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Question about SCT probes

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Hello guys, please forgive me if my question is not meant to be here. I’ve read a lot about SCT-013-000 probes and I happened to have a SCT-013-030 (the 30A model with the burden resistor built in) and all seems to work fine except that I see around 4-5 watts at no load. I am wondering if this is normal. Also, what would happen if I buy a SCT-013-015 probe? I assume it would have greater sensitivity so I should even be able to see just 2-3 watts at no load? Not that this silly problem is very severe to me, I’m just trying to understand how it works :slight_smile: Thanks!

I think you mean the SCT-013-000 current transformer.

There are many factors that affect the reading you see when here is no load.

Very much depends on what you are using to convert the output of your current transformer to power. Also, You do not say what you are using. Experience tells us that the analogue input circuit built on a “breadboard” is likely to show a higher unwanted current than a carefully designed custom printed circuit board, for example.

The second factor is the quality of the power supply that you use. Unless you have a clean pure direct current supply, any electrical noise can make its way into the analogue to digital converter and appear as current.

The third factor is the mathematics. If you are calculating the rms average current and then multiplying that by a number for the voltage, the act of squaring the current values is equivalent to rectification, and that will automatically add any electrical noise to the current; whereas is you are measuring both instantaneous voltage and instantaneous current, multiplying the two to give instantaneous power and then averaging to give you the average real power, then the unwanted “noise power” will be much lower.

You are seeing 5 W in 7 kW - that is less than 0.1%, so I think that is already quite good.

The SCT-013-030 and the SCT-013-015 are identical except for the value of the internal burden. Changing to a SCT-013-015 c.t. will have greater sensitivity (i.e. the same current will give more output voltage because of the higher burden resistance), but it will hve a lower maximum current of course, and it will have bigger errors. If that is acceptable to you, then change to that one.