EmonVs show input voltage present when there is Zero Volts?

I am just in the process of setting my energy monitoring system up.
EmonPiCMS with a emonTx and EmonVs…
Now the interesting thing I have Is I supply my EmonCMS and EmonTx from a UPS supply,… that is always available ( I have Low voltage supply for my internet hardware and share this with my EmonPiCMS and EmonTx4,… Not the EmonVs,… that is supplied ( currently ) from a internal house ring socket.)
Now on Sunday I was doing some CT config stuff within the consumer unit, (reversing one of the CTs etc). So for safety I turned off House power at the consumer unit,… ( killing all sockets within the house , but EmonCMS/Tx4 continue on UPS supply, and EmonVs was connected to a dead socket).

When power was restored I was looking a Vrms for EmonTx V4,… and noticed this was changing state during the Off period. as shown in attached Picture.
Am I making some incorrect assumptions, about how my voltage is referenced,… and how do I force EmonPiCMS use the TX4 voltage measurement rather than the EmonPicms voltage measurement,… which is no longer used.?
Your slightly Baffled…

From those snippets:
Is the emonTx and the emonTx4 one device or two?
Does the emonTx4 get its power from the emonVs or from “Low voltage supply” (which is what? 5 V ?)

I first suggest you check that you didn’t have a backfeed from your UPS into the rest of the house wiring. It might seem unlikely, but it has to be considered - judging by the picture of the voltage, it looks very much like there was an intermittent connection as you were rummaging around in the consumer unit. If you do have a backfeed, it is potentially dangerous and must be rectified promptly.

Do you mean “use” or “show”?
Neither the emonPi nor the emonTx (any version) can use a voltage value, as a number from an external source, for their real power calculations, a sample of the actual voltage must be available to each unit individually. The voltage value is only useful for apparent power calculation and for indication and recording.

Is the emonTx and the emonTx4 one device or two?

Yes this is one device sorry for confusion, 1x EmonTx V4 with 6 CT inputs powered directly via usb-C/ comms from RaspberryPiCMS,… which in turn gets its supply from a UPS unit.

check that you didn’t have a backfeed from your UPS into the rest of the house

Yes I understand where you are coming from as backfeeds are very dangerous to oneself as well as DNO work force when there is a real power failure. This unit has no ability or capability to backfeed the mains,… as the ‘UPS’ runs from a mains input to 24v DC output SMPS,… or a 12V battery ( under mains supply failures). Back feeding 24v to the frame power supply is most unlikely,… and I don’t see how a 24V DC SMPS could work in reverse.
Not to mention I didn’t get any ‘mains belts’, or tingles. as the graphs might infer.

And to clarify the EmonVs is powered from the House ring circuit,… so will loose ( or should loose all power), when the consumer unit is switched off.

force EmonPiCMS use the TX4 voltage measurement

I was hoping the system would use the EmonVs voltage measurements,… as I believe these to be more accurate than the EmonPiCMS, with external AC transformer. or am I missing something. Or do I need to use the external 9vac transformer for EmonPiCMS and power measurements.

There was absolutely no indication above that this was the case, and it was not outputting mains voltage.

Neither do I.

The only explanation I have here is something isn’t as you think it is.

Clearly you need to re-read what I wrote above. The emonPi isn’t at present capable of accepting the voltage sample from the emonVs, because of the different input circuitry. This might happen at some future date - but not without a fairly substantial redesign of the emonPi. You do need to use the Ideal a.c. adapter with the emonPi if you require accurate real power measurements.

If you don’t have a 9 V a.c. voltage input to your emonPi, it will automatically assume a fixed mains voltage (probably 230 V, but check) and use the measured current to show the apparent power. You can, in the emonCMS Input processing, divide that “power” by the voltage to give the current, then multiply by the emonTx4 voltage to give you a hopefully slightly more accurate value of apparent power. You however cannot have the real power, which is what your supplier measures and you pay for.

Many Tx for the feedback Robert,… OK I take onboard your comments about the voltage measurement,… I was hoping,… that the more accurate measurements for the voltage from the Emontx V4 could be shared,… but wishful thinking,… I do have a 9vac transformer that I have been using,… I was just hoping to retire it,… and save a small amount of power.

Regarding my apparent strange voltage measurements,… whilst power is off,… nothing definitive,… but it did occur to me that I had been modifying rings, fitting/reversing CTs (space within my consumer unit is tight to say the least, not to mention a wiring mess from the late 1980 where electricians were not a ‘tidy’ as they are today)… etc,… and disconnecting ‘the voltage measurement supply ring’ from the dead bus bar, (which has all other house loads connected),… so with that in mind this ring was effectively floating,… as this ring circuit is only lightly loaded,… I believe it may be that the ring maybe be floating up through some induced voltage, and with no serious resistive loads on to ‘ground it’, it simply floats to 240v or so,…

It would be nice to verify this for science you might say,… but I do not feel inclined to mess with the consumer unit unnecessarily,… ( if it ain’t broken don’t fix it ), and as I said,… its a bit of a rats nest inside my consumer unit to say the least, with little spare cable length should a cable break.

One final question/point,… I have reconnected the 9Volt ac transformer,… but as yet the emonpi does not see it?,… do I need to reboot, or can I force a rescan of inputs some other way?

You will need to do a complete, controlled, power-down. Rebooting the Pi does NOT reboot the “emon” front end processor. After power-up, it will see the a.c. as the front end restarts.

You could if you have my “emonPiCM” software, but it’s never been officially adopted - so you’re almost guaranteed not to be running it.

Insulated splice or butt crimps are a good answer to that.

Tx Robert,… ( was thinking ‘Reboot’,… and assumed this would reinitialise input board,… )
But did a shutdown,… and pulled power plug,… count to 10,… and powered back up.
I now have two voltage measurements,… and not surprisingly there different,…

I assume the emonVs gives a more accurate measurement,… as this does tie closer to DVM measurement, ( yes appreciate DVM has not been recently calibrated ).
for reference:-
emonVs:- 239Volts
emonPi:- 231Volts
DVM:- 238Volts

To this end I have added a 1.03x Uplift on the emonPi voltage,… to closer reflect what I believe is the actual voltage…
But many tx guys

That is exactly the voltage tolerance on the UK a.c. adapter - ±3%, so when you add in the tolerances of the emonPi, it was well within what you should expect.

Ah,… ok,… that’s nice to know… Tx…