Frequently Asked

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Why is the reading not zero when there is no current?

Also answers Why is the CT not showing zero's when not connected?

This is due to the RMS calculation. To have perfect zero would imply the waveform is a perfect straight line. The CT sensor and the signal chain on the RPICT is analog. This means that noise is also measured. Therefore it reports zero + noise when not connected or without any current flowing across.

For a board setup with the SCT-013-000 and 100A fitted burden resistor one might find a background noise around 0.2A (50W for 240V). Do take into consideration the range of the board versus the level of noise. In the example the reading range is 100A with a background noise of 0.2A. This is a 0.2% ratio.

What if I need to read something below or around the noise level?

One way to have a lower background noise is to opt for a lower range at purchase time (rating).

Another possibility is to use a voltage sensor and measure Real Power instead of Estimated Power.

The best way we offer to reduce the background noise to a minimum is using the RPICT7V1 Version 4.

Can I reduce the background noise with a calibration? No.

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 (or near 0).

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 applied to be clearly visible. Anything in the vicinity of 50 watt would not stand out much from noise.

Why is Real Power sometimes 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.

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.

Which pins are used on RPICT series?

The Raspberrypi pins the RPICT board uses are listed below.

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

For some of the newest board the following pin is also used for auto-reset:
7 - GPIO4 (to arduino reset)

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.