CT output current and maximum input

Hello, I have two questions related to ordering custom CTs.

  1. For the output current, is 50mA suitable?
  • Manufacturers like Echun produce a range of outputs from 20mA - 5A. See this echun example.
  • Application is I need to order some “special” CTs for busbars, etc.
  1. What happens if I exceed the rated input?
  • So I clip a 600A CT (say 50mA output) and it gets 900A, what happens? Maybe it outputs 75mA? So we get a voltage V=75mA * 20 ohms=1.5V? Does it break the ADC or ESP8266?
  • An application example: I have my 600A CT on a 1200A cable but never expect >600A and don’t need a valid reading >600A. Can I use a 600A CT?

Cheers, Brett

Yes, the inputs are designed for 0-50mA RMS.

I can’t speak for what happens to the CT. Probably OK but that’s a question for the manufacturer.

For IoTaWatt, the working limit should be 117%, or 58mA RMS. That’s within specs and should measure completely within range.

The safe electrical limit, per the component data sheets, is about 160%. That’s based on the ADC absolute limits of Vdd (3.3VDC) +/- 0.6V or 4.5VDC. 4.5VDC = 3.18VAC RMS. Because we read as DC with a bias, the value is half or 1.59VAC RMS / 20 Ohms = ~80mA RMS or 160% of 50mA RMS.

When you go above that, the excess voltage goes to the power rails and could begin to cause trouble if excessive and long in duration, but transient spikes would probably not be an issue.

The TVS diodes will protect the ADC somewhat as they start to conduct at 0.5V, ahead of the ADC which goes to the rails at 0.6V. Although beyond the range of this problem, the TVS diodes will also short beginning at 6V and fully saturate at 10V.

I do have customers using Echun 800A and 1200A CTs with IoTaWatt.

Thanks Bob, most helpful.