Frequently Asked

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1 - Why do I see a constant small value while no CT is connected?

This is commonly asked for RPICT8 and RPICT3T1 units.

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 when no current is flowing across.

For a board setup with the SCT-013-000 and 100A rating one might find a background noise around 0.1A (25W for 240V). Do take into consideration the range of the board versus the level of noise. In this example the reading range is 100A with a background noise of 0.1A. This is a 0.1% 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 (see option 'rating').

Another possibility is to use a voltage sensor and measure Active Power (also called Real Power) instead of Estimated Power. This might require to acquire a new board that support voltage reading.

Note that using additional hardware on the Raspberrypi like video monitor / keyboard and additional hats will increase the risk to have higher noise.
The power supply unit quality has an impact on the level of noise. Make sure you are using a 2A minimum PSU.

The noise level also depend on the RPICT board used. RPICT units with atmega328p mcu (RPICT8 RPICT7V1 etc) do have a more consistent noise across channels compare to RPICTs with a attiny84 mcu (RPICT3T1 RPICT3V1).

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

2 - What if the reading is different from my multimeter?

RPICT series are sold configured with theoretical calibration values. These provide good results in most cases. If you do trust the instrumentation device you are comparing against and want both of them to match then you should carry out a calibration of V and I. See this article. Calibration_VCAL_ICAL_PHASECAL.

Please consider these points below.

  • The device compared against could be inaccurate.
  • If you are measuring the power of an apparatus that states its nominal power do not expect to read that number exactly. For example the label of an apparatus may indicated 2kW. Nevertheless do not expect to read 2000.00W but rather something between 1500 or 2500 approximatively. Also note some devices like ovens will be on and off and consume nominal power only for a short period of time.

3 - CT is clamped and load is applied but reading does not change

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.

Do not clamp on whole wire 01.png

2. If using RPICT7V1 or RPICT4V3 or RPICT3V1 make sure the voltage sensor is plugged in and voltage reading is correct. Although even if the voltage sensor is not connected it is still possible to read Irms.

3. Make sure the load applied is sufficient and adapted. If using a 100A range board then a load of minimum 100W should be applied to be clearly visible. Anything in the vicinity of 25 watt would not stand out much from noise (with a 100A units).

4 - 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.

Also when power is near zero you will then just read the noise which might be randomly positive or negative.

5 - Do I need to use a Voltage sensor?

Yes if you need to do more accurate readings on power.

Yes if you are measuring for current direction (e.g. import/export in solar panel).

For any other readings using an estimated voltage (RPICT8, RPICT3T1) is usually sufficient.

6 - Can I use a voltage output CT with RPICT series?

The vast majority of RPICT series only support current output CT's. The only units able to support voltage output CT's are RPICT7V1 RPICT8 and RPICT4V3 in version 4.

For information the SCT-013-000 is current output. All other SCT-013-030 SCT-013-005 and other SCT-013-0xx are all voltage output.

If you wish to read a range of 30A we recommend to use the SCT-013-000 with a 30A rated RPICT board.

"...but the sct-013-000 is for 100A. Is precision lost here?"
These are 2 different specifications.
The rating given by the manufacturer for the SCT-013-000 tells where the sensor provide a reading in spec with the datasheet. In this case if going beyond 100A this sensor will not respond linearly.
The rating indicated for the RPICT is the range on which we scale the ADC. In other terms we have 1024 or 4096 points of resolution and we distribute these resolution points over the chosen rating. The 30A rated board will have 4096 resolution points from 0 to 30A.

7 - Which pins are used on RPICT series?

Rpi gpio 01.png

The Raspberrypi pins used by the RPICT board 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)

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

If you 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.

To find out which program uses the serial port use the command below

fuser -v /dev/ttyAMA0

9 - The board is being reset every x seconds

This is commonly due to something using GPIO4 on the Raspberrypi. This could be both a physical connection or a software action.

  • Make sure there is no hardware connected to GPIO4.
  • Make sure there is no software using GPIO4.

Some unit have 1-wire enable which is using GPIO4. You can disable it with the raspi-config command. Choose option 5 Interfacing options. Then P7 1-Wire. Then disable it.

If you do need to use 1-Wire along side RPICT then you might have to remove a resistor on the RPICT to allow that. Note that you will loose the auto-reset feature.

10 - The cat command outputs nothing

  • First of all make sure you have religiously followed all steps in the Raspberrypi first setup.
  • Did you connect the RPICT board to an emonpi image at any point? See here
  • If this happen it might be a good idea to restore the default configuration with the command.

11 - Why do I get nan's?

The 'nan' in the output indicates that a channel has reached the over limit. This could be caused by

  • The current is higher than the rating of the RPICT board.
  • A voltage port has been left unconnected. There are three possible options here.
    • Connect the voltage sensor if you have it.
    • Or remove any channels involved with this voltage port from the configuration.
    • Or short circuit the voltage input to keep tight at zero.
  • The system is placed in a noisy environment and spikes are triggering the over limit.
    • Try to isolate the Raspberrypi/RPICT stack.
  • If measuring the output of an inverter you will encounter heavy harmonic content. This results in the waveform requiring a much larger range than without harmonics. Under these conditions it is possible to see a 80A current triggering nan's on a 100A rated board. Do take into consideration this fact when choosing the rating of the RPICT board.

Note that once a channel as triggered an over limit all other channels involved in the same computation node will also indicate nan.
For example if you have configured an active power with CT1 and V1. If CT1 triggers an over limit then not only Irms will show nan but also Vrms RealPower ApparentPower ReactivePower and PowerFactor of the same node.

Issues with nan' appearing too often can be diagnosed by transforming the RPICT into a scope. See Transform_a_RPICT_into_a_web_scope.

12 - Can I change the rating of my RPICT unit?

No. Not really. The rating is a hardware setup and can not be changed from software configuration.
Please make sure you order the correct rating at purchase time.
Every board is sold fixed for a given sensor and given range of reading. For example SCT-013-000 in 100A.

Maybe Yes. However it might be possible if you are able to solder/desolder smd resistors.
Note that we will refuse returns for unit that have been modified using the following steps below.
Resistors are 0805 type and located close to the jack socket port.
The required value of the resistor can be obtained from these links below.

  • For RPICT3T1 RPICT3V1 RPICT4T4v2.5 and also RPICT8/RPICT7V1/RPICT4V3 in version 3

Gen3 Passive Component Setup

  • For RPICT7V1 RPICT8 and RPICT4V3 in version 4

Gen4 Passive Component Setup

  • For RPICT7V1 RPICT8 RPICT4V3 RPICT4T4 in version 5

Gen5 Passive Component Setup
After changing the resistor the calibration value in kcal must be changed also.