor. Another alternative is to use a voltage sensor to compute Real Power which provide much lower background noise.

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 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 200W should be measure to be clearly visible. Anything in the vicinity of 50 watt would be hidden in noise.

Why is Real Power negative

Real power will be either positive or negative depending on the current direction. If the negative number is not desired flip the CT clamp the other way.

Which pin are used on RPICT series?

1 - 3.3V (RPICT3T1, RPICT3V1, RPICT4T4)
6 - GND
8 - TXD
10 - RXD

Python 'device reports readiness to read but...' error

You might encounter the following error when running a Python program.

Traceback (most recent call last):
  File "/usr/local/bin/", line 564, in <module>
    L = wait_and_read(options)
  File "/usr/local/bin/", line 59, in wait_and_read
    r =
  File "build/bdist.linux-armv6l/egg/serial/", line 501, in read
serial.serialutil.SerialException: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

This is usually due to a duplicate access to the serial port.

To solve this find the other program using the serial port and stop it.

Common causes are:
- Emonhub is already running. Stop it using sudo /etc/init.d/emonhub stop.
- Another python program (or cat) is already running.
- The Raspberrypi has been misconfigured. Make sure you have correctly followed all the steps in Howto setup Raspbian for serial read.