So I finally have both the time and the necessary hardware to start experimenting with energy monitoring. I’m starting with the CT-sensors tutorial and after some hic-ups, I think I’m finally seeing correct values (current and apparent power).
I thought I’d share my mistake here so that others might avoid it (and perhaps the tutorial can be improved).
My main mistake was that I calculated the value of the burden resistor not based on the max. current of the CT (a YHDC SCT-013-000) but the maximum current that will flow through the wire around which the CT is clamped. My thinking was that since each of the three phases coming into our house is fused with a 25A fuse, so that I will never measure more than 25 Amps on any single phase. This is, of course, correct, but it was apparently not correct that I could/should use a different burden resistor because of this.
I have not quite fully understood, why this is so, but I think it has to do with the fact that the CT produces 50mA at full primary amperage and that will, of course, not change just because I never measure such high currents. It seems that my thinking-mistake was that in order to increase my measurement accuracy, I need to get a CT whose max current is closer to my max current. But then again, isn’t the accuracy already maxed out by following the turtorial, i.e. by using a 33 or 35 ohm resistor?
I think what confused me in the tutorial is that it speaks of “max. primary current” and not “max. CT clamp current”. Also, it doesn’t mention the 50 mA max. secondary current of the CT at all even though this seems to be really what the calculation of the burden resistor is about, right? I now realize that the 50 mA is indirectly represented via the 2000 CT turns, but doesn’t this complicate things more than necessary? Who cares how may turns the thing has if we know that it outputs a maximum of 50mA? Or maybe I’m just looking at it from the wrong angle?
Anyway, while I’m at it I might as well mention another little mistake I made: In
Burden Resistor (ohms) = (AREF * CT TURNS) / (2√2 * max primary current)
I misread 2√2 as the square root of two, rather than two times the square root of two. I think it would be more consistent with the notation used if this were written as 2 * √2 instead.
Oh, and another question: I created my 33 Ohm burden resister with three 100 Ohm resistors in parallel. Is there any disadvantage with this? To me it seems like the obvious solution but parallel resistors are not even mentioned in the turtorial, so I’m wondering whether there is a reason for this.