Question regarding precision of CTs (SCT006)


#21

Since I was also curious I did another test with the hot air gun on the second level and the waveform indead looks much better (I also calibrated my scope, doh):

I also checked the reported power in the Iotawatt and my power meter and I would say it was reporting inside an acceptable margin of error. I also hooked up the hot air gun on level 1 with the SCT006 and about 10 Ohm resistance but it did not really look different to the 30 ohm (speaking about the shape of the waveform).

Now I am wondering how common are such kind of appliances? I want to have a fairly accurate system but I am planning to do per room measurements with the SCT006 in a 3-phase system with 3 SCT013 on the main phases coming in in addition. So from the findings here the measurements of the main phases coming in should be quite ok, but depending on the kind of load the per-room measurements may not. The question is how likely is it to see this type of load as the hot air gun. I am not keen to buy a complete new set of CTs, but seeing an error of over 50% on the SCT006 does have a bitter taste to it.

@Giraffe: nice findings, I dont get everything, especially in the first one, but I clearly see the diode in the second one, in line with the lower power mode.


#22

Based on your measurements you heat gun will have a different but similar circuit. I expect the diode is in series for the low setting then switched out on high.

The little current spike in the off cycle is caused by some inductance from the motor. The spike will have lots of high frequency components, the higher sampling rate and bandwidth of the scope will capture these but not the Iotawatt.


#23

This is a very timely thread for me, I was about to order a whole bunch of SCT006s for monitoring my low power circuits.

Are these still the best choice for 50Hz, <10A circuits? Perhaps sticking with the SCT013 is the way to go even though it would only use <10% of their range?


#24

I guess it depends on the type of load you have if the SCT006 are suitable to use or not. If you have loads that may behave similar to the one described in this thread you better look for alternatives. I am stil not sure if I want to change to other CTs for my installation. I would love to have a look at some solid core CTs, but it seems like I have to source them from china again which will take over a month again. I would also prefer to do some testing with them before I buy a whole bunch of them, but that would delay the project even further.

Does someone have access to a similar load as my hot air gun and owns e.g. the DL-CT08CL2? It also seems to be hard to find the DL-CT08CL2, the DL-CT08CL5 are much more common. What is the difference between these, in a hurry I could not find any data sheets on these.


#25

The unusual characteristics of this load is just that, unusual and not typical of almost any other appliance.

You say you want to measure each room. How will you do this ? Usually lights and power and other items like oven for example are on different circuits and further each circuit runs from the switchboard to each outlet in a daisy chain fashion all over the house so there is nowhere to actually measure each room.


#26

Another name for the diode that chops off half of the wave is rectifier, and another way to look at the resulting signal from the CT is to call it DC current. There is no “alternating” as in AC, but rather pulsating DC, as in pulse width modulation (PWM). It’s not even clear that the SCT013 or the plug in meter measure this accurately, only that they agree with each other.


#27

Half-wave rectification circuits are fairly rare. I have one on my kitchen vent fan light, which is fairly common. But they generally aren’t used that much, because they make lousy dimmers due to the flickering. But, many dimmers do something similar. They are just more symmetric.

Many modern power supplies, which are used in almost everything, have somewhat unusual current waveforms and will cause some inaccuracy. But, the question is how important is it? What are you going to do with the data?

Unless (and even if) you are willing to pay a lot of money, there will be errors in your measurements.


#28

This topic is well timed for me as I just purchased a bare IoTaWatt and am looking for current sensors.
HWCT004 was mentioned a bit further up and I don’t mind popping the line off each breaker to install. Are these accurate over a wide range? (I want to monitor my washing machine to alert when it’s finished, so I’ll want to be watching various levels down to almost off. )
Maybe accurate isn’t quite the right word, more like can it sense a very low (a few watts) current?


#29

I’ve had HWCT004’s in my main panel for over a year now on several circuits, including my heat-pump which I’ve tracked for a year now against a revenue grate digital meter and within 0.5%. Now that the heating season is over, the jeat-pump draws about 11 watts on standby:

That said, “a few watts” is very difficult to separate from the noise, and there are other challenges to accuracy at very low current. I think 5-10 watts at 120V, or 10-20 watts at 230V would be safe. IoTaWatt will report zero if it senses 0-2 watts. Many electronic meters will not record less than 10 watts.


#30

Thank you, those should be just fine, can’t imagine the washer taking less than 10 watts even if it’s just sitting there thinking about the laundry.

What’s the upper end while still accurate for the HWCT004?
I’m mostly going to be monitoring normal appliances or the dryer but also want to keep an eye on the AC unit and the 80-ish amp furnace.
HWCT004 for most and SCT013 for the big amp units?
(will also need something for the mains, the house has a 200 Amp service)


#31

They maintain their accuracy until saturation, which occurs near 50 Amps at 60Hz, which is what I think you have. Here is the test data:


You can see that linearity is very good. Phase shift is also very small. All that said, they are not UL Listed and the insulation on the leads is not code, so FYI.


#32

Looks good for almost everything in the house.
Now, the HWCT004 doesn’t have any protection whatsoever, yes? Just a coil?
When I wire these up to the 3.5mm jacks, should I include something? Resistor or diode?
There a handy howto around? :slight_smile:


#33

That’s right - no diode to protect against open circuit. So you would need to add a TVS diode or short the connector.

If you are in the USA, I have echun ECOL09 UL Listed solid core CTs with TVS diodes, 1.5m cords and 3.5mm jacks 9mm hole. They are essentially the same specs as the HWCT004 at 800 turns ratio and rated for 50A. They are $8 plus shipping ($7.20 USPS flat rate box any qty).

Also have
echun ECS10-50 which is a 1000 turns ratio split core 50A 10mm opening - $9.00
echun ECS16-100 2000 turns ratio, split core 100A 16mm opening $11.50 (equiv to SCT013-000)

When you add up the cost of the HWCT-004, leads, TV diodes, 3.5mm jacks, these may start to look like a reasonable way to go.


#34

How much for USPS to Canada? (R3E2L8)
I’m thinking I’ll need 8xECOL09 and 4xECS16

Regarding actually installing… I’ve seen folk stuffing the monitoring unit inside the breaker box (not great for the wifi), just bolted to the wall beside with the sensor wires running through a hole in the breaker box, and a plastic electrical box bolted beside with plastic conduit.
Is this one of those “depends where you are” things or a bit of lack of regulation for such things?


#35

Sorry, I can’t do business in Canada yet. Happy to ship anywhere you like in the USA.

The local codes, interpretation, and enforcement can vary. I can only recommend that you have a local electrician install the CTs.


#36

@overeasy I’d be interested in some of the ECOL09 UL listed there, but can’t seem to PM you (probably cause I’m a new user… Have come from openenergymonitor :slight_smile:)


#37

You should be able to PM now.


#38

Where is the link to the store?
Are you still using the one in OEM or something else?
I think I want to purchase some Cats.


#39

Be another couple weeks for an online store. Sales will be restricted to US for now. OEM is still selling IoTaWatt but not the echun CTs. Not sure if that’s what you meant by Cats. I’ve been accepting orders for shipment to US via PM. I invoice via PayPal and ship USPS.


#40

Janecker, I compared readings from a PZCT-02, SCT013-000 and DL-CT08CL5 on the same 120v/15 amp circuit (all at the same time). I found with the CL-CT08CL5 I can select the CL-CT08CL2 CT as the CT type, but had to “2x” the reading for it to match the reading of the other two CTs.

DL-CT08CL5 = 20A/10mA 2000/1