Installing a 3rd party 3 phase meter

piggyitm · Sep 13, 2011

Hi has anyone installed their own 3 phase meter into a metal clad board?I have a customer that wants to keep an eye on the consumption of a unit that he has let out.It's wired in swa from the board so I'm trying to find something with CT coils wired back to a display that's either cut in to the board lid or bushed out to a separate enclosure.I saw one at RS-online and it was £440.00.Is there any other method anyone can think of?

Cheers

spark 68 · Sep 13, 2011

Hi,
What size is the supply in Amps per phase ?, as I believe anything over a 100A per Phase requires the use of CT's.

Iam sure you can buy multi-function meters (Nemo or Socomec) and suitable CT's cheaper than £400.

Knobhead · Sep 14, 2011

Just Google polyphase meter

UKMeterman · Sep 29, 2011

Hi,
The Nemo 96 can be a interesting bit of kit for this job. Has anyone got experiance of the THD measument using the NEMO 96 I have had some strange readings.

spark 68 · Sep 29, 2011

Has anyone got experiance of the THD measument using the NEMO 96 I have had some strange readings.

In what way ?

Is it real distortion ? , you may need an oscilloscope or analyser to see this.

Depending on the meter and which configuration you are using, some require the S2's to be earthed and some don't (in fact with some Nemo meters, in some configurations it says do not earth the S2's).
Also depending on the meter some can correct, automatically for mis-connected/phased CT's, but can affect the accuracy of the measurements.
The CT wiring should be at least 2.5mm
Are the CT's big enough current wise ? (if they are saturating) and is the meter set with the correct CT ratio ?

If it is real distortion,it could be down to many things, like unbalanced loads, or highly inductive/capacitive loads, if it is the latter then PFC correction, or active harmonic filtering can help, but this subject really is outside of my payscale, someone like Tony or E54 may be the better people to ask.

UKMeterman · Sep 29, 2011

Hi,
I am using a 15:5 ct and using the unit single phase to measure single appliances, or small groups of appliances. One instance was measureing the power to a 700W audio power amp on and off load. The power amp had a linear power supply, this causes the display to alternate 98%, 99% and 0%. I also tried it with a desktop PC, one PC got an figure of 0%. Other PCs I have seen distortion about 68%.

Thanks

spark 68 · Sep 29, 2011

So it's only a baby Ct then,

On linear PSU's you will have a large inductive load (the transformer), and then a bridge rectifier followed by a large capacitor, so this probably will cause some distortion.

With the computer PSU, these are Switched mode PSU's and so will have a bridge rectifier followed by a large capacitor, so again this will distort the mains waveform.

Some SMPSU's are better than others distortion and electrical noise wise, despite CE marking, and more modern types may have active PFC fitted as standard.

This would suggest your meter is correctly monitoring the THD, it may be that the meter is relatively slow at updating the THD display,and nothing to unduly worry about.

Cheers.